Question about surface impedance

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Re: Question about surface impedance

Postby bert stoltenborg » Mon Oct 15, 2012 6:51 pm

http://www.homerecording.be/forum/t33605.htm

I asked Prof Desart, he confirms it is a difficult topic.

He did some programming though:

http://www.homerecording.be/forum/t28576.htm

This also is nice:

http://www.geluidforum.nl/viewtopic.php?t=1337
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Re: Question about surface impedance

Postby Scott R. Foster » Tue Oct 16, 2012 3:28 am

Indeed.
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Re: Question about surface impedance

Postby ChrisW » Tue Oct 16, 2012 6:36 am

Thanks Bert.

Its a good job my sister-in-law is Dutch because Google translate pretty much butchers that page...

Nonetheless, if Eric has written it (in whatever language), its worth reading.

Chris W
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Re: Question about surface impedance

Postby bert stoltenborg » Tue Oct 16, 2012 9:38 pm

I have a second print of the book on my stick you know well, Cris.
Is this quote of yours removed?
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Re: Question about surface impedance

Postby ChrisW » Wed Oct 17, 2012 8:37 am

Hi Bert

Yes, I got a copy when I visited you last year. But to be honest, I haven't had any time to jump into the details again - my other job has hijacked all my time.

What quote of mine are you referring to?

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Re: Question about surface impedance

Postby bert stoltenborg » Wed Oct 17, 2012 11:58 am

Sorry for being unclear; I meant the quote from page 18 of the book, I cannot find it, maybe I should look better, but no time now.
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Re: Question about surface impedance

Postby ChrisW » Wed Oct 17, 2012 12:16 pm

Ah yes. I haven't actually checked the second edition to see if the same statement is there.
Even if it isn't, all I know is that the imaginary part of a surface impedance value somehow represents a phase change, but exactly how is not clear to me.

Still wondering, but with no time to look into the details at the moment...

Chris W
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Re: Question about surface impedance

Postby Ido » Thu Oct 18, 2012 5:54 pm

I still don't know dutch, and I can't see well all those million colored lines of Eric's,
could someone please give a brief summary line on all those absorption graphs? conclusions?
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Re: Question about surface impedance

Postby Icecube » Thu Oct 18, 2012 7:02 pm

Hi Chris

Thanks for answering.

I have given this a lot of thought. A negative imaginary impedance is clearly associated with a capacitive reactance. However, there are certainly some references amongst people way more intelligent than me that there is also an inductive reactance induced by porous absorbant!? Bert mentioned above this concept of a transfer back and forth between potential and kinetic energy, which makes sense, and would be a capacitive reactance effect. On balance since the absorber models generally give large negative imaginary impedance at low frequency then I am certainly coming down on the side of an equivalent capacitance (or compliance in acoustic terms). So how do we use this....?

Z=pc in air ~1.21x343=415, with no imaginary part. For absorbant material we use a typical absorber model and calculate a "real" part of impedance. As far as I understand then this is equal to "pc" in that material. The crucial bit that many simple analyses don't then take account of is that the speed of sound in the material is not the same as air. If you substitute (Omega/Real part of the wave number) for the speed of sound in the absorber, then by substitution you arrive at the "effective" density within the absorber. Now, if we were to assume the equivalent RC type circuit for an absorber, with R equivalent to the real part of the impedance and C the capacitance calculated from the imaginary part then I believe the phase as well as magnitude would be taken into account? What do you think? (In general at very high frequencies the imaginary part tends to zero, as does the capacitance, and the impedance becomes just resistance)

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Re: Question about surface impedance

Postby ChrisW » Fri Oct 19, 2012 6:49 am

Hi Gary

Yes, that certainly makes sense - although I'm having to dredge my mental archives to remember the theory here...

So then if I summarise your summary(?!), as the frequency decreases, the observed acoustic behaviour bears an increasingly strong equivalence to the electrical idea of capacitance - and this is represented by the imaginary part of the impedance becoming increasingly negative.

Chris W
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Re: Question about surface impedance

Postby Icecube » Fri Oct 19, 2012 10:57 am

Yea, exactly Chris, that's the way I see it.?

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