Ummm... if you want to do something about transmission through speaker walls, you do not want an airspace inbetween. You want something that will convert the sound energy to something else... what would that be? That would be heat.
That's how it works...
The thing to put between cabinet walls is something like sand or glass beads. That works.
For box wall transmission you want to thing two things: damping and absorption. Stiffness works only in concert with the those two as a system.
You can make a curved surface for a cabinet without creating any tension on the curved bit of whatever it is... plywood for example you could cut kerfs and bend, you could steam bend it, you could mill it out of solid wood, or mill it out of MDF. So unless you deliberately create some tension, there won't be any.
Tension, what does it do to resonant frequency?
Is that good?
Internal "early reflections" do not bounce around enough to make any real world difference at frequencies of interest - comparing a "right angle" box with a curved surface box...
Again, you have to take a minute to calculate the wavelength (dimension)of the wave, ie. frequency, and see if that 1/4 wave or 1/2 wavelength (or other multiple) is of any scale near the dimensions inside the cabinet - and then figure out what effect your internal treatment will have.
chris661 wrote:
I'm not sure what you are referring to here. But if you are talking about internal cabinet pressure effecting the tweeter (diaphragm), then the problem is not with the cabinet per se, but that the tweeter has not been isolated from the cabinet volume.
The tweeter should always be isolated from the cabinet volume!! Tweeters are not designed to be "air tight" at all. One should always put them in a sealed section or place a "cup" or "box" behind them that is very very well sealed!
I would think that using the case of a tweeter with a vented pole piece that this idea would be made be self-evident?
That's how it works...
The thing to put between cabinet walls is something like sand or glass beads. That works.
For box wall transmission you want to thing two things: damping and absorption. Stiffness works only in concert with the those two as a system.
You can make a curved surface for a cabinet without creating any tension on the curved bit of whatever it is... plywood for example you could cut kerfs and bend, you could steam bend it, you could mill it out of solid wood, or mill it out of MDF. So unless you deliberately create some tension, there won't be any.
Tension, what does it do to resonant frequency?
Is that good?
Internal "early reflections" do not bounce around enough to make any real world difference at frequencies of interest - comparing a "right angle" box with a curved surface box...
Again, you have to take a minute to calculate the wavelength (dimension)of the wave, ie. frequency, and see if that 1/4 wave or 1/2 wavelength (or other multiple) is of any scale near the dimensions inside the cabinet - and then figure out what effect your internal treatment will have.
chris661 wrote:
I'm not sure what you are referring to here. But if you are talking about internal cabinet pressure effecting the tweeter (diaphragm), then the problem is not with the cabinet per se, but that the tweeter has not been isolated from the cabinet volume.
The tweeter should always be isolated from the cabinet volume!! Tweeters are not designed to be "air tight" at all. One should always put them in a sealed section or place a "cup" or "box" behind them that is very very well sealed!
I would think that using the case of a tweeter with a vented pole piece that this idea would be made be self-evident?
I just noticed that the Nautilus design was referenced... not sure that the tweeter is open in the back, and if it is, if this is significant? ... to the extent that it is sitting on the end of a reverse cone that is not stuffed at all... given that the length of the back cone is then long compared to the wavelength, I'd expect that it looks somewhat like an infinite baffle, or at least a transmission line of sorts... and in that case there would be very little HF energy to reflect back directly, except in the case where the volume at large would resonate - which presumably is below the F3 of the driver by some margin...at least that's what I thought about that... dunno if that matches the reality or not?
A long tapered pipe...eh?
_-_-bear
A long tapered pipe...eh?
_-_-bear
cirrus18 said:
Not me on the 1st point...
Instead of reiterating my build stiff, high frequency panel resonance box methodology it is explained in detail here http://www.diyaudio.com/forums/showthread.php?s=&threadid=98834
On the 2nd you are describing the major panels on the Fonken s(or any Onken for that matter)
They aren't perfect but they do work very well.
dave
planet10 said:
Agreed re internals, sphere is the worst shape. Olsen found that the sphere and the (like Dave's Fonkens, but with another big bevel on the top edge) were the best baffle shapes. Article is here. Mods - if you don't like the picture posted (from the above article), please remove it.
