Yes, I was waiting to describe this until I had examples that show these points. I have those examples, I just am short on time.
Aren't you, in essence, measuring two different things here: real-world linear volume displacement of the subwoofer (which is to say, in a 4th order BP the driver/airspring/port) and real-world amp output before clipping?
Or is it more complex than that?
I think it would be interesting for you to confirm by comparing your OBF measurements (using a known high-power amp) to Klippel xmax measurements of the drivers.
Correct, its those two things that I am measureing. When the sub has a plate amp installed, thats all that I CAN measure. And the combination is the most important thing - sometimes it can be seperated and sometimes not. Because seperating them is not always possible, comparisons can only be made on the unseperated results.
Kliipel only measures the driver, while my test is measuring the system. There is no reason to believe that the two things would be the same.
Leaving out the amp, I think one may want to see if there's a correlation between linear xmax as measured by something like a Klippel or DUMAX. Assuming enclosures roughly similar in size and tuning, and of course drivers of similar surface area, the big variable would intuitively seem to be linear throw. (True, that is a lot of assumptions to make.)
If Earl's method is working it would be very welcome because it would tell you if a particular subwoofer in a particular setup for a particular application has adequate performance.
This doesn't replace good engineering and established measurement methods. It just helps the user to get the performance he's looking for. I'm curious about it.
Best, Markus
This doesn't replace good engineering and established measurement methods. It just helps the user to get the performance he's looking for. I'm curious about it.
Best, Markus
This correlation or lack of, has been thrown arround a lot. It would be very nice to know, but it would require measureing a lot of different drivers and subs and I'm only making one. My guess is that there would be little correlation between the numbers and the perception.
Earl, what you are suggesting as your "fart test" is not so new. The speaker reviews in High Fidelity back in the 70s perhaps even the 60s would increase the bass test level until 10% distortion or buzzing was reached. Keele, in his reviews for Audio, tested for displacement limited max SPL using a shaped burst and he also increased level until 10% distortion or buzzing was encountered. He used a bridged Crown amp and some speakers would handle very short term bursts of up to 10KW - not at low bass frequencies.
It is clear that some drivers are usable well beyond the linear Xmax, depends on the driver and the nature of the non-linearities.
Pete Basel
It is clear that some drivers are usable well beyond the linear Xmax, depends on the driver and the nature of the non-linearities.
Pete Basel
Wow it's been a long time since I looked back here or what has been posted. I'm glad to see that Dr. Geddes is starting to put out information on his sub testing method. This is a little bit more advanced version of what I've been doing, and very close to what I've been trying to do with the new software I've been using.
Anyway, another thing I found that worked (at least sometimes) was that I could also figure out what aspect of the port was overloading by taking a series of measurements around the port and looking at the spectrograph. Basically (as long as the mic isn't being overloaded) close measurements at different angles about the port can, to a point, differentiate if the overload is from turbulence around the outside of the port, or outright core turbulence. I mean, you can calculate this too, but I'm not so sure the correlation is there. When the mic is placed at a strong oblique angle to the port mouth, if I see a lot of higher order spectrum noise, that is less prevelant fully onaxis, that seems to jive with it being mostly turbulance from edges. If on the other hand it basically looks the same nomatter where and what angle the mic is placed, and is also higher order, it seems to be core overload as well. I've not done extensive experiments to see how accurate this is.
