Measuring Loudspeakers, Part Three Page 3

A final matter should be discussed. It is a hidden assumption when measuring a loudspeaker's amplitude response that the microphone is in the far field; ie, is more than a couple of wavelengths away at the lowest frequency of interest. An alternate way of looking at the matter is that the microphone should be at least as far away as the largest dimension of the loudspeaker to be measured. With my standard microphone distance of 50", this assumption will no longer be true for large loudspeakers. With big speakers, such as the various kinds of panel speakers, there will be a proximity effect [55] that tilts up the response at low frequencies. This, of course, will also be true when the loudspeaker is listened to at the same distance.

Assessing the acoustic performance of big panel speakers is therefore an undertaking fraught with difficulty. Some years ago, for example, I had to measure a loudspeaker that had a small dome tweeter that radiated sound only in the forward direction, a large ribbon midrange unit that behaved as a dipole, and an omnidirectional woofer. Both the measured response and the perceived balance of this speaker varied according to how far away the listener and microphone were, rendering meaningless any discussion of this speaker's "frequency response."

How meaningful is a loudspeaker's on-axis amplitude response? Stereophile's founder J. Gordon Holt argued over a decade ago [56] that a loudspeaker with a measured flat on-axis response won't sound correct: "Many times in past years I have been impressed by the incredible flatness of the measured high-end response of some speakers...In every such case I have been equally amazed at how positively awful those loudspeakers sounded—so tipped-up at the high end that I could not enjoy listening to them," he wrote, adding that "Audiophile loudspeakers which measure nearly flat through the lower middle range seem to have a penchant for sounding sucked-out and gutless through that region...loudspeakers that measure flat in my own listening room sound thin at the low end, while those sounding flat at the bottom measure as having a low-end rise."

However, another Stereophile reviewer, Martin Colloms, disagrees [57], citing the results of single-blind listening tests: "The favored speakers were those which possessed very even axial responses over 100Hz to 10kHz when measured by third octave and octave averaging." As a design goal, he felt that the engineer "should be aiming for a deviation of ±0.25dB or less in the forward directed response, while according rather less importance to narrow band deviations of greater amplitude..." Audio reviewer Don Keele [58] adds that "the on-axis response should be smooth, because it defines the spectral balance of the sound that first arrives at the listener and so is of greatest subjective importance in judging timbre."

An analysis of 74 loudspeakers that I performed in 1991 [59] also showed a good correspondence between flatness of measured on-axis response and listener preference. Grouping loudspeakers by the log-frequency-weighted standard deviation of their response between 170Hz and 17kHz—the weighting was to compensate for the linear spacing of the frequency bins produced by the FFT process—I discovered a clear correlation between flat on-axis quasi-anechoic response and the tendency for the loudspeaker to get a positive review in Stereophile. This correlation also appeared when the overall results of blind listening tests performed by the magazine were analyzed [60].

I suspect that the measured responses referred to by Holt were taken in-room. In this case, the measured data will include contributions from both the speaker's anechoic axial response and its power response (see later). The presence of absorbent room furnishings and drapes will dramatically affect the balance at high frequencies; the power response of a typical forward-firing loudspeaker tends to slope down with increasing frequency. It is likely, therefore, that a flat in-room response can be achieved only by equalizing the on-axis response to tilt up. This will be much more significant in large than in small rooms (see later).

Fig.24 shows the anechoic or quasi-anechoic response of a good-sounding loudspeaker taken on its tweeter axis with the MLS technique described above, averaged across a 30 degrees horizontal window centered on the tweeter axis and corrected for both the microphone's departure from flatness and the response error introduced by the system's anti-aliasing filter. The curve is actually a composite, consisting of the seven spatially averaged responses taken with a 30kHz bandwidth from 1kHz to 30kHz, a separate on-axis measurement taken with a 5kHz bandwidth from 312.5Hz to 1kHz (this makes the graph look more presentable, but the true frequency resolution is unchanged), and the complex sum of the speaker's nearfield port and woofer responses below 312.5Hz (the complex sum adds the two amplitude responses taking the phase responses into account). A big, high-Q, hence narrow, peak can be seen at the metal-dome tweeter's ultrasonic resonant frequency, but the response through the midrange and treble is otherwise pretty flat: it fits within ±2dB limits over almost the entire audio band.

Fig.24 Typical MLS-derived response of a good-sounding loudspeaker at 50", averaged across a 30 degrees horizontal window on the tweeter axis, and spliced to the nearfield LF response.

Note that the frequency resolution between 300Hz and 1kHz is limited, the data points being spaced quite wide apart. But the graph is still informative. A small and probably benign dip can be seen just below 1kHz. There's a small peak at 3kHz that may add a little "bite" to the sound, or it might add a little bit of presence-band emphasis that will make the sound "vivid" without it being so much so that the sound will become aggressive. On the other hand, the peak may be compensating for something else the speaker does, meaning that the perceived sound balance might actually be flat (see later).