The Science of Speaker Cabinet Design - Page 2 - AVS Forum | Home Theater Discussions And Reviews
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post #31 of 179 Unread 04-14-2017, 10:40 AM
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Driver Mounting Screws...

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Originally Posted by Mpoes12 View Post
CLD approaches don't actually short out in this way, so no you don't need to do that. It wouldn't be the case that screwing through the entire baffle suddenly kills the benefit of the damping. However, for decoupling purposes alone, yeah it would potentially have some benefit to find a way to isolate the screw mounts from the baffle. Reducing speaker vibrations in this way is already dealing with the last 10% of sound quality. Isolating the screw mounts is now into the .5%. I wouldn't lose sleep over it, the CLD process in the first place is a big step in the right direction (or any heavy application of damping to a rigid massive box), this is just icing.

My earlier point about the way my cabinet was made was that I see value in mounting the speaker on to a CLD sandwich because that will actually help dissipate some of the energy that it transfers into the front baffle.
Thanks.
Interesting subject. I think that if we could Visually see the sound waves emanating from the cabinet, we'd all be quite surprised (horrified?). Something like what an infrared camera does for heat "loss" studies.


I haven't read the articles you've linked to yet but I do know that once you start getting into those last few percentiles, (esoteric) speaker manuf have historically applied a lot of Voodoo indeed.


J&H
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post #32 of 179 Unread 04-14-2017, 11:16 AM - Thread Starter
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Originally Posted by Jekyll & Hyde View Post
Thanks.
Interesting subject. I think that if we could Visually see the sound waves emanating from the cabinet, we'd all be quite surprised (horrified?). Something like what an infrared camera does for heat "loss" studies.


I haven't read the articles you've linked to yet but I do know that once you start getting into those last few percentiles, (esoteric) speaker manuf have historically applied a lot of Voodoo indeed.


J&H


Your right but in this case, no voodoo here. It's sound science. CLD is a well established practice. We are talking about silver wire and gold/Teflon capacitors with spurious benefits at great cost. We are talking about thoroughly studied topics using rigorous methods (with regard to cabinet radiation) and really good reason to believe CLD is a good option. Part of why I wanted to post this is that it is dealing with the last little bits but it's also real Sound science. The speaker could become a more ideal speaker with better decay behavior and lower distortion. It would also have a slightly smoother response potentially, but the papers point out most of the issue is in the time domain, not steady state.

If someone wants to fund the effort id be happy to build a well know and we'll regarded kit speaker with two cabinets. One using all of these principles and one not. Then measuring it and seeing what we get. I just think we know the answer, it will have better cad, slightly lower distortion, and depending on the basic enclosure construction, a flatter response in the midrange. The differences will be a few dbs in the response at best and a bit more drastic decay in the CSD. Most would probably dismiss it as too small to matter, but I already linked papers discussing the audibility of these issues.

To quantify it in concrete terms I would say that this is at least as big a difference as using an AE driver over a good Eminence driver. In fact because of the results of the auralization distortion research I think a greater impact since the driver would have little impact on decay relative to the cabinet.


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post #33 of 179 Unread 04-14-2017, 11:24 AM - Thread Starter
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Originally Posted by Vince_B View Post
If cld is the biggest help why not just build a cabinet with two layers of 1/2" mdf and green glue? The main issue I see is that you need real glue at the edges so you might have to use a router to thin one layer in the middle leaving an edge that you can titebond. Green glue is not an adhesive and would likely be detrimental to finishing the cab. I do wonder though if you could lay up two layers of 1/2" as a whole sheet and then cut and join as normal. Veneer might be sufficient to prevent green glue coming out at the edges. Haven't played with green glue yet but have read of potential issues with it due to it staying squishy forever.

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That's what you basically do. You don't glue the entire box together with green glue. You adhere the layers together with it. The panel then is glued and screwed as normal to the other panels. While decoupling is somewhat important CLD can provide roughly the same damping as it would if no screws go through. It just changes the boundary behavior slightly and has little impact on damping overall. Certainly the whole box can't be a fully decoupled set of six sides, it wouldn't stay together.

