AUDIO QUESTIONS and ANSWERS: Advice on readers' technical problems
by LARRY KLEIN, Technical Editor
Electromagnetic Speakers
Q. The use of electromagnetic speakers is virtually unheard of in the stereo industry. However, I have recently become aware that their efficiency is 30 to 40 percent greater than that of the permanent-magnetic types generally used today. Thus, to reach a given decibel level, an electromagnetic speaker would require less amplifier power. I feel that you would be doing a service for your readers if you would publish a list of manufacturers of this kind of speaker, as it is an elusive product to track down.
R.S. DAVIS Syracuse, N.Y.
A. For those unfamiliar with the type of speaker Mr. Davis is referring to, a few words of background would be helpful. The electromagnetic speaker was popular from the late 1920's until the late 1930's. It very much resembled a modern speaker except that in stead of having a ceramic or metal-alloy mag net, it used a heavy "field" coil of copper wire as an electro-magnet. This coil was energized by d.c. taken from the associated equipment's power supply.
There are at least two things wrong with Mr. Davis' suggestion. I don't know where he obtained his efficiency numbers, but I doubt that they are correct. In addition, it may come as a surprise to many readers to learn that as far as a woofer is concerned, the designer does not necessarily choose the strongest possible magnet available to him. At a given point, depending on a number of other design factors, adding more magnetic flux will reduce the bass performance of a woofer-because the moving system becomes overdamped.
The designer therefore chooses the magnetic flux level that provides the best compromise between efficiency and bass response-which for a hi-fi speaker system is very seldom the point at which the magnetic flux is as strong as possible.
As far as the magnetic flux in mid-ranges and tweeters is concerned, although the de signer usually does try for the strongest flux field possible (within his cost restrictions), the problem is to deliver the energy from the magnet to the desired area in the magnetic circuit of the speaker. An electromagnetic field coil provides no advantage over a conventional permanent magnet in this regard either.
In addition to the factors mentioned above, the field coil would require a power source, and given today's copper cost, a field coil would probably be as expensive as a good-quality permanent magnet. So, Mr. Davis, I am unable to supply you with a list of manufacturers of field-coil speakers, simply because, for the reasons given above, there aren't any.
Doctoring Drivers
Q. Several years ago I read about a technique to improve the performance of a woofer. As I recall. it entailed cutting slots in the outer edge of the cone and coating it with a special substance. Do you have any thoughts on this?
MARVIN SPATHE; Fairlawn. Ohio
A. Over the years I have also read articles on improving the performance of drivers which involved everything from gluing eggshell halves to the apex of the cone to doping the cone edges with photographic chemicals. And, as a matter of fact. I recommended such techniques myself in the days (over ten years ago) when I was designing build-it-yourself speaker enclosures.
It may be instructive to examine exactly what such speaker-cone modifications are in tended to achieve. Any technique for loosening the cone suspension (such as adding slots or substituting rubber segments) is aimed mostly at lowering the mechanical resonance of the cone. The speaker will then be able to reproduce somewhat lower frequencies than it could before the treatment. But the speaker's ability to go down lower, without an ac companying redesign of the voice coil and magnet assembly, means that it will run into distortion if it is played loudly at those low frequencies. In other words, the resonant frequency of the cone is just one of the limiting factors determining a speaker's low-frequency response and distortion level.
Speaker engineers are constantly making "trade-offs" in their designs, and the factors being juggled at the bass end are distortion, efficiency, cone resonance, and box size. Any attempt by the amateur to second-guess the professional by changing one of the parameters is likely to work out badly. If you dis agree with the specific trade-off choices of a speaker-system designer, it's safer to do your second-guessing electronically via a multi-band equalizer than to work directly on the drivers or the enclosure.
If you are starting from scratch with a build-it-yourself enclosure and you want to experiment with cone modifications, do your doctoring on cheap drivers rather than on expensive ones, because the better the speaker, the worse the odds that your efforts will achieve anything worthwhile.
Open-Reel Reference Level
Q. I note that Julian Hirsch refers to a “standard Dolby level” tape with a 200 nano-weber-per-meter flux level in his tests of open-reel tape recorders. However, the recognized "Dolby level" for open-reels is 185 nano-webers per meter, and S/N measurements referred to a 200-nanoweber-per-meter level will be incorrect. Would you care to comment on this discrepancy?
HUBERT THEIGAL; San Francisco, Calif.
A. You are correct in your statement. The reason Mr. Hirsch uses 200 nanowebers per meter in his tests is simply that the Dolby calibration tape he uses was made in the early days of the introduction of the Dolby B system, before the change was made to 185 nanowebers per meter. The measurement of tape-recorder noise levels, at least insofar as consumer products are concerned, involves visually averaging a constantly fluctuating meter reading whose uncertainty is somewhat greater than the 0.68-dB difference between the two tape-reference levels. To correct the readings to the current standard would really not be warranted by the precision of the measurement, but anyone who cares to "split decibels" can easily do so from the test data in our tape-recorder reports. And, of course, 200 nanowebers per meter is the standard Dolby reference level for cassette recording.
Adding Turntable Capacity
The turntable I would like to buy (because of its excellent rumble and flutter figures) is apparently intended for CD-4 use and has been designed with low-capacitance tone-arm wiring and leads. The cartridge I am considering is one of the several types on the market that require a relatively high capacitive load for flattest frequency response, and I'm wondering whether it is compatible with a CD-4 turntable.
GARRY WOODS; Crown Point, N.Y.
A. This is a problem that, as far as I know, has not yet been dealt with satisfactorily by the manufacturers. It is difficult to optimize the capacitive load on the phono cartridge (even if you know the desired value) when the capacitive loads presented by the turntable cables and the preamplifier's phono inputs are unknown. A simple trial-and-error solution to the problem--pending the day the designers get together--would be to try additional cable lengths at the output of your turn table (which will introduce additional capacitance) or replace the low-capacitance leads with conventional ones. This is easy enough to do since most turntables use cables with phono plugs that insert into jacks beneath the turntable chassis.
If the upper mid-range and highs sound better to you with a given cartridge and phono cable assembly, then that's the combination to use. I suspect, though, that in most cases the audible differences will be so subtle that the effort may not be worth making.
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Also see:
AUDIO BASICS: Shopping for Speakers, RALPH HODGES