The way I validated the method was that I figured if the noise was higher order and thus higher in frequency, that it would probably be a bit more directional and should change with angle. If the source was the edge of the tube, such as from a lack of flaring, then the point at which the angle is most "off-axis" to the source should be the middle of the tube. If there is no core turbulance, that should be the lowest amount of extra noise. I happened to have a speaker that I had experimented with a very low tuning, basically a series of 3" drivers tuned to 35hz instead of the more optimum 135hz. In order to get a .15 cubic foot box tuned to 35hz I ended up using a 1" port, and because this was such an intentionally off design, it ended up making overloading really easy. In fact, if I vary the number of drivers operating, I can vary if the driver or port would overload first, and all of this happens at a low enough volume to make testing good. This was actually designed as a on wall flat panel tv speaker. I had calculations of what level of flow would cause edge turbulence, which would cause core turbulence, and what should cause the speaker to overload. I then cross-validated this to the measurements, and low and behold it seemed to all work ok. Where I became less certain was at bigger and better designed setups. My ported sub will reach it's limits from the port before the amp or speaker. The speaker is capable of 38mm one way xmax, in excess of 4000 watts rms, and at those levels would be well in excess of 130db's in my house, I simply can not test that. Cant test it outside because wind screws up the turbulance measurements. Also, at those levels, most of my mics, placed up close, are overloading. I have one mic capable of over 190db's, but its for spl measurements and doesn't have a flat response.
Anyway, another thing I found that worked (at least sometimes) was that I could also figure out what aspect of the port was overloading by taking a series of measurements around the port and looking at the spectrograph. Basically (as long as the mic isn't being overloaded) close measurements at different angles about the port can, to a point, differentiate if the overload is from turbulence around the outside of the port, or outright core turbulence. I mean, you can calculate this too, but I'm not so sure the correlation is there. When the mic is placed at a strong oblique angle to the port mouth, if I see a lot of higher order spectrum noise, that is less prevelant fully onaxis, that seems to jive with it being mostly turbulance from edges. If on the other hand it basically looks the same nomatter where and what angle the mic is placed, and is also higher order, it seems to be core overload as well. I've not done extensive experiments to see how accurate this is.
The way I validated the method was that I figured if the noise was higher order and thus higher in frequency, that it would probably be a bit more directional and should change with angle. If the source was the edge of the tube, such as from a lack of flaring, then the point at which the angle is most "off-axis" to the source should be the middle of the tube. If there is no core turbulance, that should be the lowest amount of extra noise. I happened to have a speaker that I had experimented with a very low tuning, basically a series of 3" drivers tuned to 35hz instead of the more optimum 135hz. In order to get a .15 cubic foot box tuned to 35hz I ended up using a 1" port, and because this was such an intentionally off design, it ended up making overloading really easy. In fact, if I vary the number of drivers operating, I can vary if the driver or port would overload first, and all of this happens at a low enough volume to make testing good. This was actually designed as a on wall flat panel tv speaker. I had calculations of what level of flow would cause edge turbulence, which would cause core turbulence, and what should cause the speaker to overload. I then cross-validated this to the measurements, and low and behold it seemed to all work ok. Where I became less certain was at bigger and better designed setups. My ported sub will reach it's limits from the port before the amp or speaker. The speaker is capable of 38mm one way xmax, in excess of 4000 watts rms, and at those levels would be well in excess of 130db's in my house, I simply can not test that. Cant test it outside because wind screws up the turbulance measurements. Also, at those levels, most of my mics, placed up close, are overloading. I have one mic capable of over 190db's, but its for spl measurements and doesn't have a flat response.
Thank you, I too appreciate your posts though. They help me make sense of the measurements I take. While you rely on a solid fundamental understanding of how acoustics work in these scenario's to interpret the spectrographs, I rely on Sesame Street:
"One of these things is not like the others. One of these things just doesn't belong."
"One of these things is not like the others. One of these things just doesn't belong."
Matt,
"The speaker is capable of 38mm one way xmax, in excess of 4000 watts rms"
Curious what driver that is.
Re close-miked measurements of ports, seems that the air velocity would be a real issue, like blowing on a mike.
OT, but this reminds me of the time I had an engineer friend (not versed in speakers) put is hand near the port of a sub I had just built while I ran a sine wave at Fb. He looked all around the box and asked with a baffled look "Where's the air coming from?!" because it doesn't feel like AC.
That could possibly confound your measurements because the core velocity will be higher than at the tube wall because of viscous friction.
"The speaker is capable of 38mm one way xmax, in excess of 4000 watts rms"
Curious what driver that is.