But this is about the rear shapes, and yes, it does improve standing wave performance, but it also allows for a larger speaker to sit in the same position (ie. good SAF).
Attachments
bear said:
Linkwitz Pluto
SL measured a 40dB return loss on the mid/bass enclosure after determining an appropriate stuffing quantity and density.
Some place there is or was a description of the bursts he used to measure it and the resulting graph.
Perhaps more noticeable as a time domain stored energy aberation.
Pluto's baffle step starts around 2.5KHz. The 1.7" tweeter is getting directional at that point so it doesn't diffract too much at higher frequencies.
The ABS sewer pipe is also round, so pressure waves attempting to deform the enclosure are trying to stretch it instead of deflect it. There's an AES paper where the abstract says the round enclosure was as stiff as 4" concrete; I should renew my membership and read it for fun as a $5 download.
Absorbing 99% of the internal energy is close enough to zero for an engineer who is thinking about masking effects.
Any one with a soldering iron and screw driver could build a pair. Easy.
Driver cost is about $140/pair. The six (woofers are bridged) channels of amplification and active cross-overs are in line with what you'd spend on passive cross-overs and a used mid-fi stereo amplifier. Inexpensive.
It's really a wonderful piece of work. I like my Orions more than my Plutos, but doing so much with so little impresses me more as an engineer.
The B&W Nautilus shares the same small baffle, damped transmission line, active cross-over concept. While its SAF is probably a lot higher, output limits from the 4-way higher, and tapered transmission lines "better" the price tag and need for external amplification make it less accessable than the Pluto or average German sports sedan.
Attachments
A while back I posted an inquiry for links to software that could generate diffraction patterns for any given design like the following:
http://www.acoustics.hut.fi/demos/diffr-visual/IIndorder.html
To an extent you can see trends in evolution in enclosure shape.
Set-back baffles were the norm in cabs in the past. Then baffles became flatter. Now virtually every cab has at least a little round-over.
With advances material science and fabrication, non-rectilinear enclosures are more common.
I don't have the means to do the measurements but would the sound transmit through the panels be substantially lower (e.g. 40-60dBdB) than the direct sound (unless your panels are paper thin)? hence transmission becomes a non-issue? Point me to some solid measurements to convert me if otherwise.
Also, would the effects of the box internal reflections be magnitude higher than box / panel resonances? I would assume so. Again, point me to some solid measurements to convert me if otherwise.
Regards,
Bill
Also, would the effects of the box internal reflections be magnitude higher than box / panel resonances? I would assume so. Again, point me to some solid measurements to convert me if otherwise.
Regards,
Bill
The amount of output reduced from a cabinet's walls is very frequency dependent. At higher frequencies much shorter than the length of the cabinet dimensions, it will be reflected internally, with virtually no transmission through the cabinet. At lower frequencies much longer than the cabinet dimensions, the sound doesn't really even see the cabinet. Yes the walls have some effect on lowering the level, but usually not all that much.
Now from a physics of waves stand point (someone please correct me if I'm wrong), I believe that because the enclosure is denser than the air, the waves travel through it faster than through the air. However, I believe it doesn't take such a straight path, given the internal structure of the panels, and so some is dissipated throughout the enclosure walls. As the sound waves use up energy to vibrate the walls and wall particles, the waves are converted to heat, and thus dissipated.
Also, as I understand things, braces inside enclosures do a few things. Some argue they divide the enclosure into smaller panels which vibrate at higher frequencies that are more easily dissipated. Others say they link the mass of multiple sides, thus raising the frequency of resonance. This implies to me, but again I could be wrong, that lower frequencies have greater energy to dissipate, and so less of the energy is dissipated in the panels, as compared with higher frequencies. From my own measurements, adding bracing is one of the most effective ways to reduce panel resonances. However they have very little effect on the upper midrange and treble area, which are typically dissipated quicker anyway.
As to curved sided enclosures having lower panel resonances to begin with, this may be true. I've only measured the Parts Express curved .75 cubic foot enclosure again the regular one, and the resonances were very similar. There was ever so slightly less in the mid/lower mid-range area. However dissipation time was almost identical (decay).
