Acoustics Help: Resonate frequency between 320-360 HZ

 I just wired a room for tracking and have found a resonate frequency between 320-360HZ. The room is 11x13x7' tall, estimated, and has few wall treatments. One wall is concrete, covered with a set of curtains, another wall is wood covered drywall, 1"x8" wood pieces, another wall is drywall with two 2'x4' 703 panels, and the last wall is drywall with a glass panel. The ceilings are untreated.

 Where do I start? What are the chances that parallel walls are the problem?

Parallel untreated walls are the problem. Taught Acoustics years ago and forgot the formula to calulate room modes. You can do a google search. You use the room dementions to plot the room modes. This will tell you  what frequencies build up. Starting or changing  the  room size can smooth out the response. Other than that you'll have to use treatments. Use can send your room plans to one of the room treatment companies and they will show you what to buy to treat your room or you can then do it yourself. I know you need a least 4" thick of owens corning 703 to absorb freqs below 500 hz and so many sq. feet of it for a room. You can also use holms resonators to defuse the freqs. There are many articles on line to help you figure this out.

to help the googling- i think winetree is referring to a helmholtz resonator, as i'm unfamiliar with 'holms resonators' (sorry if i'm putting words in your mouth there  :-X ).  curtain + concrete isn't going to help much in the low to mid bands, it just helps kill the flutter echo that would exist with no treatment whatsoever.  i've been stuck with an almost identically sized room and did the entire thing with a mosaic of 2'x4' mineral wool panels, varying between 2" and 4" thickness.

you might want to hang the panels in all the corners including ceiling to wall. 705 is also useful
francois

ditto to everything said so far

corners the most effcient place for trapping

modes?  check this out  http://www.bobgolds.com/Mode/RoomModes.htm

I just wired a room for tracking and have found a resonate frequency between 320-360HZ.

Click to expand...

Room treatments can be problematic: too much absorption (wide band kinds, auralex foam style) and your room became lifeless and opressive and sometimes just make the problem worst ( foam in 10cm depth can't do many things for a 1meter wave length... but can kill all medium/high freq leaving your nasty freq more proeminent).

Well Helmotz resonator could help you if only 320/360hz is the problem. Basically it's just a box with a tuned port (as a bass reflex enclosure without loudspeaker).

Sometimes i use some 'big' nasty chinese PA speaker to do this when recording heavy bass instruments... Not really scientific but it work... sometimes!    ;D

You know the frequency band so you probably may try this application:

http://www.mh-audio.nl/user/acoustic%20calculator.asp

Much more scientific and accurate that chinese pa speakers!  

You can simulate room mode ( axial tangential and oblique) as well as different kind of acoustic treatments... Try a panel absorber too, it can be efficient with your room ( you'll probably need some of them...)

As much as possible, I'm trying to address the issues of the room without decreasing the decay time of the room. Also, I forgot to post in the first post that the celling is untreated.

Panel absorber_ vpr fraunhofer/modex plate is probably a good idea if you  want to control only the low end of the spectrum

you can use a resonant panel over an air gap with insulation to treat a specific frequency. also the helmholtz resonators can be tuned to specific frequencies. calculators/ equations can be found here in the 'original recording manual' on the linked page http://www.johnlsayers.com/

> a resonate frequency between 320-360HZ. ... wall is wood covered drywall, ...another wall is drywall with two 2'x4' 703 panels, and the last wall is drywall....

Bump your fist on the drywall. I bet you hear 340Hz thump.

how have you tested the room? possible open conduit

What's a good way to check the room? I noticed the resonate frequency just by clapping in the room. I guess what I'm noticing is a lot of early reflections. Parallel walls and right angles are all over. There is a piece of conduit between the rooms, but it's stuffed with a piece of foam.

Pictures to come.

Thanks for the help,

-P

try an oscillator. do as quick as possible on and off. by ear check how long it decays. you can do ranges of frequencies. Works well for me.

In addition to the great help here, there's also John Sayers site:

http://www.johnlsayers.com/phpBB2/index.php

The room has a volume of about 1000 cubic feet. 2500 cubic feet and larger is a good starting point.

My recommendation is to make it as dead as possible by first making corner traps out of 4" 703 (floor to ceiling). Also do walls with 2" 703 and a 2" air gap behind the 703. Space is a premium. This will take 4" off each dimension unfortunately. This will make the room dead but you can add in some diffusion to liven it up a little.

A 7 foot ceiling doesn't leave much room for treatments there. You may want to do a couple clouds. 2" 703 panels hung from the ceiling.

