Effect of Rigid Fiberglass Absorbers On Surround Speakers

klh · « **on:** 9 Sep 2004, 07:08 pm »

Ethan et al,

I have a question. It is generally known that monitors (or any speaker for that matter) usually sound better when placed away from walls. Often times there isn't enough space in a room for the correct placement of surround speakers, thus close proximity to the walls would often times be preferred (WAF). If one placed a sheet of 2" 705 FRK behind surround speakers (monitors) that are, for example, 8-12" from the wall, would that make them sound essentially as good (if not better) than if they were further out into the room? Obviously the sound isn't as critical for surround speakers as it is for mains, and since monitors generally don't go that low in fequency, I thought this might work. Please give your comments.

Thanks,

Krister

MaxCast · « **Reply #1 on:** 9 Sep 2004, 07:15 pm »

Yes. By placing sound absorbtion near the speaker you would be reducing the reflection of the wall. You should also see if the point of first reflection can be treated as well. Of course the whole wall will reflect sound so you might as well do the whole room.

Ethan Winer · « **Reply #2 on:** 10 Sep 2004, 11:46 am »

Krister,

> If one placed a sheet of 2" 705 FRK behind surround speakers <

There are two issues at play here. One is Speaker Boundary Interference Response (SBIR) which is fancy word for the series of peaks and deep nulls at low frequencies caused by the speaker-wall distance. The key here is "low frequencies" which are not affected much by only 2 inches of 705 rigid fiberglass. SBIR is partly offset if you have a subwoofer, and your surround speakers are defined as "small."

The other issue is first reflections, as Max pointed out, and 2 inches of fiberglass is fine for that. In that case you won't put it directly behind the speakers, but rather at the actual reflection points.

--Ethan

klh · « **Reply #3 on:** 10 Sep 2004, 09:12 pm »

Thanks... both of you.

Ethan... Since we're dealing with monitors they would be crossed over probably somewhere in the range of 60-80 Hz, I think the interferance wouldn't be such a big deal. Hypothetically, if the cutoff was set at 60 Hz, how far should they be from the wall (what should the minimum distance be)? As for the first reflection point, that makes a lot of sense.

Thanks,

Krister

Ethan Winer · « **Reply #4 on:** 11 Sep 2004, 02:40 pm »

Krister,

> if the cutoff was set at 60 Hz, how far should they be from the wall <

There's no one correct distance. Put them a foot away and you have a problem at some frequency. Put them two feet away and the problem moves to another frequency. No matter where you put them you'll still have peaks and dips.

If you have ETF or a similar analysis program you can measure at many different distances and see which skews the LF response the least. But I have to tell you, I've done that. The response is pretty terrible no matter how far you space them from the wall.

The only way to really flatten things out is with bass trapping.

--Ethan