“In your absense”, I didn’t know Elvis Had Left The Building ;^)
In post #5, 19th April 2011, I wrote:
“If you compare the graphs of the 12 Pi to a JBL SRX 728 ( 98 dB one watt one meter at around 80 Hz) you see the 728 actually puts out more level at 50 Hz.
You could put almost two of the 728 in the same truck space as the 12 Pi, and two 728 would give another 6 dB over one.
My single 18" Keystone cabinet, 45” x 26.5” x 22.5”, about exactly half the volume of the 12 Pi, is very close to the same sensitivity and frequency response of the JBL SRX 728, the 728 has a bit more output below 37 Hz, the Keystone a bit more above 80 Hz.”
I also posted a Smaart graph showing the dual Lab 12 ported, the BC18SW125-4 loaded Keystone, and a JBL SRX 728 all using the same 25/125 BW 24 filters and drive level.
Also posted was one of your tests which show the 12Pi and a JBL SRX 728 to be quite close in output. The 728, being a standard, commercially available sub provides a good benchmark for comparison.
I also wrote:
“As far as the reduction in distortion due to push pull, a brief look at the 12 Pi distortion compared to the Lab 12s in ported or TH cabinets shows that the 12 Pi has perhaps a bit more distortion than the normally mounted ported 12”, and a bit less than the normally mounted 12” in the TH.”
After closer inspection, yesterday, that statement is incorrect, except at 30 Hz.
I had not taken the time converting the LMS distortion line into % distortion.
I regret the error, but the rest of the post still stands, and as you can see if you read it through, I suggested the BC18SW125-4 loaded Keystone to be similar in output to the 12Pi. The Smaart graph clearly shows the 2x12 ported box to be much lower output.
The extender makes the Keystone sub 3 dB more sensitive.
Cheers,
Art Welter
If you are referencing the measurements made at the 2006 Prosound Shootout, I must say you are really reaching here. The chart shows a little sliver of less than a decibel over the span of maybe 5Hz where the response curve of a single 12Pi hornsub dips ever so slightly below the JBL box. Over the rest of the range, the 12Pi is quite a bit higher. And once you move to duals, the difference is significant. Get to larger blocks, like quads or more, and the difference is huge. That's all shown in the data, yet you choose not to mention that.
You know, I kind of hate arguing against the 728 because I like it a lot. It's great for medium sized venues. But in larger arenas or outdoors, it just can't keep up with a group of real basshorns.
At any rate, I'll talk to the track owner this week or next, and try to lock in a date in October when the track has a weekday opening. We usually do weekdays, because people most often do gigs on the weekends and need to be back. Besides, even in October, the track is still in use for racing on the weekends. October is nice because it has cooled a little bit, generally around ~70° in the afternoon.
I look forward to meeting you there, honestly. We'll raise a glass to our creations, no matter how it goes.
do both subs move in the same direction together ? with 1 wired out of phase?
or towards/away from each other ? (makes more sense to me)
after looking at the plans/photos a few times, im seeing 2 separate boxes with a driver in each, the coupling of output seems to be at the beginning of the horn, one driver outputting from the front and the other from the rear of the cone
but i might be wrong?
or towards/away from each other ? (makes more sense to me)
after looking at the plans/photos a few times, im seeing 2 separate boxes with a driver in each, the coupling of output seems to be at the beginning of the horn, one driver outputting from the front and the other from the rear of the cone
but i might be wrong?
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The two drivers are wired so their pressures are additive, i.e. acoustically in-phase. This means they are wired with one driver red terminal connected positive and the other driver black terminal connected positive. Since one is facing the throat and the other has the magnet side to the throat, they both generate pressure in the same phase.
The idea is that where one driver is slightly "weaker" because of electro-mechanical asymmetries, the other is slightly "stronger". Of course, the front and back chamber sizes have to be matched, so acoustic asymmetries aren't introduced, which would reduce the effectiveness of the push-pull distortion cancelling mechanism.
The idea is that where one driver is slightly "weaker" because of electro-mechanical asymmetries, the other is slightly "stronger". Of course, the front and back chamber sizes have to be matched, so acoustic asymmetries aren't introduced, which would reduce the effectiveness of the push-pull distortion cancelling mechanism.
There is a board separating them. They each have (same size) separate front chambers that drive individual throat orifices in apex of the horn. They also have separate (same size) rear chambers. One has its front chamber on the magnet side, the other has its front chamber on the cone side.
Both magnets face outward out so their cooling plugs attach the access plate for a heat sink. At full tilt, the plug is very hot, as is the center of the plate. But by half the plate diameter, the heat has dissipated, even if there is no gap between the plate and anything that might be beside it.
Both magnets face outward out so their cooling plugs attach the access plate for a heat sink. At full tilt, the plug is very hot, as is the center of the plate. But by half the plate diameter, the heat has dissipated, even if there is no gap between the plate and anything that might be beside it.
No driver will break mechanically over a couple of millimetres beyond a 9.6mm Xmax (and that is still way below Xmech). However, they can easily burn at even a couple of millimetres excursion (and that is way below Xmax) at the point where excursion is minimum in a tapped horn.
On Wayne’s website you can find almost everything in detail how that works. Earlier his method was described as unrealistic since sine-waves don’t compare to complex music signals. True but it would result in the same conclusion if he would run noise over longer periods of time. The reason is simple; heat builds up over long time and don’t disappear in 60seconds, even if the signal completely disappears that long. That means over a long period’s even music signals can drive a motor into critical temperature ranges.
Now, you can border yourself over a couple of mm at Xmax but that does not harm anything (except for crappie or unbalanced drivers) but realise that excursion is the best method for releasing heat from the motor! High excursion means lots of air movement in and out the motor system which is still the most effective way to cool down your motor (besides modifications)
But the reality is even you feed them up to their AES power rating and prediction says it will even pass Xmax, it won’t. That’s where power compression comes in and THD is showing its effects. If you push the driver beyond that point on continues base, the driver won’t break either but VC does burn. That’s why you need to keep the driver under its rated AES power.
Just a few rules you need to consider; the Fs of the driver must be below the Fb of the system and use specialised (extended) LF drivers since they are designed for the task.
