Frosted plexiglas doubling as acoustic panels

[Andre Majorel]

I'm thinking of fixing frosted plexiglass plates to the ceiling
of my cellar to improve the lighting.

Is there a way to position the plexiglass that would also
improve the acoustics, however modestly ? Concrete floor, solid
plaster ceiling, headroom 2.20 m.

Diffusion : if the panels are at an angle (say, 10° from the
horizontal), I expect it'll help diffusion at medium
frequencies.

Absorption : I can't make conventional bass-traps out of them
because rock wool is opaque but maybe I could use a "viscous"
mounting to dissipate the acoustical energy.

--
André Majorel <URL:http://www.teaser.fr/~amajorel/>
(Counterfeit: )
First rule of Usenet : if we don't have an answer, it's not a
good question.

[Richard Crowley]

"Andre Majorel" wrote ...
> I'm thinking of fixing frosted plexiglass plates to the ceiling
> of my cellar to improve the lighting.
>
> Is there a way to position the plexiglass that would also
> improve the acoustics, however modestly ?

You could consider those panels that are molded to
look like an array of pyramids (rather than just a flat
panel). That would add some detailed break-up to the
reflection landscape.

Or you could weave some sort of collage out of glass
fibers to try to combine light transmission and some
minor amount of absorption. (Just kidding.)

[William Sommerwerck]

To get dispersion, you'd have to adjust the length and depth of the panels
to mimic the diffusion panels currently in use.

[WillStG]

Andre Majorel wrote:
> I'm thinking of fixing frosted plexiglass plates to the ceiling
> of my cellar to improve the lighting.
>
> Is there a way to position the plexiglass that would also
> improve the acoustics, however modestly ? Concrete floor, solid
> plaster ceiling, headroom 2.20 m.
>
> Diffusion : if the panels are at an angle (say, 10° from the
> horizontal), I expect it'll help diffusion at medium
> frequencies.
>
> Absorption : I can't make conventional bass-traps out of them
> because rock wool is opaque but maybe I could use a "viscous"
> mounting to dissipate the acoustical energy.


Well - track lighting is nice too,no? And traditional in studios.
Then you can throw up some Owens Corning 703 type stuff in 2"x3" frames
on the ceiling for absorbtion...

Will Miho
NY TV/Audio Post/Music/Live Sound Guy
"The large print giveth and the small print taketh away..." Tom Waits

[Andre Majorel]

On 2007-01-24, Richard Crowley > wrote:
> "Andre Majorel" wrote ...
>
>> I'm thinking of fixing frosted plexiglass plates to the
>> ceiling of my cellar to improve the lighting. Is there a way
>> to position the plexiglass that would also improve the
>> acoustics, however modestly ?
>
> You could consider those panels that are molded to
> look like an array of pyramids (rather than just a flat
> panel). That would add some detailed break-up to the
> reflection landscape.

Thanks, I'll look for those.

--
André Majorel <URL:http://www.teaser.fr/~amajorel/>
(Counterfeit: )
First rule of Usenet : if we don't have an answer, it's not a
good question.

[Chris Whealy]

Andre Majorel wrote:
> I'm thinking of fixing frosted plexiglass plates to the ceiling of my cellar to improve the lighting.
>
Is your existing lighting too direct or too harsh? Would it not be
better to get low voltage track lights? Then you can position the
lights at any angle you require. Also many of the LV light fittings can
have Mylar diffusers mounted in front of them.
> Is there a way to position the plexiglass that would also improve the acoustics, however modestly ? Concrete floor, solid plaster ceiling, headroom 2.20 m.
Not unless you break up the surface sufficiently to create diffusion.
Given the description of your room, it sounds like your working in a
reflective box in which case it would be more useful to have the
diffusion on the walls, not the ceiling.
> Diffusion : if the panels are at an angle (say, 10° from the horizontal), I expect it'll help diffusion at medium frequencies.
They won't really provide enough diffusion to be worth the construction
effort. You would be far better applying absorption to the ceiling and
absorption/diffusion to the walls.

Diffusion works most noticeably for a listener when sound is scattered
in the horizontal plane. The cheapest and simplest way to do this is to
buy some long cardboard tubes that are between 8" and 12" in diameter
(look up packaging manufacturers in the yellow pages). The tubes
normally come in about 8' lengths.