I can't speak for all options but my own speaker decoupled the driver from the front baffle to a point. The front baffle from the side and top to a point. The rear panel is separate and removable and sits on a urethane rubber gasket. It is however screwed through all around. Most of what o have is damping rather than decoupling.

You are right that green glue isn't really glue but it is. It does setup and it does bond well. It doesn't get hard or stiff but pulling to adhered panels apart after thirty days is difficult.

Green glue may not be a good option. As I said my speakers were done with casting urethane which created a permanent bond. I have some green glue left over from my theater and could try some test panels. I k own the stuff is tried and true and my experience using it was that it's nasty and sticky so I thought it might be a good readily available option. Most CLD uses something with a higher durometer value and that truly sets up such that it can't be pulled apart without damaging it. However I see it as no worse than bitumen paste which is often used.


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post #34 of 179 Unread 04-14-2017, 11:56 AM - Thread Starter
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This is essentially a picture of the speakers radiation from the cabinet. It was a polar map of the cabinet walls radiation of frequency by angle. I know this isn't quite the same as the infrared camera idea, I couldn't find such an image, but this certainly gives the same information. Just wrap this around a speaker enclosure and you get the idea.
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post #35 of 179 Unread 04-14-2017, 12:16 PM
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Quote:
Originally Posted by Mpoes12 View Post
Your right but in this case, no voodoo here. It's sound science. CLD is a well established practice. We are talking about silver wire and gold/Teflon capacitors with spurious benefits at great cost. We are talking about thoroughly studied topics using rigorous methods (with regard to cabinet radiation) and really good reason to believe CLD is a good option. Part of why I wanted to post this is that it is dealing with the last little bits but it's also real Sound science. The speaker could become a more ideal speaker with better decay behavior and lower distortion. It would also have a slightly smoother response potentially, but the papers point out most of the issue is in the time domain, not steady state.
Thanks.
Apologies. I only meant that, to the average Joe this kind of science just seems like Voodoo. Kinda like what the I.T. network pros do down in their Batcaves.


I understand what you're getting at (tho not the entire science of CLD yet!). I've gone to some fairly long lengths myself in the past trying for those last few percentile. Kimber cable, oxy free interconnects, TipToes under the electronics (not just the speakers), Tweek contact enhancer, Sorbothane turntable mats (and turntable LP clamps), cartridge stylus lube, better Caps, 4 AWG 120VAC power feeds, on and on down the wormhole.


I like the idea of the CLD though, as it doesn't cost a heck of a lot to implement. Mostly time, care, and science.


Happy listening!
J&H
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post #36 of 179 Unread 04-14-2017, 02:09 PM
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I have had the opportunity to hear a sweep of a tower loudspeaker in an anechoic chamber. Standing behind the loudspeaker at a distance of about 2 meters, you can barely perceive any sound with a slow descending sweep, until the sound 'wraps' around the cabinet, and it becomes 'very' loud. Baffle step frequency was approximately +/- 456 hz in this case. Where the horn was dominating the pattern control, there was nil for sound behind the cabinet, but above 500 hz to about 2khz, there was 'some' radiated sound rearward.

The loudspeaker in question has a horn of approximately 8" square, and two 8" woofers, approximately 2.5 cu ft net, with two simple shelf braces. The larger the speaker, the lower in frequency you have problems.

Time domain measurements are helpful in determining problems, too much emphasis is placed on the frequency response in my opinion. With a well braced, stiff enclosure, problems are moved high in frequency, where they are more easily dealt with.

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post #37 of 179 Unread 04-14-2017, 03:43 PM - Thread Starter
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Quote:
Originally Posted by michael hurd View Post
I have had the opportunity to hear a sweep of a tower loudspeaker in an anechoic chamber. Standing behind the loudspeaker at a distance of about 2 meters, you can barely perceive any sound with a slow descending sweep, until the sound 'wraps' around the cabinet, and it becomes 'very' loud. Baffle step frequency was approximately +/- 456 hz in this case. Where the horn was dominating the pattern control, there was nil for sound behind the cabinet, but above 500 hz to about 2khz, there was 'some' radiated sound rearward.

The loudspeaker in question has a horn of approximately 8" square, and two 8" woofers, approximately 2.5 cu ft net, with two simple shelf braces. The larger the speaker, the lower in frequency you have problems.

