Measuring Room Modes

Posted: Mon Apr 28, 2008 2:57 am

Hi all.

I currently have two rooms that I want to improve the acoustics of: the small attic room that is due to become my home studio, and the largest room at the venue / nightclub I look after.

Sweeping a sine wave in the small attic room reveals obvious problems right up to 500 Hz and higher, while the room at the club has a few sub-bass dead spots, and a tendency to sound messy in the mid-range when empty.

I want to take as scientific an approach as possible to treating the rooms (especially the club) but as they are both very irregular in shape I don't know what dimensions to put into any room modelling software... so I decided to take the advice given in a book I read on the subject, and measure the actual problems instead of trying to predict them.

The book recomended playing a series of sine wave test tones into the room at 1Hz intervals, measuring the levels with a SPL meter, and logging all the results manually.... this seemed like a lot of work to me, so I used Synthmaker to knock up a little app to automate the process and plot a graph of the results.

However, this just gives a graph of the frequency response at the position of the test mic, and dramatically different results are obtained with the mic in different places in the room. I figured that if I took measurments from several different places in the room and then compared them, I could plot a graph of the differences between these measurements, which (I assumed) would correlate to the room modes: frequencies which varied wildly in amplitude in different parts of the room would be the ones most affected by standing waves, while those that varied least would be least affected... that make sense?

Here's the app as it currently stands. This version will take a total of 8 measurements, average them, then plot a graph of the variation from that average at each frequency. Screenshot

However, I still get quite different readings on the black graph when I repeat the whole test using different mic positions... which leads me to my questions:

1. Is there some scientific way I can standardize the mic positions I use for the test? I've been using the mix position for test one, the 'producer's couch' area at the back of the room for test two, then basically choosing two more random spots for the others... should I always take one reading from a corner of the room for example, or should I concentrate on the places where I might actually sit or stand?

2. Is there a better way to analyse the results? Currently I average all the readings, then plot a graph of the sum of the differences from that average.

3. Any other suggestions? Basic errors or assumptions I have made?

4. Any other affordable or free apps out there that do this better (I know, I should have checked that before I dived in with this project Rolling Eyes

)

TIA

Posted: Mon Apr 28, 2008 6:56 am

Well, for a fre app for acoustic measurement I'd recommend Room EQ Wizard (www.hometheatershack.com/roomeq), but as the author I am somewhat biased Smile

. Other measurement apps include ETF and R+D for PCs (both from www.etfacoustic.com) and FuzzMeasure for Macs (www.fuzzmeasure.com), neither is free but they offer free evaluation versions.

To find the modal responses measure with the mic in the corners of the room. Elsewhere in the room you can be at a node for one or more of the modes, so you won't see it in the measurement, and you will be affected by dips and nulls due to the effects of reflections from room surfaces. Those corner measurements let you see what resonances you have to deal with, whilst measurements elsewhere in the space help with establishing whether you have a suitable balance of absorption for reasonably uniform decay times.

Posted: Mon Apr 28, 2008 8:42 am

That Room EQ Wizard program looks pretty cool. I'm going to look into that!

Posted: Tue Apr 29, 2008 2:59 am

That does look good. Will give it a try, thanks.

This topic sponsored by: