Absorption coefficient

[The Dudster]

I studied sound engineering for two years some years ago and one of the instructors there presented us with a lot of measurements of the absorption vs frequency for a lot of various materials. As far as I can remember, none of them have any significant absorption at lower frequencies. So why are you guys insisting of adding porous absorbents inside a subwoofer when it has no absorption at the frequencies the sub reproduces? If a sub is filtered with a second order filter at 80Hz, the 160Hz is attenuated by 15dB...Or am I missing something here?

And, does anybody have any freqency vs absorption plots for the various materials?

BAF

[sreten]

Simply put :

You are correct, a subwoofer does not need to be stuffed.

But it doesn't seem right, and stuffing will attenuate
resonances above the nominal passband of the sub.

In addition the stuffing increases apparent sub volume,
up to ~ 20%, so you deal with two things at once.

sreten.

[The Dudster]

Originally posted by sreten
Simply put :

You are correct, a subwoofer does not need to be stuffed.

But it doesn't seem right, and stuffing will attenuate
resonances above the nominal passband of the sub.

In addition the stuffing increases apparent sub volume,
up to ~ 20%, so you deal with two things at once.

sreten.

Yeah, well, not too sure about the second statement. Where does a subwoofer resonate? If you filter it at 80Hz, any harmonics will be very attenuated before they reach any area where the stuffing has any relevant absorptio, rigth?

The third statement: Stuffing will give you a smaller box, agreed, but that cannot be why people primarily stuff? And will the absorption material reduce the airspeed at lower frequencies enough to validate the +20% rule?

Not trying to pick a figth, just curious and not too sure everybody out there knows about absorption vs frequency.

BAF

[Svante]

Sreten is right in that the apparent volume of the box *increases* with added stuffing. The reason for this is thermal exchange between the air and the stuffing material. The air does not get as hot at pressure maxima or as cold at pressure minima as if there is no stuffing present. In theory, the box can appear up to 40% larger, but Sreten's 20% could be a practical maximum.
The apparent volume increase has nothing to do with reduced airspeed, but with thermal exchange, or if you wish isothermal vs adiabatic compression.

However, for the ported box, the Q of the resonator will also drop in the presence of stuffing. The net effect of this is that the box will have to be made larger to maintain the frequency response. This volume increase would typically have to be larger than what is gained by the stuffing as outlined above.

So if stuffing is needed or not comes down to how much the crossover filter has attenuated the first standing wave inside the box, and what effect this first standing wave has on the unfiltered response of the sub.

[CeramicMan]

Originally posted by Svante
Sreten is right in that the apparent volume of the box *increases* with added stuffing. The reason for this is thermal exchange between the air and the stuffing material. The air does not get as hot at pressure maxima or as cold at pressure minima as if there is no stuffing present. In theory, the box can appear up to 40% larger, but Sreten's 20% could be a practical maximum.
The apparent volume increase has nothing to do with reduced airspeed, but with thermal exchange, or if you wish isothermal vs adiabatic compression....

Hmmm, I think the speed of sound does in fact drop if stuffing is added, just as it drops significantly outside in foggy weather due to water droplets in the air. I think that pressure and temperature differences are only part of it. The effective mass of air also changes.

Since the pressure differences are smaller when stuffing is added, the load on the cone is lower (as with a larger volume), but the load becomes more resistive and less reactive. One thing I'm not sure about: in Subwoofer Simulator there are 2 properties regarding (I think) the air in the box - Ql and Qa. What exactly do they refer to?

CM

[Svante]

OK, maybe the velocity of sound is lowered. Depending on how you look at it the explanation will be different. The factor 1.4 I mentioned is theoretical and corresponds to the difference between isothermal and adiabatic compression, so definitely the effect can be that large from temperature effects only.

And yes, the load becomes more resistive due to friction losses, and less capacitive due to the apparent increase of volume. The increased resistance raises the volume requirements in the BR design.

I don't know about Subwoofer Simulator, but it is common to model two losses in BR designs, one corresponding to stuffing in the box and one for the friction losses and radiation resistance of the port. Could that be it?