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Membrane absorbers
Old 25th September 2012
  #1
Lives for gear
 
🎧 10 years
Membrane absorbers

Below I have copied a typical absorption coefficient curve for a resonant (membrane) absorber from a well known text book. However, this type of curve is displayed in countless papers and text books, so, I hope there is not a copyright issue!! The curve clearly shows that there is some “useful” absorption either side of the resonant frequency.
There is a huge amount of information on the internet about membrane absorbers (some of it, I have to say, inaccurate and even plainly wrong).
Recently I have seen a few references, in my internet research, that state “absorption only takes place above the resonant frequency” of the membrane absorber?
Can anyone out there comment on this as clearly if this statement is true then many text books are misleading? Also, if the statement is false then where did they get this misinformation from?


Membrane absorbers-membrane-absorber.jpg
Old 25th September 2012
  #2
Gear Guru
 
Glenn Kuras's Avatar
 
🎧 15 years
Quote:
Recently I have seen a few references, in my internet research, that state “absorption only takes place above the resonant frequency” of the membrane absorber?
It is either side not just above. I have tested tuned trapping not only in the lab but also in our test room. Both show this.

Lab testing on this page.
Scopus Tuned Bass Traps : GIK Acoustics

Room testing here
https://gearspace.com/board/7597560-post146.html
Old 25th September 2012
  #3
Lives for gear
 
🎧 10 years
Thanks for that Glenn!!

It is also as I understood from both a physical point of view and the theoretical texts.

Just not sure where they were coming from....
Old 25th September 2012
  #4
Lives for gear
 
🎧 10 years
While we are on the subject.

I have read many posts from you Glenn and greatly appreciate the knowledge you give away freely. I am sure I am not alone in being grateful for your valued expertise.

Regards
Gary
Old 26th September 2012 | Show parent
  #5
Gear Guru
 
Glenn Kuras's Avatar
 
🎧 15 years
Quote:
Originally Posted by Icecube1 ➡️
Thanks for that Glenn!!

It is also as I understood from both a physical point of view and the theoretical texts.

Just not sure where they were coming from....
I have no idea either. No unit, to my knowledge, can be made to cut off like that. Some may cut off quicker then others but all of them have a slope of some kind.
Old 26th September 2012
  #6
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boggy's Avatar
 
🎧 10 years
Quote:
Originally Posted by Icecube1 ➡️
........
Can anyone out there comment on this as clearly if this statement is true then many text books are misleading? Also, if the statement is false then where did they get this misinformation from?
..........
Absorption graph is not misleading.
It is probably only problem that you don't have clear and practical guide how to build it to work as absorber and not resonator, firstly. Second question is tweaking resonant frequency.

Tim Farrant tried recently to give more information here:
Tim's Limp Mass Bass Absorbers
Old 26th September 2012 | Show parent
  #7
Lives for gear
 
🎧 10 years
Quote:
Originally Posted by boggy ➡️
Absorption graph is not misleading.
It is probably only problem that you don't have clear and practical guide how to build it to work as absorber and not resonator, firstly. Second question is tweaking resonant frequency.

Tim Farrant tried recently to give more information here:
Tim's Limp Mass Bass Absorbers
Firstly I was quoting what someone else had stated. I am now quite clear about how the resonator/absorber works, and believe the graph data above. Second I was not referring to anything I have built.

I saw a statement about membrane absorbers only absorbing above the resonant frequency on another internet site, supposedly written by someone of knowledge. I just wanted clarification that I wasn't missing something

I think Glenn answered the question adequately and maybe we can draw a line under that particular question. Thanks
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