Without context, this is not true. One must consider the rest of the system and how many other problems exist. A poor system will not be sensitive to enclosure vibrations, but an excellent one probably will.
The pressure inside of the enclosure is not likely a major aspect of the problem. It would likely be the structure borne forces from the driver.
And, as I said, most materials will not show nonlinearity in the situations that we are discussing and certainly NOT in musical instruments.
When the cabinet radiates due to the back wave of the driver (as in my plot above), it will be out-of-phase with the front wave. Although I don't know how that phase is transformed as it moves through the wall and reintegrates with the front wave, it seems to me anything that uncontrolled is unlikely to have a good result?
The AES paper that Augerpro attached in post 97 is a very good paper. I had not seen it before. I notice it only has 2 views so far, but I hope more people read it.
B&W build a FEM for their 800 Diamond bass cabinet, and then test the various simplifying assumptions which allow the FEM to run in a reasonable amount of CPU time.
Several times over the last year I have stated that modelling a cabinet in order to predict (and hopefully reduce) cabinet resonances would be a hard complicated problem. I have experience in aerospace structural FEMs, and I know how complicated it is to model seemingly simple structures. I got some pushback from people who claimed that vibrations are simple physics, and analyzing a cabinet would be rudimentary engineering. Their implication was that the work that Augerpro is doing is nearly worthless because simple analysis can tell us what we need to know.
After studying the B&W paper, I am confident they did a thorough job of building a sophisticated model. It was a lot of work, and anything but "simple and rudimentary". Even with that, when they tested their model results against real structure, they found "The simulation was able to reproduce these three regions fairly well but systematically over-estimating the modes frequency values by 10 to 20%. Predicted peak amplitudes and Qs are realistic"... So they got the mode shapes right, but just as I suspected, the effectiveness of glue bond line stiffness and the variation in material properties are variables which are hard to control and analyze.
So I feel somewhat vindicated. Empirical data is crucial in this area, because analysis by itself can only take us so far.
One thing that B&W did which was very innovative was their test method of isolating cabinet acoustic radiation from the much larger driver radiation: They first demonstrated that above Fs, the cone is decoupled from the motor structure, and then by analysis they showed that the virtually all of the force transmitted to the cabinet comes from the motor alone. This allowed them to cut the cone away from the voice coil. The motor, voice coil, and spider remain intact.
Now they have a driver which produces very little acoustic radiation, but which can induce vibration forces into the cabinet. This solves what I believe is the biggest hurdle with measuring cabinet radiation. Very well done...
Jim
B&W build a FEM for their 800 Diamond bass cabinet, and then test the various simplifying assumptions which allow the FEM to run in a reasonable amount of CPU time.
Several times over the last year I have stated that modelling a cabinet in order to predict (and hopefully reduce) cabinet resonances would be a hard complicated problem. I have experience in aerospace structural FEMs, and I know how complicated it is to model seemingly simple structures. I got some pushback from people who claimed that vibrations are simple physics, and analyzing a cabinet would be rudimentary engineering. Their implication was that the work that Augerpro is doing is nearly worthless because simple analysis can tell us what we need to know.
After studying the B&W paper, I am confident they did a thorough job of building a sophisticated model. It was a lot of work, and anything but "simple and rudimentary". Even with that, when they tested their model results against real structure, they found "The simulation was able to reproduce these three regions fairly well but systematically over-estimating the modes frequency values by 10 to 20%. Predicted peak amplitudes and Qs are realistic"... So they got the mode shapes right, but just as I suspected, the effectiveness of glue bond line stiffness and the variation in material properties are variables which are hard to control and analyze.
So I feel somewhat vindicated. Empirical data is crucial in this area, because analysis by itself can only take us so far.
One thing that B&W did which was very innovative was their test method of isolating cabinet acoustic radiation from the much larger driver radiation: They first demonstrated that above Fs, the cone is decoupled from the motor structure, and then by analysis they showed that the virtually all of the force transmitted to the cabinet comes from the motor alone. This allowed them to cut the cone away from the voice coil. The motor, voice coil, and spider remain intact.
Now they have a driver which produces very little acoustic radiation, but which can induce vibration forces into the cabinet. This solves what I believe is the biggest hurdle with measuring cabinet radiation. Very well done...
Jim
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Two woofers onopposing sides of a speakerbox would reduce effect of this by cancelling at least some of it?
Hi Jim
I agree. Structural analysis of an enclosure is no small feat if accuracy is required. Joints and glues, screws and nails, are all very difficult to model.
And that's just the structural vibrations. Modeling the actual sound radiation is much much more difficult.
And then there is the perception effects, which can be almost impossible to do correctly.
Thank goodness that it is not a critical aspect of a speaker design!!
What a surprise! On another thread found a video showing DIY-anechoic tests of the same driver in several different kinds of box - some made from poor stuff and one with concrete-panel walls.
