Measuring Loudspeakers, Part Three Page 5

It's also apparent that a speaker with a basically smooth, flat response but a single area of unevenness—due perhaps to a strong woofer-cone breakup mode or a cabinet resonance—will be downgraded accordingly. The same holds true for crossover or drive-unit integration problems. Similarly, if a speaker has a flat response but limited high-frequency dispersion, its appeal will be limited due to the room reverberant field tending to sound too lifeless. And no matter how flat a speaker's midband, a somewhat loose, underdamped bass is an almost unforgivable fault for many listeners, except when balanced by a degree of HF emphasis or a tilted-up treble. Conversely, the worst thing a speaker designer can do with a speaker that has an overdamped, lightweight bass alignment is to give his brainchild a tilted-up treble balance. The lack of across-the-band balance is musically disturbing, and the emphasized treble exaggerates problems elsewhere in the system.

Interestingly, if a speaker is rolled-off at both extremes, it seems to sound more acceptable than might be expected from its flatness quotient as calculated here. If this somewhat midrange-forward balance is not smooth, however, the ear seems to detect the unevenness as a cupped-hands coloration. And the opposite response trend, a lack of energy in the upper midrange, can lead to a musically uninvolving presentation.

Fig.27 shows the responses of the 20 loudspeakers with the lowest standard deviation, 1.375dB or less. These can all be seen to be superbly flat (justifying the use of the log-frequency-weighted standard deviation to characterize the speakers). The best performers are, in fact, flatter than my B&K measurement microphone! By contrast, Fig.28 is the "rogues gallery" of the speakers with the largest response deviations. (It doesn't include panel speakers, which tend to have large standard deviations because their measured responses slope down with frequency, due to the proximity effect described earlier). The high standard deviation is affected by large dips and peaks in the response, or by overall tilts up or down. One small satellite loudspeaker from Bose that I measured for Stereophile Guide to Home Theater, for example, smoothly but persistently slopes up by 10dB from 300Hz to 10kHz.

Fig.27 MLS-derived responses at 50" of 20 loudspeakers with lowest standard deviations, averaged across a 30 degrees horizontal window on the tweeter axis.

Fig.28 MLS-derived responses at 50" of 13 loudspeakers with highest standard deviations, averaged across a 30 degrees horizontal window on the tweeter axis (not including panel speakers).

The responses of the panel speakers reviewed in the time period are shown in fig.29. The graph is a little confusing, but it should be apparent that the 50" microphone distance does result in a tendency for the responses to slope down from low to high frequencies. This, as mentioned earlier, is due to the microphone still effectively being in the nearfield at low frequencies but in the farfield at high frequencies.

Fig.29 MLS-derived responses at 50" of 11 panel loudspeakers, averaged across a 30 degrees horizontal window on the tweeter axis.

The data for seven professional monitors are included in this survey. Their responses are plotted in fig.30, and it is interesting to note that their on-axis responses are no more flat than the group average. Studio monitors have other tasks to fulfill, however, including offering a much higher dynamic range than is necessary for a domestic design. Nevertheless, one of the flattest overall responses was that of a small active studio monitor from Finnish manufacturer Genelec included in this group.

Fig.30 MLS-derived responses at 50" of 7 studio monitors, averaged across a 30 degrees horizontal window on the tweeter axis.

There are five speakers included in fig.26 where the designer has used DSP chips to equalize the on-axis response [61, 62, 63] and in one case to implement the crossover filters in the digital domain. Somewhat surprisingly, while all had low-response standard deviations, there were many conventional passive designs that were as flat or even flatter.