Absorption coeficients for Hemholtz type absorbers

Post and discuss acoustic topics, Studio design, construction, and soundproofing here

Absorption coeficients for Hemholtz type absorbers

Postby Guy Staley » Fri Mar 05, 2004 7:02 am

Hi all!

I'm new here, but am quite happy to have found this forum as acoustics is a new passion of mine! I've been working professionally as a recording egineer since 1990, but have recently made the transition to studio owner. Woo hoo...er, I mean darn!

At any rate, I'm looking to build hemholtz type absorbers (i.e. - slat and/or perforated panel) and can't seem to find any data on calculating absorption coeficients. My instincts tell me that it's somehow directly proportional to perforation percentage, but that's as much as I can infer. Any help in this area would be most appreciated.

Cheers from wet and beautiful Seattle, WA.
Guy Staley
 
Posts: 36
Joined: Tue Mar 02, 2004 6:01 pm
Location: Seattle, WA USA

Postby Scott R. Foster » Fri Mar 05, 2004 11:34 am

Guy:

Here's some free advice guaranteed to worth what you paid to get in here:

Don't build a resonating absorber.

Use broadband absorption instead. Start with thick panels of cloth covered mineral fiber mounted across the diagonals of all corners running full hiegth [from floor to ceiling].

Good Luck!
SRF
Scott R. Foster
 
Posts: 3854
Joined: Mon Feb 02, 2004 12:41 pm
Location: Jacksonville, FL USA

Re: Absorption coeficients for Hemholtz type absorbers

Postby Eric.Desart » Fri Mar 05, 2004 2:27 pm

Hi Guy

Guy Staley wrote:Hi all!

At any rate, I'm looking to build hemholtz type absorbers (i.e. - slat and/or perforated panel) and can't seem to find any data on calculating absorption coeficients. My instincts tell me that it's somehow directly proportional to perforation percentage, but that's as much as I can infer. Any help in this area would be most appreciated.

Cheers from wet and beautiful Seattle, WA.


The only thing is searching for analogue applications.
You will not find easy formulas on the net. Calculating resonance frequencies is known, the influencing phenoma in function of Q and absorption coefficient are partly known and understood, but are complicated to model.

While this is a frustrating answer:
Your instinct tells you wrong. While not having tested this for slat type Helmholtz resonators, I did test huge series of perforated steelsheet on top of rockwool, in order to investigate the acoustic transparancy of those steelsheets.
This perforation ranged from ca 4.7% to 70 %.
Without going in detail: there was no relation whatsoever with this %, only with the size of the close parts between the holes versus wavelength in function of scattering, reflection.

As per Everest, Riverbank tested a single (well isolated) 10 ounce Cola bottle giving 5.9 Sabines at the resonance frequency of 185 Hz.

Eric

PS: But check Scott's response. Why do you look for slat type Helmholtz resonators? Scattering, reflection or just absorption?
Eric.Desart
Moderator
 
Posts: 2461
Joined: Tue Feb 10, 2004 4:29 am
Location: Antwerp, Belgium

Postby Guy Staley » Fri Mar 05, 2004 4:50 pm

Scott R. Foster wrote:
Use broadband absorption instead. Start with thick panels of cloth covered mineral fiber mounted across the diagonals of all corners running full hiegth [from floor to ceiling].



Hi Scott,

Could you explain further why you discourage hemholtz type absorbers?

My reasons for using hemholtz type absorbers is to conserve space, and target specific frequencies.

For instance, in the control room, I have already applied broudband absorbers to target hi-low, and low-mid modal resonances; but I now need to tackle very low frequencies, and I don't particularly have a free corner in my somewhat smallish control room without either getting rid of some gear or getting rid of some clients(!) :) Obviously, neither option is attractive. Additionally, I'm trying to preserve acoustic energy, as the room is already on the verge of being too dead; so the thought of applying anything which will absorb mid & high frequencies is unappealing.

In the second instance, I'm treating a small vocal booth. Until now, my approach has basically been to simply "kill the room" whith large masses of broadband absorption. However, I have been displeased with the "damped" quality of the room, so my new approach is to basically diffuse everything above 500 Hz, and then target the specific modal resonances below 500 Hz with slat and/or perforated panel absorbers. Once again, this is to conserve space, and to preserve acoustic energy. I'm leaning towards slat absorbers for asthetic reasons and because I can use home brew "Flutter Free" mouldings (ala RPG) to further diffuse high frequencies. My goal is a very bright & live booth, with controlled low end. Maybe this is a fantasy...

Cheers!
Guy Staley
 
Posts: 36
Joined: Tue Mar 02, 2004 6:01 pm
Location: Seattle, WA USA

Postby Scott R. Foster » Fri Mar 05, 2004 11:49 pm

Guy:

You are such a bitch... for asking a brilliant question. That said please allow me freedom to invert that question and thus provide a less than stupid answer that is less than 5 pages in length.

Why do you think you can dampen an unconstrined system [the reverberations of your room] with a constrained treatment?

Why not not dampen a wide class of reverberations by way of treatment... do you think you have a narrrow range of problem frequencies?

I bet you don't... move the measurement mic and watch the graph shift.

My $0.02
SRF
Scott R. Foster
 
Posts: 3854
Joined: Mon Feb 02, 2004 12:41 pm
Location: Jacksonville, FL USA

Postby knightfly » Wed Mar 10, 2004 1:20 am

If you want a brighter booth, you could first wrap your absorptive batts in garbage bags - keeps the fibers under control, loses some of the high absorption - John Sayers has had good results in a few rooms doing this, improves the "air" in the room somewhat... Steve
knightfly
 
Posts: 215
Joined: Tue Feb 17, 2004 11:35 pm
Location: West Coast, USA

Postby Guy Staley » Wed Mar 10, 2004 6:45 am

knightfly wrote:If you want a brighter booth, you could first wrap your absorptive batts in garbage bags - keeps the fibers under control, loses some of the high absorption - John Sayers has had good results in a few rooms doing this, improves the "air" in the room somewhat... Steve


Great suggestion Knightfly! I may try thay in my control room and/or live room. As for the vocal booth; what I'm looking for is bright and diffuse: I want to minimize the deleterious effects of very short early reflection times while still preserving high frequency acoustic energy, thus retaining a certain amount of "liveness". I am moving forward with my original plan, but have modified my approach somewhat based on Scott's comments. I will be employing several phase grating diffusers, limited amounts of broadband absorption, AND several hemholtz resonators targetd at very low frequencies. As stated previously, my intent is to diffuse everything above 500 Hz, and absorb everything below 500 Hz.

I will report back when I've finished treating the booth, and post pics so we'll all know that it happened!

Right then...
Guy Staley
 
Posts: 36
Joined: Tue Mar 02, 2004 6:01 pm
Location: Seattle, WA USA


Return to Discussion

Who is online

Users browsing this forum: Majestic-12 [Bot] and 2 guests