At 21'50", the test guy says the tests show not to waste your energy on concrete panels. Maybe not the definitive test of panel vibrations but definitely meaningful. Certainly more meaningful than the endless theoretical jaw-jaw of this thread.
DIY Speakers and Acoustic Panels - YouTube
B.
At 21'50", the test guy says the tests show not to waste your energy on concrete panels. Maybe not the definitive test of panel vibrations but definitely meaningful. Certainly more meaningful than the endless theoretical jaw-jaw of this thread.
DIY Speakers and Acoustic Panels - YouTube
B.
I like that channel, but I don't think his method will show much difference, since the driver SPL will dominate. I put my driver inside a box and just measure what the box transmits.
Jim> thanks for the kind comments on my efforts! I have been somewhat surprised by the lack of comment on my construction methods thread, since it is all measurement, little theorizing. Maybe theorizing is better for jaw jaw as Ben observes, sort of like fake news gets the clicks.
Jim> thanks for the kind comments on my efforts! I have been somewhat surprised by the lack of comment on my construction methods thread, since it is all measurement, little theorizing. Maybe theorizing is better for jaw jaw as Ben observes, sort of like fake news gets the clicks.
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Yes we have that to be thankful for. I believe that 95% of speaker goodness is frequency response (on-axis and through various off-axis). There have been countless well-regarded speakers that do many things wrong, but they get the frequency response part right... But imagine a speaker with no cabinet resonance, perfect waterfall plots, no harmonic or IM distortion, and an on-axis frequency response curve with dramatic +/- 10 dB peaks and valleys... it will not sound good at all. The excellent cabinet and low distortion will do nothing to redeem this speaker.
As I said in post #50,
And while this is true, it is also worth considering that an audible cabinet resonance can be very hard to correct after the fact. Much more difficult than a crossover problem, or a bass vent tube that is the wrong length.
For me, the value in Augerpro's work is discovering simple easy-to-implement construction methods which give a DIY builder high confidence in achieving a low-signature cabinet. I currently believe that a high stiffness highly braced cabinet is the best way to do this... but I am open to new ideas. I am also aware that the highly braced high stiffness cabinet is very hard to construct for people who do not have access to table saws, band saws, router tables, and other wood shop equipment.
If a simple box with moderate bracing can be built which has a high probability of being inaudible, this will be a big step forward for most DIY folks.
I hope that you don't mind if I play devils advocate here ( purely for the sake of discussion ), but full range speakers often have a ragged frequency response but very light weight paper cones that presumably don't transmit much vibration to the box.
In which case the energy pumped into the cabinet by the drivers hammering away on it will not be dissipated except by radiation away as sound. (Broadly).
If resonances are present within the passband of the driver then the speaker cabinet requires effective damping to dissipate the energy in the cabinet motion and reduce the cabinet resonances by a few tens of dB to make them inaudible. This can be done in more than one way by DIY folk. An alternative is to passively isolate the driver from the cabinet but this has pros and cons for a midrange, isn't really practical for a woofer (moving to fixed mass ratio too small) but no problem for a tweeter.
Wise when the science is agin yer.
A text is always crystal clear to the person who wrote it. But rather conceited of you to think others will understand it without so much as a summary evaluation from you. That goes double for looking at pictures as if they "say" the same things to any two people.
Wonderful that you've done this extensive and seemingly reliable testing. Thanks. But as far as cab wall vibration, I think your results remind me of the decades of wall acceleration testing that proves that walls really do vibrate although at perhaps inaudible levels.
If I understand your wall vibration charts, the reader is seeing (1) a plot of the FR of the speaker at some unspecified distance away and (2) what a mic picks up at a half-inch from the wall. That makes the wall sound look vastly more audible (at a half-inch) than it would be at normal listening distance. And we can't tell how much of the wall mic pick-up really is air-borne sound coming from the driver. At the least, I'd like to see the speaker mic at a half-inch plotted in comparison to the wall sound at a half-inch.
Your plots do show the FR of the wall although not obvious what value that has to audiophile builders esp since all the different panels you tested look about the same FR for practical purposes although slightly different in amplitude.
I may have that all wrong and if so, I hope you'll want to correct me.
Unfriendly as this post may seem to some features of your testing, I think what you've done is wonderful and admirable experimentation. Everybody should read it (esp after you add a summary of findings). Thanks again.
B.
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After reading carefully the somasonus website, I did not have too much trouble following what Augerpro was doing. The data he is presenting is a work in progress, not a completed research project distilled down into a paper... although I have been forced to endure many many ASTM and ICAF papers which were so poorly written they might as well have been encrypted... In comparison to some (allegedly peer reviewed) papers, the somasonus website is a model of clarity...
Ben> In the beginning of that webpage I note the SPL has been adjusted so the relation between the bare driver in the far field, and of the wall near field, are corrected to be absolute. So if you see the main box resonance as loud as the bare driver, it really is that loud (and sounds like it to my ears during the test).
