Perforated absorbers - experiences

SaSi_SiDi · 14th September 2009

I was reading about perforated absorbers yesterday considering a large absorber panel for the front wall (between the speakers) to double as a projection screen.

I was originally considering a large mineral wool panel for that location (320x180cm) but am thinking that it would cause too much high frequency absorbption as well.

It appears that an 8" (20cm) thick box with a 5/32" (4mm) front panel drilled to 0.2% perforation and using 4" thick mineral wool in the back would tune well below 100Hz.

I am considering this, perhaps with a thicker mineral wool layer inside (say 6") and a breathing cloth covering the front panel - serving as the projection screen.

As I understand it, the mineral wool inside and the cloth covering the panel, would serve so as to decrease the Q factor, making it a wider band low frequency tuned absorber.

There is a dominant 30Hz mode in the room and it's harmonics as well as a 49Hz mode from the room width and a 58Hz from the height. "Ignoring" that 30Hz mode, I am trying to find ways to fight the 50 ~ 150Hz range. Would that perforated absorber do the trick? Or would it be too much of an effort for too little effect? Could it be that a plain 4" panel of the same dimensions (~10ft x 5ft) at 4" distance from the front wall perform better?

SaSi_SiDi · 18th September 2009

Guess I will have to spend some time and plywood and find out doing some tests.

bpape · 18th September 2009

Based on the issues you're having, I think using a sealed membrane type absorber of the same thickness would do a better job for you.

In either case, the cloth would have to be held off of the front face by at least 1/2".

Bryan

Tomer1 · 20th September 2009

Quote:

the cloth would have to be held off of the front face by at least 1/2".

Does this has to do with unwanted dumping which may be coused by the cloth?

SaSi_SiDi · 21st September 2009

Quote:

Originally Posted by bpape

Based on the issues you're having, I think using a sealed membrane type absorber of the same thickness would do a better job for you.

In either case, the cloth would have to be held off of the front face by at least 1/2".

Bryan

As I've read, cloth is usually designated as touching the perforated absorber. I've read that the porous absorber behind the tuned panel absorber needs to have some distance from the membrane to have effect.

Can you please explain the distance?

Brainchild · 21st September 2009

I was wondering the same thing. If the decorative cloth were placed directly against the front of a sealed-membrane panel, it could potentially damp the vibration of the panel. It seems like it would have to be pretty heavy cloth to have a significant effect, though? If there were significant porous absorbent like fiberglass pushed up against the back of a sealed membrane, it could very easily over-damp the vibration of the panel, keeping it from building up enough acoustic energy to then be dissipated by the fiberglass for the absorber to be effective.

For perforated panels, though, the fibrous absorbent is exponentially more effective at damping the frequency of interest when it is abutted directly against the ports/perforations. Those tuned Helmholtz traps are ridiculously sensitive to the properties, quantity and placement of the absorbent within the cavity.

14th September 2009	#1
SaSi_SiDi Gear maniac Joined: Mar 2009 Location: Athens, Greece Posts: 172 Thread Starter	Perforated absorbers - experiences I was reading about perforated absorbers yesterday considering a large absorber panel for the front wall (between the speakers) to double as a projection screen. I was originally considering a large mineral wool panel for that location (320x180cm) but am thinking that it would cause too much high frequency absorbption as well. It appears that an 8" (20cm) thick box with a 5/32" (4mm) front panel drilled to 0.2% perforation and using 4" thick mineral wool in the back would tune well below 100Hz. I am considering this, perhaps with a thicker mineral wool layer inside (say 6") and a breathing cloth covering the front panel - serving as the projection screen. As I understand it, the mineral wool inside and the cloth covering the panel, would serve so as to decrease the Q factor, making it a wider band low frequency tuned absorber. There is a dominant 30Hz mode in the room and it's harmonics as well as a 49Hz mode from the room width and a 58Hz from the height. "Ignoring" that 30Hz mode, I am trying to find ways to fight the 50 ~ 150Hz range. Would that perforated absorber do the trick? Or would it be too much of an effort for too little effect? Could it be that a plain 4" panel of the same dimensions (~10ft x 5ft) at 4" distance from the front wall perform better?

18th September 2009	#3
bpape Lives for gear Joined: Jul 2005 Location: St. Louis(Wildwood), MO Posts: 764	Based on the issues you're having, I think using a sealed membrane type absorber of the same thickness would do a better job for you. In either case, the cloth would have to be held off of the front face by at least 1/2". Bryan __________________ I am serious, and don't call me Shirley Bryan Pape Lead Acoustical Designer GIK Acoustics

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21st September 2009	#6
Brainchild Lives for gear Joined: Jan 2009 Location: USA Posts: 988	I was wondering the same thing. If the decorative cloth were placed directly against the front of a sealed-membrane panel, it could potentially damp the vibration of the panel. It seems like it would have to be pretty heavy cloth to have a significant effect, though? If there were significant porous absorbent like fiberglass pushed up against the back of a sealed membrane, it could very easily over-damp the vibration of the panel, keeping it from building up enough acoustic energy to then be dissipated by the fiberglass for the absorber to be effective. For perforated panels, though, the fibrous absorbent is exponentially more effective at damping the frequency of interest when it is abutted directly against the ports/perforations. Those tuned Helmholtz traps are ridiculously sensitive to the properties, quantity and placement of the absorbent within the cavity.