For Wayne; I do appreciate all your efforts in your dedicated research and willingness to publish all findings openly but from a commercial point of view you sure know how to pick a lousy driver.
Thank you for taking the time to read and digest my work. And also, thanks for your comments about it.
To expound on what Djim said, when a driver fails, the failure mode is almost always thermal. That's not to say mechanical failures aren't possible, of course they are, especially if a subwoofer is driven hard at frequencies well below what it is designed to reproduce. But mechanical failures are preceded by noisy interference and massive distortion. This problem announces itself, usually giving the operator time to react and save the driver.
Nothing tells you when thermal failure is imminent. A driver quietly enters the conditions that causes it to overheat. You'll never know its dying until it happens. Once the voice coil adhesive breaks down from heat, it's permanently weakened, and on the road to destruction. The motor gets hot, weakens the adhesive, and the coil unwinds or deforms. At that point, the driver is irreversibly damaged, and must be reconed or replaced. Sometimes, the coil won't open in this condition, but the rubbing will cause it to buzz, eventually wearing through or sometimes an unwound coil will get caught on something in or near the gap, and will break open. Sometimes, a localized hot spot in the coil will cause it to fuse open. This is less common that adhesive failure, but I see it when high-power high-frequency energy causes the failure, because the excursion is so small. The cooling vent doesn't work well at high frequencies, because of the lack of pumping action. The hot spots are usually at the edge of the coil, or sometimes (like JBL SFG), in between cooling vents.
A little bit of history is in order, showing what we've learned in the industry about driver failure modes. If you look at drivers made before the 1970s, you'll see lots of maximum power ratings less than 100 watts. Early on, that was fine because tube amplifier power was not all that high but as solid state amplifier power levels went up, the speakers became more vulnerable. Back then, the most common speaker failure mode was thermal.
Then manufacturers began to put vents in the magnets, which used the pumping action of the cone to pump air through the gap. This was a breakthrough, and gave an immediate increase in power handling. You began to see drivers rated over 100 watts, some that could handle a few hundred watts even. This greatly reduced the thermal stress, and inspired by that success, some manufacturers began to optimize their forced air cooling mechanisms for even greater thermal control. The speaker and its cooling vent can be thought of as sort of a lossy pump, and the size, shape and geometry can be optimized for a given frequency range. Large orifices tend to work well at low frequency, and smaller ones work better at higher frequency. This is sort of like engine tuning, where you balance velocity and volume to maximize flow. The goal is to get as much air passing through the gap by the voice coil as possible.
In the 1980s and 1990s, and still to some degree today, we see a trend towards higher excursion, higher power woofers. Prosound woofers tend to be tuned for a little less excursion, trying to optimize flux around the gap. But they're still moving more than the drivers of 30 or 40 years ago, and they definitely handle more power. But some cabinets put a lot of stress on cones. One of the design goals (requirements) of a powerful high-efficiency loudspeaker system is that it matches the (relatively high) impedance of the cone motion to the (relatively low) impedance of the air motion. An example of a loudspeaker that does this very well is a horn, which presents a high impedance to the cone, and transforms the impedance by way of volume expansion to match the low impedance of the air at the mouth. So back in the day when a loudspeaker was using 50 watts, it wasn't under a lot of stress, even under the conditions of a horn. Put this same paper cone in a horn and push it ten times harder than that, and sometimes the cone will actually fold or rip.
We had entered a time in the industry where thermal failures were not the only failure mode. Cone deformation became a common failure mode in basshorn speakers, and as driver manufacturers increased (thermal) power handling faster than they increased excursion limits, direct radiating (front loader) subs often could be driven to exceed xmech, where the voice coil former strikes the back plate or the spider or surround tears.
The focus shifted away from thermal limits towards mechanical limits. Better cone materials were developed that could handle horn loading. Excursion limits were increased, which allows deeper, more powerful bass with less chance of excessive distortion. Not only does increased excursion capacity help prevent mechanical failure, it also allows subwoofers to be designed that are capable of deeper bass extension. Excursion capability is an important parameter in subwoofer design, because no matter what cabinet is used, as frequency goes down, excursion must increase to keep SPL constant. Horn enthusiasts sometimes place less emphasis on excursion because horns reduce excursion at a given frequency and SPL. But even in a horn, excursion rises as frequency drops.
As we entered the new millennium, we saw the rise of extreme excursion drivers. They trade efficiency for excursion, because they need a long coil which reduces the flux density by virtue of area. The flux in the gap cannot be concentrated in a small area, but instead must be spread out to surround a long coil. But they do offer large excursion, and since power is relatively cheap, the efficiency penalty is sometimes overlooked. Another side effect is that with lower efficiency comes higher power requirements for a target SPL. So we have begun to revisit the problems of excessive heat.
A good engineer, wanting to make his loudspeaker design produce the most clean SPL it can possibly make, will tend to choose components and configure the cabinet synergistically. The limits should be reached nearly at the same time, so that no one thing is optimized at the expense of others. It doesn't make much sense to use a super high power woofer and a dinky tweeter, for example. One will blow when the other is loafing. The undersized part will be distorting badly just before it goes. This in unbalanced system, one that just doesn't make sense. Likewise, when building a subwoofer, you don't want to focus solely on (thermal) power handling if the excursion limits the performance long before heat becomes a problem. The opposite is true too, there's no sense in using a large xmax part in a configuration that will cause excessive heat and burn it up.
Most builders will use the power handling and xmax/xmech specs provided by the driver manufacturer when designing a system. This is good practice, and can assist the loudspeaker designer to achieve a balanced, synergistic system. However, it is important to understand that these single unit values can't describe everything. It is like rating the SPL for a speaker with a single number. Without having an amplitude response curve, you don't know what SPL is at every frequency. Likewise, power handling is not a single value, but instead it is different at different frequencies, and also at different durations. It's even different with different acoustic loads.
A loudspeaker designed for bass will probably have a cooling vent that works well from just a few Hertz up through the midbass, where it starts to lose effectiveness because of the naturally occurring reduction of excursion at higher frequencies. By the midrange band, the woofer vents aren't generally doing anything at all. For a subwoofer, this may not matter but for a midwoofer, it can be an issue. What is also an issue is the duration and content of the music material. The power handling is derated as a function of time and the reciprocal of crest factor.