Keep in mind the driver is effectively inside the box. Well, it is attached to the outside of the test box, then a small box encloses the back of it. See the very first picture on my webpage. Certainly some radiation will leak through the little backside box, but I suggest it is low enough that overall, we are seeing what the box(es) are radiating, not the driver itself. So even if we want to debate just how high the S/N is, it is obviously better than trying to measure the box radiation with the driver firing into air on the other side of the box. Given that, we just need the S/N to be high enough to see relative differences between materials.
And since you won't read my webpage before you ask the question: the SB15 is just a dummy woofer for lining/fill testing to see what comes out of the cone. For all the other testing, there is no dummy woofer, and no hole cut in the box for it.
Keep in mind the driver is effectively inside the box. Well, it is attached to the outside of the test box, then a small box encloses the back of it. See the very first picture on my webpage. Certainly some radiation will leak through the little backside box, but I suggest it is low enough that overall, we are seeing what the box(es) are radiating, not the driver itself. So even if we want to debate just how high the S/N is, it is obviously better than trying to measure the box radiation with the driver firing into air on the other side of the box. Given that, we just need the S/N to be high enough to see relative differences between materials.
And since you won't read my webpage before you ask the question: the SB15 is just a dummy woofer for lining/fill testing to see what comes out of the cone. For all the other testing, there is no dummy woofer, and no hole cut in the box for it.
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Yep, I should have said that for most people, frequency response is 95% if speaker goodness... but not for everyone. Fans of full range speakers are clearly hearing something which I am not.
Not true. The energy is dissipated by the internal damping of the MDF / plywood. The resonance persists only as long as the forcing function is present. When the speaker moves on to other frequencies, the cabinet resonance quickly subsides.
I don't think that "the science is against me"... The B&W paper clearly showed that the mechanics of cabinet vibration is very complex. Saying that a cabinet must use structural damping to avoid an acoustic signature is way too simplistic. If it were that simple, why would B&W spent hundreds of engineering man hours building this complicated FEM?
Interesting that B&W utilize a highly braced high stiffness cabinet for their 800 Diamond, which is the subject of the paper. The fact that they were able to model their multilayered plywood wall material as a single orthotropic material shows that there is no structural damping other than that inherently present in the plywood.... if it had any significant damping layer (a la CLD), they could not have modelled it this way.... just say'n
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Are you saying that if i made a big wall out of the stuff you used for the cabs, the sound would be almost as loud on the far side (or as per your plots) as on the speaker side at some frequencies?
And I honestly can't figure out what "the SPL has been adjusted so the relation between the bare driver in the far field, and of the wall near field, are corrected to be absolute" means. Perhaps you could give some rough figures or what you did to make whatever it is you are saying you did less abstract.
Again, great to see your test data and still hoping I can digest it.
B.
I can't quote you a source, but I am pretty sure I read it in Martin Colloms' book: If a speaker cabinet resonance is severe, the cabinet output can equal the driver output at that frequency.
The result would be a 3 to 6 dB variation in frequency response.
How is this possible? Well remember, the efficiency of a loudspeaker driver is typically between 0.5% and 2%. So if 1 watt of 200 Hz sine wave power is consumed by the driver, something like 0.005 to 0.02 W of acoustic power is radiated from the diaphragm... the rest is wasted as heat (hopefully) or as motion (unfortunately)... With so much excess energy needing to be wasted away, it is not really surprising that a cabinet wall resonance could radiate 0.005 to 0.02 W on its own.
So Augerpro's results do not strike me as unrealistic.
The result would be a 3 to 6 dB variation in frequency response.
How is this possible? Well remember, the efficiency of a loudspeaker driver is typically between 0.5% and 2%. So if 1 watt of 200 Hz sine wave power is consumed by the driver, something like 0.005 to 0.02 W of acoustic power is radiated from the diaphragm... the rest is wasted as heat (hopefully) or as motion (unfortunately)... With so much excess energy needing to be wasted away, it is not really surprising that a cabinet wall resonance could radiate 0.005 to 0.02 W on its own.
So Augerpro's results do not strike me as unrealistic.
Yes. I mean the driver would have to be attached the other side of the wall and other technicalities to make your example apples to apples, but yes that is correct.
Now you're just being obtuse. What you see is what you get. Sitting on the couch. Getting a beer from the fridge. Licking the cabinet. Wherever. If the bare driver measures 87dB at 600hz, and cabinet radiation is 87dB at 600hz, it will sound the same to you, wherever your imagination places you in the room.
Why would I waste everyone's time plotting two responses taken under different measurement conditions on the same graph without correcting for the differing conditions?
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Many thanks for " Box construction methods ", I found it very interesting, however rather disappointing; although the bracing, filling and damping made worthwhile improvements none seemed to solve the problems, it's like we're tickling the problem rather than getting it sorted once and for all. Do we have to totally rethink box construction and materials ( epoxy granite ? ) to make an inert cabinet to hear what difference/improvement can be achieved, and at what cost and to see if it's worth the effort ?