Dear Editor (Apr. 1971)

Doppler Distortion

Dear Sir:

I read with interest Professor Greiner's comments on Doppler distortion in the December issue. According to his thesis, any long throw woofer whose response extends to 800 Hz or above should exhibit the hollowness in the voice range that he heard. However, there are numerous long throw woofers that cover such a range which do not sound hollow in the voice range; conversely, there are many speakers that theoretically have low Doppler distortion that do sound hollow-suggesting that the cause of the hollowness is something other than Doppler distortion.

Professor Greiner is correct in concluding that the problem occurs in the 500-1000 Hz area. Had his listening tests included a wide sampling of speakers he would have found that most of them sound hollow in the voice range, regardless of design. We have been able to ascribe this audible phenomenon to too much relative energy in this octave caused either by improper matching of octave to octave balance or more often by cone break-up products of the woofer introducing spurious energy in this octave. I believe he would find that this hollowness would still persist on test speakers if he used his electronic crossover to chop off the low end of the speaker's response, (that is, by using the crossover network as a high pass filter with a cut-off frequency of 100 or 200 . Hz.), thereby proving that the cause of the hollowness cannot be ascribed to Doppler distortion.

If Doppler distortion isn't responsible for the hollowness, what then are its audible effects? The comments of Audio's editor on this question on page 28 of the August 1970 chronicle some of the long standing, wide-spread skepticism that indeed any exist! How can we explain the lack of audibility of the measured distortion Professor Greiner reported in his letter and others, most notably Paul Klipsch, have reported else where; is the human ear that insensitive? Perhaps the answer lies in the spectral content of music. Music is not flat! That is, it does not radiate equal power across the audio spectrum. It s acoustic power is typically 4 to 5 times less at frequencies below 100 Hz than at lower mid range frequencies . Therefore, the lab situation is strictly academic in nature and does not exist in real life.

One could go further into a discussion of the numerous discrepancies between the simulated situation in the lab and real life (such as the sound pressure levels required in the home at various frequencies for realistic reproduction of sound) , but suffice it to say that hollowness is not the result of Doppler distortion. If there are any audible effects, they have not yet been identified by this writer nor I suspect by thousands of musically sensitive owners of long throw woofer systems.

-Andrew G. Petite National Sales Manager Advent Corp., Cambridge, Mass.

The Mud Factor Revisited

Dear Sir:

With all due respect to Dr. Klipsch and his Magnificent Audio Machines and accepting the proposition that modulation distortion is something less than a good thing-one wonders if he has not inadvertently loaded his argument as well as his horns (The Mud Factor, October, 1970) . While a 100 decibel level at two feet might prove truly scintillating to a 'test rig, do we really listen at such outputs? Particularly at 42 Hz? Without getting lost in the wilds of the Klipsch-Villchur Factor (KVF), and regardless of the intentions of learned discussions-as well as salesmen's clumsy demonstrations-the message seems to come through is that almost any sound reproducer will do better if the burden on it is kept light. Further, one's own listening preferences have to be considered- i.e., on the one hand, almost any sort of speaker will do for rock, or for that matter the well recorded but very pallid background music, but full symphonics seem to remain a problem.

All this leads me to wonder whether a . collection of (probably small) individually boxed speakers, with amplifiers tailored to deliver various segments of the range to the different outlets, might not be better than further harangues about the relative merits-and sizes of two-way and three-way packaged "systems." By extension, this could be carried to the point of idiocy, but from discussions in your publication, it would seem the required idea men, engineering, and equipment are on hand for a whole new shake, in the name of clarity.

-Robert Cummins Philadelphia, Pa.

Stokowski's Baton Dear Sir: I had to drop you a line after reading my January issue of AUDIO. Mr. Weingarten states on page 58 that the music from Fantasia is played by the "Philadelphia Orchestra under the baton of Leopold Stokowski." I make no claims as a great musical authority, but it is well known that Mr. Stokowski does not use a baton.

-William P. Fink, Westminster, Md.

Dolbyization for FM

Dear Sir: Judging from recent articles in your magazine and others, it appears that the Dolby noise reduction system in the type "B" format ·greatly improves the tape medium for home use.

It should also be noted that Dolbyization could improve FM broadcasts to the same degree. With this in mind, I propose that the Type "B" Dolby unit be incorporated into FM receivers of the future for use both in FM reception and tape playback. Some cost savings could be passed on to the consumer by elimination of power-supply duplication, although an automatic FM switching detector would undoubtedly add to the cost to some degree.

-Marshall J. Grimm; Detroit, Mich.

(Source: Audio magazine. Apr. 1971)

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