Time domain measurements are helpful in determining problems, too much emphasis is placed on the frequency response in my opinion. With a well braced, stiff enclosure, problems are moved high in frequency, where they are more easily dealt with.
I'm wondering what your suggesting here? Are you trying to suggest that your experience proved for you that sound does not radiate through a speaker cabinet and out again? If that is the case, I think that is well proven to be the case given the studies I linked.

It does make sense that at frequencies covered by a horn/waveguide little sound was traveling back through the cabinet. Very little sound would radiate through the horn or through the compression driver into the cabinet and what little sound does do so would be quickly absorbed by the polyfill. This issue is (as shown in the measurements of the papers I included) happening in the midrange and upper bass range.

You wouldn't be able to hear the difference in delayed sound of that which "wraps" around the speaker and that which travels through the speaker. They are both delayed. 1 foot equals roughly 1 ms. That means that if a speaker cabinet is 12" deep, it would only be delayed 1ms in wrapping around the speaker. You couldn't easily perceive that. Sound delayed through the cabinet of the speaker is actually greater than 1ms. What you heard could just as easily (if not more likely) been sound traveling through the cabinet.

With regard to bracing raising the problem, I think you would need to read the articles I've posted to understand, or reread what I've written. The sound traveling through a cabinet does not happen only at the resonance frequency of the panel. It just happens that the resonant frequency is where the panels become very efficient transducers themselves with high Q peaks with little decay, thus creating audible problems. In this case, yes its easier to deal with those when they are high Q and higher in frequency, but only if you deal with them. That's why I'm suggesting CLD, its a very efficient and effective means of dissipating the energy. The first article I linked to from AES, the study looking at two versions of an NHT speaker cabinet found that the better braced cabinet had higher Q lower amplitude resonances but had more overall radiated sound than the unbraced cabinet. It showed the importance of addressing the energy with damping.

In reality what I'm saying here is you need to have damping, not just bracing and mass. The CLD happens to be a great way of doing that which is known to be more effective than other means of damping, but...any damping is better than nothing. Polyfill inside a cabinet is technically providing some damping, but it's not providing much. Gluing fiberglass, cotton, wool, etc. to the panel walls also provides damping, but still not as much as a proper damping compound.
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post #38 of 179 Unread 04-14-2017, 04:13 PM
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Quote:
Originally Posted by Mpoes12 View Post
I'm wondering what your suggesting here? Are you trying to suggest that your experience proved for you that sound does not radiate through a speaker cabinet and out again? If that is the case, I think that is well proven to be the case given the studies I linked.

It does make sense that at frequencies covered by a horn/waveguide little sound was traveling back through the cabinet. Very little sound would radiate through the horn or through the compression driver into the cabinet and what little sound does do so would be quickly absorbed by the polyfill. This issue is (as shown in the measurements of the papers I included) happening in the midrange and upper bass range.

You wouldn't be able to hear the difference in delayed sound of that which "wraps" around the speaker and that which travels through the speaker. They are both delayed. 1 foot equals roughly 1 ms. That means that if a speaker cabinet is 12" deep, it would only be delayed 1ms in wrapping around the speaker. You couldn't easily perceive that. Sound delayed through the cabinet of the speaker is actually greater than 1ms. What you heard could just as easily (if not more likely) been sound traveling through the cabinet.

With regard to bracing raising the problem, I think you would need to read the articles I've posted to understand, or reread what I've written. The sound traveling through a cabinet does not happen only at the resonance frequency of the panel. It just happens that the resonant frequency is where the panels become very efficient transducers themselves with high Q peaks with little decay, thus creating audible problems. In this case, yes its easier to deal with those when they are high Q and higher in frequency, but only if you deal with them. That's why I'm suggesting CLD, its a very efficient and effective means of dissipating the energy. The first article I linked to from AES, the study looking at two versions of an NHT speaker cabinet found that the better braced cabinet had higher Q lower amplitude resonances but had more overall radiated sound than the unbraced cabinet. It showed the importance of addressing the energy with damping.