Re close-miked measurements of ports, seems that the air velocity would be a real issue, like blowing on a mike.
OT, but this reminds me of the time I had an engineer friend (not versed in speakers) put is hand near the port of a sub I had just built while I ran a sine wave at Fb. He looked all around the box and asked with a baffled look "Where's the air coming from?!" because it doesn't feel like AC.
That could possibly confound your measurements because the core velocity will be higher than at the tube wall because of viscous friction.
This driver was based on the the TC Sounds TC-3000 driver, but was pretty aggressively modified for an OEM. It has a different motor, cone, and voicecoil, but they claimed it was most similar to the TC-3000. The driver had roughly twice the rms power handeling due to the quad coils, had slightly more excursion, a slightly lighter weight cone with greater rigidity at the center than the edge, standard titanium cone, standard rubber surround, dual spiders. This was supposed to have been designed for use in a Horn loaded sub enclosure for PA use. I've mentioned this driver before and probably reported different numbers, I never knew the truth until recently as TC wasn't very forthcoming. Having said all that, it has a pretty high inductance (like 6mh's if all are wired in series) and I've long been skeptical of that RMS power handeling number. I don't really consider it a great driver, I spent fairly little on it, and since then have been unwilling to sell it, even though I probably should. I used to have a Klippel measurement of it's mechanicals, but lost that durring a recent move. As I recall it did meet the mechanical excursion it claimed but not by Klippels Xmax defenition. As long as it's used below 50hz it is a very linear low distortion driver and seems to outperform it's size compared to other current high xmax drivers. At frequencies below 50hz, the better thought of low inductance drivers don't seem to be capable of as high an SPL, so while over 50hz it rolls off some, it isn't quite as loud, below 50hz seems to be capable of greater spl, as long as you can feed it the power. Keep in mind that even in room to exceed 120db's means more than 1000watts, in fact it means way more. Realistically taht is more than I can supply it, but it would never need to hit more than 115 db's and only for brief brief moments, so in practice, it's not been an issue.
I can't really speak to your comment on the air on the mic issue. I mean, a mic is a pressure transducer no? I would think the air from the port would be no different than the pressure it's typically measuring. As I said, you have to make sure the mic isn't overloading, and it's not hard to see that happening. Also, I don't place the microphone directly in front of the port at like 1 inch, it's a few inches away, and I use a windscreen at the tip. I figured that it would only filter the really high order noise, well out of the range of the subwoofer, and reduce that effect. It was really a test I just tried, no idea if it would work, and it seems to. I don't consider it an accurate way to measure port distortion, I consider it a way to evaluate the source of port turbulance, being that of the core or edge, to help fix it. I mean, you can take a brute force tactic and just enlarge the port, but that isn't often an option. It's better to carefully engineer exactly the right port for the job to maximize the port dampening and minimize the problems associated with port size, i.e. it may have to stick out of the box.
As I understand it, smaller ports have greater dampening, larger ones have less, but also have less turbulance issues do to the greater flow ability. Larger ports can be made to have great flow and good dampening by virtue of foam, but the foam can also cause the port to unload the speaker, defeating it's purpose. This would seem to imply, to me, a need to balance the size for the design, and not just put the largest possible port you can. Especially when you are talking about a Bandpass box.
At these very low frequencies I think Dr. Geddes is basically right, it's just an air pump and a lot of these factors are less important. I mean, at a tuning in the teens, I'm not sure any of this matters anymore, and then it's just a matter of getting the port large enough to handle the volume of air pumping through it.
I can't really speak to your comment on the air on the mic issue. I mean, a mic is a pressure transducer no? I would think the air from the port would be no different than the pressure it's typically measuring. As I said, you have to make sure the mic isn't overloading, and it's not hard to see that happening. Also, I don't place the microphone directly in front of the port at like 1 inch, it's a few inches away, and I use a windscreen at the tip. I figured that it would only filter the really high order noise, well out of the range of the subwoofer, and reduce that effect. It was really a test I just tried, no idea if it would work, and it seems to. I don't consider it an accurate way to measure port distortion, I consider it a way to evaluate the source of port turbulance, being that of the core or edge, to help fix it. I mean, you can take a brute force tactic and just enlarge the port, but that isn't often an option. It's better to carefully engineer exactly the right port for the job to maximize the port dampening and minimize the problems associated with port size, i.e. it may have to stick out of the box.
