Audio & Electronic Research

background In my present listening room, I am unfortunately forced to listen very close to the rear wall. The strong reflections from the rear wall cause interference effects that are audible and negatively affect the perceived stereo image. If the listening position cannot be altered, the next best thing is to try to eliminate the rear wall reflection. A suitable acoustic absorber was therefore needed. The main listening area is a 3 seater couch about 2m wide that can seat people over a width over about 1.3m. Thus a suitable absorber should be at least 1.3m wide plus a bit extra as well, say 1.7m. The height also needs to cover the seated head height with some margin, so something at least 0.5m high is needed.

absorber design

The idea is pretty straightforward: A thick piece of acoustic foam to absorb all frequencies from high, to as low as we can reasonably go. As the figure below indicates, the low frequency performance is proportional to the thickness of the absorber. The thicker the better for absorbing low frequencies. The figure also shows that for each doubling of thickness, the limit of low frequency absorption halves. An absorption coefficient of 1.0 corresponds with all of the incident sound being absorbed, and zero corresponds with a completely reflective material. Absorption coefficients above 0.9 (-10dB) or so are needed for the absorber to be considered effective. So for a 50mm acoustic foam the useful low frequency limit of absorption occurs at about 400Hz. So in order to be effective to 100Hz a piece of foam needs to be about 200mm thick. That's a lot of foam!

The reason that thicker pieces are more effective at low frequencies is because when the sound wave strikes the wall, it's speed reduces to zero at the instant it changes direction. Conversely, particle velocity within the sound wave is a maximum at 1/4 wavelength. Also 3/4 wavelength and 1/2 wavelength multiples thereafter as shown below.

Acoustic foam absorbs sound by converting the energy of the moving air particles into heat. Where the particle velocity is greatest the absorption is most effective. So to place the absorptive material at the surface of the wall is actually the worst possible location from a low frequency point of view. Far better to move the material away from the wall to a region where the particle velocity is higher. Of course different wavelengths will have different 1/4 wavelength distances from the wall, but the spacing doesn't have to be too exact. So a simple way to increase the effective of any acoustic absorber at low frequencies is simply to space it away from the wall as far as possible. In fact, for a 50mm acoustic foam, it's performance becomes very close to 200mm foam when it's spaced 150mm away from the wall. A 75% reduction in the amount of foam needed. A very big saving!

It should be clear now why 25mm foam placed directly onto the wall is pretty much useless except at very high frequencies. As indicated in the section on acoustics, it's important to keep the reverberation times reasonably consistent at all frequencies. Adding a lot of absorption only within a small range of high frequencies will often make the room sound worse rather than better!

Aeropanel design

For this design it was convenient to use 50mm acoustic foam spaced 100mm from the wall giving a total depth of 150mm and effective absorption of 10dB or more down to about 150Hz. At very low frequencies other types of absorbers, especially Helmholtz resonators are more effective.

The width was chosen as 1.2m as this is almost large enough to cover the seated positions and also is the standard width of a piece of plywood. The height of the absorber only needed to be 0.5m, but in this case I decided to make it 1m high to add a bit of extra absorption to the room. 

The plywood sheet acts as support and decorative cover for the acoustic foam. Obviously holes need to be cut into the panel to allows incident sound to interract with the foam. Conventional wisdom says that about 5-10% open area is all that is needed for a solid panel to be acoustically transparent. The measurements that prove this to be the case are performed in an anechoic chamber at distances of several metres. However in this instance the listener is situated only tens of centimetres in front of the panel. Significant reflections occur close up with a small open area, and so it was decided to make the open area as large as possible while maintaining the appearance and structural integrity of the panel. The end result is a panel with approximately 27% open area.

The panel is 9mm thick which is enough to make the panel reasonably stiff with the cut-outs. The pattern of 3 slots was chosen for appearance reasons. I think the end result is very attractive. The natural grain of the timber is enhanced with a sanding sealer and top coat of polyurethane. The pictures don't do the panel justice. The sleek modern appearance reminds me of the best Scandinavian bent-wood furniture!

In order to provide a hanging point, an extra piece of plywood is connected perpendicular to the main panel about 2/3's of the way up. This also serves to space the panel away from the wall by the required 150mm. Additional fillet pieces of timber are installed to strengthen this join and help stop the panel bending visibly under it's own weight. A couple of strips of 50mm foam are installed near the bottom edge to push the bottom edge of the absorber squarely away from the wall.

performance

The panels work extremely well. The strong rear reflection is completely gone. The stereo image is greatly improved, remaining steady and coherent as the head is rotated and moved forwards and backwards. It doesn't sound totally dead either. Earlier experiments with just the foam on it's own gave a disturbing sense of deadness. As long as I faced forward the acoustic environment seemed OK. But when the head was rotated, there was a confusing contradiction between what was heard- no rear wall, and what was seen- a very real rear wall! The remaining wood facing of the panel adds enough of a sense of space to provide a coherent acoustic and visual environment.

conclusion

I can highly recommend this style of absorber for people is a similar situation and forced to listen at a location close to a rear wall. They are highly also recommended for the control of reverberation time when used in larger numbers within a room.  

If there is interest I can make these panels available as complete manufactured items. Contact details: aeropanel@aeronet.com.au