In reality what I'm saying here is you need to have damping, not just bracing and mass. The CLD happens to be a great way of doing that which is known to be more effective than other means of damping, but...any damping is better than nothing. Polyfill inside a cabinet is technically providing some damping, but it's not providing much. Gluing fiberglass, cotton, wool, etc. to the panel walls also provides damping, but still not as much as a proper damping compound.
What I am suggesting is that yes, there is some radiation from the enclosure. Yes, I am stating that the sound I heard was traveling through the cabinet ( between 500 and 2khz ). IIRC, the chamber was anechoic down to ~80hz from what I was told.

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Quote:
Originally Posted by michael hurd View Post
I have had the opportunity to hear a sweep of a tower loudspeaker in an anechoic chamber. Standing behind the loudspeaker at a distance of about 2 meters, you can barely perceive any sound with a slow descending sweep, until the sound 'wraps' around the cabinet, and it becomes 'very' loud. Baffle step frequency was approximately +/- 456 hz in this case. Where the horn was dominating the pattern control, there was nil for sound behind the cabinet, but above 500 hz to about 2khz, there was 'some' radiated sound rearward.

The loudspeaker in question has a horn of approximately 8" square, and two 8" woofers, approximately 2.5 cu ft net, with two simple shelf braces. The larger the speaker, the lower in frequency you have problems.

Time domain measurements are helpful in determining problems, too much emphasis is placed on the frequency response in my opinion. With a well braced, stiff enclosure, problems are moved high in frequency, where they are more easily dealt with.
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What I am suggesting is that yes, there is some radiation from the enclosure. Yes, I am stating that the sound I heard was traveling through the cabinet ( between 500 and 2khz ). IIRC, the chamber was anechoic down to ~80hz from what I was told.
Ah I see. Ok makes sense.
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Originally Posted by Mpoes12 View Post
In reality what I'm saying here is you need to have damping, not just bracing and mass. The CLD happens to be a great way of doing that which is known to be more effective than other means of damping, but...any damping is better than nothing. Polyfill inside a cabinet is technically providing some damping, but it's not providing much. Gluing fiberglass, cotton, wool, etc. to the panel walls also provides damping, but still not as much as a proper damping compound.
See this is what makes it seem like you are mixing things up. The fiberglass filling is to absorb internal reflections. Not to dampen the cabinet walls.

Im not sure I agree cabinet damping is as important as you think. I may build two sets of boxes and try for myself. Its a fairly large undertaking though.

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I didn't mean to suggest no DIYer does this.
I wasn't suggesting you were. Most DIYers don't know much about design/acoustics and so want to build a kit or someone elses design. Building anything else other than a rectangle enclosure is difficult for most people. Woodworking is not my forte so I find it very difficult. I'm really testing my skillset with one design now as it's a wide baffle design like the SF Stradivari and I want it curved not the ripped and champhered look like these. I'm also wondering whether instead of using a flat side (where the Strads use the black blocks) shaped felt with a fabric covering would be of benefit.

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That is true, but not what I was talking about.

When I suggested lining the panels with something denser like fiberglass, cotton, or felt it was in addition to batt insulation like polyfill in the center. The reason is discussed in the VS white paper. When sound waves hit a barrier they are effectively hitting an abrupt change in acoustical impedance. This abrupt change causes some or all of the wave to reflect off the barrier with the higher acoustical impedance. If instead the wave progressively travels through a higher acoustical impedance that allows for a gradual change, then less of the wave will reflect back. We want none of the wave to reflect back, all of it to transfer all of its energy either into the damping material itself or into the damped enclosure. The cabinet filling is very important, but it can't absorb all of the sound inside the cabinet.
A good way to achieve this is steal the idea B&W used in the Nautilus; a progressively tapering tube fill with various grades of damping as you suggested. I do this for the mids using a rectangular Xsection bent to fit into the cabinet shape. Works very well.

The other thing I stole was matrix bracing which I use in 6" C-C.

Edit: I'm reading the papers in post 25 or so. Thanks. Interesting thread.

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Quote:
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A good way to achieve this is steal the idea B&W used in the Nautilus; a progressively tapering tube fill with various grades of damping as you suggested. I do this for the mids using a rectangular Xsection bent to fit into the cabinet shape. Works very well.

The other thing I stole was matrix bracing which I use in 6" C-C.

Edit: I'm reading the papers in post 25 or so. Thanks. Interesting thread.


