The No.1 Website for Pro Audio
Multilayer Absorber Calculator and Limp Membrane Traps
Old 21st July 2020
  #1
Multilayer Absorber Calculator and Limp Membrane Traps

I'm not getting expected results using acousticmodelling.com multilayer absorber calculator for limp membrane traps.

Putting porous absorbent *infront* of the air layer results in much better performance vs behind the air layer. Is the calculator just not good for limp membrane calculations? Example: here

Shouldn't it be the other way around? Isn't the absorbent glued to to the back of the sealed box?
Old 22nd July 2020
  #2
Here for the gear
 

Porous absorbers do nothing when particle velocity is zero - in this case, near the back wall, so the calculator is correct. This is also why you want an air gap behind open absorber panels.


Quote:
A rough figure sometimes quoted is that the absorbent needs to be at least a tenth of a wavelength thick to cause significant absorption [2]
Acoustic Absorbers and Diffusers, Cox & Antonio, page 131. Which quotes U. Ingard, Notes on Sound Absorption Technology, Noise Control Foundation (1994).


All fluffy stuff need to be in front of an air layer or be super thick so it acts as its own air layer.
Old 22nd July 2020
  #3
I’m specifically talking about limp membrane absorbers, where it’s a sealed box. Every thread I’ve read on them say you need to pack a bit of absorption into the back of them..
Old 22nd July 2020
  #4
Here for the gear
 

Well, the physics doesn't change inside a box. At the front, particles move. At the wall, with infinite resistance and maximum pressure, it doesn't. Acousticmodelling.com uses the same Allard and Champoux (1992) model for the porous parts no matter what you put in front of it. Or the original Delany & Bazley (1970), or Komatsu's improved D&B model (2008), you can switch between them.

They all agree that thin porous absorbers do nothing right next to rigid backing. Limp membrane, Helmholtz, or just open, all models require you to place the porous absorber at the front for maximum effect, but sometimes with a minimum air gap so it doesn't touch the front.


If you blow air through a cloth it gets warm by friction; air flow turns into heat. Place the cloth against the wall though, and you'll have a hard time blowing through it.


Fully stuffing the box does work as the front layers does the work. Better designs use chicken wire to hold the stuffing near the front, no need to pay for air.
Old 4th August 2020
  #5
Gear Addict
 
Storm Mastering's Avatar
 

Just stuff it with fluffy, with wire so it doesn't touch the membrane.

http://www.acousticmodelling.com/mli...3=225&v43=6300

Absorber 3 : stuffed with fluffy
Absorber 4 : critically damped if you can find a 6300 Pa.s/m2 material
Old 8th August 2020
  #6
Lives for gear
Quote:
Originally Posted by Storm Mastering View Post
Just stuff it with fluffy, with wire so it doesn't touch the membrane.

http://www.acousticmodelling.com/mli...3=225&v43=6300

Absorber 3 : stuffed with fluffy
Absorber 4 : critically damped if you can find a 6300 Pa.s/m2 material
Bloded by me.

Can you elaborate please? How do you calculate it?
Old 8th August 2020
  #7
Gear Addict
 
Storm Mastering's Avatar
 

Quote:
Originally Posted by JayPee View Post
Bloded by me.

Can you elaborate please? How do you calculate it?
From wiki :

Quote:
The damping ratio is a system parameter, denoted by ζ (zeta), that can vary from undamped (ζ = 0), underdamped (ζ < 1) through critically damped (ζ = 1) to overdamped (ζ > 1).
Quote:
Between the overdamped and underdamped cases, there exists a certain level of damping at which the system will just fail to overshoot and will not make a single oscillation. This case is called critical damping. The key difference between critical damping and overdamping is that, in critical damping, the system returns to equilibrium in the minimum amount of time.
At the bottom of the MLAC, there is the damping calculated.
In resonant absorbers, underdamping means risk of oscillation, overdamping means taking a hit in efficiency, and critical damping is the sweet spot.

It's better to be close to 1 or a bit overdamped than underdamped. That also depends of the membrane. If something limp or dampened material to begin with, it will less be a problem to be under 1 than with a steel or hard wood membrane.

You can see it a bit like a filter, if too damped (Q very low) it will eat too much, if Q=0.707 you are in a butterworth case (maximally flat pass band) and if Q > 0.707 you begin to exhibit a resonance.

edit : a bit more on the subject
Is the latency of resonant absorbers a real problem?
Let's discuss membranes touching the absorber
📝 Reply

Similar Threads

Thread / Thread Starter Replies / Views Last Post
replies: 1075 views: 216972
Avatar for fustrun
fustrun 1 week ago
replies: 50 views: 28098
Avatar for Deleted d78e603
Deleted d78e603 9th July 2017
Post Reply

Welcome to the Gearslutz Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…
🖨️ Show Printable Version
✉️ Email this Page
🔍 Search thread
🎙️ View mentioned gear
Forum Jump
Forum Jump