Editor's Choice: Stereophile's Sampler & Test CD:
Tracks 20-23
[20] 1kHz 1/3-octave warble tone at -20dBFS
[21] Bass Decade 1/3-octave warble tones at -20dBFS: Center Frequencies: 200Hz, 160Hz, 125Hz, 100Hz, 80Hz, 63Hz, 50Hz, 40Hz, 31.5Hz, 25Hz, 20Hz
[22] Midrange Decade 1/3-octave warble tones at -20dBFS: Center Frequencies: 250Hz, 315Hz, 400Hz, 500Hz, 630Hz, 800Hz, 1kHz, 1.25kHz, 1.6kHz, 2kHz
[23] Treble Decade 1/3-octave warble tones at -20dBFS: Center Frequencies: 2.5kHz, 3.15kHz, 4kHz, 5kHz, 6.3kHz, 8kHz, 10kHz, 12.5kHz, 16kHz, 20kHz

These warble-tone tracks roughly illustrate the extent of the terms bass, midrange, and treble. They were recorded from the output of an Old Colony Sound Lab analog warble-tone generator, the frequency quoted being the approximate center frequency of each. The generator contains a sinewave oscillator that is frequency-modulated at a rate of 5Hz or so; this is fast enough that the perceived effect of low-frequency room resonances will be minimized, the test tone changing sufficiently quickly that the resonance doesn't have time to fully develop.

What you should hear: The bass warble tones, each of which (except the last) lasts 10 seconds, can be used to give a good idea of a loudspeaker's subjective bass extension in the listening room, either by listening or by using an SPL meter. Set a reference level with the 1kHz tone (track 20), then note by how much the sound level drops with each successive warble tone. (Whereas the warble tones on the succeeding tracks increase in frequency, those on track 19 decrease, to make it easier to judge bass extension by ear.)

The 200-100Hz bands can be considered the upper bass, 80-40Hz the midbass, and the remaining bands the low bass. If these bass warbles sound or measure uneven, with some either sticking out more than others or missing in action, then try moving the speakers or your listening chair around the room. The object is to get the tones to sound and measure as evenly as possible.

Tracks 22 and 23 offer sets of 10-second warble tones covering the midrange and treble decades, so that you can measure the in-room response of your loudspeakers without using an expensive spectrum analyzer. The 1kHz warble tone (track 20) can also be used to get a relative idea of a loudspeaker's sensitivity: Measure the sound-pressure level with a loudspeaker that has a sensitivity you know; then, without changing the playback level, measure the SPL of an unknown loudspeaker substituted into the system.