A speaker can handle content with a high crest factor easier than low, because it has more instantaneous energy with time in between bursts to cool down some. Conversely, when high power signals are sent for a long time, heat builds up in the magnet and pole piece, causing the local ambient temperature surrounding the voice coil to rise. Another thing to consider is the acoustic load. Cabinets that offer higher impedance to the driver (like horns) reduce their excursion, limiting the vent's cooling ability. Their increased efficiency offsets this some, but not nearly enough to prevent heat soaking at high power levels. After all, even the most efficient horn will never be able to convert all electrical energy to acoustic energy, so what remains is trapped in the motor as heat, unable to be removed by the stalled vent.
Mechanical limits are a little more simple, but even there, the single value figures xmax and xmech cannot tell the whole story. The one that is most unambiguous is xmech, which is the safe distance of cone travel, after which damage will occur. Movement past this distance causes interference, either in the form of voice coil former striking the back plate, or suspension parts (spider or surround) reaching their limits. Beyond this limit, movement causes deformation. The xmax figure though, is a little more ambiguous, because there isn't uniform agreement as to what should define it. In principal, though, it is a figure that describes the maximum excursion level where the device is most linear; Beyond which, the voice coil begins to travel out of the gap and motor strength is reduced. At this point, motor strength is rapidly reduced and cone motion becomes rapidly (symmetrically) nonlinear.
The xmax/xmech relationship gives an indication of the driver's mechanical tolerance. If xmax is considerably smaller than xmech, then it is possible that the driver cannot be driven to destruction mechanically. Once xmax is reached, the motor loses strength and may not be able to move it far enough to reach its mechanical limit. Of course, it could still be driven to the point of excessive distortion. And if the cone is unloaded, then inertia is more likely to able to carry it through to xmech, even without acceleration from the motor. In fact, since the motor has less influence on the cone past xmax, it loses electrical damping as well. The only thing that remains to damp the cone is the suspension and acoustic load. So suspension characteristics and acoustic loading influence the drivers mechanical limit, in addition to the xmax/xmech relationship.
Nothing tells you when thermal failure is imminent. A driver quietly enters the conditions that causes it to overheat. You'll never know its dying until it happens. Once the voice coil adhesive breaks down from heat, it's permanently weakened, and on the road to destruction. The motor gets hot, weakens the adhesive, and the coil unwinds or deforms. At that point, the driver is irreversibly damaged, and must be reconed or replaced. Sometimes, the coil won't open in this condition, but the rubbing will cause it to buzz, eventually wearing through or sometimes an unwound coil will get caught on something in or near the gap, and will break open. Sometimes, a localized hot spot in the coil will cause it to fuse open. This is less common that adhesive failure, but I see it when high-power high-frequency energy causes the failure, because the excursion is so small. The cooling vent doesn't work well at high frequencies, because of the lack of pumping action. The hot spots are usually at the edge of the coil, or sometimes (like JBL SFG), in between cooling vents.
A little bit of history is in order, showing what we've learned in the industry about driver failure modes. If you look at drivers made before the 1970s, you'll see lots of maximum power ratings less than 100 watts. Early on, that was fine because tube amplifier power was not all that high but as solid state amplifier power levels went up, the speakers became more vulnerable. Back then, the most common speaker failure mode was thermal.
Then manufacturers began to put vents in the magnets, which used the pumping action of the cone to pump air through the gap. This was a breakthrough, and gave an immediate increase in power handling. You began to see drivers rated over 100 watts, some that could handle a few hundred watts even. This greatly reduced the thermal stress, and inspired by that success, some manufacturers began to optimize their forced air cooling mechanisms for even greater thermal control. The speaker and its cooling vent can be thought of as sort of a lossy pump, and the size, shape and geometry can be optimized for a given frequency range. Large orifices tend to work well at low frequency, and smaller ones work better at higher frequency. This is sort of like engine tuning, where you balance velocity and volume to maximize flow. The goal is to get as much air passing through the gap by the voice coil as possible.
In the 1980s and 1990s, and still to some degree today, we see a trend towards higher excursion, higher power woofers. Prosound woofers tend to be tuned for a little less excursion, trying to optimize flux around the gap. But they're still moving more than the drivers of 30 or 40 years ago, and they definitely handle more power. But some cabinets put a lot of stress on cones. One of the design goals (requirements) of a powerful high-efficiency loudspeaker system is that it matches the (relatively high) impedance of the cone motion to the (relatively low) impedance of the air motion. An example of a loudspeaker that does this very well is a horn, which presents a high impedance to the cone, and transforms the impedance by way of volume expansion to match the low impedance of the air at the mouth. So back in the day when a loudspeaker was using 50 watts, it wasn't under a lot of stress, even under the conditions of a horn. Put this same paper cone in a horn and push it ten times harder than that, and sometimes the cone will actually fold or rip.
We had entered a time in the industry where thermal failures were not the only failure mode. Cone deformation became a common failure mode in basshorn speakers, and as driver manufacturers increased (thermal) power handling faster than they increased excursion limits, direct radiating (front loader) subs often could be driven to exceed xmech, where the voice coil former strikes the back plate or the spider or surround tears.
The focus shifted away from thermal limits towards mechanical limits. Better cone materials were developed that could handle horn loading. Excursion limits were increased, which allows deeper, more powerful bass with less chance of excessive distortion. Not only does increased excursion capacity help prevent mechanical failure, it also allows subwoofers to be designed that are capable of deeper bass extension. Excursion capability is an important parameter in subwoofer design, because no matter what cabinet is used, as frequency goes down, excursion must increase to keep SPL constant. Horn enthusiasts sometimes place less emphasis on excursion because horns reduce excursion at a given frequency and SPL. But even in a horn, excursion rises as frequency drops.
As we entered the new millennium, we saw the rise of extreme excursion drivers. They trade efficiency for excursion, because they need a long coil which reduces the flux density by virtue of area. The flux in the gap cannot be concentrated in a small area, but instead must be spread out to surround a long coil. But they do offer large excursion, and since power is relatively cheap, the efficiency penalty is sometimes overlooked. Another side effect is that with lower efficiency comes higher power requirements for a target SPL. So we have begun to revisit the problems of excessive heat.
