John Sayers' Recording Studio Design Forum Archive

This might sound funny but:

I have a sofa that acts as a bass trap. It peaks at 100Hz (cancellation) and it is most noticeable in the middle (exactly where one would want to sit). At 100Hz the cancellation is complete! I originally thought it was the room, but moving the sofa/speakers around I realized it was the sofa.

It is an IKEA KILLPAN sofa:
http://www.ikea-usa.com/webapp/wcs/stor ... 0114*10294

Frame: Solid wood, Particleboard
Seat/ Back/ Armrest: Polyurethane foam, Polyester filling
Width: 70 7/8 "
Depth: 34 5/8 "
Height: 27 1/8 "

Underneath there are two openings:

1.Under the seat – 70x27 inches and about 4in deep.
2.The back – 70x4 inches and about 25in deep – this part gets larger inside to probably about 10 inches, thus creating a sort of opened canister.

I thought of covering these with plywood, only to realize that it might create a better bass-trap in the process. I don’t know which of the openings is responsible for the 100Hz dip. If it were the back opening, it would be relatively easy to cover it with some sheet steel. I don’t know about the larger one.

Help!

M,

> At 100Hz the cancellation is complete! I originally thought it was the room <

It is the room. Bass traps do not reduce the level of bass in a room, they almost always increase it. Plus, a sofa is not likely to absorb much bass anyway unless it's in a corner. And if it does absorb bass, what it absorbs is the reflections off the walls, floor, and ceiling that cause a reduction in bass levels. So what you need are real bass traps that provide more than just the small amount of trapping you get from a sofa.

Besides all the great info on this site, also see my Acoustics FAQ. It's the 10th article in the list here:

www.ethanwiner.com/articles.html

--Ethan

you might try moving the sofa away from the wall by a foot or so and see what happens then.

if by some strange process of physics it IS the cavities in the sofa causing problems, it's more likely a problem with them being open, rather than closed. i would try sealing the cavities and perhaps filling them with something absorbant (fiberglass, mineral wool, foam, etc).. and see how that effects the sound. it's always 'undoable'...

but i think ethan is probably right. it's more likely the position of the sofa in the room, convienently places your head at exactly the right spot to cancel out that freq... if you move away from the boundy (wall) it might decrease this effect.

having a bass trap behind the sofa would probably resolve the situation, as this would both push the sofa forward and absorb enough bass to minimize the effect..

what are the room dimensions? if you put those through a room mode calculator, i'll bet you'll find your answer there.

_illium

Thank you for the replies.

I further investigated and:

1. As I sated previouslly, moving the sofa forward 3 feet has no effect. Moving one's heat forward 2 feet while still sitting changes everything.

2. I moved the sofa completely away from the listening area and placed a chair. The problem is not remarkable. There is some dip but nothing comapred to the sofa.

3. Sitting on the sofa, now 6 feet to the right of the listening position gives the same effect.

This would seem to indicate that the sofa is doing something.

I will seal the main cavity and post the results.

M

MF,

> moving the sofa forward 3 feet has no effect. Moving one's heat forward 2 feet while still sitting changes everything. <

Which proves my point - it's where you are, not where the sofa is.

Again, the sofa could absorb a little. Maybe even a lot if it's big and made of absorbent materials. But sofas are not usually considered as viable acoustic treatment!

--Ethan

Thanks, but

1 .I ment to say that moving my head forward while sitting on the sofa had a huge effect.

2.If I move the sofa forward and keep my head in the usual sitting position, that means my head is now in the same place as 1. The cancellation is present.

In this 2 cases my head is at exactly the same spot. The sofa is in a different position in relation to my head.

As soon as I have time I will start closing one of the cavities.