Room modes

Hi, Wasn't sure if this should go in the design forum or the acoustics forum, so please correct me if I've posted in the wrong forum. Here's the situation: I am designing a control room using an inside-out wall design. The rear of the room is slot absorbers with a 3 foot cavity between the slats and the inside-out wall. The front of the room consists of OC706 faced absorbers with about a 2 foot cavity between the fiberglass panel and the inside-out wall. My question is this: when doing room mode calcs, should I use inside dimensions from slats to absorber, or from dry wall to dry wall? Thanks for any help.

To clarify, I have attached a JPEG of the basic room geometry. So for instance: should I be using the 20' measurement(from absorber to absorber) or the 24' 6" measurement(from drywall to drywall) to calculate axial modes for the length. My instinct tells me to go from drywall to drywall, but as you all know, instinc doesn't count for much when it comes to acoustics. Thanks again! -Guy Staley

Hi Guy; not a "cut and dried" answer, but close - for higher frequencies (not generally modal in nature) you would figure mainly the boundaries that affect those frequencies; IOW, to the faces of slats. Not sure just how much the OVERALL dimensions would affect highs, generally not much though. Modal frequencies in smaller rooms tend to only go up to 2-300 hZ, and these will penetrate most traps including slat resonators; main effect on those frequencies would be to the first SOLID mass, in your case drywall or concrete. Same with any suspended ceilings; if bass gets through (using the ceiling for trapping) then the FULL distance will be at least as influential on modal frequencies as the shorter distance to the suspended ceiling. General rule - if it's a SOLID barrier, use it for calculating lower frequencies (modes) if it's REFLECTIVE, whether it's solid or not, it will reflect/diffuse/diffract higher frequencies; in that scenario, the SIZE of the object will determine the LOWER frequency cutoff of the disturbance; larger object, lower frequencies affected. Also, the RADIUS of any corners of interior objects will change the way mids/highs "wrap around" that object, changing the amount of diffusion that object causes... Steve

Thanks Steve. That's how I figured it would work. -Guy

On the subject of modes, is this graph close enough to what i should expect? When i try bigger and bigger rooms the most frequent frequency moves to the right. this only makes sense to me if i think: "these frequencies have small wavelengths and thus there is a more likely chance of finding them in the room, and thus the graph is misleading in that it does not tell me the most problematic frequencies" is my graph misleading or does it have any relevence to the frequencies i should be looking out for? i had to make it a lot smaller to be able to attach it, so it might havea few bugs in it. thanks a lot.

Your graph seems wrong to me. In general, you should see always increasing modal content with frequency. What you're looking for is a good average spacing between modes, and fairly low'ish standard deviation. This would indicate that your modes are well distributed (i.e. no holes, or big buildups). In general, bigger rooms give better results because there are more modes and that tends to average things out. I've attached a mode chart for my room (approx 14X26X9), and that shows what you'd expect. The hole at 50 Hz will probably be my biggest problem, next I'll need some damping around 100-180 Hz since theres some little gaps in there that could be evened out. By about 175 Hz and up the graph is dense (typical of bigger rooms). What you don't want to see on the mode chart is a combination of big holes, and dark clusters (look on my chart right at 150Hz, if the hole was bigger I'd be very concerned, or if the mode strength was 2...even stilll I'll need some damping tuned to 150Hz to deal with this). Hope this helped. Kris

beautiful layout. is that theoretically predicted with formulae or hard data? if it was generated by a formula i want that formula! thanks for the help btw. if you experiment with different size rooms, cube, golden dimension you do seem to get characteristic curves so i don't know. i have onlw allowed for the first four harmonics anyway. i really like your plot, very in your face and informative, almost like a line spectrum for diffraction.

i was thinking about putting shock absorbers underneath my floor, they would be quite stiff and rigid. does this sound like a good idea? i thought it was a good idea cos it'd be cheap and the parts are readily available you know? plus they would be controlled hydrolically so would be very adaptable to level the floor you know? Question: Good or bad idea?