A good engineer, wanting to make his loudspeaker design produce the most clean SPL it can possibly make, will tend to choose components and configure the cabinet synergistically. The limits should be reached nearly at the same time, so that no one thing is optimized at the expense of others. It doesn't make much sense to use a super high power woofer and a dinky tweeter, for example. One will blow when the other is loafing. The undersized part will be distorting badly just before it goes. This in unbalanced system, one that just doesn't make sense. Likewise, when building a subwoofer, you don't want to focus solely on (thermal) power handling if the excursion limits the performance long before heat becomes a problem. The opposite is true too, there's no sense in using a large xmax part in a configuration that will cause excessive heat and burn it up.
Most builders will use the power handling and xmax/xmech specs provided by the driver manufacturer when designing a system. This is good practice, and can assist the loudspeaker designer to achieve a balanced, synergistic system. However, it is important to understand that these single unit values can't describe everything. It is like rating the SPL for a speaker with a single number. Without having an amplitude response curve, you don't know what SPL is at every frequency. Likewise, power handling is not a single value, but instead it is different at different frequencies, and also at different durations. It's even different with different acoustic loads.
A loudspeaker designed for bass will probably have a cooling vent that works well from just a few Hertz up through the midbass, where it starts to lose effectiveness because of the naturally occurring reduction of excursion at higher frequencies. By the midrange band, the woofer vents aren't generally doing anything at all. For a subwoofer, this may not matter but for a midwoofer, it can be an issue. What is also an issue is the duration and content of the music material. The power handling is derated as a function of time and the reciprocal of crest factor.
A speaker can handle content with a high crest factor easier than low, because it has more instantaneous energy with time in between bursts to cool down some. Conversely, when high power signals are sent for a long time, heat builds up in the magnet and pole piece, causing the local ambient temperature surrounding the voice coil to rise. Another thing to consider is the acoustic load. Cabinets that offer higher impedance to the driver (like horns) reduce their excursion, limiting the vent's cooling ability. Their increased efficiency offsets this some, but not nearly enough to prevent heat soaking at high power levels. After all, even the most efficient horn will never be able to convert all electrical energy to acoustic energy, so what remains is trapped in the motor as heat, unable to be removed by the stalled vent.
Mechanical limits are a little more simple, but even there, the single value figures xmax and xmech cannot tell the whole story. The one that is most unambiguous is xmech, which is the safe distance of cone travel, after which damage will occur. Movement past this distance causes interference, either in the form of voice coil former striking the back plate, or suspension parts (spider or surround) reaching their limits. Beyond this limit, movement causes deformation. The xmax figure though, is a little more ambiguous, because there isn't uniform agreement as to what should define it. In principal, though, it is a figure that describes the maximum excursion level where the device is most linear; Beyond which, the voice coil begins to travel out of the gap and motor strength is reduced. At this point, motor strength is rapidly reduced and cone motion becomes rapidly (symmetrically) nonlinear.
The xmax/xmech relationship gives an indication of the driver's mechanical tolerance. If xmax is considerably smaller than xmech, then it is possible that the driver cannot be driven to destruction mechanically. Once xmax is reached, the motor loses strength and may not be able to move it far enough to reach its mechanical limit. Of course, it could still be driven to the point of excessive distortion. And if the cone is unloaded, then inertia is more likely to able to carry it through to xmech, even without acceleration from the motor. In fact, since the motor has less influence on the cone past xmax, it loses electrical damping as well. The only thing that remains to damp the cone is the suspension and acoustic load. So suspension characteristics and acoustic loading influence the drivers mechanical limit, in addition to the xmax/xmech relationship.
I designed the 12Pi hornsub after some discussions on a hifi forum about the LABhorn. The LABhorn was initially designed as a DIY horn for prosound use with an intended passband of 30Hz to 100Hz. But as with many other things, someone proposed using it for home theater, and using EQ to boost the deepest bass to match the passband level. I responded to that saying I thought it was a really bad idea, because distortion would be through the roof. This cascaded into a fairly heated discussion about hornsub power, distortion and general bravado.
My position was that a horn tuned for 30Hz should probably only be used down to 30Hz, and that even at relatively low power levels, a 20Hz signal presented to it would create triple digit distortion. Then add to that the fact that they were going to boost the band below 30hz by 10dB+, it seemed to me to be an ugly proposition. I mean, I am a horn enthusiast, but this just wasn't a good idea to me. I even understand the spectacle of huge basshorns, how cool that is and all. I like big powerful motors and fast cars and motorcycles, all that good "guy" stuff. So I get the col factor of basshorns in the man cave. But still, my position was that direct radiating subs tuned appropriately for the band, like reflex boxes tuned to 20Hz, would be better. I had begun to study and implement the Welti multisub approach around the same time, so I felt the best approach was multiple direct radiating subs, per Welti.
The LABhorn proponents were fairly zealous, and I equally so. Neither side really relented. They said the basshorn was so powerful, that at home theater levels it would be loafing. I said that at 5 watts of passband power, the (10dB+) EQ below 30Hz would bump that range up to 50 watts, making distortion rise to the triple digits, over 100%. They said I was just a naysayer, and I should be tarred and feathered. Then they asked me a key question, which was "what would you do?"
I had already told them what I would do in a home theater, which was multiple direct radiating subs, bass-reflex probably. Each box wouldn't have the output of a hornsub, but it would be more than adequate for the environment. They would be tuned appropriately for the passband, so distortion wouldn't be bad at moderate power levels. And of course, each box is much smaller than a hornsub, easy to place and quite good for a multisub configuration.
But then the question was rephrased, and I was asked, "what would you do to improve the LABhorn?"
At first, I disregarded the question, knowing it was rhetorical. It was just another argumentative forum post from a LABhorn lover.
As days passed, though, I realized that I could actually answer this question with a new design that addressed some of the weaknesses of the LABhorn. The 12Pi hornsub is that design.
