Panel trap experiment design

[Brian Dayton]

this relates to this:

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=7053

which relates to discussion falling out of this thread:


http://www.johnlsayers.com/phpBB2/viewtopic.php?t=6895


Attached is a sketch to accompany this description of the proposed test.



Test 1:  reference wall 1

5/8" drywall
2x4 studs 24" OC, R13 fiberglass
5/8" drywall



Test 2:  reference wall 1 with panel traps added per ethans basic designs, including the "high bass trap"

5/8" drywall
2x4 studs 24" OC, R13 fiberglass
5/8" drywall
Ethan panel trap recommendations


Test 3:  repeat of 2, sans high bass trap


Test 4:  reference wall 1 with panel traps tuned to lower frequencies added
5/8" drywall
2x4 studs 24" OC, R13 fiberglass
5/8" drywall
Low frequency panel traps


Test 5:  repeat of test 4 with a second layer of drywall added to the non-trap side, and both layers of drywall on the non-trap side not attached to the structure

so

5/8" drywall
5/8" drywall
no attachment to studs
2x4 studs 24" OC with R13 fiberglass
5/8" drywall
low-frequency panel traps

Test 6:  repeat of test 5 with low frequency traps unsealed


Test 7:  repeat of 5 and 6 with Ethans traps


Test 8:  repeat of 5 and 6 with no traps, as a second reference, so:

5/8"
5/8"
no attachment to studs
2x4 studs 24" OC with R13 fiberglass
5/8" drywall



This protocol is subject to change at my discretion.  That is an ambitious and potentially time consuming set of tests.  The tests are competing with some important-to-GG Company work, and some possibility that the entire test protocol cannot be done exists.  

It seems very clear to me that part of how destructive panel traps will be to TL relates to the rleative locations of the resonances in the wall and the reosnances in the traps, and as such it is necessary to add the lower frequency traps as well.  I presume that <100hz panel traps are not uncommon anyway.  And further, the potential for contesting whatever results arise from the test almost mandate a broader set of tests lest whatever 1 or 2 tests are run wind up of use to nobody and simply the subject of additional debate.

Low frequency trap shall consist of 3.5" deep cavity with R13 fiberglass insulation and double 5/8" drywall.

[Bob]

Brian:

Looks good.

I presume that <100hz panel traps are not uncommon anyway.

Have a look at fig 20, fig 21, fig 22 (page 16) in
http://www.bbc.co.uk/rd/pubs/reports/1992-10.pdf  (hi Avare!)
for a 63hz panel trap design.

Interesting that for testing they raised all 28 of them off the floor vertically on 20mm battons, and separated them horizontally by 20mm as well (580mm units mounted/placed at 600mm centers gives 20mm horizontal separation).
Presumably the floor under them (the reverb room floor) qualifies as a relatively infinite mass floor.

[Brian Dayton]

first results are in.

more discussion is at sayers

the band-by-band data:

frequency reference all traps minus mid/high
31.5 20.0 19.4 19.5
40 13.5 14.8 14.9
50 18.7 19.4 18.8
63 16.5 14.7 13.9
80 14.6 14.8 14.0
100 14.4 16.1 16.1
125 23.3 23.6 24.4
160 34.1 31.5 32.9
200 35.6 37.1 37.2
250 38.6 38.6 39.2
315 36.2 39.6 39.7
400 33.6 40.9 40.4
500 40.8 44.0 43.2
630 40.9 44.9 44.1
800 43.9 48.1 46.5
1000 45.6 51.2 48.9
1250 47.3 53.0 49.5
1600 45.8 52.5 47.7
2000 37.6 49.8 42.6
2500 36.7 49.9 42.1
3150 42.7 55.1 48.4
4000 49.3 60.7 54.9
5000 54.0 63.9 58.7

[bert stoltenborg]

Who's gonna make an exerpt of  this for me?

[Zaphod]

Yeah please, some should comment, or we may
start guessing  :mrgreen:

[Bob]

A couple of updates today at http://www.johnlsayers.com/phpBB2/viewt ... 3&start=15

more tests to come

The same traps will be installed on hopefully at least one more wall before weeks end

[Brian Dayton]

basically, adding wood panel traps to a wall (forming a triple leaf, but making the wall deeper and heavier) didn't have much of a noteworthy negative effect on transmission loss at all.

[Zaphod]

Wasn't the HF TL increase due to the mid/high absorber to be
expected?

After all, some of the sound is converted into heat and this will
help also TL.
If i recall, even eric always said that the worsening of TL related
to mid/high narrow band absorption was to be related to the user
cranking up the volume to compensate for the sound absorbed,
hence letting out more bass from the room.

[Rod Gervais]

Wasn't the HF TL increase due to the mid/high absorber to be
expected?

After all, some of the sound is converted into heat and this will
help also TL.

Zaph,

Although it makes sense that some variance would be expected - I had no idea that the effect would be as drastic as it was.

It would be great if one could predict this to the point where it could also become a part of the room's equation for isolation.

It would be interesting to see this test performed with only broadband treatments - maybe it's as simple as determining if the absorption coefficients of a product are always equal to the amount of  increase in TL values at  each particular frequency.

Remember - even though there was a slight (abiet VERY slight in this case) reduction at some frequencies in the LF range using Panel "Traps" - I would expect that wth a broadband approach (i.e. 703 - 705 fabric wrapped) that the increase in TL would be evident at all frequencies.

 

If i recall, even eric always said that the worsening of TL related
to mid/high narrow band absorption was to be related to the user
cranking up the volume to compensate for the sound absorbed,
hence letting out more bass from the room.