So that brings me back to my original point. The primary reason that companies make curved enclosures is for looks. Be it as simple as the PE boxes or as complicated as the KEF MUON, all of that is for looks. Other methods are far more effective at eliminating panel flexing, energy transfer, and diffraction.
Now from a physics of waves stand point (someone please correct me if I'm wrong), I believe that because the enclosure is denser than the air, the waves travel through it faster than through the air. However, I believe it doesn't take such a straight path, given the internal structure of the panels, and so some is dissipated throughout the enclosure walls. As the sound waves use up energy to vibrate the walls and wall particles, the waves are converted to heat, and thus dissipated.
Also, as I understand things, braces inside enclosures do a few things. Some argue they divide the enclosure into smaller panels which vibrate at higher frequencies that are more easily dissipated. Others say they link the mass of multiple sides, thus raising the frequency of resonance. This implies to me, but again I could be wrong, that lower frequencies have greater energy to dissipate, and so less of the energy is dissipated in the panels, as compared with higher frequencies. From my own measurements, adding bracing is one of the most effective ways to reduce panel resonances. However they have very little effect on the upper midrange and treble area, which are typically dissipated quicker anyway.
As to curved sided enclosures having lower panel resonances to begin with, this may be true. I've only measured the Parts Express curved .75 cubic foot enclosure again the regular one, and the resonances were very similar. There was ever so slightly less in the mid/lower mid-range area. However dissipation time was almost identical (decay).
So that brings me back to my original point. The primary reason that companies make curved enclosures is for looks. Be it as simple as the PE boxes or as complicated as the KEF MUON, all of that is for looks. Other methods are far more effective at eliminating panel flexing, energy transfer, and diffraction.
Maybe this will be a subject of discussion with no resolution, because of individual perception:
I have a pair of Focal "Eggs" and was so pleased with the sound, that I continue making non-rectilinear enclosures whenever possible.
In the Jan 2009 issue of Stereophile is a review if the Burmester B25 ( $12K pr ). on page 71 John Atkinson's includes a cumulative spectral-decay plot using an accelerometer. it shows a very conspicuous ridge @ 172Hz and side wall flex between 50 - 70 Hz.
The listening notes imply that there are some issues in those frequency ranges.
I am not aware of any definitive study on the differences in flex of flat boxes vs curved.
I guarantee I could fix those spikes in the response doing something that would have little impact on the actual side wall flexing. I showed graphs I took of my accelerometer measurements of the Summa Abbey's and showed about a 10db reduction in energy between 50 and 250-300hz. I did this by adding the Sonic Barrier 1.5" foam. I know I know, it had to reduce flexing, the accelerometer is only picking up vibrations, so a reduction in vibrations must equate to less flexing. None the less, my point is that it wasn't accomplished via a more rigid box.
Curved, flat, round, etc all seem to show a lot of similar amounts of flexing in my measurements. I'm not saying their are no benefits, just that it requires a whole system approach of extensive internal bracing. I also believe that I could create an enclosure that is completely dead, without relying on curved sides. We aren't talking about outside pressure on the box, it's inside pressure. We also aren't talking about a sphere or egg, we are talking about a box in which the only curvature is the side walls. Even if you make the equate to a bridge, the pressure is coming from the wrong place. If you pushed a bridge up from the center it's much weaker than down from the top.
Now, as for diffraction, thats a whole different issue. If the egg shaped speaker sounded better, and it was shape related, I would argue it was because of a reduction in diffraction, rather than an improvement in rigidity.
Another issue to consider is, are there any definitive experiments equating box resonances with audibility using reliable experimental methods. I don't believe there are, and from what I've read and seen, its a very poorly defined area of science. Companies all use different methods for measurement, with a great deal of argument over what is the best method to use, or what the data even actually means. In my experience, finding a peak in the box response doesn't mean it's box related, the box, being denser than air, is just really good at transmitting it. Finding a peak in the box response that isn't seen in the response might indicate that, especially if it shows up in the impedance (That, to me is a very important part of the issue). However, a peak in the box response and in the frequency response is probably being caused by the simple fact that the speaker has more energy in that area of the response.
