John Sayers' Recording Studio Design Forum

Triple leaf
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Author:  Eric_Desart [ Sat Aug 19, 2006 7:40 am ]
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:( Everytime I edit the above message the picture description in the titlebar of the uploads is lost (dissapears).

Anyhow, this comparison is rather unique on the net I think.
Hence the curious ones can study how those resonance frequencies behave by changing the use of those materials.

The Tables are build up in columns, with different buildups from the same material.

This are strict theoretical calculations based on the mass-spring system, ignoring other parameters as mounting method, flanking, or whatever other phenomenon.

Author:  ozymandias [ Sun Aug 20, 2006 3:00 am ]
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Wow, very interesting stuff - thanks! :)

Author:  Eric_Desart [ Sun Aug 20, 2006 4:44 pm ]
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A stupid question: If I make such model in US units how do USers express such thicknesses in fact (for plywood).
I can imagine that drywall goes in fractions to x/8", but plywood can go much thinner wher x/8 of an inch is too course.
When recalculating metric measures I come to 1/32 resolutions?

How do you guys express those thin thicknesses for plywood or hardboard? Searching the net gives me a lot of Metric stuff, but in Imperial it's not very clear to me.


Author:  John Sayers [ Sun Aug 20, 2006 9:05 pm ]
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Eric - I think the standard sizes and their equivalents rounded out are:

1/8" = 3mm
1/4" = 6mm
3/8" = 10mm
1/2" = 12mm
5/8" = 16mm
3/4" = 20mm
1" = 25mm

but they do use 3/16" 7/16" etc.


Author:  Eric_Desart [ Mon Aug 21, 2006 12:43 am ]
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What surprices me if I go to a woodshop here I find much more thin panel thicknesses.

I searched further and found some links which look interesting (I still must look myself in detail): ... s_book.asp (look at links on page) ... mation.htm

Author:  Brian Ravnaas [ Fri Aug 25, 2006 1:26 pm ]
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is this the thread that lead to the other thread where Ethan asked about adding a panel trap?

I would be honored to volunteer Audio Alloy - Green Glue Company - to test that... should be fairly fast / inexpensive to test. We'll just add a 4' x 4' panel trap to a 64 square foot test wall and report. That will take until the end of september.

That's always, always, always the best way - to test. I'd be grateful if somebody would email or post (and remind me!!) how to build the panel trap and attach it.

Author:  z60611 [ Fri Aug 25, 2006 1:58 pm ]
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I believe the membrane should be as large as possible, to make it more membrane-ish, and less structurally-damped-by-the-2x4s. So 4x8 sheets of plywood are good, and 2x8 sheets of plywood are less good.

The traditional formula is simple enough (ignores 2GOF):
d = 28900 / (f ^2 * m)
d = depth in inches
f = resonance frequency in hz
m = surface density in pounds per square foot

1) e.g. 50hz = 8.4" out, using 4x8 sheets of plywood and 2x10's on 4' centers (i.e. the edges of the plywood), with the 2x10's cut to 8.407".

8.407 inches = 28900 / ((50hz ^2) * 1.375 lb/ft^2)
where 1.375 is the surface density of typical 1/2" plywood.

2) e.g. 100hz = 2.1" out, again using 4x8 sheets of plywood and 2x3's on 4' centers, with the 2x3's cut to 2.1"

2.1 inches = 28900 / ((100hz ^2) * 1.375 lb/ft^2)

3) e.g. 50hz, but with Gypsum/Drywall instead of plywood:

5.25 inches = 28900 / ((50hz ^2) * 2.2 lb/ft^2)
where 2.2 is the surface density of typical 5/8" drywall/gyspum.

Author:  z60611 [ Fri Aug 25, 2006 2:03 pm ]
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Ethan's Bass Trap link is &

Author:  Tonio [ Sat Aug 26, 2006 7:23 am ]
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At this point, I surmise my 1st dumb question isn't all that dumb?