There were three areas I was focused on. First was the lumpy response when used in singles or even small groups. I've designed a lot of horns, and I knew I could reduce the big dip the LABhorn had just above cutoff with careful front and rear chamber size, and possibly by making the mouth just a little larger. Second was to use push-pull drive to reduce the distortion at very low frequencies. And third was to improve cooling. The LABhorn suffered from thermal stress, but the common conception was that failures were excursion related. I knew I could make the drivers more robust by improving their cooling.
The LABhorn guys saw this effort as an attack, and the adversarial posture of the two "sides" widened even further. Pity, because there is so much overlapping development. I actually made the cooling plugs so they would fit the LABhorn, making easy retrofit possible. I initially had planned to use an entirely different layout, but by maintaining some of the same basic shape, it allowed cross-fitting of parts.
Changing the response to make it a little smoother was relatively easy. Hornresp is a mature program, and allows easy development of basshorns. You can expect acoustic measurements of your physical model to match the Hornresp model's predictions. So I was able to sim up an improved design within a short period of time.
The push-pull drive was kind of a no-brainer too. The LABhorn used two drivers, so it seemed natural for me that they be configured in push-pull. My thoughts were that passband distortion levels of a good basshorn were already very low, but push-pull couldn't hurt. Might shave a little more distortion. Down near cutoff, where the horn starts becoming more reactive and can't load the cone as well, I thought it might help even more. And below cutoff, where distortion skyrockets, I thought push-pull might keep distortion in check. There's not a lot of VLF content in a prosound event that would cause this distortion, where it might occur, high-pass filtering is probably wise. But whatever might be down there, push-pull will reduce distortion. So if someone just has to run a basshorn below cutoff, this is the one to do it with.
The cooling system was where I put the most effort. At first, I was under the impression, like many other people, that the best way to remove the heat was with forced-air convection. My assumption was that the air in the sealed rear chambers was getting hot, causing the cooling vents to lose effectiveness. So my first efforts involved ducting the vent to a heat exchanger to remove heat from the box. My biggest design problem for that first system was to introduce unidirectional flow so I could have a hot side and a cool side, but to do it in a way that didn't create an asymmetrical load on the cone. Couldn't valve it in a way that caused any pressure differential on alternating half cycles, or in fact, anywhere in the cycle. The acoustic load of whatever ducting arrangement I might create had to be perfectly uniform at all frequencies and all displacements.
The cooling system was where I put the most effort. At first, I was under the impression, like many other people, that the best way to remove the heat was with forced-air convection. My assumption was that the air in the sealed rear chambers was getting hot, causing the cooling vents to lose effectiveness. So my first efforts involved ducting the vent to a heat exchanger to remove heat from the box. My biggest design problem for that first system was to introduce unidirectional flow so I could have a hot side and a cool side, but to do it in a way that didn't create an asymmetrical load on the cone. Couldn't valve it in a way that caused any pressure differential on alternating half cycles, or in fact, anywhere in the cycle. The acoustic load of whatever ducting arrangement I might create had to be perfectly uniform at all frequencies and all displacements.
In truth, that was the easy part. I was able to make a "valve" that succeed in introducing unidirectional flow without causing any pressure difference in any part of the cycle. It used two orifices, each allowing laminar flow through it in one direction but causing turbulence in the other direction. One orifice was oriented to flow one way, the other allowed flow in the other direction. This made unidirectional flow without any change of pressure at any part of the cycle.
So that part was done, but the problem was that the air flowing through the vents wasn't all that hot. There was no energy to remove. The bursts of air were strong, so it isn't as though the air passing through the gap was too slow. That's really the key to forced air convection cooling - airflow. And the driver has plenty of that, of course, assuming excursion isn't limited by the acoustic load. But when the driver is moving, the venting is working well, and there just isn't a lot of heat to remove.
But what was hot was the pole piece. It would get hot enough to cook on during my tests. Before I gave up on it though, I tried putting the speaker in different ambient air temperatures, to simulate the heating of the air in a small rear chamber and also going the other way, refrigerated air just to see what effect there was. Nothing. The range of ambient temperatures matters if it changes by hundreds of degrees (as I later discovered), but not if only by a couple dozen.
So that's when I focused on that super-hot pole piece. I decided to make a plug that would contact the pole piece and wick heat away. I put holes in the plug so it wouldn't restrict the cooling air vent. The improvement was striking. I tested this extensively, going through a few drivers in the process, to find out exactly what the limits were, both on the bench and in a loudspeaker system. I eventually patented this system, because it worked so well.
What happens is the voice coil magnetism creates eddy currents in the steel pole pieces, which cause direct heating. It also radiates heat into the magnet structure and some heat is transferred by convection too. The motor core, its ceramic magnet and pole pieces being a thermally insulating material surrounding a metal core, form the perfect heat storage device, just like a Thermos bottle. Even if the voice coil is being cooled from cone motion, the motor core temperature will still rise to temperatures approaching the boiling point. This makes the local ambient temperature surrounding the voice coil be very high, shifting its operating point. The situation is made even worse when the driver is used in cabinets that limit excursion, like basshorns.
In the end, the cooling plug approach proved very effective. It is fairly simple to manufacture and implement, and especially in the case of the 12Pi hornsub, fits perfectly between the magnets and access panels. The panels then serve dual purpose, being both a way to access the drivers and also as a heat sink. Tests have shown that it increases power handling over 225%, making the 12Pi hornsub significantly more robust, particularly during long high-power events.
My position was that a horn tuned for 30Hz should probably only be used down to 30Hz, and that even at relatively low power levels, a 20Hz signal presented to it would create triple digit distortion. Then add to that the fact that they were going to boost the band below 30hz by 10dB+, it seemed to me to be an ugly proposition. I mean, I am a horn enthusiast, but this just wasn't a good idea to me. I even understand the spectacle of huge basshorns, how cool that is and all. I like big powerful motors and fast cars and motorcycles, all that good "guy" stuff. So I get the col factor of basshorns in the man cave. But still, my position was that direct radiating subs tuned appropriately for the band, like reflex boxes tuned to 20Hz, would be better. I had begun to study and implement the Welti multisub approach around the same time, so I felt the best approach was multiple direct radiating subs, per Welti.
