Warble Tones In Cool Edit

Warble tones are just sinewaves swept over a small range, usually 1/3, 1/6, or 1/10 octave. The sweeping should be done in a linear fashion (that is, the modulating waveform should be triangular) but a sinewave can be use with much the same effect (which is the only option in Cool Edit). Due to the FM sweep, and due to the integration over time of the measuring device, warble tones can be used to determing the smoothed frequency response of a speaker in a reverberant environment. The warble makes measurements performed somewhat less immune to room modes, and certainly they are easier to use than pure sine waves for this purpose because of this. The following details how to create warble test tones in the shareware program CoolEdit:

1. From the menu selection "generate" select "tones...", then select a 44100 or 48000 sample rate (whichever is the highest that you soundcard will accept) mono channels, and 16-bit resolution. Click "OK".
2. Click on the "lock to these settings only" check box so that it has a check in it. Double click on the "A 440 (default)" preset. Click on the "dB volume" "L" slider arrow buttons until it is set to -10. The "R" slider should be missing. The "frequency components" sliders should all be set to zero except for the first, which should be set to 100. The "flavor" should be set to "sine".
3. Calculate your base frequencies by multiplying 20Hz repeatedly by the tenth root of ten (1.2589254118). After calulating them, round each of these frequencies to the nearest whole number. This should give you (for one decade): 20, 25, 32, 40, 50, 63, 80, 100, 126, 159, 200.
4. Enter 20 (the first number calculated in step 3) for the "base frequency", and enter 0.125 of this base frequency in the "modulate by" entry area (for 20 this would be 20 * 0.125 = 2.5). Enter 2.5 in the "modulation frequency" entry area. Enter 5 in the "duration" entry area. Click "OK".
5. Click to the right of the waveform display so that the waveform is deselected (not blue), then click on the "generate" menu option, pick "silence", enter 0.5 in the "silence time" entry area. Clikc "OK". Click to the right of the waveform again so that the waveform is deselected. This inserts 1/2 second of silence between the tones, kind of handy if you can't hear them but *can* see your SPL meter reading something.
6. Repeat steps 4 and 5 for each of the base frequencies listed in step 3. Remember to multiply the "modulate by" number by 0.125 of the base frequency. All other settings should stay the same.
7. Save the file as "low", then repeat steps 3, 4, 5, and 6 for the base frequencies multiplied by 10 and 100. Label these files "mid" and "high" respectively. All done.

I also like to insert 10 seconds of "reference" 1kHz warble tone at the beginning of each file. This lets me calibrate the volume control to the meter "0dB" point. Different calibration points help here (inserting a -20 dB reference is handy for calibration when analyzing those readings that go off-scale because they are too small).

The instructions above were for 1/3 octave warble tones. If you want to make 1/6 octave warble tones, just multiply 20Hz by the twentieth root of 10 (1.1220184543) repeatedly to get the base frequencies, then use 0.0625 of the base frequency as the "modulate by" figure. You should get twice as many base frequencies as for the 1/3 octave case above (actually you end up with the geometric mean in between each value given by the 1/3 octave case): 20, 22, 25, 28, 32, 36, 40, 45, 50, 56, 63, 71, 79, 89, 100, 112, 126, 142, 159, 178, 200. I think you can safely double the modulation frequency (to 10Hz) for 1/6 octave warble tones, since the relative sweep rate will remain the same.

I'll leave the calculation of 1/10 octave base frequencies as an exercise to the reader, but it is easy to do knowing that they are approximately related through the 33rd root of ten (1.072267222 -- an approximation to the tenth root of two which is 1.0717734625, but this doesn't give clean decade endpoints, so we use the 33rd root of ten instead). Use 0.036 of the base frequency for the "modulate by" figure.

These tones work well, but they drive everyone in the house crazy when you are using them. Filtered pink noise is less obnoxious, but I haven't quite figured out the filtering options of Cool Edit yet.

Click here to download a spreadsheet with all of the calculations in it.