Standing Waves

Posted by: David Hobbs-Mallyon on 02 October 2001

I wondered whether anyone could shed any light on this - room acoustics always seems to be a bit of a black art. Anyway, my system has a resonance which seems to occur somewhere at the bottom of the frequency range of a cello. Despite lot of attempts to solve this, I've had no success (apart from putting the speakers in another room). How would I calculate the wavelength of a note of a cello - might give me some indicator as to which dimension to dampen.

David

Posted on: 02 October 2001 by John Channing

There are a few things you should remember:
1) The speed of sound is dependent on humidity and temperature, but is roughly 331m/s.
2) The eigen modes of a room are in half wave lengths. Think of it like a guitar string, with the wall being where the string is attached. The first resonant mode is when there are nodes (points of no displacement) at each end and a single anti-node in the middle. Therefore a room with one length of 20ft will have its first resonance at about 27-28hz.
3) However, it is not quite that simple, because a room is a 3 dimensional space and you cannot work out its modes simply by teating it as a 1-d resonator along its width, length, and height. To work out the modes exactly you need to model it in k-space (I had this exact question in the finals of my physics degree. It has an exact analogy with the problem of a particle confined to a box in quantum mechanics, see Particle in a Cubical Box )
John

Posted on: 02 October 2001 by David Hobbs-Mallyon

John,

I thought my maths was alright, but not to that level. Is 1-d a good approximation?

David

Posted on: 02 October 2001 by Greg Beatty

David -

Are you sure that what you are hearing is, in fact, a room resonance? It may be, but possibly not.

* Do you hear the frequency peak through headphones?

* Do you hear it with all sources?

* Did you hear it when the kit is setup in a different room?

* Does moving the speakers help/hurt/or have no effect?

* Have you tried different supports for the kit and/or speakers? This isn't so hard for your purpose. Move the kit to a traytable or nearby shelf or the floor. Plop your (standmounted) speakers on a table or put a board under your (floorstanding) speakers. You're listening for a significant difference here - not necessarily a drastic improvement. If stands are the issue, then you can try different stands or a different setup (loosening/tightening/change in mass) of your current stands.

Play around and see if you can isolate the problem. Of course, it could be a room resonance but you can drive yourself mad trying to get rid of it if it isn't.

- GregB

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Posted on: 03 October 2001 by David Hobbs-Mallyon

Thanks for the replies guys,

The dimensions of my room are 15.5 ft x 10.5. Currently I have the speakers firing down the longer dimension. The speakers are about 15 inches from the wall and about 6 ft apart. On the outer side of each is a bookshelf and a cupboad, so clearance on either side is only about 1.5 ft.

Re: supports - I use Hutter, and this is light years ahead of my previous stand - this will not be changing.

As mentioned previously - the sound does not occur if you move the speakers (but not the system) to another room.

The reason I was asking about the wavelength, is that I spoke to someone who mentioned, it could be a smaller dimension, such as the distance between fireplace and back wall or distance between shelf struts.

Firing down the other dimension of the room is not really practical. I have tried the speakers at the other end of the room, but this sounded too clinical in comparison.

David

[This message was edited by David Hobbs-Mallyon on WEDNESDAY 03 October 2001 at 12:20.]

Posted on: 03 October 2001 by John Channing

quote:
I thought my maths was alright, but not to that level. Is 1-d a good approximation?

Actually, without working the numbers out I don't really know how good the 1-d approximation is. At some point I will attempt to write some VB or Java code that works out the resonant modes of a regular shaped room (i.e. a rectangular box).
John

Posted on: 03 October 2001 by Martin Payne

quote:
Originally posted by David Hobbs-Mallyon:
On the outer side of each is a bookshelf and a cupboad, so clearance on either side is only about 1.5 ft.

David,

I suggest that you check that any back-boards of the bookshelf/cupboard are not resonating. Try hitting them with a closed fist and listen for the frequency of the 'thump'.

Check also for similar from the sofa, and any other large furniture.

cheers, Martin

Posted on: 04 October 2001 by David Hobbs-Mallyon

Thanks for the replies.

I have carpet on the floor so don't think there are so many problems there. I also quite recently filled the floor void with sound insultation material which has helped to tighten the bass up, although the resonance is still there.

One thing I forgot to mention, is that at the back of the room, there is a false wall, which seems to consist of very thin board. I'll look into this in a bit more.

David