Some bookshelf speakers are more "bookshelf" than others, it seems. The confusion stems from the term "bookshelf," which was assigned some years back to the first air-suspension systems that were, in their time, much smaller than the "full size" or flooring-standing speakers deemed necessary for full bass response. Even so, to call a speaker "bookshelfc when it required a shelf two feet deep and two feet high was overly optimistic, as many a buyer soon discovered.

Speaker system size is a relative thing, depending on one's frame of reference. I recall one hi-fi show, wher Edgar Villchur was proudly demonstrating his a/s speaker as it pumped out bass tones from what was, to audio insiders, an impossibly small box. A lady in the audience rendered the usually verbal Villchur speechless when she asked: "Why must it be so large?"

"Large" or "small," a so-called bookshelf speaker generally needs some kind of support to raise off the floor. One reason is to avoid an overly heavy bass often accompained with unwanted reinforcement of room resonances. Another is to get the tweeter (and midrange driver, if used) at a height more in line with the height of listeners' ears while, at the same time allowing those treble reproducing elements to clear possible obstructions of furnishings. Another reason, in some instances, would be to avoid pumping annoying bass tones to people on the floor below.

For all this, the typical "bookshelf" speaker should be raised off the floor-if not actually on a shelf, then on some kind of pedestal, stand, bench, or even the top of an extra-long cabinet. As a rule, in such a position the speaker will produce a more musically balanced tone.

Q: A friend advises that I need more than a grapic equalizer to correctly "tune" my listening room. Is he right?

A: He may be, depending on how accurately you want to tune your room. With an octave (or partial-octave) equalizer you can do a fair job of adjusting the many controls by ear, using ordinary musical program material. Better than that would be to plot the system's response on frequency graph paper, using test tones (from a tape, record, or-better-yet-a signal generator). Still better is the professional technique that employs a "real time analyzer." This test instrument once was so expensive and arcane that few installers owned one and they had to charge a hefty fee for using it in your home. But now it appears that the real-time analyzer may be more generally available. There's a model by Crown International (priced around $2,000), and one just released by Scott tha is priced at about $550.

Q: Is the "Dolby 25" switch on some receivers the same as a Dolby decoder?

A: Only if the description for a receiver specifies that it has a Dolby decoder built in. Otherwise, the switch is "half" of what is needed for optimum reception of Dolbyized FM. That is it will provide the requisite 25-microsecond time-constant in the deemphasis action, but it will not in itself "decode" the Dolbyized signal. You still need a regular Dolby decoder (available as a separate unit or included in some cassette recorders) to get the correct high-frequency rolloff. CAPTION: Illustration, no caption.