Cut the tubes in half down their length and lean them up against the
walls at roughly 30" centres and you will immediately notice that the
modal resonances of the room are significantly reduced without affecting
the reverberation time. Now add absorption to lower the RT as required.
> Absorption : I can't make conventional bass-traps out of them because rock wool is opaque but maybe I could use a "viscous" mounting to dissipate the acoustical energy.
A "viscous" mounting as you describe is really a panel absorber, where
you use the natural resonance of a panel to act as an absorber (much
like when a drum skin rings sympathetically). What would probably be
more effective is to make corner traps. These work on the basis that
you can get extra depth to your absorber by utilizing the corner areas
of a room.

Chris W

--
The voice of ignorance speaks loud and long,
But the words of the wise are quiet and few.
---

[Mogens V.]

Andre Majorel wrote:
> I'm thinking of fixing frosted plexiglass plates to the ceiling
> of my cellar to improve the lighting.
>
> Is there a way to position the plexiglass that would also
> improve the acoustics, however modestly ? Concrete floor, solid
> plaster ceiling, headroom 2.20 m.
>
> Diffusion : if the panels are at an angle (say, 10° from the
> horizontal), I expect it'll help diffusion at medium
> frequencies.

Dunno how much it'll diffuse, but it /will/ reduce standing waves due to
parallel walls.

> Absorption : I can't make conventional bass-traps out of them
> because rock wool is opaque but maybe I could use a "viscous"
> mounting to dissipate the acoustical energy.

I don't get the rockwool and opaque mix... If you have rockwool behind
the plexi, I can understand if you mean the wool isn't opaque :)

Anyways, if you can arrange the plexi and rockwool to an acceptable way
of light disperser, you can still have a kinda Helmholtz resonator
sucking thingy.
I would line those plexis at, say one meter (3 feet) distance to one
another, leaving the nessesary 'crack' running vertically between them,
to have the resonator behind the plexi.
You may have to break each plexi plate surface up in two between the
plate and wall, to create the cavity.

Not too easily explained in a text mail :-D

If math is needed, I have some Brüel & Kjäer books on controlling sound
in theaters et al, with theory and formulas, but need to dig 'em out.

--
Kind regards,
Mogens V.

[Mogens V.]

Chris Whealy wrote:
> Andre Majorel wrote:
>
>> I'm thinking of fixing frosted plexiglass plates to the ceiling of my
>> cellar to improve the lighting.

...snipped..

>> Diffusion : if the panels are at an angle (say, 10° from the
>> horizontal), I expect it'll help diffusion at medium frequencies.
>
> They won't really provide enough diffusion to be worth the construction
> effort. You would be far better applying absorption to the ceiling and
> absorption/diffusion to the walls.
>
> Diffusion works most noticeably for a listener when sound is scattered
> in the horizontal plane. The cheapest and simplest way to do this is to
> buy some long cardboard tubes that are between 8" and 12" in diameter
> (look up packaging manufacturers in the yellow pages). The tubes
> normally come in about 8' lengths.
>
> Cut the tubes in half down their length and lean them up against the
> walls at roughly 30" centres and you will immediately notice that the
> modal resonances of the room are significantly reduced without affecting
> the reverberation time. Now add absorption to lower the RT as required.

Nice idea! Cheap too. Do you leave the cut surface or the curvation
against the wall? I guess it'll be the curvation, so you can fill the
half-cavity with padding, right?

>> Absorption : I can't make conventional bass-traps out of them because
>> rock wool is opaque but maybe I could use a "viscous" mounting to
>> dissipate the acoustical energy.
>
> A "viscous" mounting as you describe is really a panel absorber, where
> you use the natural resonance of a panel to act as an absorber (much
> like when a drum skin rings sympathetically). What would probably be
> more effective is to make corner traps. These work on the basis that
> you can get extra depth to your absorber by utilizing the corner areas
> of a room.

Big Nod. It also takes away one of the most annoying problems in rooms
not directly constructed for music: Room modals, which works especially
well in corners, often in the 60-70 to 130-150hz range.


How often I've wished everyone would apply corner traps in their
appartments; it would take away a lot of that annoying bumbing bass,
especially combined with proper speaker isolation from the floor.

--
Kind regards,
Mogens V.

[Chris Whealy]

Mogens V. wrote:
> Nice idea! Cheap too. Do you leave the cut surface or the curvation
> against the wall? I guess it'll be the curvation, so you can fill the
> half-cavity with padding, right?
You place the convex surface outwards. By using a range of diameters
and spacings, you can create a very diffuse environment that is
excellent as both a performance/recording/practice space and a listening
space.

