DEAR SAM: Could you advise me about the use of granular vermiculite to soundproof the walls in my duplex condominium? There is about a one-inch space between the finish wall and the cinder block that is the thickness of the furring strips, into which the severmiculite could be poured. Would this do a good job?

ANSWER: The subject of "acoustical perfume" such as background music in a restaurant to mask the sounds of voices may also be utilized for the privacy of a doctor's office adjoining his reception room with patients.

However, if you are referring to a common wall in a two-story duplex condominium, this is another type of soundproofing in which you want to provide "quieting" for yourself or your neighbor and sound control.

Using vermiculite to fill the spaces will insulate to a maximum of 3/4 inch (the thickness of furring strips); also, you may have to open the top edge of each space between furring strips to fill the vertical section but still have to use fiberglass to stuff the top manually. Patching the filler holes will necessitate additional labor.

A more satisfactory method would be to build new stud walls of 2 x 3s at 16-inch centers, not touching the existing common walls. This will allow the installation of full thich fiberglass blanket or batt insulation (3 1/2 inches). Install resilient metal channels, horizontally at approximately 24-inch spacing; then apply 4 x 8-foot sheets of 1/2-inch gypsum board with self-tapping screws into the channels.

Screw holes and joints in the gypsum board must be spackled, using Elmer's Red-i-Spack which does not contain hazardous asbestos; however, the top and bottom spaces of the gypsum board should preferably be filled with non-setting caulking for tightness at ceiling and floor, respectively. Painting or wall-papering can then be applied.

A substitute wall finish of "wood-grain" gypsum board, with beveled edges, can be used. The self-tapping screws can be painted to match the color of the wood-grain; similarly, the caulking can be painted, unless you prefer to install a ceiling molding and a baseboard.

Sam Fishlyn welcomes your questions on home improvements but can include only those of general interest in this column. Write to him in care of this paper or to P. O. Box 62, Newton Centre, Mass. 02159.