Effect of acoustic foam on Speaker enclosure Volume

[kchilaka]

Folks,
There have been some posts regarding the effect of stuffing on speaker enclosure volume but this regards acoustic foam rather than wool or polyfill stuffing. Here is my situation

Raw internal speaker volume = 92 litres (46x8.5x14.5 inches)

I used two inch wedge shaped acoustic foam on all internal surfaces.. Since these are wedge shaped I am estimating average thickness at 1 inch for the acoustic foam.
So that leaves me a net internal volume = 58.6 litres (44x6.5x12.5 inches).

I initially did the lining of the speaker walls assuming that the foam would not effectively alter the speaker volume. I assumed(probably incorrectly) that the reduction in volume would be compensated by the porosity of the open-cell acoustic foam..

Initially, my enclosure was tuned to 40Hz for my drivers which have a Fs of 39Hz and my ears tell me that the tuning was spot on..

However, after adding the acoustic foam lining, I seem to have pushed the tuning up but about 8 to 10 Hz..

Have other people seen this in their designs?

[BlueWizard]

Others may disagree, but the insulation is really part of the cabinet tuning. I and a couple other people got the bright idea that the original manufacturer did not put enough Poly batting/quilting into our bookshelf speakers. So we added more, and that completely messed up the tuning of the cabinet.

In my case, I ran some basic low end plots and discovered that when I added more Poly batting, it created a peak in the bass, which one might think would be nice. However, just above that bass peak, it created an even greater bass DIP or drop.

In the end, I discovered that the manufacturer had indeed put exactly the right amount of insulation into the speaker.

If it is possible, I would start with the Foam on the back of the cabinet. Then using Audio Test Tone and an SPL Meter, plot the frequency response between say 30hz and about 200hz. Lay that out on a graph on Single Log Graph paper. Though a spreadsheet or normal graph paper would also be fine.

Now add additional Foam to the bottom (nearest the woofer), and plot again. Then add Foam to the Top (nearest the Tweeter), and re-plot. Now add Foam to each side and re-plot again. Compare the plots. Does one give a better smoother response?

There is an ideal amount of internal Foam (or other) Damping for any given speaker.

Also, you may find that Foam at the back or alternately Foam at the top and bottom, and Poly quilting on the surfaces gives you the correct tuning.

So, what I'm speculating is not a massive change in internal volume, but rather over-damping or under-damping. Though with 2" wedge foam all round, I suspect over-damping.

For what it is worth.

Steve/bluewizard

[454Casull]

Originally Posted by BlueWizard

Others may disagree, but the insulation is really part of the cabinet tuning. I and a couple other people got the bright idea that the original manufacturer did not put enough Poly batting/quilting into our bookshelf speakers. So we added more, and that completely messed up the tuning of the cabinet.

In my case, I ran some basic low end plots and discovered that when I added more Poly batting, it created a peak in the bass, which one might think would be nice. However, just above that bass peak, it created an even greater bass DIP or drop.

Sealed or BR?

[BlueWizard]

In this case, of myself and other people I know, they were Bass Reflex.

But that's a good point. I'm sure the Damping material does matter in Acoustic Suspension or Sealed cabinet, but I suspect (or speculate) that it matters in different ways.

Steve/bluewizard

[454Casull]

Originally Posted by BlueWizard

In this case, of myself and other people I know, they were Bass Reflex.

But that's a good point. I'm sure the Damping material does matter in Acoustic Suspension or Sealed cabinet, but I suspect (or speculate) that it matters in different ways.

Steve/bluewizard

In sealed boxes all it does is increase the effective internal volume. In many cases you can get up to 30% extra volume from the same space.

BRs are a different story but damping may help those too.

Volume filling a reflex box