AUDIOCLINIC (Jul. 1988)

Measuring Speaker Impedance

Q. How can the nominal impedance of a speaker be determined with a d.c. ohmmeter? Speakers are usually described as having a nominal a.c. impedance.

-Name withheld; San Leandro, Cal.

A. To measure the impedance of a speaker at some given frequency, you need a source of that frequency, an a.c. voltmeter, and a variable resistor having a low ohmic value, somewhere in the range of the expected speaker impedance. This is in addition to the ohmmeter you have already mentioned. The resistor is connected in series with the speaker, and this series combination is connected to an audio amplifier. The frequency source is then fed into the amplifier at some level which can provide a convenient reading on the voltmeter. Loud levels are not necessary and would only be annoying to listen to. The value of the variable resistor is adjusted until the voltage developed across it exactly equals that of the voltage developed across the speaker terminals. You then disconnect the resistor and measure its value with the ohmmeter. This resistance value will be equal to the impedance of the speaker at the particular frequency used for the test.

Binaural Sound

Q. What is binaural sound? As I understand it, microphones are placed where a person's ears would be, and a stereo recording, preferably on tape for maximum separation, is made. This is played back through headphones, in order that an accurate image, as would be heard if actually present, is received. Is this correct?

-Name withheld; San Leandro, Cal.

A. You seem to have a good understanding of binaural sound. Mikes are mounted in a dummy head where a person's ears would be. When properly done, a model of the external human ear is actually present on the head.

This gives us the proper front-to-back filtering at high frequencies, plus the presence peak which is produced in our own hearing mechanism. The output of the mikes is fed into a tape recorder in the conventional way.

Sound is reproduced through a set of good phones, optimally at the same relative sound level was "heard" the microphones. When done correctly, the effects are astounding, much better than most of the quadraphonic systems that I have heard. When properly processed, good speaker-based quad can even be recovered from these binaural tapes, but the effects are not as good as with phones.

Some tricks in playback equalization are required when binaural material is played through loudspeakers. These have to do with the reduction of the presence peak introduced by the head and external "ear." (Editor's Note: For a more detailed discussion of binaural sound, see "A History of Binaural Sound" in the March 1986 issue.)

Tone Quality and Speaker-Cabinet Finish

Q. I have a pair of speakers which are finished in walnut, but I want to refinish them in a painted polyester and lacquer. Will doing this distort or change their tonal qualities?

-Shane McGinnis; Morgan City, La.

A. Speaker enclosures are designed to be rigid. They must not vibrate because, if they did, they would radiate unwanted sound along with the desired sound from their cones. The effect of this added output would be to color the sound that you hear. Because adding a new finish will not change the rigidity of the enclosure, it will not affect the sound quality produced by your loudspeaker system.

Distortion on Phonograph Records

O'Neal Douglas' problem with inner groove distortion ("Audioclinic," November 1987) may not be a problem with his cartridge or tonearm. The problem could be caused by a dirty stylus, which can lead to severe mis tracking. If he always listens to an entire record side, by the time the stylus has reached the inner grooves, it has accumulated 15 to 20 minutes worth of dust and dirt. The problem is exacerbated by playing a record which has been poorly cleaned or by using an album or stylus that is still damp from cleaning.

After much experimentation, I have found that I get the best results from using a dry brush on the stylus. In extreme cases, however, I do use a drop of fluid to remove encrusted residue. Similarly, use dry, or almost dry, record-cleaning brush and make sure that the disc is thoroughly dry before playing. Also, if you keep the dust cover on your turntable, your records will seldom need more than a quick once-over with a brush.

-David Delisle; St. Paul, Minn.

Deterioration of Nonpolarized Electrolytics

Q. I have wondered for some time about the possible deterioration of the nonpolarized electrolytic capacitors used in loudspeaker crossover networks. I understand that ordinary electrolytics have a finite shelf life but that they can be rejuvenated by subjecting them to polarizing voltage. Is this the case with the nonpolarized units? What would be the audible warnings of deterioration?

-Kenneth Beers, Jr., Tremont City, Ohio

A. I have not noticed deterioration of any nonpolarized capacitors in crossover networks I have owned. If you believe that the characteristics of a midrange driver or tweeter have changed, it is certainly possible that a change in capacitance values might be the cause. I would think that the values of such capacitors would decrease. In the case of the tweeter, this would probably mean the high-pass cut-off frequency would increase. In the case of a midrange or woofer, more highs would be fed into it as the value of the associated capacitor decreased in value.

You are right about shelf life of electrolytic capacitors that are not in service. In the case of the nonpolarized units, they will be in service whenever you use your loudspeaker systems.

Capacitors of this type are nothing more than two electrolytic capacitors back to back, with two terminals of the same polarity wired together. The two free terminals would both be opposite in polarity from the first two. When a.c. is applied to a nonpolarized capacitor, one portion of it will charge while the other conducts. Their roles reverse with each reversal of the input wave form's cycle.

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(Source: Audio magazine, Jul. 1988, JOSEPH GIOVANELLI)

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