Recent turntable designs and the promotions for them have stressed direct-drive mechanisms, quartz-controlled speed, and some other fairly exotic items such as complex tone-arms that look like turret-gun installations. While all this may be well and good (though not necessarily better than simpler designs), one aspect of turntable design that has not received much notice is the suspension of the platter and motor under the fancy dress plate.

This particular design feature becomes more important these days with the new "super discs" that are being released, since it relates to the perennial problem of acoustical feedback whereby loud sounds from the speakers are sensed by the record-player, amplified with the music, reproduced, re-sensed and re-amplified, and so on.

The result is a strong noise that obliterates the music and can drive the system into dangerous overload. Since the louds are louder than ever on the new records, with signal peaks exceeding those of older discs by 20 dB or more, the danger of acoustic feedback is greater than ever.

To a degree, acoustical feedback can be eliminated by locating the loudspeakers as far away as possible from the turntable. But, given the reality of a room's dimensions, that distance may not be far enough in all instances and for all records. The other factor involved here is the turntable's inner suspension. If too stiff, it will give rise to feedback at some distance from the speakers and at some volume level. Even when the feedback noise itself is not audible, it can distort the sound you do hear.

The way to test a turntable for this potential problem is to switch on your amplifier or receiver, and turn the selector knob to "phono." Keep the volume control at minimum. Place a record on turntable, but do not switch it on. Place the tone-arm so that the stylus rests on the portion of the record just outside the label. Now start tapping the top surface of the player (not the platter) while slowly raising the volume control. If the system is prone to acoustic feedback you will begin to hear a definite noise from the speakers at some setting of the volume control. Turning this control up more will make the noise sound worse, like a loud motor. That's acoustic feedback.

If, on the other hand, you do not hear this noise as you raise the volume control, the system is free of acoustic feedback.

If you have acoustic feedback it may take extraordinary measures to overcome it, such as cushioning the entire record-player in thick foam plies, or even suspending it, cradle-like, from the ceiling. Or, you might simply find it impossible to play some of the new "super discs" -- except perhaps at listening levels far below their inherent capability. You might even decide to get a new turntable. Actually, this is only one of several possible problems (others, for instance, being the ability of the cartridge in the arm to track some of the heavy passages on the new records, or the overload capability of the amplifier, or the dynamic range of the speakers) that could arise when playing these records. Which means, in sum, that if the new records are going to win wide audiences, the makers of playback equipment had better reassure the public of certain aspects of their products' performance that have not been too widely publicized up to now.