General question about acoustic damping...

[kesslari]

Now that my O10 is done, there's a question I've been wondering.
I was pretty darn thorough about putting damping throughout (hooray for mattress topping...)
And I've read here that damping is an area where commercial speaker mfg's sometimes skimp.

I can see how damping would prevent the soundwaves from bouncing hard off hard internal cabinet surfaces. That in itself seems pretty significant.

But I read somewhere a line about "acoustic damping 'tricks' the driver into acting as though the cabinet had a greater internal volume" (or something like that).
Is that a somewhat accurate description, or a bunch of hooey?
And if it's accurate, how does that work?
And more generally, what all (besides damping down unwanted vibrations/reflections) does damping do?
Thanks.

Bill/Mods - Not sure if this is the right place for this question, if not, please move it.

[Sydney]

I can see how damping would prevent the soundwaves from bouncing hard off hard internal cabinet surfaces. ...

If you look at the acoustic absorption properties of various materials ( like foam ) you will notice that high frequencies are absorbed to a much greater degree then low frequencies. In most cases such materials are used to take out the "hollow ring"
In the original T24 article: It was tested with and without stuffing damping material in to the rear chamber - There was no difference with or without it so no reason to use it. In NON Sub cabs - DR's for instance it is used.
Hopefully Bill will verify the reason, but I believe it is to "de-ring"
Being a different denser medium than the surrounding air it converts acoustic energy to low grade heat through friction.

But I read somewhere a line about "acoustic damping 'tricks' the driver into acting as though the cabinet had a greater internal volume" (or something like that).

In some designs ( transmission lines for example ) the damping material is critical to the design providing resistance and tuning the line.
The Loudspeaker Design Cookbook demonstrates the effect that box damping ( basically stuffing ) has on closed boxes, and bass reflex cabs, But that is different from simply lining a cab with foam to suppress higher frequency standing waves.

[Bill Fitzmaurice]

But I read somewhere a line about "acoustic damping 'tricks' the driver into acting as though the cabinet had a greater internal volume" .

Myth. But as with most myths there is a factual basis to it. Over-stuffing a cab with damping will lower the box Q, and that can tame a resonant peak, giving a response curve that emulates that of a larger box. But the average sensitivity and f3 will still be that of the small box.

[Frankenspeakers]

When you you 'overstuff' (using about half to three quarters pound per cubic foot) you change from isothermic to adiabatic loading. Basically what is happening is that the additional stuffing converts the pressure waves to heat. Some sealed box designers use this function to reduce box volume by ten to twenty-five percent. However there is a finite limit to the amount of extra stuffing that can be used- beyond that point the stuffing begins to merely take up space and start actually reducing box volume.

HOWEVER, comma, most of this is not applicable to horn design inasmuch as the stuffing we use is to damp excess high frequencies. (to use your word: to 'de-ring' the enclosure)

Additionally, in a transmission line there are two competing theories regarding stuffing- one is that the stuffing merely attenuates high frequencies. the second is that in addition to HF atenuation, stuffing 'slows down' the speed of sound through the media and allows the line to appear longer than it's actual length. John Cockroft has some intersting articles in the now defunct SpeakerBuilder magazine.

<now stepping aside to duck the slings and arrows of controversy>

[Bill Fitzmaurice]

Basically what is happening is that the additional stuffing converts the pressure waves to heat.

It does, but only anecdotally, and you're not going to keep your house warm with the calorie or two of heat created. This claim is mainly made in advertising for damping materials, and as such most of the heat is hot air of the Madison Avenue variety.
What damping actually does is present a higher impedance load to the driver than air alone, with attendant alteration of response. You can model this in HornResp, which has the ability to model a box with damping of any desired thickness and density. You can see the results not only in the SPL charts but also in the impedance. Once you do so how stuffing works becomes very obvious.
As for the heat, that's created in every speaker via the friction that results from the compression of air by the motion of the cone. Very basic physics there. When damping material is added more mass is being compressed, and the heat increases. But even if you multiply the heat tenfold it's still infinitesimal.
In a TL stuffing absorbs unwanted higher frequency resonances, which also can be seen in an impedance chart. Inadequate stuffing won't reduce them sufficiently, while too much stuffing density can suppress the desired 1/4 wavelength resonant peak, and along with it the vent output.

Modeling software makes it possible to see how damping functions, but most of the theoretical treatises on damping predate modeling software by three or four decades, and more than a few designers of the modern era are still using these outmoded references.

[Frankenspeakers]

Basically what is happening is that the additional stuffing converts the pressure waves to heat.

It does, but only anecdotally, and you're not going to keep your house warm with the calorie or two of heat created. This claim is mainly made in advertising for damping materials, and as such most of the heat is hot air of the Madison Avenue variety.
What damping actually does ... ...
As for the heat, that's created in every speaker via the friction that results from the compression of air by the motion of the cone. Very basic physics there. .

Very true, I was merely mentioning the how, not the how much. As for heating the house- isn't that what those banks of 3W SET's are for?

[kesslari]

The house gets heated from the indignation of people seeing piezo's in high quailty speaker cabs...

[Bill Fitzmaurice]

The house gets heated from the indignation of people seeing piezo's in high quailty speaker cabs...

They're only indignant about how good we sound for so little, and how bad they sound for so much.