depth or area covered?

[Pier]

So, what does absorb low frequencies better?

For the same number of acoustic panels, for example roxul rw60:

It's better having a trap of 8'' of acoustic material and 2'' gap of 2 feet width x 4 feet high? (less area but deeper)

Or having a trap of 4'' of acoustic material and 2'' gap of 2 feet width x 8 feet high? (more area but )

[avare]

Your examples are not practical alternatives acoustically. The low end absorption is controlled the most by the depth of the absorber overall. For 4" to 12" deep traps, use 4" RW60, with the balance of the trap being a gap.

Andre

[Pier]

mmm Ok

So let's say I'm using 4'' of rw60, then making the gap of 4'' instead of 2'' would make a difference?

[Mulmany]

Yes.

It has to do with the acoustic energy being greater the farther away from the wall you get (lows). Acoustic energy is at a minimum at a boundary (not good for a panel absorber placed on the wall) while the pressure is at a maximum at a boundary (good for a resonator).

PS. There are a lot of good threads on the topic.

[Pier]

Originally Posted by Mulmany

Yes.

It has to do with the acoustic energy being greater the farther away from the wall you get (lows). Acoustic energy is at a minimum at a boundary (not good for a panel absorber placed on the wall) while the pressure is at a maximum at a boundary (good for a resonator).

PS. There are a lot of good threads on the topic.

Thanks I will research that.

But isn't acoustic energy the same as pressure?

[SAC]

Velocity is at its greatest at a distance of 1/4 wavelength (note that this position is not constant as it is relative to frequency).

Velocity is zero at a boundary surface, but pressure is greatest.

Porous absorption works by dissipating particle velocity as heat, and is thus most effective when located in regions of highest velocity..

Resonant absorbers work by dissipating pressure as heat.Thus resonant absorbers function most effectively in regions of highest pressure.

...hope that simplifies the relationships just a bit...

[Mulmany]

I always forget that the term is velocity. Its in my head it just does not always come out right.

Thanks SAC!

[Pier]

Originally Posted by SAC

Velocity is at its greatest at a distance of 1/4 wavelength (note that this position is not constant as it is relative to frequency).

Velocity is zero at a boundary surface, but pressure is greatest.

Porous absorption works by dissipating particle velocity as heat, and is thus most effective when located in regions of highest velocity..

Resonant absorbers work by dissipating pressure as heat.Thus resonant absorbers function most effectively in regions of highest pressure.

...hope that simplifies the relationships just a bit...

1/4 of the wavelength? I'd say it's 1/2 as maximum velocity happens when pressure is 0.

If you say the wave starts when pressure is maximim/minimum, then maximum velocity happens at 1/4 and 3/4.

But, what happens when a wave hits a wall and bounces back?
Does it reset it's phase?
Or does it simply bounce with no phase distortion?

[SAC]

Generally speaking, in a bounded space:

The velocity of a sound wave is greatest at odd ¼ (pi/2) wavelength multiples. Conversely the pressure is at a minimum at these points.

The velocity of a sound wave is at a minimum (0) at the boundary and at even multiples of ½ wavelength (pi). Conversely, the pressure is a maximum at these points.


For sound transmitted through a medium that encounters a boundary medium exhibiting a greater acoustical impedance, the wave reflection has no change in phase.

If the encountered boundary medium exhibits a lower acoustical impedance than the medium of transmission, the wave is reflected and the phase is reversed.