Measuring Loudspeakers, Part Two Page 7

It must be a good idea in general to reduce the amplitude of resonant modes, but how audible a high-Q, narrow resonance will be depends not only on its level and the area of the radiating panel, but also its frequency. If the resonance lies in the upper bass, say between 100 and 200Hz, it will likely coincide with specific musical notes as well as being excited most of the time. An octave or so higher, and a narrow-bandwidth, high-Q resonance might well "fall in the cracks" between the discrete frequencies of musical notes. (With equal-temperament Western music, the notes are farther apart in frequency the higher they lie on the musical staff.) In addition, the higher in frequency these resonances are, the more subjectively benign they appear to be, probably because they decay faster.

While a higher-frequency resonance would still be excited by wide-band, noise-type signals like drums, the resonance might remain lurking undetected for most of the time, in which case it might as well not exist. On the other hand, if it coincides with the frequency of a musical note, it can be excited continually. Thus if you examine two similar loudspeakers, each of which has a strong panel resonance present in the midrange, one might sound colored while the other sounds clean.

This also suggest a mechanism for some "tweaks" that are claimed to improve the sound of loudspeakers. By mass-loading the panel and adding damping, the frequency and Q of any resonances present may be shifted by only a little, but enough to move them into the musical gaps [45].

But what if you damp such a resonance only a little? This might have the paradoxical effect of making it more audible, as it will now be more likely to be excited more often, due to its lower Q. (A broad, shallow peak covers more frequencies than a narrow, sharp one.) However, any detectable change in sound tends to be reported by audiophiles as being an improvement!

However, looking at the behavior of the 300 or so loudspeakers that I have measured, several common factors emerge from the auditioning that correlate with the presence of strong cabinet resonances between 100Hz and 500Hz. (Remember that other "objective" factors will also contribute to the same subjective perceptions.) The clarity in the lower midrange can be disappointing. Tenor instruments like cello or trombone lack clarity or acquire a "woody" character. The bass can sound muddy, diffuse, one-note, blurred, or lacking in power, rather than tight, articulate, and extended, as it does in real life. Music can seem to drag, in rhythmic terms. Male voices can "boom" and female voices "hoot" at some frequencies and not others, with the result that the little inflections of tone that are characteristic of real voices become diluted. Centrally placed images, particularly of voices, can smear toward the speaker positions at some frequencies.

Harmonic Distortion
There is considerable discussion in the literature of nonlinear (harmonic) distortion in loudspeaker behavior [46, 47, 48]. All loudspeakers have nonlinear distortion, and small, inexpensive loudspeakers tend to have more nonlinear distortion than large, expensive loudspeakers. Perversely, I don't think this is that important a factor in loudspeaker performance. I have measured loudspeaker harmonic distortion spectra when listening tests had suggested that it was unusually high or low [49, 50]. I have also investigated distortion when I have found a loudspeaker producing audible sub-harmonics, tones whose frequencies are an integral fraction, one half, one third, one quarter, of the fundamental [51]. In a presentation at the 1989 Audio Engineering Society Convention in New York, the mathematician Manfred Schroeder postulated that the production of subharmonics is often related to the presence of chaotic behavior in a diaphragm. This latter phenomenon can be heard on Stereophile's Test CD 2, Track 25.

But of all the loudspeakers that have been reviewed in Stereophile in the past eight years, there are only a few in which noticeable levels of harmonic distortion have been associated with negative review findings. However, I do conjecture that listeners use overall distortion to set a comfortable playback level. If a loudspeaker has high intrinsic distortion, hence a limited dynamic range, it won't be played as loud. Once the level of harmonic distortion rises above a threshold (probably one that is different for each listener), the listener reaches for the volume-control knob. I realize, of course, that my opinions on this subject will be controversial.

Part 3 of this series will examine what is meant by a loudspeaker's frequency response and how loudspeakers behave in rooms.

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