Theory question for the experts: Depth vs Surface of absorption.

[Northward]

Originally Posted by DanDan

Alistair, panels of rigid material start to behave a little like a board. Thus one could tend towards a panel trap behaviour.
Whealy doesn't have this behaviour in it.
If this behaviour is understood and worked out carefully it can be used to advantage.

^^ This ^^

[UnderTow]

Hi Thomas!

Originally Posted by Northward

^^ This ^^

And this is somehow related to what I see in the impedance graphs, right?

Does anyone know what the best place is to look for information about panel absorbers? (Websites, papers or books or all three).

Thanks in advance,

Alistair

[Dange]

Originally Posted by UnderTow

How important is impedance? What is the effect of the Real and Imaginary components?

Very important, it tells more about a material than the absorption coefficient, which is an 'absolute' value.

The real part of impedance is the resitive part

The imaginary part of the impedance is the reactive part.

Unless you're versed in electronics then that's not too useful though.

A way to look at it is (working in a single plane), material that has a large real component and zero imaginary component would offer reduction of a plane sound wave passing through it and not reflect any sound back towards the source.

Material that has both a real and imaginary component would too reduce the sound wave passing through it but also reflect some of the sound wave back to the source direction. The amount of reduction and reflection is dependant on the size of the real and imaginary values respectively.

So what are the optimum real and imaginary values? Also from electronics, you get maximum power transfer when impedances are matched. Air has an impedance too, which is generally just resistive. This is equal to around 410 N·s·m^3 . Sound travels in air, so the ideal absorber (that with an absorption coefficient of 1 at all frequencies) would have real value equal to 410 and an imaginary value of zero at all frequencies.

[johndykstra]

What a great thread. Watching intently.

[DanDan]

Alistair, I think you will find panel DIY problematic.
Here's two unfortunate stories of failure. Panel straddling
The successful use of semi rigid boards as partially resonant traps is even more of an unknown.

When you get the book, you will see that Newell uses thin layers glued to hangers etc. Light material would be impractical and inefficient when this thin. He also provide a link to a white paper on that waveguide effect (hanger angles).

Everest states that 703 is a pretty much perfect absorber from 125 to 4K.
Perhaps this was the start of a myth. The music community are very lemming like. e.g. The death of high quality sound in favour of loudness. MP3. Elixir Strings....
Andre, the Librarian, has kindly shown us some NASA research on lining a wind tunnel. Clearly he is also a spy.

Tne conclusions can be summarized as follows:
Sound absorption in the low to mid-frequency range can be increased by the use of PF 3350 fiberglass (nominal flow resistivity 4100 mks rayls) instead of Owens Corning 703 fiberglass (27,000 mks rayls) and by an increase in lining thickness from 6 inches to 10 inches.
When PF 3350 fiberglass is used, the acoustic absorption coefficients predicted for 10 inches of material are essentially the same as those for 8 inches of material plus a 2-inch air gap.

NB the very high GFR of 703 of 27K. It is commonly taken as 16,500 I think.
This leaves a question mark, but I kinda lean towards NASA here.

DD

[ritelec]

Originally Posted by johndykstra

What a great thread. Watching intently.

What a great thread. Watching confusedly.

[UnderTow]

Thanks Dange and DanDan,

Your posts are very useful and have helped clarify a few things in my mind.

As long as I don't try to build a Hubble Telescope, I should be OK with the NASA information. ;-)

I've added this book to my order: Amazon.com: Acoustic Absorbers and Diffusers: Theory, Design and Application (9780415471749): Trevor J. Cox, Peter D'Antonio: Books

That, together with Philip Newell's book, should help me understand the basics a bit better.

I'm sure I will be back with more questions in due time. For now, thanks again to everyone that contributed their knowledge, experience and time. Feel free to add anything that comes to mind. :-)

Alistair

[avare]

Originally Posted by UnderTow

I've added this book to my order: Amazon.com: Acoustic Absorbers and Diffusers: Theory, Design and Application (9780415471749): Trevor J. Cox, Peter D'Antonio: Books

That, together with Philip Newell's book, should help me understand the basics a bit better.

I'm sure I will be back with more questions in due time. For now, thanks again to everyone that contributed their knowledge, experience and time. Feel free to add anything that comes to mind. :-)

What comes to mind is that you received hundreds, or thousands of dollars/euros/pounds worth of advice in:

If I were you with that space available and chose to use it for absorption, I would go with the lightest fibre type absorbent material and use a thickness that would yield a ~1,600 Rayls (4 times the impedance of air) surface impedance with rest being air gap

.