Can you share pictures of the tapered tube?

I think most people know this about me but I'm a big adherent of Earl Geddes ideas and designs, including owning kits of his Abbeys. Mine are not identical to his standard kits of the time as I made minor modifications to the crossover and enclosure. I've also made a few smaller designs of my own based on his approach. His crossovers use a notch in the crossover at the crossover frequency (slightly lower typically) to achieve a slope that matches what he wants, is steeper at about an octave below, and provides better phase tracking through this region. In fact he has made some crowsovers that behave much like a first order in phase but with steeper slopes. I've adopted this approach in mine as well. With active there are similar or better options but with passive this is the best way I've seen to achieve high order slopes with the least parts and least impact on phase. I believe versions of these are called elliptical crossovers.

I tell you this because one or my challenges becomes how to create a tapered tube for a 12" driver. He likes larger drivers because he can match the directivity to that or his preferred waveguides and lower crossover points more readily. Again I've adopted the same basic approach in my own designs were feasible and this makes tapered tube enclosures somewhat difficult.

I think the matrix brwcing is a cool idea. It's a lot of work and I've always wondered if other approaches might achieve similar if not better results with less work, but I don't have the wood skills or time to mess with it. Heavily braced or not, still seems like good damping is important, hence this post. Instead of using matrix bracing I've mostly used dowel or stick bracing with ribbing and damping plates. The dowels or sticks are easier to do because you just cut to the width of the box. The ribbing is the same thing but it just doesn't tie panels together, it lies along panels. I have an accellerometer and have tested the effectiveness of this and found it to be fairly effective. Nothing I've ever done has achieved the results of something like a Wilson Audio enclosure.


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Quote:
Originally Posted by tuxedocivic View Post
As for bracing vs CSL damping, I understand that rigidity and damping work against each other.

How so?

Increasing stiffness raises resonant freq, and higher freq are easier to damp than lower.



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Ive never known the sound to pass through the cabinet walls to be a problem, but I may experiment with this. Or did I misunderstand you?

Many years ago (forget where) I saw a sobering study of how much sound came out of a sealed enclosure that had another smaller speaker playing inside of it.

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How so?

Increasing stiffness raises resonant freq, and higher freq are easier to damp than lower.
My understanding of it goes something like this.

As stiffness goes up, resonance goes up. As damping goes up, resonance goes down. Talking about the frequency here.

As stiffness goes up, Q goes up. As damping goes up, Q goes down.

Higher frequency is easier to hear. Lower Q is easier to hear.

The common approach is to be stiffen until the resonance is in the stop band. Im assuming here that wall thickness stays the same.

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Originally Posted by noah katz View Post

Many years ago (forget where) I saw a sobering study of how much sound came out of a sealed enclosure that had another smaller speaker playing inside of it.
Im gonna try this. I need to make a large enclosure to measure Vas on a 24" sub anyways. Ill stick a speaker in there and measure it!

Ryan
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Just want to throw out my offer again to any Denver residents willing to construct the boxes, I can do measurements to wade through some of the possibilities. I'll do frequency response, IR, and harmonic distortion.
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With increasing stiffness frequency of resonance and Q go up, but amplitude of vibration for a given input force goes down. In other words, it resists vibration better (from one perspective), just rings at a higher frequency. And that's ignoring mass. If you increase mass and stiffness such that you have the same end resonant frequency, you are still better off as amplitude of vibration will be lower for a finite impulse.
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My responses braqcketed by *

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Originally Posted by tuxedocivic View Post
As stiffness goes up, resonance goes up. As damping goes up, resonance goes down. Talking about the frequency here.

* No; damping has a minor effect on res freq, lowering it only slightly, unless you approach critical (Q = .5), but that's not going to happen unless you install discrete dampers. *


As stiffness goes up, Q goes up. As damping goes up, Q goes down.

* Not 100% sure on this, but I believe that's the case - higher stiffness means reduced displacement, and that may mean less damping because stiction may not be overcome, or simply because of lower internal hysteresis (i.e. friction within the material itself). *


Higher frequency is easier to hear. Lower Q is easier to hear.

* Yes; and sure - lower Q means higher amplitude = louder *


The common approach is to be stiffen until the resonance is in the stop band.

