Unlike the advertising for home-stereo equipment, the promotion for car-sound systems is not subject to a "disclosure" ruling by the Federal Trade Commission.

As a result, car-stereo systems have been touted with claims of power output that often are highly inflated Vis-a-vis their realistic levels. This may change to a marked degree starting in June of this year, when a number of car-sound equipment manufacturers reportedly will begin to adhere to a voluntary agreement intended to bring power claims down to earth, based more or less on the guidelines now followed for home components.

So, what once may have been advertised as a "200-watt system" could become an honest 25-watts-per-channel system. The "200 watts" refers to 100 watts per channel, which probably is a peak power rating (about 3 dB above the clipping level). Now we are down to 50 watts per channel. But put a ceiling on the distortion level, and specify the power across the entire audio band rather than only at mid-frequencies, and we are down another 3 dB -- that is, to 25 watts per channel.

Is that much power really needed for sound that is reproduced in a relatively small enclosed space of even a big-sized car or station wagon? Oddly enough, it may well be needed despite the fact that reproduced sound in the much larger home listening room generally requires no more, or sometimes even less, amplified power most of the time.

The answer lies in two factors relevant to car-sound systems. One is the ambiant noise level that must be dealt with in vehicles. Driving with the window(s) open can result in a prevailing "noise floor" of 50dB to 90 dB, depending on traffic sounds and the noise of your own car. With the windows closed, the noise level drops of course, but you still must contend with such noise sources as the car's engine, heater or air conditioner, and whatever sound filters through from the outside.

Exact noise level figures are impossible to cite for all possible driving situations, but it has been estimated that at best, the improvement in ambiant noise when the car windows are rolled up while crusing at 55 mph in normal highway traffic comes to only 15 dB.

This still leaves the interior of the car actually "noisier" than the average home listening environment. In order for the sound of the stereo system to get sufficiently above that "noise floor" to permit you to enjoy it with a modicum of fidelity to both frequency response and dynamic range, the system must provide proportionately higher power. Add to this fact the relatively low efficiency of the better car loudspeakers (their efficiency is deliberately traded off in an effort to provide decent bass response from what have to be really small enclosures) and you have upped the power requirements by another factor.

Keep in mind too the particular mathematical relationships involved. Doubling the power represents an increase in sound level of only 3 dB. That much increase is heard as a "somewhat" louder sound. For a sound to appear "twice as loud," an increase of 10 dB is needed -- and that much of an increase calls for 10 times the amplifier power.

As a result, an honestly rated 25-watts-per-channel for a car stereo system seems reasonable enough. And actually, that is all you may be getting from many of today's so-called 200-watt systems. FEEDBACK

Q: Can two lengths of speaker leads of narrow guage by combined (in parallel, of course) to provide the equivalent of one lead of thicker guage?

A: Yes, they can. But the guage numbers do not combine arithmetically as you might suppose. For instance, two lengths of No. 22 guage do not equal one length of No. 11 guage. You should consult a standard wire table that gives the diameter in mils for various wire guages. Some of the most commonly used in hi-fi hookups are No. 14, which is 64 mils in diameter; No. 16, 50.8 mils; No. 18, 40.3 mils; No. 20, 32 mils; and No. 22, 25 mils. n

You can see that two No. 22 wires will provide about the same thickness as one No. 16 wire. Two No. 20 wires will provided the equivalent of one No. 14 wire.

When using double lengths this way, it is best to twist them tightly around each other and solder the ends.