Hi smart and funny people - I'd like bat around an acoustical problem of a music mix room, if anyone's interested.

Some details about the room to be discussed, questions 1-6 at bottom:

Dimensions (obviously not great) 27'L X 14'4" W X 9'2" H)  The entire room is
sub grade and
surrounded by concrete 6" or
double brick on all sides  The exposed surfaces are concrete floor, concrete ceiling
with dense absorber
panels applied to 60% of the surface area and E-mounted about 12" (adjustable), all
drywall walls except
one end wall exposed brick.  The walls (1/2" drywall, are not decoupled, regular
wood stud 16" o/c, 5" air
space) have little insulation in them and sound "boomy" to a punch, WHY NOT TUNE
THE WALLS AS BASS TRAPS?

______________

I am a  recording engineer and studio owner, just closed a commercial facility, setting
up one in residence.  We have worked with several acoustics consultants in the past and at the early stages of this project.  Would like ot work with someone, but really haven't met a smart, accountable, undistracted / available person yet. IN the meantime, we're down to getting it done using our many years of exposure to the subject, with a bit of light help here and there. For testing we
are using pink noise,  pure tone sweeps, familiar recordings, Logic Audio on Apple  1.25 gig with the PAZ frequency analyzer plug-in and a Behringer 8000 reference mic.  Several  questions are up in the air:

1) What is a respectable "ideal' room curve to aim for, when measured with pink
noise or sweeps (no weighting on the measuring device).  Is it realistic or even desirable to try for +/- 3 db from 40 Hz to 20k with a bit of roll off from 16 & up?  At first we had a huge wide bump of perhaps 12 db peaking at 125 Hz, still fairly present from 80 to 150 ..  Since then speaker positioning and sub woofer tuning have more or less
evened out that average (a big factor was moving the upper and sub speakers toward the back wall and up of the console).  At this stage there is a basic average from 50Hz  to 16Khz but it is
rather jaggedy.  We will deal with the mids / upper area later but in the lower end, should we still be looking at bass absorption - will  it even out the jaggs a bit but reduce the low end too much?

 To see a graph of our current room response (steady state pink noise no weighting), please see the home page of www.revsound.ca

2) What is a simple, efficient way to test the decay times of various frequencies, and how to specifically target tightening up low frequency decays, as opposed to reducing the level of low frequencies. I believe the Honorable Mr. Ethan Weiner shared an observation that evening out the lows with absorption would increase the percieved loudness of low end).

 
3) How can I test whether our existing, standard construction walls are absorbing bass
or amplifying  it? What can we do to make these walls bass traps? Take them off, add lots of insulation, put wood back on in a diffusive shape? Put the drywall back on?  Or just add 2X4 braces to stiffen the wall first see if that helps?

4) Diaphragmatic absorbers seem to specify a rigid frame and a front membrane
which vibrates.  At the same time I have been advised to "stiffen" our walls to reduce bass response in the room.  This seems to contradict.  We had built four 2'X4' 125 Hz diaphragmatic bass absorbers from Everest's somewhat vague plans and one of them appeared to do nothing when a high pressure area was tested with the reference mic, then the unit affixed to the wall.  No difference in reading

a) Was ours a valid method of testing?
b) We used 1X4 wood for the frames, it seems customary to use 2X4's - was this an
issue?
c) How can one qualify the properties of the 3/16 plywood to be used as the
membrane? Ours looked like
2 layers of mahogany veneer with something "wood-like" in the middle.  We used 2"
3.3 lb "rigid"
fiberglass inside, not touching the membrane.
d) Is it a limitation to make these as small as 2'X4'?  Would this be so small that one
unit won't measure?
e) Altogether is this so few that it is an unrealistic starting point?
 
 

5) Upon stopping live signing or pink noise or music program, after about 30
seconds an oppressive,
low velocity 80hz / approx. Eb undulating signal fills the room, lasting about 2
minutes. Should we build 80
Hz absorbers just for this - but , if 80 Hz isn't particularly showing as a big bump in a
pink noise test or
sweep - then what?

6) Other than speculating bu looking at dimensions, etc., how can one determine if
the standing waves are
occurring along one path or another, so as to decide where to place treatment.  What
is the difference in
the remedy of treating mode-caused low freq. bumps compared to "1st reflection"
caused bumps?