Anyway, Mr Desart I don't follow the graphs that you graciously posted.
:oops: could you - or anyone splain(explain) please?


Author:  z60611 [ Sat Aug 26, 2006 8:40 am ]
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The day Eric posted it, at my home woman (no relation to Paul's woman) came in. Smiled at me curious about what I had found.
Woman said, "What's so interesting." She looked closer, frowned, "Oh, acoustics." and left.
She came back 15 minutes later and asked "How can you stare at that chart for half an hour?"
I replied, "Different chart" and scrolled up to the first one, and then back down to the second one.
She handed me a bowl of watermellon and strawberries, each cut up into 3 cubic centimeter pieces and said "Eat." and she sat down beside me and opened her book with her own bowl of the same on her lap.

Some people learn by formulas.
I learn by examples -- and then use the formulas grudgingly later to fine tune to specific situations I'm interested in.

The charts are a bunch of different examples of wall systems. Where it gets interesting is when you start comparing them, and discovering trends.


This is relevant to Ethan's question because the traditional formula (1DOF - assumes infinite mass behind other leaf)
d = 28900 / (f ^2 * m)
d = depth in inches
f = resonance frequency in hz
m = surface density in pounds per square foot

75mm = 2.952 inches
surface density of 1/2" drywall is 1.8 pounds per square foot.

for the top left wall would be
f = sqrt( 28900 / (m * d) )
f = sqrt( 28900 / (1.8 * 2.952inches) )
f = 73hz.

same as Eric calculated for 'First Wall'.

If that second 'wall' is a membrane absorber you've just built, but you stick it on a similarly constructed current wall, then the resonances you end up with are not the target one you wanted. i.e. not 72hz, but rather something that's pretty good at 44hz and 117hz.

Within the limitations of Eric's disclaimers. namely
This are strict theoretical calculations based on the mass-spring system, ignoring other parameters as mounting method, flanking, or whatever other phenomenon.

Author:  Tonio [ Sat Aug 26, 2006 9:31 am ]
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Thank you, great splaining :D Now I feel dumber :evil: Just kidding
You know I get the same respond from the female gender about acoustical things :wink:


Author:  Andrew Steel [ Sat Aug 26, 2006 10:14 am ]
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very quick and simple output from WinFLAG, hope it helps


Author:  Eric_Desart [ Mon Aug 28, 2006 1:24 pm ]
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Do you have this graph also in numbers?
Exact mass, exact wool, exact whatever.

Can you also make the same without wool? (also with numbers if possible?)


Nice conclusions you entered here. :) I expected those tables to trigger you.
But it were not just an arbitrary bunch, but I entered them as logical series.

If you want to check certain combinations just give them.

Warm regards

Author:  z60611 [ Mon Aug 28, 2006 1:56 pm ]
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Do you have this graph also in numbers?

Have you ever heard of William Playfair (1759 - 1823). He invented charts and graphs. In 1786 he created the first published bar chart, which illustrated Scotland's 1781 trade surplus with America and its simultanious trace deficit with Russia. The reason I meantion it is you asked for numbers, and from American Scientist May-June 2006 pg 275:
pictures created to represent observations were necessarily less accurate and more interpretive than were numbers themselves. A "general mistrust of pictorial representation" in British science continued into the early 19th century (and a cautious skepticism about graphic representation remains part of scientific culture today). Watt tactfully urged Playfair to provide tables to support his arguments, pointing out that "the charts now seem to rest on your own authority, and it will naturally be enquired from whence you have derived your intelligence."

Graphical representation , he [Playfair] argues in the Breviary, can "facilitate the attainment of information, and aid the memory in retaining it." ... It conveys its message to the eye in an instant.

Author:  z60611 [ Mon Aug 28, 2006 1:58 pm ]
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I have the formula for 1DOF with one wall infinite mass.

I've been meaning to see if one of my books answers these questions:
What's the formula for 1DOF with two normal masses?
What's the formula for 2DOF with two normal masses?

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