absorption behavior of a panel trap

[Bob]

Paul:

No ideas from me just yet on your topic.



Brian:

we have higher absorption and a NARROWER bandwidth, which is 180 degrees contrary to what every single comment i've ever seen about adding insulation to panel traps says

Cox says that the absorbant (mineral wool) absorbs most of the energy, so without it I guess it's a less absorbing panel trap (i.e. taking that idea to extremes, the peak absorbtion coefficient may be less than the peak of one with some absorbant).
Also, and this is probably more relavent, A and B are not only different in their addition of absorbant, A is also 60% deeper, which might move the peak lower than 100hz which is the leftmost on the shown graph.

[Brian Dayton]

I can tell you that the drawing of your understanding is correct...

Need proof?

When I tested wood panel membrane traps at IBM's acoustics lab the
traps achieved 100% absorption at their center frequency. If adding damping
to lower the Q also lowered the peak absorption, then they would have
absorbed much less than 100%.

As best I can tell, the panel does most of the absorbing, and the
fiberglass damps the panel motion. And lowers the Q too, of course.



those are EW's comments, i enter them to ensure that i don't misinterpret.

[Brian Dayton]

....

this would seem to imply that the panels motion is at the core here

I dunno how related this is.....


Yesterday when I had my almost finished computer silencign cabinet, I did some Isolationtest with the fan on full speed.

The scenario....

The box was sealed on all sides except the top.

then I laid ( not fastened ) a sheet of 18mm MDF on top of the box, and noted how loud I could hear the fan noise. ( pretty quiet )

then... as a further test, and actually in order to press the sheet of MDF tighter over the box to IMPROVE TL, I put a heavy'ish weight ( A full 300' reel of power cable ) on the MDF sheet in the centre.

To my suprise, I noted the TL was LESSENED. Soon as I lifted the weight off the top panel the TL Improved.


MY first though was the resonant frequeny of the panel was changed ( Lowered ) to more match the freqency of the fan noise, and perhaps let more sound through. But I'm not entirely sure this is the case.

Any thoughts, as to what this phenomen might be due to, and whether it's related to this discussion?




Paul

how big was the roll relative to the panel? did it overlap the thing and sit over the side walls, or did it sit in the middle?

[Bob]

Paul
Is this a bit like
a) put a penny on top of an otherwise quietly vibrating box, and suddenly the penny makes a racket
b) hit a cymbal and the cymbal makes noise

[Paul Woodlock]

....

this would seem to imply that the panels motion is at the core here

I dunno how related this is.....


Yesterday when I had my almost finished computer silencign cabinet, I did some Isolationtest with the fan on full speed.

The scenario....

The box was sealed on all sides except the top.

then I laid ( not fastened ) a sheet of 18mm MDF on top of the box, and noted how loud I could hear the fan noise. ( pretty quiet )

then... as a further test, and actually in order to press the sheet of MDF tighter over the box to IMPROVE TL, I put a heavy'ish weight ( A full 300' reel of power cable ) on the MDF sheet in the centre.

To my suprise, I noted the TL was LESSENED. Soon as I lifted the weight off the top panel the TL Improved.


MY first though was the resonant frequeny of the panel was changed ( Lowered ) to more match the freqency of the fan noise, and perhaps let more sound through. But I'm not entirely sure this is the case.

Any thoughts, as to what this phenomen might be due to, and whether it's related to this discussion?




Paul

how big was the roll relative to the panel? did it overlap the thing and sit over the side walls, or did it sit in the middle?

The panel is about 4' x 4'. The roll was abrounf 10 inches in diameter and sat in the centre of the panel.

Pal

[Paul Woodlock]

Paul
Is this a bit like
a) put a penny on top of an otherwise quietly vibrating box, and suddenly the penny makes a racket

THe reel of cab;le wasn't rattling, as a penny would.

b) hit a cymbal and the cymbal makes noise

I'm not quite sure how tahat coudl be related at all. There's no impact going on.

Paul

[Bob]

Hi Paul

'm not quite sure how tahat coudl be related at all. There's no impact going on.

Not impact. Excitation.
The wire spool is the cymbal.
A 1/2" insulated wire at 90 degrees to another 1/2" insulated wire isn't going to make much noise.
But with a spool (300' of power cable) there would be lots of surface impacting. It might add up to something audible.

You were there. I'm just wild ass guessing here.

Have you tried
a) putting some unused Sylomer on top of the box
b) putting a 50lb anvil on top of the Sylomer (same weight as the wire)
c) (please do not calculate the exact amount of Sylomer to decouple it with resonance lower than the excitation frequencies)

[Paul Woodlock]

Hi Paul

'm not quite sure how tahat coudl be related at all. There's no impact going on.

Not impact. Excitation.
The wire spool is the cymbal.
A 1/2" insulated wire at 90 degrees to another 1/2" insulated wire isn't going to make much noise.
But with a spool (300' of power cable) there would be lots of surface impacting. It might add up to something audible.

