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Help! Frequency response measurement driving me nuts! (LONG)

208.180.29.48

Posted by Allan Moyse on November 20, 2000 at 20:52:20

For a while now, I've been trying to make measurements of my system's freqency response so that I can better set my active crossover controls and parametric EQ.

Here was my game plan:

1. Make an audio CD containing tracks which are sine waves at various freqencies.

2. Play this CD and measure the volume of each freqency with my Radio Shack sound level meter.

3. Graph the resulted data points to get my freqrency response.

So, off I go:

I wrote a program in MatLab (a mathematics software package that I use for work) that generated simple 44.1 kHz, 16-bit, mono WAV files at various different freqencies. All files were generated with the same amplitude.

I used Adaptec EZ CD Creator to burn a CD containing these WAV files as tracks. This program automatically converts the WAV files to normal CD audio tracks.

So, now I have my CD. I set up my Radio Shack analog sound level meter on a microphone stand at my listening position pointed straight forward between my speakers.

I started off by playing the 5 kHz track and adjusting the preamp volume so that the meter read ZERO on the 80 dB range. My air conditioner, etc, was turned off, and there were no extraneous noise sources.

I then started playing the other tracks, starting with the lowest, 10 hz, and going upward to 10 khz.

I was expecting a nonlinear response--especially since I know my room has some bass boom problems. However, what I measured shocked me. I don't know wether my room's acoustics are to blame, or if there is something wrong with my test setup or methods....

Example data:

700 hz = +0 dB
800 hz = +3 dB
900 hz = +9 dB
1000 hz = -4 dB
2000 hz = -5 dB
3000 hz = -6 dB
4000 hz = +9 dB
5000 hz = +0 dB
6000 hz = -7 dB
7000 hz = -1 dB

What do you think? Is it possible for a room to cause these kinds of readings (assuming that my CD & meter are OK), or is something else suspect?

My listening position (and therefore the meter position) is rather close to the back wall of the room. I know that's bad, but it is a smallish square room, and really can't be helped.

Thanks for helping me out,

Allan

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