After you are done with that, add in some poly-cylindrical diffusers stuffed with dense fiberglass. They are cheap and pretty easy. 1/8" ply is easiest to bend for poly's but may be hard to find. Try perspex or something. It bends well and can be made to be semi-translucent (make them into low wattage lamps for dual purpose fixtures).

Plan to spend $1k-$2k on 703, burlap style fabric, and installation materials. Get someone to help install it if you can.

Looks like you have about 350 square feet of wall space. That's about 8 cases of 2'x4' 2" panels plus a couple extra's for clouds.

Best,
jonathan

At the risk of feeling like an idiot.....

I read this thread a week ago and PRR's comment about bumping a fist on the wall has kept me thinking and feeling like I missed something a long time ago. Please someone put me straight.

The way I see it (and I know I am simplifying things by not mentioning harmonics and modes etc):

Sound is absorbed when the sound energy sets something moving and the very mechanical movement of that something ends up dissipating the energy as heat. As a general rule - things like to move when they are excited at or near their natural (resonant) frequency. Thus.....

A curtain absorbs high frequencies because the little fibres are small enough to be excited by the short wavelength of the high frequencies but the low frequencies go straight through. Even the fibres of a curtain must have a frequency that they like to naturally vibrate at but they dont get excited by low frequencies.

A Helmholtz resonator is at the other end of the scale. A cavity tuned to a particular frequency absorbs sound energy at the intended frequency by the air inside resonating. The wave goes in, resonates, and in doing so the minutely compressing and expanding waves of air in the cavity loose energy in the form of heat.

A panel absorber is somewhere in the middle. A membrane (usually wood or similar) and the volume of the cavity behind is sized (or tuned) to absorb a particular frequency (or a band of frequencies centered around a middle frequency). Put wool in the cavity and it widens that band out, but reduces the absorbtion overall (particularly at the middle frequency). But in the end the result is the same as the curtain and Helmholtz resonator. Sound at a frequency that matches the natural frequency(s) of the panel sets the panel in motion and as a result the sound energy is converted to heat. High frequencies (and low ones) are simply reflected away un changed as they do not in any way effect the motion of the panel.

Herein lies my issue with the fist on the wall. If you hear a 340Hz thump when hitting the drywall, wouldnt that wall be good at absorbing 340Hz and as a result this would not be a problem frequency? You get a problem frequency when hard reflective walls face each other setting up a resonant frequency in the air in between. If one of those walls is not reflective, and actually resonates at 340Hz, I would have thought that wall would stop the problem frequency - the problem frequency would not even be able to establish itself.

On a similar vein.. i was once told by an expreienced engineer (recording) that contrary to popular belief, recording drums on a stage made with floorboards is not nessesarily the best way to get a big drum sound. If the stage (really just a big panel absorber) is at the frequency that gives the drum sound its oomph, then it will absorb the oomph. What oomph is left to get to the mic? Better to get a lump of concrete to set the drums up on and reflect all that oomph right into the mic.

Now the argument might be "but why does the resonant cavity in an acoustic guitar make the guitar loud?" or "why do those drums sound so good to me on the stage made with floorboards?" This would require the introduction of the concept of the difference between forced vibration (something mechanical making the vibration happen - the guitar string or person hitting the drums) and simple resonance set in motion by a sound wave. Perhaps this is a discussion for another time.

Thoughts?

Cheers,

Tim

> If you hear a 340Hz thump when hitting the drywall, wouldnt that wall be good at absorbing 340Hz and as a result this would not be a problem frequency?

You may be right.

But that's a LOT of drywall in a TOO small room. I'm not sure the anechoic-chamber tests fully apply. Or what he's really hearing. Or how stiff his studs are.

If your thinking was true, and IF his loaded studs resonate ~~340Hz, then he can't be having a problem. Yet he does. Perhaps he really has excess absorption 100Hz-300Hz, so it seems-like 340Hz is strong.

I just want him to thump. Maybe it's "Hey, that's it!" Maybe it's "Nah, PRR is full of crap again."

But I think the real answer on a room that size is to fill it with as many dead cats (fluffy wool, foam, velour, groupies) as possible, and sit close to the speakers.

Un-parallel would be a nice touch, but not a cure. Assuming adjacent rooms can't be eaten into, false walls would reduce the already too-small space. Whereas 4"-12" of pyra-foam from 36" to the ceiling and into the corners would make it very dead without reducing floor space much (if you use short racks).