As I understand it, smaller ports have greater dampening, larger ones have less, but also have less turbulance issues do to the greater flow ability. Larger ports can be made to have great flow and good dampening by virtue of foam, but the foam can also cause the port to unload the speaker, defeating it's purpose. This would seem to imply, to me, a need to balance the size for the design, and not just put the largest possible port you can. Especially when you are talking about a Bandpass box.
At these very low frequencies I think Dr. Geddes is basically right, it's just an air pump and a lot of these factors are less important. I mean, at a tuning in the teens, I'm not sure any of this matters anymore, and then it's just a matter of getting the port large enough to handle the volume of air pumping through it.
Interesting driver, thanks for the info.
"I use a windscreen at the tip."
How big of a wind can it take? 20 m/s, which I believe is below where turbulence starts, is 45 mph.
"As I understand it, smaller ports have greater dampening, larger ones have less, but also have less turbulance issues do to the greater flow ability. Larger ports can be made to have great flow and good dampening by virtue of foam, but the foam can also cause the port to unload the speaker, defeating it's purpose. This would seem to imply, to me, a need to balance the size for the design, and not just put the largest possible port you can. Especially when you are talking about a Bandpass box."
The right size port is the minimum w/o significant compression, but with today's most capable drivers in a reasonably sized box that is often not achievable (the port length and volume become impractical), and the only solution is PR's.
"I use a windscreen at the tip."
How big of a wind can it take? 20 m/s, which I believe is below where turbulence starts, is 45 mph.
"As I understand it, smaller ports have greater dampening, larger ones have less, but also have less turbulance issues do to the greater flow ability. Larger ports can be made to have great flow and good dampening by virtue of foam, but the foam can also cause the port to unload the speaker, defeating it's purpose. This would seem to imply, to me, a need to balance the size for the design, and not just put the largest possible port you can. Especially when you are talking about a Bandpass box."
The right size port is the minimum w/o significant compression, but with today's most capable drivers in a reasonably sized box that is often not achievable (the port length and volume become impractical), and the only solution is PR's.
Hi Noah, sorry I didn't see the response. I'm really not sure, I lack the technical knowledge to adequately answer your questions. I came about this through a mix of the acoustic understanding and trial and error. As for precise measurements of wind speed and it's effect on microphone compression, I really don't know enough about the topic to tell you.
The driver was a serious piece of pride for me when I got a hold of it, basically very few existed and it seemed quite unique. However, anymore, I dont know, maybe it's complacency, maybe there is something better, but I just want to try something else. I have Dr. Geddes subs coming, but those aren't to replace this, those are to supplement the 50-150hz bass range, not the 50 and below range.
I think my biggest issue with the driver is the lack of efficiency. A lot of people over on the AVS forum, even around here, will have no problem buying a big PA 2000, 4000, 10,000 watt amplifier. They are actually readily available and affordable. Thing is, that is far more wattage than my wall can deliver, and I realized that more clean output was going to happen through system efficiency, not bigger amps.
Having said all that, there seems to be no such thing as high efficiency and below 50hz. I mean, lots of claims, but when you look at the efficiency in just that range, short of a massive massive box, nothing seems to offer more than 90db's of sensitivity at 1 watt. I mean, maybe tapped horns can, I've built a few for fun with different drivers. I did built a 30hz tapped horn for a 6.5" Sub driver I have from Mpyer, but it's not any more efficiency in that low range than the same driver in a ported box (at that range, it's considerably more efficient above that point). Might have been my design, I'm not an expert. Might be reality too, who knosws.
