Quantity of damping material in cabinet

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Damping material is used for eliminating the unwanted waves radiated from rear of driver to the cabinet.

I think the waves radiated from the rear of driver is desirable to eliminate completely for reproducing sound with pure and accurate.

For that, IMO it is desirable to fill the damping material in as much as possible (but it should not block the air flow to the port), especially for the mid chamber in 3 way system.

Then, how much do you fill the damping material and why ?
 
You can calculate the wavelength of standing waves using formulas and
if your box is not treated with stuffing material, you should be able to see
small spike on impedance plot meaning it is alive and kicking. You would
also see it on FR plot too as an inconsistency around the same frequency.

You know you have put enough of the damping stuff when the spike and
FR issue dissapears. Dampen it further more if you feel you need to lower
the Qtc of the driver in the box which means you are reducing the spl around
Fb. Decide for yourself when it's enough. Simulation helps.
 
Certain programs (MJK worksheets, Hornresp) can simulate filling/stuffing and actually tell you how much this should be in real life in grams of polyfill or similar material. Others like WinISD have a 'Qa' parameter, which represents absorption losses, you just have to guess at how much material this really is.

Lots of stuffing in a vented box can quickly kill the bass, so better use somewhat thick padding on the walls, to start at least, and then adjust as desired. Stuffing a vented box with a high Qt driver can tame the peaking response a bit, but this is only a band-aid on an already compromised situation.
 
First you need to understand that acoustic absorption varies with frequency as well as with stuffing material, density and thickness. See here for the absorption coefficients of various materials. In a nutshell, to absorb increasingly lower frequencies, you need higher density material and more of it but what you want to use is going to depend on what driver it's for and where the xo is placed. Personnally, I like rigid rockwool of about RHT 60 if I can get my hands on it.

So for a sealed alignment, use a bigger box if you can get away with it so that Qtc ends up between .5 and .7 . This will allow for more stuffing but still allow enough volume to leave some nice breathing space behind the driver. Also if your design allows it, think about increasing the distance the backwave has to travel from the driver to the back wall and then back again by making the box longer and narrower so that, without increasing the internal volume, you have increased the effective thickness of your stuffing and thereby also increased the amount of absorption taking place.

I haven't worked that much with ported so I will leave that to others, but since more stuffing drops the bass response, you may again want to start off with a bigger box which will give you some peaking in the low bass to compensate for that absorption loss. And a bigger box means more stuffing again. And more absorption with a longer pathlength still applies as well.

But in both cases, I agree that you have to experiment to see what sounds best to you.
 
Then, how much do you fill the damping material and why ?

Since you are specifically talking about internal standing waves, know 2 things.

Standing waves in a box (or equally inside a listening room) need a treatment equal to roughly 1/4 any cabinet dimension to absorb the lowest standing wave of that dimension. Treatment is more effective off the reflecting surface than it is on the reflecting surface.

BAF or Dacron materials are very crappy at absorbing standing waves. Fiberglass or rock wool are much better.

Okay, maybe that's 3 things.

David
 
Since you are specifically talking about internal standing waves, know 2 things.

Standing waves in a box (or equally inside a listening room) need a treatment equal to roughly 1/4 any cabinet dimension to absorb the lowest standing wave of that dimension. Treatment is more effective off the reflecting surface than it is on the reflecting surface.

BAF or Dacron materials are very crappy at absorbing standing waves. Fiberglass or rock wool are much better.

Okay, maybe that's 3 things.

David

Totally agreed re: spacing off walls. Stuffing is most effective where pressure is lower- the middle of the box. Of course, you don't want to restrict the vent action overmuch in a tuned system, so what's to do? I used the bracing structure, and the ribs on the inside surface of the box hold the stuffing off the surface, while being held in place by the inner bracing structure, so the stuffing is closer to the middle of the box but still doesn't impede the pressure path for the vent operation.

https://imageshack.com/i/31img0530xfj

https://imageshack.com/i/jyimg0529rj
 
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Since you are specifically talking about internal standing waves, know 2 things.

Standing waves in a box (or equally inside a listening room) need a treatment equal to roughly 1/4 any cabinet dimension to absorb the lowest standing wave of that dimension. Treatment is more effective off the reflecting surface than it is on the reflecting surface.

BAF or Dacron materials are very crappy at absorbing standing waves. Fiberglass or rock wool are much better.

Okay, maybe that's 3 things.

David


Dave is very right. For some reason most diy speaker builders seem to think that BAF Dacron and other Poly fills are gods gift and they are not they are very poor absorbers at low frequencies. HD fiberglass/rockwool are the best bang for the buck. S.A.E. rated Acoustical felts are excellent but costly. Dave is also correct about material application having a big impact upon how well different materials will work.

Thanks Dave for pointing this out. I make this case myself from time to time but always seem to end up with someone telling me how wrong I am and it gets to be a drag trying to tell people who don't want to hear. Best regards Moray James.
 
I've looked, but I can't seem to find it. There was a paper, seems like it was JBL, that went into the thermodynamics of why fiberglass was vastly superior to polyfill for energy absorption in speaker boxes. Nowdays people seem to equate fiberglass with asbestos as a breathing hazard and go to great lengths to use something else.
 
I'm coming round to the feeling that aperiodic or damped reflex might work best. :)

See, most reflex suffers from standing waves. But damp it with some stuffing behind the woofer and in the middle of the box, and it becomes BW3. And that seems to fall nicely between BW2 closed box, and BW4 or LR4 reflex. And works well with room gain.

It also keeps stuffing away from the crossover, which keeps it cool. So it could be the best of all worlds. :cool:

Bit of Allison Effect dropoff there at 150Hz with the Nomex 164. PEERLESS-NOMEX-164

The Poor Man's Strad with a lower woofer (causing an Allison Effect at 250Hz) seems to support the idea too: Poor Man'
 

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