Audioclinic (Sept. 1979)

Recording Engineering Schools

Q. I recently graduated from high school and have become very interested in the recording engineering field. Can you give me a list of institutes that offer courses in this field?

- M. Kuc; Vancouver, B.C., Canada.

A. The Institute of Audio Research, 64 University Place, New York, N.Y. 10003, offers advanced courses dealing with recording and related fields. You might also look into: (1) Cleveland Institute of Electronics, 1776 East 17th Street, Cleveland, Ohio 44114; (2) Bell & Howell Education Group, National Offices, 2201 West Howard, Evanston, III. 60202; (3) Technical Center Institute, 320 West 31st Street, New York, N.Y. 10001; (4) Teccart Institute, Montreal, Canada; (5) Eastman School of Music, Rochester, N.Y. 14604; (6) Loyola University, Montreal, Quebec, Canada, and (7) Brigham Young University, Provo, Utah 94602.

Billboard magazine, 1515 Broadway, New York, N.Y. 10036, has an "International Directory of Recording Studios" for sale.

You might also get helpful information from such journals as db, 1120 Old Country Road, Plainview, N.Y. 11803 and Recording Engineer/ Producer, P.O. Box 2449, Hollywood, Cal. 90028.

Measuring an Unknown Frequency

Q: I should like to find out the frequency of an unknown audio signal.

How can I hook up the unknown signal and a calibrated signal generator, sweep until I hear a beat or null, and then take my reading from the calibrated generator?

- F. Alexander; Maspeth, N.Y.

A. The simplest way by which one can determine a frequency generated by an uncalibrated source is to obtain a frequency counter. Certainly if you plan to do much along these lines, the investment in a counter is justified.

The uncalibrated source is fed directly into the input of the counter. If its out put level is too low for the counter to handle, an appropriate preamplifier must be used. The frequency of the unknown signal is read directly on the counter's display.

Another method for determining the frequency of the unknown signal source is more like the system mentioned in your question. In this arrangement the unknown signal is compared to a signal of known frequency. The two signals are adjusted to produce equal audio outputs. Each signal is fed to the input of a two-position mixer, whose output is connected to an amplifier and loudspeaker system.

Perhaps the best procedure is to listen to the unknown and then attempt to match that sound fairly closely to that of the known generator, by alternating listening to one or the other of the signals while you adjust the known to equal the frequency of the unknown source.

When you think they are reasonably close in frequency, listen to the two signals together. You probably will hear them, plus a third signal set up by interaction between the two signals.

Adjust the known signal till this third sound disappears. What you will then hear is a variation of the amplitude of what appears to be one signal. Care fully adjust the frequency of the calibrated source until even this effect vanishes. You will then have the two signal sources running at the same frequency. Therefore, the frequency of the unknown source can be read as though it were the known source. The accuracy of your results will depend on the accuracy of the known signal generator's calibration.

If the frequency of the unknown signal is above the range of audibility, the same basic procedure can be used, although it would be subject to error.

In this instance you would sweep the calibrated source until you hear a signal. This signal represents the beat tone created by the known and unknown signals which are close enough in frequency to produce an audible beat. Unfortunately, rather than obtaining a true beat between the known and uncalibrated signals, it is possible that the harmonics of the signal of known frequency can beat with the fundamental frequency of the un known or vice versa. Thus, when you adjust for the absence of such a beat, your reading of the known signal frequency may reflect a reading created by harmonics rather than the fundamental frequencies involved. There fore, be sure to sweep the known signal's frequency over a sufficient range to pick up more than one beat.

The beat which is the strongest will represent the beating of the two fundamental frequencies.

Where supersonic frequencies are not involved, a third procedure is possible. Instead of combining the signals, feed each of them into a stereo amplifier channel. Make the same tests while listening to the output of that amplifier with headphones.

Matching Headphone to Amplifiers

Q. I have headphones rated at 250 ohms which I would like to use with my amplifier's front panel headphone jack. Do I just install resistors in series with the jack, and what value should I use? The jack is rated at 8 ohms.

- Steve Anderson; Santa Ana, Cal.

A. A source rated for a low impedance (8 ohms) load can drive one of higher impedance (250 ohms) easily. The reverse is not true, however.

Matching them in this case will make no improvement in sound, but would lower the signal input into the phones because your amplifier already has resistors in series with its output. These are required to keep from feeding the amplifier's entire output to the phones and blowing them out (and hurting your ears).

Degrading of Electrolytic Capacitors

Q. Is it true that electrolytic capacitors wear out gradually if left in a non-operating condition? Such could easily be the case with consumer-type electronic audio gear which may not be operated for extended periods of time.

How insidious for an amplifier to wear out just sitting there--say, perhaps, in storage for a considerable length of time!

-Steven Heinisch, Prior Lake, Minn.

A. When an electrolytic capacitor is not used, most of the time its capacitance does not change much. Its volt age breakdown point, however, will gradually fall. This fact can cause trouble when a piece of equipment has not been operated for long periods. In some instances, the capacitance may also decrease.

(adapted from Audio magazine, Sept. 1979; Joseph Giovanelli )

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