Mind you that I lowered the measurable energy in the Abbey box, as I showed, but it had absolutely no impact on the impedance plot or frequency response of the speaker either nearfield or in room at listening position.
Curved, flat, round, etc all seem to show a lot of similar amounts of flexing in my measurements. I'm not saying their are no benefits, just that it requires a whole system approach of extensive internal bracing. I also believe that I could create an enclosure that is completely dead, without relying on curved sides. We aren't talking about outside pressure on the box, it's inside pressure. We also aren't talking about a sphere or egg, we are talking about a box in which the only curvature is the side walls. Even if you make the equate to a bridge, the pressure is coming from the wrong place. If you pushed a bridge up from the center it's much weaker than down from the top.
Now, as for diffraction, thats a whole different issue. If the egg shaped speaker sounded better, and it was shape related, I would argue it was because of a reduction in diffraction, rather than an improvement in rigidity.
Another issue to consider is, are there any definitive experiments equating box resonances with audibility using reliable experimental methods. I don't believe there are, and from what I've read and seen, its a very poorly defined area of science. Companies all use different methods for measurement, with a great deal of argument over what is the best method to use, or what the data even actually means. In my experience, finding a peak in the box response doesn't mean it's box related, the box, being denser than air, is just really good at transmitting it. Finding a peak in the box response that isn't seen in the response might indicate that, especially if it shows up in the impedance (That, to me is a very important part of the issue). However, a peak in the box response and in the frequency response is probably being caused by the simple fact that the speaker has more energy in that area of the response.
Mind you that I lowered the measurable energy in the Abbey box, as I showed, but it had absolutely no impact on the impedance plot or frequency response of the speaker either nearfield or in room at listening position.
I'd like to start by first pointing out that l like curved enclosures more.
Now, allow me to share some thoughts, and ask for your comments.
I am not sure how much stiffer a curved enclosure would be compared to a flat one. It depends on how it was curved and the material. For MDF, I doubt it would make a difference. Plywood might be a bit stiffer with the layers being expanded due to curving.
But I am sure that a well thought enclosure with flat panels can be as stiff as a curved one. OTOH, curved enclosure panels should be a hell of a lot of work to cut properly and seal airtight. I am certain that it can be done but when cut and formed with hand tools, there are all sorts of issues to overcome in order to make edges fit together and form a sturdy and airtight enclosure.
On internal reflections, I think that a better solution could be internal stuffing. It controls internal reflections well and can also increase the effective internal volume for the bass loading.
Secondary emmissions from the enclosure edges is something to think about and a simply curved coffin will have as pointy edges as a plain one.
I am thinking that a 3 ply plywood (3 x 20mm) enclosure can allow edge trimming to make them as spherical as possible. That combined with plenty of bracing and internal stuffing is -IMHO-a much better solution from an engineering point of view (i.e. has a higher effect / cost ratio).
Now, we do have examples of commercial loudspeakers like the KEF Muon that defy cost and is built like a sculpture. But with only 100 pairs built and a 6 figure price, this is certainly not the way to go if the goal is to get to a cost effective solution.
Now, allow me to share some thoughts, and ask for your comments.
I am not sure how much stiffer a curved enclosure would be compared to a flat one. It depends on how it was curved and the material. For MDF, I doubt it would make a difference. Plywood might be a bit stiffer with the layers being expanded due to curving.
But I am sure that a well thought enclosure with flat panels can be as stiff as a curved one. OTOH, curved enclosure panels should be a hell of a lot of work to cut properly and seal airtight. I am certain that it can be done but when cut and formed with hand tools, there are all sorts of issues to overcome in order to make edges fit together and form a sturdy and airtight enclosure.
On internal reflections, I think that a better solution could be internal stuffing. It controls internal reflections well and can also increase the effective internal volume for the bass loading.
Secondary emmissions from the enclosure edges is something to think about and a simply curved coffin will have as pointy edges as a plain one.
I am thinking that a 3 ply plywood (3 x 20mm) enclosure can allow edge trimming to make them as spherical as possible. That combined with plenty of bracing and internal stuffing is -IMHO-a much better solution from an engineering point of view (i.e. has a higher effect / cost ratio).
