Re: A really good listening room - how to make
The DIY trap looks like an excellent idea - maybe Harbeth should make something like that - but I seriously doubt the claim that it is effective at 80Hz.
To be effective at 80Hz or so, the absorber material would have to be about 1/3 wavelength deep; the wavelength of 80Hz is 4.2m and one third of that is 1.41m. Clearly this DIY absorber is about 0.3m deep which means that its efficiency at absorbing 80Hz will be negligible, but it will be effective at absorbing about 300Hz at which it is about 1/3 of the wavelength deep.
However, it could be that the combination of the three pieces of (what look to me like fibreglass) on top of the hidden inner cloth and then the small air gap under the cloth to the wall could somewhat increase the absorption, but I'd say, from appearance, that this DIY trap whilst effective in the midrange - and therefore most worthwhile - is ineffective or useless at low frequencies. It's just doesn't protrude from the wall far enough to prevent any obstacle to the low frequencies in the room. To absorb LF you need a very sophisticated absorber made from sprung sheets of heavy, roofing felt or similar held over a deep frame and tuned so that the sheets flap (very slightly) in the presence of the LF and hence energy is transferred from sound to heat at those frequencies. In other words, you need a clever combination of mass, springiness, depth and natural absorption: a few floppy pieces of fibreglass will present no resistance to a low frequency sound wave at all.
Also - I strongly recommend not to use fibreglass for health reasons. I believe Rockwool to be far less irritating when inhaled.
Alan A. Shaw
Harbeth Audio UK