Its a basic principle that is so often forgotten. It the room's
acoustics are crap, then not matter what you try to record/reinforce in
that environment, the room will always be fighting against you.
Therefore, create an acoustically pleasing (I.E. diffuse) room, and 90%
of your recording/reinforcement problems disappear.

As for packing the tubes, this is not really necessary if they are
mounted flush to the wall. However, if you want to add absorption to
the room, but don't want to have squashy Rockwool all over the walls,
then you can pack the inner surface of the tubes with Rockwool, and
mount them about 4" away from the wall. The gap will allow reflected
sound to be absorbed by the back face of the tubes, whilst the front
face produces the required diffusion.
> Big Nod. It also takes away one of the most annoying problems in rooms
> not directly constructed for music: Room modals, which works
> especially well in corners, often in the 60-70 to 130-150hz range.
To be honest, he easiest way to reduce the modal resonance of a room is
to use diffusion. The tubes break up the flat surfaces that would
otherwise sustain the resonant frequencies. Also, once the sound has
been scattered correctly, you greatly increase the path length of a
sound wave before it returns back to the point in the room from which it
originated. This has two major benefits:

1. The sound wave has been significantly attenuated by both the
increased distance it has travelled and the increased number of
reflections
2. As a consequence of point 1), you will get significantly greater
gain before feedback if you do any recording in the room.

> How often I've wished everyone would apply corner traps in their
> appartments; it would take away a lot of that annoying bumbing bass,
> especially combined with proper speaker isolation from the floor.
Corner traps are no doubt useful, but are only necessary if the room has
poor diffusion.

So many people think that treating a room acoustically simply means the
addition of absorption, but they never really consider diffusion
(probably because it is much harder to predict the benefits of diffusion
using a computer prediction than it is using simple absorption).

Chris W

--
The voice of ignorance speaks loud and long,
But the words of the wise are quiet and few.
---

[Mogens V.]

Chris Whealy wrote:
> Mogens V. wrote:
>
...snipped..
>> How often I've wished everyone would apply corner traps in their
>> appartments; it would take away a lot of that annoying bumbing bass,
>> especially combined with proper speaker isolation from the floor.
>
> Corner traps are no doubt useful, but are only necessary if the room has
> poor diffusion.
>
> So many people think that treating a room acoustically simply means the
> addition of absorption, but they never really consider diffusion
> (probably because it is much harder to predict the benefits of diffusion
> using a computer prediction than it is using simple absorption).
>
> Chris W

You're so right. My latter comments were meant for normal living rooms,
where diffusion can be a lot harder to do aestetically than fixing a
couple of corner modes.
Thanks for the other explanations.

--
Kind regards,
Mogens V.


"One thing you can say about ignorance,
it causes a lot of interesting arguments."
-- Bob Heil (from his book "Concert Sound")

[Andre Majorel]

On 2007-01-24, Chris Whealy > wrote:
> Andre Majorel wrote:

>> I'm thinking of fixing frosted plexiglass plates to the
>> ceiling of my cellar to improve the lighting.
>
> Is your existing lighting too direct or too harsh? Would it not be
> better to get low voltage track lights?

Can't stand them.

>> Is there a way to position the plexiglass that would also
>> improve the acoustics, however modestly ? Concrete floor,
>> solid plaster ceiling, headroom 2.20 m.
>
> Not unless you break up the surface sufficiently to create
> diffusion. Given the description of your room, it sounds like
> your working in a reflective box in which case it would be
> more useful to have the diffusion on the walls, not the
> ceiling.

The room is about 7 x 3.5 m with a 1 x 2 m "extension" at each
end and a pillar in the middle. It has a solid plaster ceiling
and solid concrete on all other surfaces. Not unlike the inside
of a bunker.

I can't recall exactly how it sounded when it was empty, but I
don't remember it ever sounding good. In any case, even
cluttered as it is now, it still sounds atrocious which makes me
think the low ceiling is part of the problem. (Surprise.)

>> Diffusion : if the panels are at an angle (say, 10° from the
>> horizontal), I expect it'll help diffusion at medium
>> frequencies.
>
> They won't really provide enough diffusion to be worth the
> construction effort. You would be far better applying
> absorption to the ceiling and absorption/diffusion to the
> walls.

I don't know of any material that is both translucent and an
absorber. Plexiglas in 1-to-3-mm thick sheets is probably highly
reflective above 1 kHz.