Andre

[prairiedog]

Alistair, regarding the impedance plots you posted, I would suggest you ignore what is happening below about 85Hz for the high resistivity material. Although Komatsu doesn't specify an applicable range for his model, his equations do produce some rather unexpected results outside the range of the test results from which his model was derived.

Demetris

[Dange]

Originally Posted by Dange

The real part of impedance is the resitive part

The imaginary part of the impedance is the reactive part.

I should also add that complex impedance is either stated in the form of:

Z(impedance) = Real + Imag

Or

Z(impedance) = |Z(impedance)| * phase


Not a combination of real, imaginary, magnitude and phase. It's either real and imaginary OR magnitude and phase. Both are valid ways of expressing the complex impedance. I find the real and imaginary form most intuitive as I explained in my earlier post.........

[boggy]

Originally Posted by avare

.......
Pullng some numbers from my memory, Non-Envonment studios typically use modified porous absorbers 2-4' (~600-1,200 mm) deep for "full range" absorption. Hidley has called some of these rooms as being 10 Hz rooms. I will leave th interpretation of what that means exactly, to the marketing sales types here.
......

I really don't know if these rooms are "10Hz", but in AES paper:

"Sound field characterisation and absorption measurement of wideband absorbers", S. Torres-Guijarro, A. Pena, A. Rodríguez-Molares, and N. Degara-Quintela, 126th Convention, 2009, Munich, Germany

is measured impedance of P. Newell hanged type panel absorbers, used in N-E design.

Looking at (modeled) absorption coefficient frequency characteristic graph, one can see that this type of "bass trapping", has:

1. an peak in absorption coeff. (alpha~0.9) at F(lambda/4)=70/L
2. and null at F(lambda/2)=140/L,
3. and, again, constant maximum (0.9) from F(3*lambda/4)=210/L and up,

where L is width (in metres) of hanged panel that form a waveguide (with other panels).

If overall depth of absorber is from 600 to 1200mm then we have L from 0.933m to 1.867m... or better in table:

depth     L      Peak   Null    Max
0.6m   0.933m  75.0Hz  150Hz    225.0Hz
1.2m   1.867m  37.5Hz   75Hz    112.5Hz

because absorption coefficient still works very well, even for frequencies below F(lambda/4) ... which is 37.5Hz for absorber with 1.2m depth... it's not that "impossible" to get (fairly) damped resonances even down to ... 20Hz, if not 10Hz.... with this type of absorbers.

btw, impedance was measured in situ with B&K sound intensity probes.

Best regards,

Bogic

[avare]

Thanks for the great post Bogic, including reference to the technical paper! The analysis that you did is interesting. At one level it confirms the point I have made repeatedly, and particularly in the Q4 for Avare thread, that significant absorption is achieved well below the λ/4 frequency.

OR
having not read the paper, if the measurements were taken in situ in a non-environment room, it is verification of the Invisible Alpha© design. As Newell wrote in Recording Studio Design 2nd edition:

Low-mid absorption is provided by acoustic labyrinths through which the waves are guided, diffused and diffracted, as well as being forced to drag their way through large, absorbent-lined ducts. The lowest frequencies are addressed by means of air-damped constrained-layer panel absorbers and ‘deadsheet’ membrane absorbers, which effectively line the room with a heavy, acoustically-dead, semi-limp bag.

Bolding is by me.

Fully absorbed,
Andre

[DanDan]

Great thread, caused by an intriguing question.
Thanks boggy, that is meaty stuff there.
On a personal note, it is great to see you posting here now.
DD

[UnderTow]

And the plot thickens...

Thanks Andre, Demetris, Dange and Bogic for your posts! More stuff to chew on...




Dobar dan Bogic,

Considering that my room has an equal height and width of 270cm (first mode at 63,8Hz), it might be interesting to do some hanging absorbers with a length of 1097mm (70*1000/63,8) over the back wall (as sketched in the last post on the previous page) and measure the results before continuing...

But I am getting ahead of myself. The measurement mic hasn't even arrived yet!

Thanks everyone,

Alistair

[boggy]

Originally Posted by UnderTow

......
Dobar dan Bogic,
........

Dobar dan (good day) Alistair.

[boggy]

Originally Posted by UnderTow

.......If one wants to try and control frequencies as low as possible in a room, is it best to maximize the area of absorption or the depth?
....

Both.
Look at definition of total absorption in room:

A=S1α1+S2α2+...+Snαn=ΣSiαi

If you increase alpha (this means increasing depth, simplified)... and if absorbing area is constant... total absorption is increased. If you DEcrease absorbing area, but alpha is constant ... total absorption is DEcreased.... etc. no free lunch!

avare, DanDan you are welcome!

Cheers,

Bogic