The LABhorn proponents were fairly zealous, and I equally so. Neither side really relented. They said the basshorn was so powerful, that at home theater levels it would be loafing. I said that at 5 watts of passband power, the (10dB+) EQ below 30Hz would bump that range up to 50 watts, making distortion rise to the triple digits, over 100%. They said I was just a naysayer, and I should be tarred and feathered. Then they asked me a key question, which was "what would you do?"
I had already told them what I would do in a home theater, which was multiple direct radiating subs, bass-reflex probably. Each box wouldn't have the output of a hornsub, but it would be more than adequate for the environment. They would be tuned appropriately for the passband, so distortion wouldn't be bad at moderate power levels. And of course, each box is much smaller than a hornsub, easy to place and quite good for a multisub configuration.
But then the question was rephrased, and I was asked, "what would you do to improve the LABhorn?"
At first, I disregarded the question, knowing it was rhetorical. It was just another argumentative forum post from a LABhorn lover.
As days passed, though, I realized that I could actually answer this question with a new design that addressed some of the weaknesses of the LABhorn. The 12Pi hornsub is that design.
There were three areas I was focused on. First was the lumpy response when used in singles or even small groups. I've designed a lot of horns, and I knew I could reduce the big dip the LABhorn had just above cutoff with careful front and rear chamber size, and possibly by making the mouth just a little larger. Second was to use push-pull drive to reduce the distortion at very low frequencies. And third was to improve cooling. The LABhorn suffered from thermal stress, but the common conception was that failures were excursion related. I knew I could make the drivers more robust by improving their cooling.
The LABhorn guys saw this effort as an attack, and the adversarial posture of the two "sides" widened even further. Pity, because there is so much overlapping development. I actually made the cooling plugs so they would fit the LABhorn, making easy retrofit possible. I initially had planned to use an entirely different layout, but by maintaining some of the same basic shape, it allowed cross-fitting of parts.
Changing the response to make it a little smoother was relatively easy. Hornresp is a mature program, and allows easy development of basshorns. You can expect acoustic measurements of your physical model to match the Hornresp model's predictions. So I was able to sim up an improved design within a short period of time.
The push-pull drive was kind of a no-brainer too. The LABhorn used two drivers, so it seemed natural for me that they be configured in push-pull. My thoughts were that passband distortion levels of a good basshorn were already very low, but push-pull couldn't hurt. Might shave a little more distortion. Down near cutoff, where the horn starts becoming more reactive and can't load the cone as well, I thought it might help even more. And below cutoff, where distortion skyrockets, I thought push-pull might keep distortion in check. There's not a lot of VLF content in a prosound event that would cause this distortion, where it might occur, high-pass filtering is probably wise. But whatever might be down there, push-pull will reduce distortion. So if someone just has to run a basshorn below cutoff, this is the one to do it with.
The cooling system was where I put the most effort. At first, I was under the impression, like many other people, that the best way to remove the heat was with forced-air convection. My assumption was that the air in the sealed rear chambers was getting hot, causing the cooling vents to lose effectiveness. So my first efforts involved ducting the vent to a heat exchanger to remove heat from the box. My biggest design problem for that first system was to introduce unidirectional flow so I could have a hot side and a cool side, but to do it in a way that didn't create an asymmetrical load on the cone. Couldn't valve it in a way that caused any pressure differential on alternating half cycles, or in fact, anywhere in the cycle. The acoustic load of whatever ducting arrangement I might create had to be perfectly uniform at all frequencies and all displacements.
The cooling system was where I put the most effort. At first, I was under the impression, like many other people, that the best way to remove the heat was with forced-air convection. My assumption was that the air in the sealed rear chambers was getting hot, causing the cooling vents to lose effectiveness. So my first efforts involved ducting the vent to a heat exchanger to remove heat from the box. My biggest design problem for that first system was to introduce unidirectional flow so I could have a hot side and a cool side, but to do it in a way that didn't create an asymmetrical load on the cone. Couldn't valve it in a way that caused any pressure differential on alternating half cycles, or in fact, anywhere in the cycle. The acoustic load of whatever ducting arrangement I might create had to be perfectly uniform at all frequencies and all displacements.
In truth, that was the easy part. I was able to make a "valve" that succeed in introducing unidirectional flow without causing any pressure difference in any part of the cycle. It used two orifices, each allowing laminar flow through it in one direction but causing turbulence in the other direction. One orifice was oriented to flow one way, the other allowed flow in the other direction. This made unidirectional flow without any change of pressure at any part of the cycle.
So that part was done, but the problem was that the air flowing through the vents wasn't all that hot. There was no energy to remove. The bursts of air were strong, so it isn't as though the air passing through the gap was too slow. That's really the key to forced air convection cooling - airflow. And the driver has plenty of that, of course, assuming excursion isn't limited by the acoustic load. But when the driver is moving, the venting is working well, and there just isn't a lot of heat to remove.
But what was hot was the pole piece. It would get hot enough to cook on during my tests. Before I gave up on it though, I tried putting the speaker in different ambient air temperatures, to simulate the heating of the air in a small rear chamber and also going the other way, refrigerated air just to see what effect there was. Nothing. The range of ambient temperatures matters if it changes by hundreds of degrees (as I later discovered), but not if only by a couple dozen.
So that's when I focused on that super-hot pole piece. I decided to make a plug that would contact the pole piece and wick heat away. I put holes in the plug so it wouldn't restrict the cooling air vent. The improvement was striking. I tested this extensively, going through a few drivers in the process, to find out exactly what the limits were, both on the bench and in a loudspeaker system. I eventually patented this system, because it worked so well.
What happens is the voice coil magnetism creates eddy currents in the steel pole pieces, which cause direct heating. It also radiates heat into the magnet structure and some heat is transferred by convection too. The motor core, its ceramic magnet and pole pieces being a thermally insulating material surrounding a metal core, form the perfect heat storage device, just like a Thermos bottle. Even if the voice coil is being cooled from cone motion, the motor core temperature will still rise to temperatures approaching the boiling point. This makes the local ambient temperature surrounding the voice coil be very high, shifting its operating point. The situation is made even worse when the driver is used in cabinets that limit excursion, like basshorns.