If the plexiglas is mounted as a low-frequency absorber, the LF
behaviour of the room will improve but the HF behaviour may
degrade. When standing, instead of getting reflections from
plaster 60 cm from my ears, I'll get reflections from plexiglass
40 cm from my years.

I figured that slanting the panel could mitigate the HF
aggravation without compromising the LF improvements (panel
absorber).

> Diffusion works most noticeably for a listener when sound is
> scattered in the horizontal plane. The cheapest and simplest
> way to do this is to buy some long cardboard tubes that are
> between 8" and 12" in diameter (look up packaging
> manufacturers in the yellow pages). The tubes normally come
> in about 8' lengths.
>
> Cut the tubes in half down their length and lean them up
> against the walls at roughly 30" centres and you will
> immediately notice that the modal resonances of the room are
> significantly reduced without affecting the reverberation
> time. Now add absorption to lower the RT as required.

That'll be handy if/when the time comes. Thanks for the tips.

--
André Majorel <URL:http://www.teaser.fr/~amajorel/>
(Counterfeit: )
First rule of Usenet : if we don't have an answer, it's not a
good question.

[Andre Majorel]

On 2007-01-24, Mogens V. > wrote:
> Andre Majorel wrote:
>
>> Diffusion : if the panels are at an angle (say, 10° from the
>> horizontal), I expect it'll help diffusion at medium
>> frequencies.
>
> Dunno how much it'll diffuse, but it /will/ reduce standing waves due to
> parallel walls.

Perhaps "diffusion" was not the right word. Yes, the idea was to
prevent standing waves. Thanks.

>> Absorption : I can't make conventional bass-traps out of them
>> because rock wool is opaque but maybe I could use a "viscous"
>> mounting to dissipate the acoustical energy.
>
> I don't get the rockwool and opaque mix... If you have rockwool behind
> the plexi, I can understand if you mean the wool isn't opaque :)

The sort of bass-traps I'm familiar with is a flexible panel
parallel to the wall and some distance away from it, the void
being filled by rock wool. The panel resonates with the low
frequencies. The rock wool dampens the resonance, converting the
mechanical energy to heat.

> Anyways, if you can arrange the plexi and rockwool to an acceptable way
> of light disperser, you can still have a kinda Helmholtz resonator
> sucking thingy.
> I would line those plexis at, say one meter (3 feet) distance to one
> another, leaving the nessesary 'crack' running vertically between them,
> to have the resonator behind the plexi.
> You may have to break each plexi plate surface up in two between the
> plate and wall, to create the cavity.

One meter in which direction ?

Here's what I had in mind :

/ / / / / / / / / / / / / / C E I L I N G / / / / / / / / / / / / / / /
/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
|__ __| _O O-_ |__ __|
|| _ FLUO _- --_ _ ||
|| / \ TUBE _- --_ / \ ||
|| \_/ _- --_ \_/ ||
|| _- --_ FLUO ||
|| _- --_ TUBE ||
|| _- --_ ||
__||__ _- --_ __||__
|______|O -O|______|

<----------------------- 1.60 m ------------------------>

The panels would be attached to the ceiling and the bottom of
the I beams with something soft (represented by O's in the
drawing above).

--
André Majorel <URL:http://www.teaser.fr/~amajorel/>
(Counterfeit: )
First rule of Usenet : if we don't have an answer, it's not a
good question.

[Chris Whealy]

Andre Majorel wrote:
> Here's what I had in mind :
>
> / / / / / / / / / / / / / / C E I L I N G / / / / / / / / / / / / / / /
> / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /
> _/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
> |__ __| _O O-_ |__ __|
> || _ FLUO _- --_ _ ||
> || / \ TUBE _- --_ / \ ||
> || \_/ _- --_ \_/ ||
> || _- --_ FLUO ||
> || _- --_ TUBE ||
> || _- --_ ||
> __||__ _- --_ __||__
> |______|O -O|______|
>
> <----------------------- 1.60 m ------------------------>
>
> The panels would be attached to the ceiling and the bottom of the I beams with something soft (represented by O's in the drawing above).
Try as much as possible to avoid flat surfaces. If you can put a bend
into the plexiglass (without breaking it), then a curved surface will
give better diffusion of sound than a flat panel.

Chris W

--
The voice of ignorance speaks loud and long,
But the words of the wise are quiet and few.
---

[Andre Majorel]

On 2007-01-24, Chris Whealy > wrote:

> Try as much as possible to avoid flat surfaces. If you can
> put a bend into the plexiglass (without breaking it), then a
> curved surface will give better diffusion of sound than a flat
> panel.