You were there. I'm just wild ass guessing here.

Have you tried
a) putting some unused Sylomer on top of the box
b) putting a 50lb anvil on top of the Sylomer (same weight as the wire)
c) (please do not calculate the exact amount of Sylomer to decouple it with resonance lower than the excitation frequencies)

Sadly it's all too late now. I've made the desktop ( well not finsihed cos of the router bit cock up ). I put this on and then a billion tools on top of it. It's even quiter now as teh desktop is much thicker and heavier than that offcut of MDF I tested with.

When I finish the desktop, I'll try to remember to test it again, by placing a weight in the middle. I've got loads of bricks, I'll try about 6 of 'em




Paul

[Brian Dayton]

ok, i've contemplated your experiment a bit, and i guess i don't know much, but have some thoughts


1.  the central weight may bow the MDF and worsen the seal

2.  alternately, it may strengthen the seal, but not yield actual seal, and worsen performance at some specific frequency.  seals and how they bleed sound (cracks, rather) are resonance related, like a port in a sub-box.  i do not purport to have full understanding of this, but it's possible that a smaller crack would cause some particular frequency to worsen

3.  the big weight in the middle would present an intertial resistance, and may partition the panel into small sub-panels with higher resonance points, causing some particular freq range to worsen.  see sketch

[Brian Dayton]

but WRT a panel trap mass-spring type of resonance, i'd wager no.  your panel is reasonably big, the cavity resonanby deep, and adding that weight smack in the middle would have a maximum effect on the intertia of the thing, and whatever mass-spring effect would probably be low in freq

it doesn't seem like 'puter fans have loud LF content to me

[Brian Dayton]

as threatened previously somewhere or other, i took some data on the absorption of walls relative to 12" of concrete in a lab recently

it was in the TL chamber, but FWIW, that chamber was previously an accredited C423 room until the lab opted to revoke their C423 "license" as they didn't want to be in that biz

the change in absorption of 2x4 walls peaked at resonance, again at the secondary structural resonance, and drifted down to zero by 25hz (relative to concrete)

if anyone cares, i'll post the data

[Paul Woodlock]

ok, i've contemplated your experiment a bit, and i guess i don't know much, but have some thoughts


1.  the central weight may bow the MDF and worsen the seal

2.  alternately, it may strengthen the seal, but not yield actual seal, and worsen performance at some specific frequency.  seals and how they bleed sound (cracks, rather) are resonance related, like a port in a sub-box.  i do not purport to have full understanding of this, but it's possible that a smaller crack would cause some particular frequency to worsen

3.  the big weight in the middle would present an intertial resistance, and may partition the panel into small sub-panels with higher resonance points, causing some particular freq range to worsen.  see sketch

Cool sketch, and pretty much what I thought was going on.

My vote ( from gut feeling ) is option #3


Paul

[Eric.Desart]

as threatened previously somewhere or other, i took some data on the absorption of walls relative to 12" of concrete in a lab recently

it was in the TL chamber, but FWIW, that chamber was previously an accredited C423 room until the lab opted to revoke their C423 "license" as they didn't want to be in that biz

the change in absorption of 2x4 walls peaked at resonance, again at the secondary structural resonance, and drifted down to zero by 25hz (relative to concrete)

if anyone cares, i'll post the data

Hi Brian,

I'm interested that you post it.

What do you mean with at resonace and secundary structural resonance?
One is the MSM the second the resonance of the whole wall? Some kind of panel resonances influenced by the construction?

[Brian Dayton]

Hi Brian,

I'm interested that you post it.

What do you mean with at resonace and secundary structural resonance?
One is the MSM the second the resonance of the whole wall? Some kind of panel resonances influenced by the construction?

hey eric, i have the data here, and i'm taking a closer look.  i think it's probably best if i post this that you handle the commentary after i offer what i can, as i'm sure you 110% know that this form of absorption test is question-mark prone

[Brian Dayton]

ok, folks, here is what i've got

some comments:

the 200hz thing.  all of the 2x4 walls exhibit a peak in absorption around 200 hz, i had commented the other day after first making this graph that i thought this was related to the secondary structural resonance, it's not so, that's higher in frequency around 250-400hz

note the two curves for GG walls and the big change in this area (200hz).  the only difference in those walls was modifcation to the filler wall (additional sealant, 4 layers of drywall added to each side).  