Now, we do have examples of commercial loudspeakers like the KEF Muon that defy cost and is built like a sculpture. But with only 100 pairs built and a 6 figure price, this is certainly not the way to go if the goal is to get to a cost effective solution.
Curved or nonparallel walls do not really reduce the standing wave behavior, they just make it more difficult to predict. Nonparallel walls can greatly reduce flutter echo in listening rooms, but speaker enclosures generally are much smaller and actually have damping material in them, so flutter echoes are less of a concern. Enclosure shape is mostly an aesthetic decision.
As far as the pressure vessel theory goes - how much pressure do you think is actually involved in loudspeakers? - Let's see: atmospheric pressure is ~101000 Pascals, give or take. and 0dB is 2e-5 Pascals. That means 140dB inside an enclosure dB=20*log10(P/Pref) = 10^7*Pref = 200pascals. Holy crud, that is so much pressure, NOT!
Sonotube subs are an application for curved walls that make sense. In this case the diameter and length is much less than any frequency they reproduce, and internal pressure just turns into tension in the wall. If you have a speaker that is producing wavelengths less than any box dimension, making curved sides that mate with flat panels isn't doing much...
Everyone seems to point to Olson's graphs and say: "See? Curved enclosures are better." Well, Olson used what is essentially a point source always placed exactly in the geometric center of the baffle. Hopefully you are smart enough not to do that or you deserve the results you get.
HK26147 - that visualization is nothing more than the distance from the source to the edge placed on a line from receiver to the same edge point. While it may look neat, it is simple enough to visualize without a visualization. How much difference do you think curved edges will make?
The BDS http://www.pvconsultants.com/audio/frdgroup.htm will show you the effect of cabinet and edge shape (and listening position) on frequency response.
I agree that bracing is much more effective than curved walls in other than subwoofer enclosures. I have placed a hand on a sonotube sub playing loudly and felt essentially nothing. Not so for curved walls like the subject of this article, unless built 10x thicker than the sonotube _and_ braced..
As far as the pressure vessel theory goes - how much pressure do you think is actually involved in loudspeakers? - Let's see: atmospheric pressure is ~101000 Pascals, give or take. and 0dB is 2e-5 Pascals. That means 140dB inside an enclosure dB=20*log10(P/Pref) = 10^7*Pref = 200pascals. Holy crud, that is so much pressure, NOT!
Sonotube subs are an application for curved walls that make sense. In this case the diameter and length is much less than any frequency they reproduce, and internal pressure just turns into tension in the wall. If you have a speaker that is producing wavelengths less than any box dimension, making curved sides that mate with flat panels isn't doing much...
Everyone seems to point to Olson's graphs and say: "See? Curved enclosures are better." Well, Olson used what is essentially a point source always placed exactly in the geometric center of the baffle. Hopefully you are smart enough not to do that or you deserve the results you get.
HK26147 - that visualization is nothing more than the distance from the source to the edge placed on a line from receiver to the same edge point. While it may look neat, it is simple enough to visualize without a visualization. How much difference do you think curved edges will make?
The BDS http://www.pvconsultants.com/audio/frdgroup.htm will show you the effect of cabinet and edge shape (and listening position) on frequency response.I agree that bracing is much more effective than curved walls in other than subwoofer enclosures. I have placed a hand on a sonotube sub playing loudly and felt essentially nothing. Not so for curved walls like the subject of this article, unless built 10x thicker than the sonotube _and_ braced..
That appears to be quite a strong misinterpretation of acoustic theory. In free space, waves will 'ignore' small objects, but a loudspeaker cabinet is airtight. A low frequency of one cycle/day has a very long wavelength - do you think that will pass through the walls of an airtight vessel (think propane cylinder) without resistance?
I never said without resistance, but remember how sound is propagated. The denser the material, the quicker sound passes through it. The only resistance that happens is that some of the energy is dispersed and transformed, typically into heat. My point was that a curved enclosure has no impact on standing waves at low frequencies, since the waves are much longer than the enclosure, they won't see the enclosure. I can provide graphs that show actual measured data of low frequencies freely traveling through an enclosure with almost no attenuation.
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