Diaphragmatic traps...

Jeff, > Adding "traditional" absorption typically does shift the modes down. However, considering the mass impedance of something like a typical gyp-board wall, the modes usually shift up. < This is very interesting, please explain more. I had ASSumed any change from the walls would be simple absorption at some range of low frequencies. How is this different from "mass impedance" you mentioned? --Ethan

Ethan, I think there's a discussion of non-rigid wall behavior in Kuttruff's Room Acoustics. From what I can recollect (I don't have my copy here with me), a non-rigid boundary with a spring impedance lowers the eigenfrequencies (modes) and a non-rigid boundary with a mass impedance raises them. I believe it was Morse who did most of the practical investigations.

Jeff, > a non-rigid boundary with a spring impedance lowers the eigenfrequencies (modes) and a non-rigid boundary with a mass impedance raises them. < Okay, thanks, that makes sense. I ran it by my expert friend Bill Eppler who put it into electrical terms for me. One is like a transmission line with an out-of-phase back-reflection caused by a short circuit at the far end (mass impedance, like sound hitting a brick wall), and the other is like an in-phase reflection caused by an open circuit. I'm still not sure how a wall could have other than a mass impedance, but I'll gladly take your word for it that this can occur. (Though if you'd like to elaborate, I'm all ears.) --Ethan

Ethan Winer wrote:

I'm still not sure how a wall could have other than a mass impedance, but I'll gladly take your word for it that this can occur. (Though if you'd like to elaborate, I'm all ears.)

Um, make it soft? Add foam/fuzz? 8) Perhaps this is where you got the original idea of lower modes? Porous absorption in the sense of foam/fuzz acts as a spring impedance. Membrane absorption - which is effectively what a gyp-board over studs wall provides - would be mass impedance. I should note that it's been quite a while since I've reviewed the concepts involving non-rigid walls. So, if there are any physicists lurking, I disclaim everything - I'm going from memory here and it would not be surprising if I have greatly over-simplified things! :wink: However, since the behavior we're talking about jives with what I've seen in countless measurements, I would be inclined to think it may have something to do with it. YMMV

Jeff, Thanks. I'm really looking forward to seeing you on our panel at AES this year. And I absolutely promise not to mention anything about modes versus standing waves there. :wink: --Ethan