Measuring Loudspeakers, Part Two Page 6

By contrast, fig.19 shows the CSD of a loudspeaker whose on-axis frequency response is severely unflat. The graph is dominated by a severe mode at 3750Hz. In addition to the speaker's timbral problems, this resonance could be heard as adding a hard "zinginess" to the overall sound. However, loudspeakers with such audible resonant problems appear to be very rare these days.

Fig.19 Poor cumulative spectral-decay plot (0.15ms risetime).

To produce a meaningful CSD, the time data need to be free both from noise—it helps to average as many separate impulse response measurements as the host PC can manage—and from environmental reflections, such as those from the microphone stand and its associated hardware, and the speaker support. If not, such reflections produce spurious ridges in the plot that might be interpreted as indicating the presence of resonances. Again, it is also important not to aggressively window the data and produce too short a time record. While this can produce smooth-looking plots [37], they are misleading. Version 10.0A of the MLSSA software flags the area in a CSD plot with dots where the data are invalid due to an inadequate time record (shown in the bottom-left corners of figs.18 & 19). Hawksford [38] has also suggested modifying the CSD plot by compensating for the loudspeaker's minimum-phase behavior. This should make lower-frequency resonances easier to see, but I have yet to try it.

Floyd Toole and his associate Sean Olive did considerable work on the audibility of resonances [39, 40]. It is generally held that high-Q, high-but-narrow peak resonances are less objectionable than low-Q, low-but-broad peak resonances. It is also held that dips in the amplitude response that might also be associated with resonant behavior are less audible than peaks. In my experience, the cleaner-looking a loudspeaker's CSD plot in the upper midrange and treble—above 1kHz, say—the better the chance it will receive a positive review. Loudspeakers that are praised by listeners for "good clarity," "low grain," or "excellent transparency" tend to have clean-looking CSD plots. Conversely, loudspeakers that are referred to as being "grainy" or "harsh" have hashy-looking CSD plots (although, of course, such parameters as nonlinear distortion and frequency balance also contribute to such comments).

Panel Vibrational Behavior
Generations of audiophiles have tried rapping speaker cabinets with their knuckles to see how "dead" the enclosure is. Some enclosures sound like a block of stone, others sound more like a xylophone. Lipshitz, Heal, and Vanderkooy [41] concluded from calculations of the total radiated energy that the sound of the cabinets of the loudspeakers with which they were experimenting would be audible or close to the borderline of audibility. A loudspeaker I reviewed in 1997 [42] had cabinet resonances that were so severe that if you played music through it then paused the CD player, you'd hear audible reverberation at the listening chair as the excited resonances died away.

I have not yet found it practicable to produce quantitative information on cabinet resonant behavior. However, to look at the behavior of loudspeaker cabinets in a more rigorous way than the simple "knuckle-rap" test, I make use of MLS excitation and CSD plots. An inexpensive piezoelectric-tape (polyvinylidene fluoride) accelerometer, 4" long by 1" wide and similar to an acoustic guitar transducer [43], is taped to the cabinet walls at various places, the cabinet is excited with a 2kHz-bandwidth test signal from the DRA Labs MLSSA system at a standard level, and an impulse response is calculated/captured.

Stanley Lipshitz and his colleagues noted that the accelerometer measurements of a loudspeaker cabinet's walls varied tremendously according to how the speaker was supported while the measurement was being performed. For Stereophile reviews, I support each loudspeaker with three upturned metal cones that contact the base of the speaker in the center at the rear and at the two front corners. This allows resonant modes to develop to their fullest, according to the results of a series of experiments I carried out examining this subject in detail [44]. These tests also showed that the best means of coupling a speaker to its stand—"best" in the sense of maximally reducing the amplitudes of cabinet vibrations—was to use a "lossy" coupling material, such as Blu-Tack.

Fig.20 shows a typical impulse response calculated from the accelerometer's output. The ringing overlaying the decay tail of the impulse is obvious. Fig.21 shows a cumulative spectral-decay plot calculated from the time-domain data in fig.20. Four or five resonant modes can be seen.

Fig.20 Impulse response calculated from output of PVDF tape accelerometer fastened to center of loudspeaker cabinet sidewall (100ms time window).

Fig.21 Cumulative spectral-decay plot (0.15ms risetime) calculated from the time data in fig.20.

It is hard to predict the effect of such behavior on perceived sound quality. The amplitude of the modes might be small, but a loudspeaker cabinet can represent a much larger radiating area than its drive-unit(s). A panel may be very lively, but if it faces away from the listener, its subjective effect may be minimal.

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