Good point, thanks, but another can of worms. I expect that
bending the panel will raise its resonance frequency. For a
bass-trap, you don't want that, do you ?

I'm guessing that the aborption range of a bass-trap is related
to the resonance frequency of the panel. Which depends on its
dimensions, thickness and tension. Problem is, I don't have
anything to use as a starting point other than trial and error.

--
André Majorel <URL:http://www.teaser.fr/~amajorel/>
(Counterfeit: )
First rule of Usenet : if we don't have an answer, it's not a
good question.

[Scott Dorsey]

Andre Majorel > wrote:
>
>I'm guessing that the aborption range of a bass-trap is related
>to the resonance frequency of the panel. Which depends on its
>dimensions, thickness and tension. Problem is, I don't have
>anything to use as a starting point other than trial and error.

Not really. There are two kinds of things you can build to deal with
bass problems.

First you can build a Helmholtz resonator, which is actually a resonant
chamber that is tuned to one frequency. This is useful to fix a room
problem that only exists at a single frequency.

But a bass trap is different. A bass trap is a thing that is designed
to have the same impedance as air to audio, so the wave just passes right
into it and disappears rather than being reflected back. A bass trap
acts like putting an open window up in place of the wall.

It's true that a bass trap is only effective down to a certain frequency
which depends on the dimensions. But it's not supposed to be a resonant
device like a Helmholtz resonator is.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Ethan Winer]

Andre,

> Perhaps "diffusion" was not the right word. Yes, the idea was to prevent
standing waves. <

Plexiglass and curved panel etc do nothing to prevent or even reduce
standing waves at low frequencies. For bass problems, most rooms benefit
from broadband LF absorption. Tuned traps are occasionally useful, but
broadband is almost always preferred in domestic size rooms. See this:

http://www.ethanwiner.com/acoustics.html

--Ethan

[Edwin Hurwitz]

In article . com>,
"WillStG" > wrote:


>
> Well - track lighting is nice too,no? And traditional in studios.
> Then you can throw up some Owens Corning 703 type stuff in 2"x3" frames
> on the ceiling for absorbtion...
>
>

This sounds like a good way to go, but you'll need a lot of 2x3 inch
squares to get much coverage. Sounds labor intensive. Or is this a
Stonehenge moment?

:-)
Edwin

[WillStG]

Edwin Hurwitz wrote:
> In article . com>,
> "WillStG" > wrote:
>
> > Well - track lighting is nice too,no? And traditional in studios.
> > Then you can throw up some Owens Corning 703 type stuff in 2"x3" frames
> > on the ceiling for absorbtion...
> >
> This sounds like a good way to go, but you'll need a lot of 2x3 inch
> squares to get much coverage. Sounds labor intensive. Or is this a
> Stonehenge moment?

The frames! You make the *frames" for the OC 703 with 2"x3" (or
1"x4") of course!

The actual panels would be maybe 2'x4', something like that. All the
703 I have is salvaged from designers/studios or prebuilt into aluminum
2'x2' frames, so I dunno exactly what the dimensions of virgin 703 is.
Probably the size of "Realtraps" I imagine... <g>

Will Miho
NY TV/Audio Post/Music
"The large print giveth and the small print taketh away..." Tom Waits

[Chris Whealy]

Andre Majorel wrote:
> Good point, thanks, but another can of worms. I expect that bending the panel will raise its resonance frequency. For a bass-trap, you don't want that, do you ?
>
To be honest, I think you've been rather side tracked by the theoretical
aspect of panel absorbers. It is notoriously difficult to make useful
predictions of the resonant frequency of such devices because any slight
variation in the mounting conditions (I.E. how you fix the panel to the
wall) can completely alter the resonant properties of the panel.

Such panels are usually wall mounted, and the dimensions of the panels
you're talking about around the light fittings will not be large enough
to create the effect you're after.
> I'm guessing that the aborption range of a bass-trap is related to the resonance frequency of the panel. Which depends on its dimensions, thickness and tension. Problem is, I don't have anything to use as a starting point other than trial and error.
Let me reiterate my point about diffusion. In a diffuse room, modal
resonances are significantly disrupted any way. In practical terms, the
best thing to do is build yourself a diffuse environment, then measure
the decay response of the room. If you do have any persistent
frequencies, then a waterfall plot will highlight them and you can then
build a Helmholtz absorber tuned to the offending frequency range.

Chris W

--
The voice of ignorance speaks loud and long,
But the words of the wise are quiet and few.
---