* Very doable with subs, much harder with mains *

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Im gonna try this. I need to make a large enclosure to measure Vas on a 24" sub anyways. Ill stick a speaker in there and measure it!

I can try this in a few weeks when the weather is good, and can band pass a bookshelf speaker actively to measure the amount of re-radiated sound out of an enclosure with the main woofers shorted so they do not move.

"What the heck was that crashing noise?" Me: "oh, just tossed the shampoo bottles off the shelf on the opposite side of the house. "
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My responses braqcketed by *
Q doesnt mean higher amplitude, it means wider peak, therefore more area under the curve to hear.

You say the effect depends on geometry. You have to assume the same thickness of panel. Otherwise I could just assume an infinitely thick panel and say stiffness wins.

As for the real details of this, my knowledge comes from a former speaker designer at Snell and the owner of Planet 10 hifi. The planet 10 guy is querky, but really knows his box design stuff. Ive never considered myself a good enclosure designer but he has helped me learn a lot of stuff. I dont consider myself to be in a good position to really tackle the issue until I take the time to do the research and experiment for myself. I'm only relying on these other people.

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I can try this in a few weeks when the weather is good, and can band pass a bookshelf speaker actively to measure the amount of re-radiated sound out of an enclosure with the main woofers shorted so they do not move.
It would be great if we both did this independetly and see what we come up with. Itll be a few weeks at least for me to get around to building the test box. But it is on the list.

Ryan
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I can try this in a few weeks when the weather is good, and can band pass a bookshelf speaker actively to measure the amount of re-radiated sound out of an enclosure with the main woofers shorted so they do not move.
Shorting the woofers will help, but not completely lock them, and the cones will reradiate far more sound than the box walls, which is fine if it's cone reradiation that you want to test.

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Q doesnt mean higher amplitude, it means wider peak, therefore more area under the curve to hear.
This is a common misconception, besides which if it were true, there would be no point in increasing damping.

Lower Q broadens the curve only in a relative sense, because adding damping has a much bigger effect on the peak at resonance.

As you can see from the generalized amplitude ratio curve, increasing Q (the weird symbol is xi = 1/2Q, so xi = .02 corresponds to Q = 50, xi = 1 is Q = .5, etc), increasing damping reduces amplitude at *all* freq.
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This is a common misconception, besides which if it were true, there would be no point in increasing damping.

Lower Q broadens the curve only in a relative sense, because adding damping has a much bigger effect on the peak at resonance.

As you can see from the generalized amplitude ratio curve, increasing Q (the weird symbol is xi = 1/2Q, so xi = .02 corresponds to Q = 50, xi = 1 is Q = .5, etc), increasing damping reduces amplitude at *all* freq.
I didnt say damping doesnt lower amplitude. But you did say lower Q means higher amplitude. I dont see that being the case in your chart. Also, you havent provided enough details to explain how Q is proportional to amplitude. Can you provide more information where that chart came from? I think Ive seen it before.

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Not the be all end all source, but: https://en.m.wikipedia.org/wiki/Q_factor

The first example is a pendulem. The pendulem is excited the same amplitude whether in air or oil, but have different Qs due to different dampings.

But now Im confused. Haha.

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Originally Posted by tuxedocivic View Post
But you did say lower Q means higher amplitude. I dont see that being the case in your chart. Also, you havent provided enough details to explain how Q is proportional to amplitude. Can you provide more information where that chart came from? I think Ive seen it before.
Not sure what details you want; the chart is self-explanatory, at least once you convert xi to Q, but in a nutshell, the top curve is lowest damping/highest Q etc.

The chart is a standard amplitude ratio graph found in mechanical vibrations texts; I think I might have posted it once before.

Unfortunately the origin of Q muddies the water for mechanical applications.

It was originally applied to electrical circuits where they were trying to minimize losses and get maximum output from oscillators - the exact opposite of what we want.

Noah

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I dont feel the chart is adequate because by my definition of Q, the chart just says to me that as Q goes up, amplitude goes up IF bandwidth stays the same. Thats expected.

It also looks like amplitude goes down as Q goes down, which is the opposite of what you said.

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Ill have a look in my physics texts and see if I can find anything to help me.

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