The 2011 Richard C. Heyser Memorial Lecture: "Where Did the Negative Frequencies Go?" Case Study 2: Loudspeakers
Case Study 2: Loudspeakers
During a visit to Canada's National Research Council many years ago, I noticed, stuck to the wall of the prototype IEC listening room, a page of results from one of Floyd Toole's seminal papers on the blind testing of loudspeakers. The scoring system was the one that Floyd developed and that I subsequently used for blind tests at Stereophile: "0" represents the worst sound that could possibly exist, "10" the perfection of live sound. On this scale, a telephone, for example, rates a "2." The speakers in Floyd's test pretty much covered the range of possible performance, yet their normalized scoring spread, from the worst to the best, was just 1.9 points. Other than some pathological designs, the audiophile speakers I test for Stereophile probably cover a range of 0.5 point on Floyd's scale; but, as I pointed out in my 1997 AES convention paper, our reviewers' ratings generally follow Floyd's findings: that people tended to prefer speakers with flat response and controlled dispersion.
But not always.
There are vocal advocates of high-sensitivity horn speakers. There are equally vocal advocates of low-sensitivity, large-panel speakers. The horn advocates, among whom was Stereophile's founder, the late J. Gordon Holt, enthuse about these speakers' "jump factor." I used to ask Gordon what he meant by that: was it dynamic range? No, it wasn't. Was it low distortion? No. His reply was always that there was something about the presentation of these speakers that more closely resembled what he experienced from live classical music.
What about panel speakers? Being dipoles, their interaction with the room is very different, so maybe that's why their advocates love them, not even considering the fact that, with a large panel, the listener is almost never far enough away from the speaker to be in the farfield, the result being more basswhich everyone likes, at least in two-channel audio. Of course, there are no cabinet resonances because there is no cabinet, and perhaps people's preference is due just to the fact that he very large radiating area inherently ensures more linear behavior when moving the same mass of air as a much smaller cone driver. But panel speakers' impulse responses look terrible. Well, I say "impulse response"; what I actually publish and use for my reviews is not the response of a loudspeaker to an impulse, but the impulse response calculated by cross-correlating an MLS signal or a chirp signal, based on the loudspeaker being a perfectly linear system. (John Vanderkooy has investigated the shortcomings of this assumption.)
Perhaps, then, the problem with measuring panel speakers is that the calculated impulse response does not accurately reflect the behavior of a large panel in the same manner as it does a conventional speaker's. At the 1989 AES Convention in New York, mathematician Manfred Schroeder discussed Chaotic systems in audio and mentioned, in passing, that a Chaotic system tended to produce subharmonics. And indeed, when I have measured panel speakers I have found that they produce subharmonics. You feed the panel speaker a 1kHz tone and in its output you find a 500Hz tone as well as the usual distortion harmonics at 2, 3, 4kHz, etc.
So in a large panel speaker, while the average position of the diaphragm follows the driving signal, the motions of the individual elements of that diaphragm are Chaotic. The speaker sounds much better than its measured behavior suggests, as all that randomness is integrated by human hearing's latency.
Or do people like the sound of panel speakers because the intensity of the sound they producethe sound power per unit area of the radiating surfaceis closer to what people hear live? This question was triggered by a live vs recorded demo in which I took part in 2009, in Maryland. Two pianists performed for an hour on a well-prepared Steinway D grand piano, which I recorded at 24/96. Then, after a break, the audience heard the recital again, this time played back through a system. (This meant miking the piano far more closely than I would do for a conventional recording, to avoid a double dose of the room acoustic when the recording was played back.)
The system was full-range, with low coloration and sufficient dynamic-range capability that I was able to match sound-pressure levels. But even though the SPL and the sound of the piano matched closely, listeners felt that the reproduced sound just didn't have the "bigness" of the real thing. I wasn't sure what they meant by "bigness." Yes, the piano's dispersion would excite the room very differently than would a pair of speakers, but perhaps these listeners were suggesting that they were aware that the intensity of the sound was not correct, even when the SPL and response were correct.
So when people say they like panel speakers that measure poorly, perhaps they're responding to those speakers' more accurate intensity, at least when it comes to large sources.
Food for thought?