for those who don't know, a filler wall is some wall that fills part of a test lab opening to give a more normal size, in this case the wall was small at ~9" x 10", and that can lead to odd behavior (small walls don't behave like walls, really)


first antiresonance above the primary resonance. the 160hz dip in absorption correlates to the frequency of the first anti-resonance of this type of structure (2x4 walls exhibit anti-resonance just above the primary resonance, i avoid the use of the term MSM as this isn't an air-spring resonance).  In theory, damping will REDUCE performance at this frequency, in practice we observe a minimum gain around this point, which is reflected in the data collected.  Whehter or not this mimina in absorption can be correlated to anti-resonance (anti-panel trap behavior), is clouded by the 200hz thing


secondary resonance - there seems to be a second boost in absorption around the secondary structural resonance of the undamped walls, green glue seems to wipe this out


primary resonance and MSM - EACH WALL EXHIBITS PEAK ABSORPTION AT IT'S PRIMARY RESONANCE.  MSM FOR THE ALTERNATE WALL, AND THE PRIMARY STRUCTURAL RESONANCE FOR THE 2X4 WALLS.  ABSORPTION DRIFTS DOWN, NOT PROGRESSIVELY UP, BELOW THIS.



ok, the walls were:

5/8 drywall on both sides of a 2x4 wall with 24" OC studs, no insulation.  resonance (primary) = ~95-100hz

as above, but with 3.5" mineral fiber in the cavity.  resonance, primary = ~90hz

as above, but with (in the source room) double 1/2" drywall with Green Glue.  resonance, primary = ~85-90hz

as a test to ensure that what we were seeing (the curves ARE similar, after all) wasn't just a phenomenon not related to the walls, i entered data for a third wall (the filler wall), which had strange construction, but exhibited high TL when tested, but with a partial MSM behavior at 40hz, and a TL minima at 40hz to go along with it.  I note that this wall exhibits a peak at 80hz as well, so perhaps someof the peak at 80hz IS related to an arbitary thing, or perhaps there is some reason.  NO MODAL ACTIVITY AT OR AROUND 80HZ IN THIS WALL


and i think that the inclusion of this alternate wall adds alot to the graph



ok, here it is, the data in graphical form, whatever comments you have would be just awesome

[Brian Dayton]

some more notes:

the shift down in frequency for the absorption curves is clear for the with-insulation (somewhat lower resonance) 2x4 walls

-if a false peak at 80hz exists, then the apparently broad but lower absorption on the non-insulated wall would be peaky and still lower


-this data supports that insulation raises absorption


-this data appears to show that damping doesn't lower absorption (at least at the primary resonance), which i think is odd, it should IMO.  it also didn't raise it


-the data seems to be correlated 10x more to the resonance beavhior of the walls than to TL or to mass



i intend to take (with audio alloys money, god bless 'em) a couple dozen more tests like this (with associated TL) on GG and regular walls for a variety of types.

based on experience in our own rooms (less good than this one), expect the most pleasing curves to be shown by staggered stud and double stud walls - broader resonant activity and goes lower in frequency.

i dunno about RC walls

[Brian Dayton]

[

Hi Brian,

I'm interested that you post it.

What do you mean with at resonace and secundary structural resonance?
One is the MSM the second the resonance of the whole wall? Some kind of panel resonances influenced by the construction?

i've decided to not use the term MSM with respect to 2x4 walls.  MSM sort of = Fmam it seems in most lingo, and this isn't an air spring resonance.

Lin and Garrelick predicted a series of resonances and antiresonances in the 2x4 wall, and that is exactly what's observed when measuring those guys for either TL or modal behavior

i was wrong in my earlier post i think, i elaborate above

g'night

B

[Eric.Desart]

Many thanks Brian,

I will reread it couple of times.

[Bob]

Brian:

Your graph has 5 walls: what's the "alternate wall" ?

no-insulation: {G16, WS90@600, G16}
mineral-fiber: {G16, WS90@600 with R13 mineral fiber, G16}
R13 and green: {2G13 with Green Glue, WS90@24" with R13 mineral fiber, 2G13 with GG}
alternate wall type: ????
same GG wall prior to filler mod: ???

G16 is 16mm gypsum = 5/8"
G13 is 13mm gypsum = 1/2"
WS90@600 is 2x4 Wood Studs (90mm = 3.5") @ 600mm on center (24")
2G13 is two layers of G13



Eric: I read all your posts a couple of times too, including your latest about AcousticMat (thanks).

[Brian Dayton]

some more comments:


a filler wall was present, the data for all the 2x4 walls includes this filler wall at 17% of the total area

the "alernate wall" was the full-hole filled with the filler wall.  BIG QUESTION MARK:  does a 8'10" x 8'8" filler wall behave the same as the small section of the filler wall once 8' x 8' have been cut out?

i doubt it, small wall sections don't act like whole walls.  see, the whole-filler wall didn't show this 200hz thing, but i think that the 200hz absorption peak is related to the filler wall and some strange behavior it had when used so small.  i think this because when the wall was modified it changed the absorption around 200hz.  the GREEN GLUE WALL WAS NOT MODIFIED, only the filler wall was.

the green glue walls were the exact same wall, Bob, only the filler wall was changed.

sketch below

to avoid concerns about this filler wall, in the future when testing at this lab we will have the filler wall made out of 12" solid concrete.

i'll be sure to post the data for every wall type that we test