OT - Advanced Acoustics

[lwilliam]

I'd been having a lot of trouble getting my mixes to translate well to other systems (a lot of trial and error - mostly error) and I finally did a sine wave plot of my monitor/room. No wonder...as much as 20db differences at some frequencies (I did three runs and averaged the results).

I moved the monitors around (Tannoy System 8 II), put sand in the monitors stands (BIG difference), got some Auralex, etc., so now the mids and high end are much better (smoother) from about 300hz on up..

However, it seems I have a couple of room modes that are causing some serious low-end problems. Specifically, I get a HUGE dip at around 100hz (12db) and a peak right after that at around 130hz (up about 6db). This makes the low-end sound, uh, rather...funky, at best. Otherwise, I get good response clear down to 40hz.

The 100hz range seems to be a "triple mode" where all three room axes have a corresponding wavelength that matches (or is a multiple of) that frequency - according to a couple of room mode calculaters I tried. I GUESS that there is some serious bass frequency cancellation going on because they're hitting each other out of phase.

So the question seems to be: would adding some bass traps actually help bring up 100hz by absorbing some of the low-frequencies so there is LESS phase cancellation? It seems a bit counter-intuitive to trap MORE bass in order to increase it, but if it really IS phase cancellation, it should work. What do y'all think?

[Larry Elliott]

Hi Larry

You are correct about the issue being modal. Moving your monitors will make some difference but this can be tediuos. Note testing with sine waves is not the ideal method as it exagerates the modal behaviour. Use a swept tone thru the offending frequencies and you should hear the problem one stand out.

Have you plotted out all the modal frequencies? One method of analysis is to group the modes into 1/3 octave bands. There should be more modes as the groupings go up in frequency - if not the room will sound "unbalanced".

Your situation with 100Hz and 130Hz - these will combne and create the colouration you are hearing. A bass trap centred on 125Hz would probably make a great deal of differnce, but such devices are not small.

I would be willing to run some calcs for you if you send me your room dimensions and the surface materials.

Larry Elliott
Auckland, New Zealand[/LIST]

[lwilliam]

Thanks so much!

I don't know what you mean by plotting out all the modal frequencies. I plotted out frequencies at 5hz intervals up to 150hz (40hz, 45hz, etc.), then jumped to 10hz up to 250hz and gradually got wider as the frequencies went up.

I'm certainly glad to hear that sine waves tend to exagerate frequency variations. I don't have a way to plot the frequency response with other than sine waves. I could use the digi signal generator for pink noise (I used it for the sine wave), but I don't have an RTA. I'd have to rent one. I just used a Radio Shack level meter (not an Earthworks mic and Gold Line RTA - for sure).

I then found a couple of web sites offering modal calculations and plugged my room dimensions into it. One of them said that 98.6hz was a mode of all three dimensions.

RPG makes a bass trap that seems to work at around 125hz best, but it's around $600 for a pair. I'm willing to get Auralex LENRDs which are a LOT cheaper even if they don't do it as well. As long as there is some improvement, I can get used to it.

I send you an email with more specifics.

Thanks again! [img]images/icons/grin.gif[/img]

[Kev]

You realy need to do this with an FFT type analyser.