[/quote]

This is either a measurable phenomenon - or it isn't -

In the case of this wall assembly - it is..........  - although it is not so drastic as to be a major concern.

Turning up volume does not equate to a worsening of TL value in a wall assembly.  Otherwise walls would have no real TL value that could be reported - because people with very sensitive ears would turn volume down - and thus theirs would have a greater TL value?  - and people like me (who are 1/2 deaf from years in construction) would have the same walls with worse TL values because they turn up the volume?

Nope - I don't care if you transmit a partucular frequency at 70dB or 100dB - if a wall isolates that frequency  by 40dB - then it's 40dB - and doesn't change with volume flucuations.

Sincerely,

Rod

[Zaphod]

Rod, sorry for expressing myself in such an unfortunate way  :bang

What i was going to say is that i don't recall anyone saying that
lowband high/mid absorption lowers the TL of a wall.

The TL term in the phrase you quoted is clearly out of place.  :oops:

What i was saying is that the listener, by compensating (volume knob)
for a  lack of highs, may actually cause higher sound levels in the near
room, because of the unabsorbed low frequencies.

[bert stoltenborg]

I would like to see these measurements performed with a decent measurement system.

[Eric Desart]

The TL term in the phrase you quoted is clearly out of place.  :oops:

I can't see why.  TL is TL, whether overall, weighted, per band, whatever ......
Zaphod it's good that you give a better context. It's good that Rod should wonder about that context rather than saying:
"This doesn't make sense."

I'm usually very, very carefull not to say things which make no sense.

[Rod Gervais]

 It's good that Rod should wonder about that context rather than saying:
"This doesn't make sense."

Eric,

Both I and my post - stand corrected..........    :oops:

Sincerely,

Rod

[Bob]

These traps only covered 1/3 of the wall for any given frequency, and there was a 2.6dB difference at 63hz.
Seems to me there's a possibility of a 6dB difference if the entire surface was a big absorber (*3 area, *2 power, not 2.6+2.6+2.6).

[Bob]

Rod and Zaphod:

I think what Zaphod was thinking was a variation of this

You can't use an absorber to stop sound leaving a music room.

Shaky science paragraph: Absorbtion coefficients are measured on a difuse sound field, giving an average absorbtion for that frequency for all vectors. Difuse sound comes from lots of directions, including those that go through the 2' and 4' lengths of the absorber, and not just those that go through the 2" depth. Obviously the If you have speakers on wall A, and absorbers on opposite wall B, that's sound going through the thinest direction of the absorber, and will get less absorbtion than the published absorbtion coefficient, at least in the first reflection. In other words the sound is still going to hit the wall, and probably a lot more of it than you are expecting.

An absorber will reduce the sound field within the room, effectively reducing the db volume in the room. According to "Noise control in buildings" by Cyril M. Harris, pg 4.11, the amount of noise reduction is given by level reduction in db = 10 log (A after / A before)
where A is the total absorbtion in sabins. So, if you had a room with carpet and people and cabinets with around 140 sabins,  and then covered the walls with an absorber with NRC=0.70 giving new absorbtion of 789 sabins, then you'd end up with a difference of 7.9db.

This is a good thing when you are trying to silence machinery, or screaming babies, or some other reasonably constant db(C) noise source. But you are not a CONSTANT noise source. What happens when you are playing your stereo or instrument is that you turn the volume up to where it feels comfortable for you. i.e. whatever TL gains you made by adding absorbtion, are negated almost precisely by turning up the volume 7.9db. So you've spent money on absorbtion. And either you've turned up the volume to the point where your speakers are now distorting, or you spend money on new and better speakers and amplifiers to compensate. So now you've spent a lot of money, and the result is that the same amount of noise is heard outside the room, and inside the room isn't any louder either. So all you've really done is just spent money.

Instead of putting up absorbtion to get room to room TL, why not use something that's commonly used for TL, such as gypsum or concrete or decoupling.

It's very important to understand what "NRC" means. It stands for "Noise Reduction Coefficient." This is a measure of how much sound is absorbed by a material inside a room. This number tells you how much quieter the room will be from sounds generated within the room.

This is completely different from the measurement called "STC", which is the "Sound Transmission Class." This number measures the ability of a material to block sound either leaving or entering a room.

generally speaking, that one layer of:
Acoustic Ceiling Tiles and Rigid Fiberglass and Rigid Rockwool have a high NRC (.5 to 1.0) and a low STC (0 to 1)
Sheetrock/Drywall/Gypsum has a high STC (28 ) and a low NRC (0.01).
My use of 'high' and 'low' are simply relative adjectives. For example there are STC 70 wall configurations that block a lot more than a single sheet of sheetrock.

Which I've posted a few times, and am currently considering rewording a bit in light of Brian's test in this thread.

[Zaphod]

Yeah Bob, plus the lowband thing and the fact that cranking up the
volume might left some more bass (which is difficult to absorb)
yelling in the room.

That money spending thing made me lough loud  :mrgreen:

Eric,
Both I and my correction post - stand corrected  :oops:    

love you guys  :mrgreen:

[Brian Dayton]

The next time we plan to be in a test lab is Riverbank in very early November, if i don't follow up with the panel traps then i will next trip to orfield.



If these traps were added to a staggered stud wall, as an example, the effect may be more noteworthy.  And adding these traps does add quite a bit of mass, and on the decoupled system (stagg studs) that might lower MSM and with that bring some positive effects in addition to whatever negative effects occur.  Its best not to guess too much.

But it is within the realm of feasible than additive triple leafing isn't likely to cause disaster.