In the end, the cooling plug approach proved very effective. It is fairly simple to manufacture and implement, and especially in the case of the 12Pi hornsub, fits perfectly between the magnets and access panels. The panels then serve dual purpose, being both a way to access the drivers and also as a heat sink. Tests have shown that it increases power handling over 225%, making the 12Pi hornsub significantly more robust, particularly during long high-power events.
^ this is one of the reasons i love the www, any one of the regulars here could convince me to build some design or other, because im good with tools (saw/hammer) not math. if one of these people posts something and another *smarter* more informed, more experienced person finds a flaw in what was posted, and points out the flaws in simple (ish) language
it is me who gets to reap all the benefit of years/decades of real world experience, by building what is obviously the superior box.
i hate smoke and mirrors and ABSOLUTELY LOVE transparency (and clean bass)
thanks Wayne.....
it is me who gets to reap all the benefit of years/decades of real world experience, by building what is obviously the superior box.
i hate smoke and mirrors and ABSOLUTELY LOVE transparency (and clean bass)
thanks Wayne.....
Wayne,
Reducing heat in the motor structure using the heat exchanger cooling plug, reducing distortion using push pull, smoothing the frequency response, and lowering Fc are all good things you have implemented in the Pi12 design.
I used to like watching drag races, and see the winner of two cars that use the same 426 Hemi block, and see the incremental changes that have reduced the 1/4 mile times by almost half (maybe more, have not watched much since the days of Don Garlits) from 7+ to 4+ seconds.
Just like in drag racing, to really know how two speakers compare, they need to be tested under the same conditions, with the same test gear.
Please don’t take this with any disrespect, as I have looked at your data and it is obvious you have a passion for speakers, but I can’t seem to find any side by side measurements of a well built LAB sub and the 12Pi.
I would appreciate you posting a link or the results if that has been done, to compare how the incremental changes you have made in your design compare to the LAB sub.
Art Welter
Nobody has ever brought a LABhorn to any of the Prosound Shootout events, though they have been invited. In fact, back in the day, I offered $1000 as a wager that the LABhorn would smoke before the 12Pi did. But nobody showed.
I'm not going to build a LABhorn to test - I don't feel I need to do that. I've extensively tested the cooling plugs, both on direct radiating woofers and on horn loaded woofers. So I know what the cooling plug does. That's probably the most significant feature, which is why I patented it.
I'm not going to build a LABhorn to test - I don't feel I need to do that. I've extensively tested the cooling plugs, both on direct radiating woofers and on horn loaded woofers. So I know what the cooling plug does. That's probably the most significant feature, which is why I patented it.
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Wayne, I have read your post # 50 and it’s not I want to criticize your findings but I believe I didn’t explain my earlier comment well enough.
Quote from post # 50:
“What happens is the voice coil magnetism creates eddy currents in the steel pole pieces, which cause direct heating. It also radiates heat into the magnet structure and some heat is transferred by convection too.”
With this post (and similar on your website) you are suggesting that the Eddy Currents ( 'Werbelstrom' in German which means twister-current) will only take place in Ferro Metals and that heat in the magnet comes from external radiation.
In my view all materials that conduct electricity (also non-Ferro metals such as aluminium and copper) are able to produce Eddy Currents. Many believe that magnets do not conduct electricity but I believe there are enough metals inside to produce significant Eddy Currents. Because magnets create a magnetic field it will even amplify the Eddy Currents created by the magnetic field of the Coil. Therefore in my view heat development inside the magnet can be significant and even more in Neo magnets.
Another assumption that can be questioned is that the top-plate generates more heat then magnets. Besides it depends on the relation how much material of each, the assumption is probably based on the fact that metals do conduct heat better than magnets. The top-plate will reach its high temperature much sooner until the magnet reaches the temperature of the top-plate. Eventually the magnet will heat the top-plate instead of visa-versa.
A magnet is basically a rock (with metals inside) and rocks in general don’t conduct heat very well. When heat is finally ‘stored’ in a rock it won’t be released easy. Combine this with the effect of higher Eddy Currents in Neo magnets and it seems logical why the extra cooling fins on Neo drivers are placed near the Neo magnets. Some manufacturers use special aluminium housing around their Neo magnets with extra cooling tunnels between the Neo magnets. Besides the extra air that flows near the magnets it also enlarge the aluminium surface to absorb and conduct more heat from the motor.
Also many believe that Neo magnets are more sensitive to heat. In my view they don’t but because the Eddy Currents are higher in Neo magnets and the mass is much lower they are able to produce an equivalent of the heat produced by traditional magnets.
Hopefully this explains my earlier comment in more detail and feel free to respond.
Quote from post # 50:
“What happens is the voice coil magnetism creates eddy currents in the steel pole pieces, which cause direct heating. It also radiates heat into the magnet structure and some heat is transferred by convection too.”
With this post (and similar on your website) you are suggesting that the Eddy Currents ( 'Werbelstrom' in German which means twister-current) will only take place in Ferro Metals and that heat in the magnet comes from external radiation.
In my view all materials that conduct electricity (also non-Ferro metals such as aluminium and copper) are able to produce Eddy Currents. Many believe that magnets do not conduct electricity but I believe there are enough metals inside to produce significant Eddy Currents. Because magnets create a magnetic field it will even amplify the Eddy Currents created by the magnetic field of the Coil. Therefore in my view heat development inside the magnet can be significant and even more in Neo magnets.
Another assumption that can be questioned is that the top-plate generates more heat then magnets. Besides it depends on the relation how much material of each, the assumption is probably based on the fact that metals do conduct heat better than magnets. The top-plate will reach its high temperature much sooner until the magnet reaches the temperature of the top-plate. Eventually the magnet will heat the top-plate instead of visa-versa.
A magnet is basically a rock (with metals inside) and rocks in general don’t conduct heat very well. When heat is finally ‘stored’ in a rock it won’t be released easy. Combine this with the effect of higher Eddy Currents in Neo magnets and it seems logical why the extra cooling fins on Neo drivers are placed near the Neo magnets. Some manufacturers use special aluminium housing around their Neo magnets with extra cooling tunnels between the Neo magnets. Besides the extra air that flows near the magnets it also enlarge the aluminium surface to absorb and conduct more heat from the motor.
