Audioclinic (Aug. 1980)

Distortion Problem

Q. I am a professional entertainer with a six-piece musical group in which everyone either solos or sings together in ensemble. 1 use six micro phones wired for high impedance and a six-channel, 30-watt amplifier. When the individual mike levels are adjusted to a volume such as to make each singer heard as a soloist, the result of the ensemble singing is overload and distortion. Singing less loudly produces a sound lacking in vitality. Backing off from the microphones results in noticeable loss in presence. What can I do?

-Name withheld.

A. There could be several things which may account for the distortion you have mentioned. Perhaps the result of all vocalists performing simultaneously results in driving the amplifier beyond its power output limits. It also may be that some of the stages ahead of any master volume control may be overloaded.

Assuming that you do have a master volume control, turn it up, and reduce the settings of the six individual mixing pots. This will help unless the mike preamplifiers ahead of the individual mixing pots are overloaded. In that event, you will need a pad between the mike and its associated amplifier input.

A similar result might be obtained merely by wiring the mikes for low-impedance operation. Don't worry about impedance mismatching. All that would happen is that the microphone would produce less output. There might be just a bit more noise from the electronics of the amplifier, but this would be overridden by the desired program.

Calculating the Wave Length of Sound

Q. I know that for electromagnetic waves, wave length equals the velocity of propagation through the particular medium, divided by frequency. For ex ample, at 1 MHz the wave length is:

x=

(300x10^6 meters per sec) + (1 x 10^6 Hz)

=300 meters.

Is there a formula for finding the length of an audio frequency wave?

- Name withheld.

A. The method for determining the wave length of any frequency is the same as you use for determining the wave length of a 1-MHz signal. Sound waves traveling through air move at approximately 1,100 feet per second. (This speed does vary somewhat with temperature, but this figure gives a good approximation.) By substituting this figure for the 300,000,000 meters per second, you will arrive at the wave length of a given frequency. Note that a signal having a 1,100-Hz vibration rate will have a wave length equal to one foot. A frequency of 550 Hz will have a wave length of about two feet.

Standards for Stereo Placement

Q. Are there specific standards followed by engineers during recording? I have noticed that on most recordings, a vocal is always centered but a guitar and/or organ will be recorded at left or right. Other instruments in the sound field are not easily localized. Is there a universal coding system that most engineers follow?

- Bob Hoffman, Worth, III.

A. There are no specific standards for assigning instruments to a specific channel, at least with "popular" music.

It is true that the vocal is almost always centered and that the bass is always centered. The vocal is centered in an attempt to produce a compatibility between the stereo sound and the sound which would be heard if the recording were played on a mono system, such as over an AM broadcast station. The bass is centered because of the nature of the disc-cutting process. When centered, the bass produces only lateral modulation of the groove. Because the bass displaces the groove more than anything else, it is better not to have any vertical groove displacement. Under some conditions, where the bass is not centered, such displacement might be so great that the cutting stylus would actually lift from the surface of the disc for an instant, causing a break in the grooves.

The remainder of the instruments and the background vocal placement is left to the judgment of the artist, the producer, and the recording engineer.

Much of the success of a recording may depend on this placement and upon the superimposition of special effects, such as phasing and reverberation, which are often needed.

In classical music, the engineers usually try to reproduce the sound of the musicians just as they are seated when making the recording. Bass centering is automatic, however, and in some instances there may be added reverberation or equalization.

Impulse Noise Reduction

Q. I am considering the purchase of a transient noise filter and plan to use it for general playback of discs as well as for recording from discs onto tape.

The discs I record from are often in less than optimum condition. From what I understand of these filter devices, they are non-complementary and work by eliminating noise by means of an "attack time" principle. This being the case, can such a device be used to cut down on noise from any program source--i.e., FM or tapes already re corded from scratched records? Is such a device effective only when filtering scratches, ticks, etc. from discs?

-Kenneth Lisagar, Bayside, N.Y.

A. The non-complementary devices designed to cut down on impulse noise will "look for" peak rise times. (That should be steep rise times.) Even a cymbal crash does not have the fast rise time of a scratch on a disc. The device does not "know" what is feeding into it. All that it does know is that it must act upon "seeing" steep rises of energy, whether they come from the output of an FM tuner or from that of a tape which contains this impulse noise.

Bandwidth and FM Tuners

Q. Please explain why and how bandwidth is related to frequency response in FM reception.

- R. Zimmerli, Muttenz, Switzerland

A. The bandwidth of an FM tuner has little effect on frequency response. Bandwidth does, however, play a vital role in distortion and stereo separation. If the bandwidth is restricted, the extreme ends of each modulation cycle will be depressed or completely removed. This will produce IM and harmonic distortion. In addition, side-bands may be reduced, with the result that some of the stereo information may be lost, leading to a decrease in separation.

(Audio magazine, Aug. 1980; Joseph Giovanelli )

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