SOUND ON SOUND

[Hugh Robjohns]

It seems that we are trying to get the absorber as near as possible to where maximum number of particles are moving (high pressure)

Yes, you want the absorber where the maximum number of air particles are moving... but that's at the point of lowest pressure, not highest.

if you think about it, if the air particles are moving they aren't putting up any resistance so low pressure... but if the particles can't move you have high pressure...

... rather than how fast those particles are moving.

How fast they are moving is determined purely by the frequency of the sound waves -- high frequencies require them to move quickly (say 10,000 times a second) whereas low frequencies require them to move more slowly (say 100 times a second).

Even though a porous absorber works by velocity, its best placement is determined by pressure.

No, it's best placement is definitely where the air particle velocity is greatest, which coincides with the point where the air pressure is lowest.

[DanDan]

Consider a 'standard' 100 x 600 x1200 fibre trap. Say OC703, Alpha 0.84 at 125Hz.
Place it quarter wavelength about say 0.9M from the wall. Should be maximum absorption here right? No, the 125Hz Wavefronts will sail by taking little to no notice of this relatively small impediment, impedance even. Now consider a suspended ceiling, 20mm OC705 absorbent tiles, no blocking (CAC) layer. Say the gap is 400mm. This very thin layer, in conjunction with the void, will now absorb well down to 50Hz. Area matters too.