I live in a condo complex with thin, poorly insulated walls. The wall behind my bed is common with the next door condo's master bedroom and bathroom. I can hear just about every noise. The small ones I can ignore, but the shower door banging is very startling in the middle of the night and early morning.

Is there a way to solve this problem without tearing into the common wall? Can I put in a false wall with insulation? What kind of material and construction would be best? Do I need to get my homeowner's association to approve the project? -- J.N. High-density, mineral-fiber acoustical tile helps reduce transmission of sound, and tiles can be applied to walls, as well as ceilings. However, your best bet would be to construct a second wall with no physical connection between it and the first wall using materials that reduce sound transmission.

The second wall should be constructed so that a two-inch air space remains between it and the original common wall. You can get still greater resistance to sound transmission in the added wall by using resilient clips for attaching gypsum lath to studs. The framing of the second wall should be from wall to wall and the ceiling. Place a blanket of insulation between the framing members, then finish the wall with gypsum wallboard.

Since this construction is an interior improvement, you should not have to get permission from the homeowner's association. However, there is probably no harm in checking before proceeding. I have a lovely antique table with a marble top. The marble is porous and has several yellow rings on it made by a glass or cup. Can you advise me on how to refinish the marble? -- S.L. Your best bet for removing stains on marble is to use a poultice mix made by combining an absorbent powder (baking soda, talc or plaster of paris) with a solvent or strong liquid cleaner.

Different types of stains require different types of poultice mixtures. For instance, smoke, soot and cigarette stains are best treated with a poultice made of baking soda or bleach. For coffee, tea, soft drinks and food stains, use a mixture of an absorbent powder with sodium citrate crystals and water.

For alcohol rings from drink glasses, use baking soda and a 6 percent solution of hydrogen peroxide.

First clean the marble surface with a good marble cleaner or mild detergent. Combine the ingredients of the poultice to form a paste.

Wipe the stained area with the same liquid used in the paste, then apply a one-quarter-inch layer of the paste with a wooden or plastic spatula or spoon.

Wear rubber gloves. Cover the area with a plastic wrap and seal all edges to the surrounding surface with masking tape. Let it remain on the surface for 48 hours.

Moisten the poultice with water, then dry with a soft cloth. You may have to repeat the process for more stubborn stains. Once the surface is clean, use a marble polish to protect the surface. We have a new home with a fireplace faced with old red brick. A white powdery substance is leaching from the brick facing.

What is this material and how can it be removed? -- R.M. The condition on the brick is called efflorescence. Moisture coming through the brick causes salts to come to the surface. To correct the problem, clean the brick thoroughly and then wash it down with a solution of muriatic acid, no stronger than one part of the commercial acid to nine parts of water. Be sure to pour the acid into the water -- not the reverse -- and use rubber gloves and goggles.

Prior to washing the brick with the acid solution, soak the brick in clear water. Use a stiff brush when applying the acid solution to any whitish areas. Rinse thoroughly with clear water. After the bricks have dried, apply a coat of Thompson's Water Seal to protect the surface.

Send inquiries to Here's How, Copley News Service, P.O. Box 190, San Diego, Calif. 92112-0190. Only questions of general interest can be answered in the column.