Also many believe that Neo magnets are more sensitive to heat. In my view they don’t but because the Eddy Currents are higher in Neo magnets and the mass is much lower they are able to produce an equivalent of the heat produced by traditional magnets.
Hopefully this explains my earlier comment in more detail and feel free to respond.
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Wayne,
Congrats on the patent!
I can certainly understand why no one showed for the bet you proposed:
“The signal shall be run at 800 watts RMS for 15 minutes or until one of the loudspeakers fails. Power shall be increased 10% and run for 1 minute, then increased 10% more and run for another minute and so on until one of the speakers fails. The loser pays the winner $1000.00 and the results will be published here. If there is a challenger willing to take this bet, we'll do it at the Prosound Shootout.”
That challenge is about the same as asking someone to bet that a room with a 800 watt heater will be cooler after 15 minutes with it’s door open than closed :^).
Of course, in addition to taking on a stupid bet, they get to pay their way to the test site, blow up $300 in speakers, then drive back and replace or recone them.
You wrote:
“Tests have shown that it (the cooling plug) increases power handling over 225%”
That statement is not clear to me (or the two folks I consulted in my household better in math than me ).
Do you mean 400 watts power handling is increased to 900 watts, just over 3 dB ?
Increased “power handling” is often accompanied by power compression.
In your extensive tests do you have any data showing how much the claimed increase in power handling your cooling plug provides translates to in SPL over the stock Lab 12 in the 12Pi?
Art Welter
I agree with you 100%. The confusion comes from the fact that you are speaking about speakers in general and I am talking about the LAB12 woofer in particular.
Direct heating from eddy currents is inductive, so it requires an electrically conductive structure to do it. Steel is conductive, so we know for sure that eddy currents are present there. There are also some electrically conductive magnets like Alnico that are heated inductively. I would expect Faraday rings get inductive heating too.
Ceramic magnets have powdered ferrite in them, but I don't think they're very conductive electrically. If they were, they probably wouldn't suffer from flux modulation as much as they do. I would expect them to act more like Alnico magnets, having something of a self-bucking effect, resisting flux modulation on their own.
In the LAB12, the center pole gets much hotter than anything else. The top plate also gets very hot. The magnet does too, but it seems to be a secondary effect, like from conduction from the pole piece and top plate.
Now then, a ceramic magnet might be getting direct heating from induction. It could be that each little ferrite particle is heated. In a way, that mechanism doesn't need to be fully understood to take advantage of the fact that the motor core begins to heat soak, and that wicking it away is desirable.
We know that the motor core is heated directly by induction, and indirectly by radiation and convection. The proportions and distribution are different between motors with different motor structures, and some resist heating better than others. Some are able to wick the heat out of the core, into a surrounding structure. That's the goal of the cooling plug approach, of course, to wick the heat out of the core, and to sink it into the plate.
That's right. A LAB12 driver modified to accept the cooling plug can handle 840 watts RMS continuously applied. Without a cooling plug, it will fail within a couple hours at 400 watts RMS. It can handle 200 watts continuously, but not 400 watts.
These figures are from sine wave tests, i.e. crest factor of 1.414. Eminence (and many other manufacturers) test with an EIA 426A pink noise source, which has a crest factor of 2.0, and is much easier on the speaker. My test is not to impeach Eminence or their tests, it isn't to say their speakers don't handle the power they're rated at. So don't take it that way at all. My tests were to punish the speaker, on purpose.
Music and other higher crest signals will allow higher power levels, of course, but the heat soaking from long continuous use is still the same. Subwoofers get a throbbing, powerful signal that can really heat soak the magnet after some time. Cooling plugs are very good insurance, making the driver much more robust.
Cooling plugs also reduce compression, because the voice coil's local ambient temperature is reduced.
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One more thing, an aside, actually. Djim was talking about neodymium magnets, which are electrically conductive like alnico. They're also really strong for their size. That reminded me of a story.
When I made the first prototype cooling plugs, I used some aluminum tubing to make them with. On a lark, I dropped a neodymium magnet in the aluminum tube.
The tubing acted like a Faraday ring, with the strong magnetic field of the moving neodymium magnet passing through. Its movement created enough of an eddy current in the aluminum to slow the movement of the magnet, from the induced current creating a reverse magnetic field.
It's a cool trick - try it. Goes through slowly, like the aluminum tubing is filled with molassas.
When I made the first prototype cooling plugs, I used some aluminum tubing to make them with. On a lark, I dropped a neodymium magnet in the aluminum tube.
The tubing acted like a Faraday ring, with the strong magnetic field of the moving neodymium magnet passing through. Its movement created enough of an eddy current in the aluminum to slow the movement of the magnet, from the induced current creating a reverse magnetic field.
It's a cool trick - try it. Goes through slowly, like the aluminum tubing is filled with molassas.
Thanks Wayne and sorry for the confusion. No misunderstanding about the centre pole and that’s why I didn’t mention it. Indeed, every manufacturer has there own solution for dealing with the heat from the centre pole and your modification solution proves to be very effective.
I do understand it is not of an importance to the Lab12 (or most conventional drivers) but the ‘mechanism’ can become an issue if you are going to work with higher loads or if the magnets become stronger and smaller like coated (Ni) Neo’s.
For us the ‘glues’ for attaching different components are becoming the most troubled parts. It seems very difficult to produce glues that don’t suffer from extreme temperatures over long periods. The rate of repairs under guaranty will hopefully push the manufacturers to come with better solutions.
Absolutely. The thermal problems I see are almost always the result of adhesive failure. Sometimes, I see a voice coil fuse but that's a weird deal, usually an amplifier problem or something like that. Sometimes I can fuse a voice coil when I'm testing in extreme cases, with almost zero excursion and tons of power (like heat soak a woofer and then hit it with 1kHz at 10x max-power, that'll fuse it). But in most cases, the thing that fails is the adhesive. After it weakens, stuff comes loose and bad things happen.
That's what the cooling plug is designed to do. It wasn't designed to give extra headroom, although it does, a little bit. It wasn't designed to increase power handling, per se, although it obviosly does that too. Its whole purpose in life was to keep people from blowing up their woofers when running them all night at high power levels. It keeps the core cooler, which helps prevent the adhesive from cooking.
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