The driver was a serious piece of pride for me when I got a hold of it, basically very few existed and it seemed quite unique. However, anymore, I dont know, maybe it's complacency, maybe there is something better, but I just want to try something else. I have Dr. Geddes subs coming, but those aren't to replace this, those are to supplement the 50-150hz bass range, not the 50 and below range.
I think my biggest issue with the driver is the lack of efficiency. A lot of people over on the AVS forum, even around here, will have no problem buying a big PA 2000, 4000, 10,000 watt amplifier. They are actually readily available and affordable. Thing is, that is far more wattage than my wall can deliver, and I realized that more clean output was going to happen through system efficiency, not bigger amps.
Having said all that, there seems to be no such thing as high efficiency and below 50hz. I mean, lots of claims, but when you look at the efficiency in just that range, short of a massive massive box, nothing seems to offer more than 90db's of sensitivity at 1 watt. I mean, maybe tapped horns can, I've built a few for fun with different drivers. I did built a 30hz tapped horn for a 6.5" Sub driver I have from Mpyer, but it's not any more efficiency in that low range than the same driver in a ported box (at that range, it's considerably more efficient above that point). Might have been my design, I'm not an expert. Might be reality too, who knosws.
This is certainly the case and it can be proven mathematically. It simply comes down to trying to presurize a vessel and that takes more and more power as the frequency gets lower.
I've tried simulations with all kinds of drivers, and find a similar thing. Below 40 Hz the nominal efficiency means very little. Let's say you have a typical 12" sub driver with fs of 20 Hz and 86 db 1w1m. In a vented box you get an F3 of 20 Hz and perhaps 83 db 1w1m. Larger drivers of similar design tend to simply have more excursion and lower fs which means they will have similar output and efficiency but more LF potential. An efficient 12" pro driver might output 95 db down to 40 Hz in a vented box but when used down to 20 Hz the efficiency in that range comes down to match the high excursion driver, with some difference related to box size and tuning. The efficient driver loses it's effiency and becomes excursion limited - it's working well outside it's intended application. The high excursion driver is probably thermally limited, and may not use all of it's excursion.
I'd like to see some new 18 - 21" drivers designed for 20 - 200 Hz with an fs at 20 Hz, a good motor design, high power handling (~500w) and around 20mm xmax. The trend is towards lower fs and higher excursion but I think this would be a good balance, giving higher efficiency and with a big 20 Hz tuned vented box there's no need for massive excursion. I think the market wants something different.
At one stage I was planning on building a HUGE bass horn with two high excursion drivers. It was a box running floor to ceiling (2.85m high x 2.85m wide x 0.7m deep). The same drivers sim at 118 db down to 20 Hz in a vented box with 650w to each driver, but according to hornresp about 140 db with the same bandwidth. The mouth area is equivalent to 40 12" drivers, doubling the effective piston area 4 times > 12db efficiency increase which leaves about 10 db difference probably related to comparing half space vs corner loading.
I was going to build the horn, until I walked into the room it was intended for. The idea died instantly when I realised how imposing it would be. The crazy idea got scrapped. Ok enough rambling from me.
I'd like to see some new 18 - 21" drivers designed for 20 - 200 Hz with an fs at 20 Hz, a good motor design, high power handling (~500w) and around 20mm xmax. The trend is towards lower fs and higher excursion but I think this would be a good balance, giving higher efficiency and with a big 20 Hz tuned vented box there's no need for massive excursion. I think the market wants something different.
At one stage I was planning on building a HUGE bass horn with two high excursion drivers. It was a box running floor to ceiling (2.85m high x 2.85m wide x 0.7m deep). The same drivers sim at 118 db down to 20 Hz in a vented box with 650w to each driver, but according to hornresp about 140 db with the same bandwidth. The mouth area is equivalent to 40 12" drivers, doubling the effective piston area 4 times > 12db efficiency increase which leaves about 10 db difference probably related to comparing half space vs corner loading.
I was going to build the horn, until I walked into the room it was intended for. The idea died instantly when I realised how imposing it would be. The crazy idea got scrapped. Ok enough rambling from me.
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