Whew!  Thanks for even reading this!

Gary Justice AKA Greenfield
www.revsound.ca
(Toronto, Canada)
416.410.4146

--
 
 

Gary,

You ask a lot of questions, but I'll do my best anyway. :-)

dense absorber panels applied to 60% of the surface

The 1-foot spacing helps enormously in that cement bunker, but how thick are
the panels and what are they made of exactly?

WHY NOT TUNE THE WALLS AS BASS TRAPS?

Because there's no one good frequency to tune them to. You need broadband
low frequency absorption, and plenty of it.

Is it realistic or even desirable to try for +/- 3 db from 40 Hz to 20k

Ha ha ha ha ha ha.

Seriously, you'll be lucky to get +/- 10 dB throughout the bass range unless
you have a lot of money. The good news is that's still about +/- 20 dB
better than most rooms.

At this stage there is a basic average from 50Hz to 16Khz but it is
rather jaggedy.

You need to measure with at least 1 Hz resolution to see the true response.
The graph on your web site is, um, optimistic to say the least.

should we still be looking at bass absorption

Yes!

will it even out the jaggs a bit but reduce the low end too much?

No such thing as too much.

What is a simple, efficient way to test the decay times of various
frequencies

I use the ETF program from www.acoustisoft.com and it's fabulous. But it's
PC only, not Mac.

How can I test whether our existing, standard construction walls are
absorbing bass or amplifying it?

A wall can't amplify unless it has an AC power cord. You're probably getting
some absorption, but over a limited range of frequencies. You need real bass
traps.

At the same time I have been advised to "stiffen" our walls to reduce bass
response in the room. This seems to contradict.

Yes, that's wrong. Stiff walls improve isolation but make the response
within the room worse. Of course, you could replace your sheetrock walls
with cardboard and you'll still have big problems due to the outer
surrounding cement and brick.

We had built four 2'X4' 125 Hz diaphragmatic bass absorbers

You need to pretty much line the entire room with broadband bass traps. Four
small units that absorb over only one octave will not do very much.

after about 30 seconds an oppressive, low velocity 80hz / approx. Eb
undulating signal fills the room

That sounds more like a truck is driving on a road nearby. Anything that
happens 30 seconds after you stop making sound is more likely to be UFOs
than a property of your room.

Other than speculating bu looking at dimensions, etc., how can one
determine if the standing waves are occurring

I can tell you for sure that they are. Play an 80 Hz sine wave and walk
around the room. Then do the same at literally any other frequency between
maybe 40 and 300 Hz. You will clearly hear peaks and nulls all over the
place. This is why you need broadband low frequency absorption.

--Ethan

"Ethan Winer" ethanw at ethanwiner dot com wrote in message
...

A wall can't amplify unless it has an AC power cord.

Brings to mind the scene in "Back to the Future" with Michael J. Fox
hitting that one chord while standing (momentarily) in front of that
enormous speaker cone.
--


Neil Henderson
Progressive Rock
http://www.saqqararecords.com

Neil,

Michael J. Fox hitting that one chord

ROF,L.

Have you ever seen the AniMusic DVD? It's a fabulous computer animation with
instruments that play themselves. The first track has a huge wall full of
amplifiers. It's very cool.

Look at www.animusic.com then click Future Retro, though the demo images are
too small and too low-res to do justice to this great piece of work.

--Ethan

"Ethan Winer" ethanw at ethanwiner dot com wrote in message
...
Neil,

Michael J. Fox hitting that one chord

ROF,L.

Have you ever seen the AniMusic DVD? It's a fabulous computer animation
with
instruments that play themselves. The first track has a huge wall full of
amplifiers. It's very cool.

Look at www.animusic.com then click Future Retro, though the demo images
are
too small and too low-res to do justice to this great piece of work.

LOL! That is pretty cool! When are they going on tour?

--


Neil Henderson
Progressive Rock
http://www.saqqararecords.com

Neil,

LOL! That is pretty cool! When are they going on tour?

They're on tour right now - in a DVD player near you!

AniMusic is probably my all-time favorite DVD. I've seen it at least two
dozen times.

--Ethan