Accoustic Foam.... I think I'm crazy....

Kind of but not exactly. I was thinking more along the lines of a speaker which had wide dispersion inside the axes of the side walls but not outside. Hard to explain I guess but I suppose if we thought of sound projecting out from the speakers in a wide cone, but cut that cone in half rather than having a very narrow cone focussed on the listening position (which I think is how directional speakers work - happy to be told I'm wrong).

 

You just desribed a planer type speaker ...unless I'm missinterpriting things. Like an electrostatic.

 

Trying to keep up, honest, but...

Does a tile behind a speaker that works in LF (cos that's why it's there no?) have to be thick?

 

I dunno, but I don't think so. I'm a bit of ignoramus when it comes to these things and can see what I'm imagining in my head but find it dificult to put into words.

Maybe if I used a light analogy. If I were to think of a normal set of reasonably wide dispersion speakers as like a set of headlights with quite a wide conical beam - shining into a garage with white walls - would that be a fair analogy? Light going out to the side of the headlights would reflect back in off the walls, but if you put some black light absorbent blinkers on the outside edges of the lights to catch the half of the beams going out, you wouldn't get the side reflections - between the blinkers it'd be the same just without the wall reflections ... unlike narrow beam spotlights which would I guess be more akin to the directional speakers described by Tenson. Wouldn't speakers blinkered in this way work without producing a specific hotspot so long as you sat within the space between the blinkers and without reflections off the walls?

 

Paging 7V...

Isn't this the kind of area that Steve is really into?

 

I did already discuss it with him briefly and he didn't think it daft, but thought it would be best to just try it out (needless to say I haven't ) Time, y'know. But think I'll give it a go. I was thinking some acoustic foam stuck to a speaker high board on feet, two of them. Put them in place when listening and out of the way when not, thus getting over the whole WAF problem with acoustic treatment on walls.

 

I can go for that analogy; for the purposes of experimentation this would be the same as setting your speakers up outside... which is what some speaker designers do I believe.

 

Ideally as thick or thicker than those wavelengths of frequencies that you wish to absorb. As example 100Hz has a wavelength of about three and a half meters, this is where the problems with LF porous absorption begin. If you use a porous absorber with thickness at least equal to the one-quarter wavelength of a given frequency (for 100Hz about 88cm) then you'll achieve some degree of useful absorption.

 

The frequency at which foam is effective is dependant on its thickness, due to the wavelength of the sound at that frequency. You could not make a foam tile an inch thick that is effective to 20Hz because an inch is neither here nor there compared to the wavelength of 56ft. It is very significant to the wavelength of 1kHz of 1.12ft or 10kHz wavelength of 1.3 inches.

That is why for effective LF absorbtion in a domestic environment (where several feet of foam may not be acceptable), resonant traps have to be used.

 

Beaten to the punch there...

 

This hi-fi lark is getting very complicated, think Im just going to get a Bose Wave, the guy on QVC said it'll fill any room with full sound...............

 

There are two ways to 'cheat' a little with LF porous absorption using relatively thin materials.

The first is to fix a foam tile (100mm as example) at a distance from a room boundary that equates to either the one-quarter or three-quarter wavelength of a given frequency. These are the points where wave velocities are highest and consequently where most absorption (or more properly energy conversion) occur. Hanging pairs of 100mm foam tiles from the ceiling at distances of about 88cm and 264cm from a room boundary will be useful at frequencies around 100Hz.

The second method is to increase density - essentially taking a thick material and compressing it.

Both very low WAF

 

So if we are talking about the area immediately behind a speaker, close to the rear (or do I mean front?) wall (eg. wall mounted) and assuming a small driver, are there relatively small traps that work in this area? (as opposed to corners etc.)

 

I think the bit of cheating that I'm on about is: can you make up for not having space behind a speaker by putting absorption in that area?

 

there are devices available that can go down as low as 50hz, and are 4" thick, but they cost mucho cashos

 

A pair of 2ft by 4ft by 4†thick rockwool panels mounted on 2†batons and screwed to the wall would be worth experimenting with as a minimum. You could also try removing the inner polyfil cushions of a sofa (take off the outer covers because these are reflective) and experiment with these Coda.

Helmholtz and diaphragmatic absorbers are another alternative. In short however it's a real bugger to try to 'soak up' long wavelengths using any method. EQ is the only real domestically acceptable solution at LF.

 

But EQ isn't applied in the time domain, only frequency, whereas standing frequencies take a while to build up - half a second or so - hence if you EQ down such a peak in frquency (only) you unnaturally weaken the leading end (in time) - is that not so?

 

Matty will those be the Fraunhofer Plate absorbers, AKA Modex Plates?

I downloaded the patents on them a while back and they're a blindingly simple design, also according to the patent application they give acoustic isolation as well as acoustic control in the room. I did find a $ price listed somewhere and they were hideously expensive.

 

Thats the ones, Broadband and Plates, between £300 - £400
very simple design as you say, very effective too, and very hard to get hold of, we have many on order and none coming through the door

 

So how do they work then? If they are effective down to 50Hz, that's almost a room treatment holy grail or philosophers stone isn't it?