ESL with acoustic foam

[andrelim]

Regarding bass on ESLs.
"Bass. At low frequencies, as wavelengths become large relative to panel width, the out-of-phase signals from opposite sides of the diaphragm tend to wrap around and cancel each other. So like all true dipole speakers (not to be confused with conventional-driver quasi-dipole surrounds), electrostatics are prone to weak bass. Increasing panel size helps with this, as well as with the output/efficiency problem discussed above, but there's a practical limit."
-Martin Logan website
Would it be possible to place lots of acoustic foam on one side of a electrostatic loudspeaker to absorb the out of phase signals?
Would that increase the bass or is there some problem that will crop up?

[bolserst]

Originally Posted by andrelim

Regarding bass on ESLs.
Would it be possible to place lots of acoustic foam on one side of a electrostatic loudspeaker to absorb the out of phase signals?
Would that increase the bass or is there some problem that will crop up?

In general acoustic foam is not very affective at absorbing low frequencies, below about 300 Hz.
To block the out of phase bass from the rear of the ESL you pretty much have to use a physical barrier.
For their full range model Martin Logan adds a large baffle extension on the side nearest the bass panels to increase the bass output.
Adding an additional baffle extension on the other side of the panel would have increased the bass output further,
but the response and radiation pattern of the curved mid-treble panel would be adversely affected.
Also, with two large side baffle extensions, a cavity would be formed behind the panels which would potentially add an undesirable cavity resonance.

One other possibility was mentioned in the Williamson-Walker US patent 3008013:
Replace the baffle extensions with active ESL panels that are only driven at low frequencies where the phase cancellation is most problematic.
At higher frequencies, these panels will be essentially acoustically transparent so they will not induce a cavity resonance or adversely affect the mid-treble response.

[Calvin]

Hi,

as bolserst already said, foam layers of sufficient effectiveness would be of unpractical huge physical size and terrible optics too. But besides reducing the SPL on the foamed side this could lead to a positive shaping of the distribution character towards a ´kidney´. But still it wouldn´t help with the main problem, which is that an ESL is no good bass in first place.

jauu
Calvin

[andrelim]

What about installing a ESL panel in a wall?
The out of phase signals would be blocked by the wall.

[DaveG]

Beveridge speakers absorb the rear wave. Model 2 are about 18" deep.

[Calvin]

Hi,

installing a panel into a wall would basically mean the same as to put a dynamic driver into a cloased box. But while the high mass of the drivers moving system leads only to an acceptable rise in resonance frequency the Fs of the ESL could rise undue high. Because of the diaphragm beeing virtually acoustically transparent utmost care needed to be taken that no internal reflected soundwaves could pass the membrane (something I dislike about the simple flat Beveridge casing). Peter Walker suggested rather deep shallow casings with lots of wadding and possibly open to the end.

jauu
Calvin

[andrelim]

If acoustic foam doesnt absorb frequencies below 300hz, then what about mineral wool insulation boards used in bass traps? Would around 18" of it behind a ESL panel sufficiently absorb the out of phase signals?

[andrelim]

What about micro perforated plates? I know the stators are made of perforated metal but would additional layers behind absorb sound? Since each plate is 0.5mm - 2mm, it would be possible to stack many layers without reaching unpractical sizes and making the ESL unsightly.

[bear]

Metal plates will reflect strongly at HF, and not absorb at LF... I think you are thinking of a Helmholtz resonator. That is a possibility, although problematic because of the inevitable reflection at some frequency related to the distance to the diaphragm...

Mineral wool, or any other absorptive material, sure, but you'd need about 3 feet or better of whatever that is back there to be effective down to useful bass freqs... go for it...

Or you could go listen to some Acoustats - ESL dipoles with plenty of bass.

There have been discussions of DIY Acoustats here and elsewhere...

_-_-bear

[andrelim]

I was referring to Micro perforated plate - Wikipedia, the free encyclopedia
"A Micro Perforated Plate (MPP) is a device used to absorb sound, reducing its intensity. It consists of a thin flat plate, made from one of several different materials, with small holes punched in it. An MPP offers an alternative to traditional sound absorbers made from porous materials...When the oscillating air molecules penetrate the MPP, the friction between the air in motion and the surface of the MPP dissipates the acoustical energy."
-Wikipedia