The Stereophile Test CD Tracks 20-31
[20-31] 1/3-octave warble tones at -20dB (DDD). All 0:22 except Track 20, which is 1:01
Center frequencies: 1kHz, 200Hz, 160Hz, 125Hz, 100Hz, 80Hz, 63Hz, 50Hz, 40Hz, 31.5Hz, 25Hz, and 20Hz
These warble-tone tracks were recorded on the Nakamichi 1000 from the output of an Old Colony Sound Lab 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 effect of low-frequency room resonances on the perceived level will be minimized (footnote 6). The test tone changes frequency rapidly enough that the resonance doesn't have time to fully develop. These warble tones can therefore be used to give a good idea of a loudspeaker's subjective bass extension. Those with access to a sound-level meter—Radio Shack sells quite a good one for not too much money—can set a reference level with track 20, the 1kHz band, then note by how much the response drops with each successive track. The 200Hz-100Hz bands can be considered the upper bass, 80Hz-40Hz the midbass, and the remaining bands the low bass.
Track 20 can also be used to get a relative idea of a loudspeaker's sensitivity: measure the sound-pressure level with a loudspeaker whose sensitivity you know, then, without changing the playback level, measure the spl of an unknown loudspeaker substituted into the system.
Footnote 6: After the Test CD had been mastered, I checked the modulation frequency of these warble tones with an FFT analyzer and realized that it was closer to 14Hz rather than 5Hz. Note that as well as the higher modulation frequency than specified, the warble tone is not particularly pure, FM sidebands appearing around its second and third harmonics. Although the "buzz" of higher harmonics is faintly audible on these tracks—we had considered digitally low-pass filtering these tracks when we compiled the master tape, but weren't able to due to lack of time—the strongest harmonic, the second, lies just under 30dB down from the fundamental, ie, around the 3% level.
Neither the higher FM frequency nor the lack of spectral purity renders these tracks on our Test CD any less suitable for their intended purpose. I included these tracks on the CD in order to provide audiophiles with a low-cost way both of finding the optimum positions in their listening room for their loudspeakers and of assessing the bass extension of those speakers, either in conjunction with an inexpensive spl meter like the Radio Shack or by ear alone. Even with their faults, these warble tracks work just fine in this respect. But note that cleaned-up, 5Hz-warbled versions of these low-frequency tones appear on Stereophile's 1992 Test CD 2, while the 1995 Test CD 3 extends the set of warble tones up to 20kHz.—JA