Treating Noise Sources
From a practical standpoint, noise can be divided into two basic types: airborne and structure borne. Each requires separate control strategies and products.
Airborne noise, as the term implies, consists of sound pressure waves moving in air from a noise source. These waves travel, of course, at the speed of sound (approx. 760 mph) in multiple directions. Ideally, the noise source would be contained in an enclosure lined with a noise barrier material, technically referred to as a “limp mass” or “limp mass barrier”. Basically we want a non-resonant heavy material such as lead sheets or “loaded vinyl”, which is vinyl into which is mixed a heavy, clay-like substance. This can then be made into rolls or sheets. The noise barrier can then be installed to box in the noise.
If total containment is not possible, then placement of the barrier in direct line of sight between noise source and receiver is the next best alternative. But remember that if an enclosure is 10 - 15% “open"”for noise transmission, up to 50% of that noise will escape.
Another important characteristic of airborne noise is the build-up of sound energy within a closed space. This includes engine boxes, vehicle cabs, rooms, or boat pilothouses. The noise level can increase up to 30% in a space with hard reflective surfaces. Noise absorbing materials will dissipate this sound energy by providing a rough textured or porous surface. Suitable materials include carpeting, acoustical fiberglass, open celled acoustical foam and mineral wool. Absorber materials are often bonded to a barrier to form a composite product with superior overall performance.
Structure borne noise and vibration is a major problem in mechanically connected or welded machinery, vehicles and boats. It is treated by either isolation or damping, or a combination of both. Isolation usually means engine mounts or pads to decouple the vibrating source from surrounding structures. Vibration energy is thus prevented from migrating to other locations. Vibration mounts must be of an appropriate density to match the weight of the vibration source. They may be constructed from metal and rubber, resin impregnated fiberglass, or plastic alloys.
Damping refers to the process of removing vibration energy from stiff panel surfaces like sheet metal, wood or reinforced plastics. Drumming and ringing noise is reduced by applying sheets of damping material to selected locations such as automobile door panels, boat hulls or bulkhead areas. Damping sheets can also be sandwiched between layers of plywood or particleboard to make a “quiet” construction panel. Because isolation treatments are only partially effective, damping is often required to achieve desired noise and vibration reductions.