Noise pollution has joined air and water pollution as a modern worry.

Sound is normally transmitted by sound waves traveling through the air, which, when striking a typical interior wall, cause one side of the wall to vibrate.

These vibrations, in turn, transmit the sound waves through the air inside the hollow wall cavity and cause the other side of the wall to vibrate in unison. At the same time, the studs inside the wall also start vibrating and further serve to transmit sound waves to the other side as well as to adjoining surfaces and attached structural members.

Sound waves radiate outward in all directions from their point of origin (much like ripples in a pond), traveling easily through the air and many structural materials.

They find their way into a room, bounce off the hard, smooth surfaces, and are reflected back into the room to continue reverberating until all their energy is dissipated. If, on the other hand, the surfaced encountered are soft, porous, or resilent, the sound waves are morelikely to be absorbed and fewer will be left to bounce around.

However, covering walls and ceilings with sound-absorbing materials such as acoustical panels or tiles will do little to prevent sound from coming through from the outside or from another room. Acoustical material will help absorb noise created within that room, but will be no help in keeping noise from entering the room when the noise originates elsewhere.

So how the noise be kept out? Keeping in mind that sound travels most easily through the air, the first step is to plug all potential air leaks and openings in the structure -- openings that will not only permit air to infiltrate, but also allow sound waves to pass easily.

Here, many of the same techniques used in sealing out cold air during the winter can be used. These include caulking around all window and door frames, plugging all gaps in walls with insulation, installing weather stripping around doors and movable windows, and even putting up storm doors and windows.

Of course, there are no windows on inside walls, but there are doors, and adding sponge-rubber weather stripping around each door will help cut down sound transmission. So will replacing hollow and thin-paneled doors with heavier solid wood doors -- or even putting up a second door in the same space, (so that one door opens in each direction).

In addition, remember that electrical outlets, recessed medicine cabinets and similar wall cutouts reduce the thickness of the wall, thus reducing its ability to stop the transmission of sound.

This is especially true when electrical outlets are back to back in a common wall, or when pipes run through a wall in such a way that there are sizable openings around the pipe covered only be a decorative flange or collar. s

Because the cost of building a wall resistant to sound transmission adds slightly to the original cost of the house, most homes do not include this type of construction.

But for those planning major alterations or additions, or those who find noise problems severe enough to warrant rebuilding an entire wall, the "staggered-stud" method of construction is often recommended when putting up a new wall.

This involves building a thicker wall using 2-by-6s for the top and bottom plates (along the ceiling and the floor). The studs are still 2-by-4s, but they are installed with every other one staggered -- that is, with only every other stud even with opposite edges of the 2-by-6s along the top and bottom.

When the gypsum board panels are nailed up on each side, those on opposite sides will not be supported by the same studs. In other words, each wall face is supported by a separate set of studs; thus there is no direct means of transmitting sound waves through the studs from one side to the other.

In this wall, as in any other hollow-wall construction, adding fiber glass insulation (unfaced insulation is adequate because no vapor barrier is needed) helps even further to cut down the amount of sound that will pass through.

Built this way, a wall would have enough sound-insulating quality to make loud voices almost inaudible on the other side. For even greater sound, transmission resistance, a layer of sound-deadening board could be installed under the finish layer of gypsum board.

Owners of co-op apartments or condominiums that share a common wall with a noisy neighbor, or those who live in attached houses that have a common wall, may not find it practical to tear the entire wall down to put up a new one. One solution is to rip the wall open on your side, pack fiber glass insulation into the exposed spaces between the studs, then put up a layer of sound-deadening board and finish with a layer of gypsum board.

Where ripping open the existing wall is impractical or not permissable, and where you are willing to lose about six inches of floor space along that wall, another and even more effective method of handling the problem is to build a second wall in front of the existing one, spaced at least 3/4 inch from it; (the new wall must not be in contact with the extinguisher).

The new wall can be of standard 2-by-4 construction, but it should have fiber glass insulation packed into the spaces between the studs. That, plus the layer of sound-deadening board under the gypsum board, will provide an effective barrier against sound transmission from the other side.

Noise coming from overhead (through the ceiling) is usually caused by people walking. Carpet and a cushion-type carpet pad on the floor upstairs will help, but acoustical tile on the ceiling below will not.

However, if headroom is adequate to allow for a loss of about six inches in ceiling height, a suspended ceiling, using wood or mineral fiber panels, will provide a sizable reduction in noise transmission -- especially if the space between the suspended ceiling and the existing ceiling is further insulated with batts of fiber glass.

When a suspended ceiling cannot be p ut up because of lack of sufficient headroom, the only other cure is to rip the existing plaster or gypsum board off completely so the joists are exposed. Then force three- or four-inch-thick batts of fiber glass up between each joist and nail resilient metal channels or clips to the bottom of the joists before refinishing the ceiling.

The gypsum baord panels are secured to the channels or clips with screws instead of being nailed to the joists. The result is a "floating" ceiling that will transmit much less noise to the bottom surface.