Stereophile Test CD 3 Signals for Testing Loudspeakers & Rooms
 Chromatic Scale (left channel, then right channel) (DDD) 1:30
This and the next three tracks are intended to enable audiophiles to get a handle on how their systems and loudspeakers interface with their listening rooms—even if they don't have any test equipment.
Rather than a conventional frequency sweep, I decided to use a test signal that's musically relevant. This track features sinewaves tuned to the frequencies of the modern equal-temperament scale, sweeping up from the low C1 at 32.7Hz, on the space five ledger lines below the bass staff, to the high C8 at 4.186kHz, nine ledger lines above the treble staff, and back down again. Each note has a frequency equal to the frequency of the one below it multiplied by the twelfth root of two.
The scale appears first in the right channel, then in the left, and, to aid identification of system resonant frequencies, the other track in each case features the octave C notes played on a Sound Blaster synthesizer card set to its "timpani" voice. Middle C, or C4—the note between the staves—is identified with a double note, as is the very highest C. The marker Cs have the following frequencies: 32.7Hz, 65.4Hz, 130.8Hz, 261.6Hz (Middle C), 523.25Hz, 1046.5Hz, 2093Hz, 4186Hz.
What you should hear: With small speakers, it's possible that you won't hear the lowest notes. Otherwise, the scale should sound even, without some notes sounding louder or softer than others (though in the bass, room problems will make this unlikely). If you do hear something wrong, note where it stands in relation to the marker "bonks" in the other channel. You'll then have a rough idea of the frequency range affected, whether it's due to a room standing wave (30-200Hz) or a loudspeaker crossover problem (1.5-4.5kHz).
With loudspeakers, play the scale and listen to the cabinet walls with a stethoscope (about $20 from any good pharmacy). You'll hear a cabinet resonance as an accentuation of the note. If the resonance is particularly severe when you pause the CD player while the affected note is playing, you might even hear it ringing on its own. If the cabinet has resonances, but they don't lie at the frequencies of musical notes, it's possible that they'll have less of a deleterious effect on music.
 Bass Decade 1/3-octave warble tones at -20dBFS (DDD) 2:47
Center Frequencies: 200Hz (Index 1), 160Hz (Index 2), 125Hz (Index 3), 100Hz (Index 4), 80Hz (Index 5), 63Hz (Index 6), 50Hz (Index 7), 40Hz (Index 8), 31.5Hz (Index 9), 25Hz (Index 10), and 20Hz (Index 11)
 Midrange Decade 1/3-octave warble tones at -20dBFS (DDD) 2:32
Center Frequencies: 250Hz (Index 1), 315Hz (Index 2), 400Hz (Index 3), 500Hz (Index 4), 630Hz (Index 5), 800Hz (Index 6), 1kHz (Index 7), 1.25kHz (Index 8), 1.6kHz (Index 9), 2kHz (Index 10)
 Treble Decade 1/3-octave warble tones at -20dBFS (DDD) 2:32
Center Frequencies: 2.5kHz (Index 1), 3.15kHz (Index 2), 4kHz (Index 3), 5kHz (Index 4), 6.3kHz (Index 5), 8kHz (Index 6), 10kHz (Index 7), 12.5kHz (Index 8), 16kHz (Index 9), 20kHz (Index 10)
The warble-tone tracks (which roughly illustrate the extent of the terms "bass," "midrange," and "treble") 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 effect of low-frequency room resonances on the perceived level 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 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 a sound-pressure-level meter. Set a reference level with track 18, index 7—the 1kHz band—then note by how much the sound level drops with each successive warble tone (footnote 7). (If your cassette deck came with a microphone, put the mike at your listening chair, set the recorder to "Record," and monitor the sound level with its meters.)
The 200Hz-100Hz bands can be considered the upper bass, 80Hz-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 18 and 19 offer sets of warble tones covering the midrange and treble decades, so that you can measure the in-room response of your loudspeakers without having to pop for an expensive spectrum analyzer. The 1kHz warble tone 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.
 "Caca-cophony": Stereophile's Special Burn-In Noise (DDD) 2:15
For this track, I originally thought of using the traditional pink- or white-noise signals so beloved by engineers. But why not use real noise? Accordingly, on one sunny January day, pretty much the entire staff of Stereophile and Schwann Opus/Spectrum gathered in Stereophile's back yard, armed with all manner of noise bangers, bongers, and blowers. Publisher Larry Archibald, who can be heard featured on the trashcan lid toward the end of the track, counted us in; the result is what could be politely called a "caca-cophony." For reference, the peak acoustic level was 106dB.
Caca-cophony! From left to right: Richard Lehnert, Nancy Fay, Doug Chadwick, Steve Stoner, Maura Rieland, Larry Archibald, Bruce Shortz, Gretchen Grogan, Polly Summar, and Susan Phillips let it all hang out.
To ensure that this track had enough deep bass and high treble to be useful, I digitally mixed in a low-frequency, 5Hz-150Hz swept sinewave and 16kHz and 20kHz 1/3-octave warble tones. The resulting peak spectrum (fig.4) has pretty much an equal amount of energy in each 1/3-octave band from 5Hz to 20kHz, while the impulsive nature of the signal will suitably stress speaker suspensions.
Fig.4 Stereophile Burn-in Noise, 1/3-octave peak spectrum (5dB/vertical div.).
To use this track for breaking-in loudspeakers and power amplifiers, set the CD player to Track Repeat and adjust the playback level to a moderately loud level—make sure that the 5Hz content at the beginning doesn't make the woofer voice-coils hit their end-stops! Next, switch your preamplifier to "mono," place your speakers face to face, and invert the polarity of just one of them. In this way, most of the sound will cancel. Leave the room, leaving the system to play for as long as the manufacturer of your loudspeakers recommends. If you don't have a mono switch, plug just one of your CD player's outputs into both preamplifier channel inputs using a Y adaptor.
To break-in electronic components—CD players and preamplifiers—just play the track in Repeat with the preamplifier turned up full but the power amplifier turned off.
Footnote 7: 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.—John Atkinson