Audioclinic (Q and A) (Jun. 1973)

by Joseph Giovanelli

Frequency of a "Click"

Q. What is the frequency of the "click" sound heard when a 1.5 V flashlight battery is temporarily connected to a loudspeaker (as when phasing a system)?

-F. Alexander, Maspeth, New York.

A. The voltage applied to a speaker for phasing purposes is, in this case, a DC voltage. This voltage, therefore, has no frequency.

Application of this voltage will cause the speaker's cone to move one quarter of a cycle. When the voltage is removed, the speaker returns to its rest position, completing what could be considered to be the first half of an audio cycle. (This statement assumes that we have a perfect loudspeaker.) No loudspeaker is perfect. Therefore, when the DC voltage is applied, the speaker's cone will move. Because of its inertia, the cone will move somewhat beyond the point determined by the magnitude of the DC voltage and the Strength of the magnetic field surrounding the voice coil. The cone then moves back the other way, attempting to come to an equilibrium. It may move past this proper equilibrium point and, therefore, have to reverse direction once again. The cone will, of course, finally come to its proper rest position. When the voltage is removed, the cone begins to move toward its normal rest position. Again, the inertia of the cone assembly will cause the cone to overshoot that rest position.

Once more, therefore, the cone will oscillate around that rest position. The amplitude of these oscillations will be determined by the speaker's damping.

The frequency of the oscillations will be determined by the mass of the cone and voice coil assembly, plus the springiness of the suspension of that cone.

The suspension, of course, might well include the air trapped within the speaker enclosure.

In addition to the speaker's own resonant frequency, you may also hear frequencies which are determined by resonances within the enclosure which were excited by the sudden movement of the cone.

This cone's sudden movement may also excite resonances in the listening room, giving rise to added sound.

What you hear, then, is a composite sound. not at all related to DC voltage.

Disc Rotation or Groove Displacement

Q. From my calculations, the outer grooves of a 12-inch LP travel at just over 53.8 cm/sec. If I am correct, why is my Shure RM901E rated at 6.2 mv/channel at 5 cm/sec? Even trackability ratings are at speeds of 25 cm/sec and below. Is the output voltage directly in line with increase or decrease of the disc's speed?

-Art Cook, Indianapolis, Indiana.

A. There is no direct relationship between disc speed and groove velocity.

We are not interested in how much groove material passes under the stylus during one second. We are interested solely in the displacement of the groove. If we could add up the total groove displacement, vertically and horizontally, during one second, the result should be equal to the figures you see on the charts for output voltage versus cm/sec. It is not the amount of material passing under the playback stylus which results in the production of a signal; it is the displacement of the groove by the modulation imposed on it during the cutting process in which we are interested when we specify the parameters for a phonograph cartridge.

Matching a Four-Channel System

Q. I have gone four-channel. I have a JBL amplifier and two JBL speakers.

I added the EV 1244X decoder/amplifier and two KLH 32 speakers.

Is that a good match of equipment? I sometimes wonder if I am really hearing four-channel sound.

-Charles N. Bailey, Pawtucket, Rhode Island.

A. I cannot determine from here whether or not you really have a good match between the two brands of speakers you are using. I can say only that I believe that the safest way of matching speakers is to use the same make and model of speaker for all four channels. Not doing so can result in shifting of the image, tied with changes of musical pitch or instrumental textures.

If you do not seem to hear four channel sound, it may be that you do not have your speakers properly phased. Even if you have the front pair properly phased, and the rear properly phased, it may be that the phase of one pair must be reversed with respect to the phase of the other pair. In other words, you may have to reverse the leads of each speaker in one pair of speakers only.

In addition, check the balance between front and rear channels.

It can also be that your program is not four-channel, and does not have enough difference information to produce the desired spatial effect.

(Audio magazine, Jun. 1973)

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