Acoustic foam - a quick and totaly non-scientific test

[SaSi_SiDi]

I recently purchased 18 sheets (2x4) of ~2" thick eggcrate looking acoustic foam. The moulding is pyramid looking.

I bought them to use for inside dressing of several loudspeakers, not for acoustic treatment, although one can say they are there (in the enclosures) to do exactly the same thing, fight standing waves in a very very small room...

So, I thought it was a nice opportunity to spread them around the room or even superchunk them to see what they do.

Test 1: stacked against the wall forming a 2x8 panel 6" deep. They had no effect. The 4" floor to ceiling bass trap on the opposite corner had a very notable effect once placed there.

Test 2: Spread them around the walls and on shelves to maximise covered surface. The difference in the highs was noticeable. Even the difference in room luminance was noticeable with all this grey material spread around. I felt the need to boost the treble a bit. However it was not bad. After a few moments of getting used to it, the sound appeared a bit "cleaner" and that probably was reduced flutter echo (of which my still untreated room with just a single bass trap has in abundance).

However, there was no noticeable difference in the low frequencies. I used a test piano passage with frequencies not exceeding flat A (440Hz) and the short notes are muddy, especially at higher volume levels.

I really wonder what good foam does inside a loudspeaker but I guess I already have the answer as I also bought two rolls of 4mm bitumen sheets as well as two bags of dacron to fill up the enclosures. I only wish I could make rockwool get inside the enclosures but I need to break it up and then chunk it inside. Messy business.

[Ethan Winer]

Originally Posted by SaSi_SiDi

I really wonder what good foam does inside a loudspeaker

The purpose of insulation inside a sealed (acoustic suspension) speaker cabinet is to lower the resonant frequency of the box. The same thing happens when bass traps are added to a room. The mode frequencies are lowered so the room becomes acoustically larger than its dimensions imply.

--Ethan

[SaSi_SiDi]

Originally Posted by Ethan Winer

The purpose of insulation inside a sealed (acoustic suspension) speaker cabinet is to lower the resonant frequency of the box. The same thing happens when bass traps are added to a room. The mode frequencies are lowered so the room becomes acoustically larger than its dimensions imply.

--Ethan

Indeed, and KEF have been pioneering in building lightweight enclosures (18mm chipboard for a 75lt box) and reducing resonance by mild bracing and bitumen sheets on the inner walls.

There is so much common ground between a small listening room and the inside of a loudspeaker enclosure that I wonder if the behavior of the enclosure has been modeled like a listening room - to tame modes. It strikes to me that if we have 20dB variances in the output inside a room, there is bound to be similar behavior in a loudspeaker enclosure causing similar feedback to the woofer cone.

But come to think of it, the techniques are similar, save for fluffy wool stuffing of the entire volume. That would make the room a bit unfomfortable, but I guess it would help the listening conditions a lot.

[Ethan Winer]

Originally Posted by SaSi_SiDi

There is so much common ground between a small listening room and the inside of a loudspeaker enclosure

Yes, except for one very important difference. A speaker box is much smaller than the low frequency wavelengths being produced. So inside the box there are no modes at woofer frequencies. Instead you have only changes in air pressure.

--Ethan