David A. Collings PE
Barriers and Enclosures: (Containing the Noise Source)
Installing a solid barrier to interrupt the direct radiation of sound to the listener is a frequently used method for controlling many noise sources. The most effective form of barrier, the full enclosure, is widely used on fixed noisemakers such as generator sets, compressors, heavy machinery, engines etc. In many cases, where ventilation is required, openings must be provided in the enclosure walls. These sound paths must be treated with some kind of a silencer. Access door and viewing windows must be essentially airtight and the construction of all parts of the enclosure must be sufficiently heavy to prevent transmission of sound through the walls.
For some noise sources a partial enclosure or barrier may be the only practical solution. Sound barriers are increasingly appearing along major highways with varying degrees of success. To be effective, an open barrier must be positioned close to the noise source and be as high as possible. Noise reductions of 15 to 20 decibels are possible with an open barrier but there are a number of practical problems that must be considered: -
1. Reflections from a solid-faced barrier can increase the sound levels on the source side up to 3 decibels.
2. Temperature inversions in the atmosphere can reduce a barrier's far-field effectiveness.
3. A tall barrier may block light and be esthetically undesirable.
The two challenges facing the highway barrier designer are a). to develop a low cost, maintenance-free material that will absorb sound and b). to find a configuration that will effectively block sound waves with a lower height wall. Some progress has been made with sound absorbing porous cement aggregates as an alternative to perforated metal panels filled with fiberglass or mineral wool. Solid wood or concrete barriers, however, are still the most commonly used despite the problems with reflected sound. In some parts of the world, transparent barriers have been installed along urban freeways to reduce the visual impact of tall walls. Various designs with serrated, rounded or split tops have been proposed that claim to increase the effectiveness of the barrier at a lower height. Few of these concepts can justify the "hype" with which they have been launched!
While codes and standards for highway sound barriers have been largely left in the hands of the State Transportation Departments, a number of development projects have been sponsored by the Transportation Research Board of the National Academy of Sciences.
Architectural Enclosures: (Containing the Listener)
Many of the outdoor noise sources in our towns and cities cannot easily be controlled by containment and we have come to rely on the soundproofing of our homes and offices to provide us with a comfortable acoustic environment. Building design and construction must not only isolate the occupants from outside noise but should minimize their exposure to noise from internal sources such as air conditioning, office equipment and the activities of other occupants.
Noise pollution of interior spaces can result from: -
· Airborne noise from open windows, ventilation ducts etc.
· Structure borne noise transmitted through the walls or floors.
· Interior noise from sources within the room.
Modern air-conditioning and ventilation systems have helped to eliminate airborne noise from outside sources. Air ducts however, if not properly designed, can provide a path for room to room noise transmission or provide a noise conduit from the air-moving device itself.
Walls and partitions may act as diaphragms that relay noise from one side to the other. In general, an airtight partition with properly sealed doors and penetrations will exclude normal high frequency sounds. Lower frequency sounds, however, generally excite some structural response in the facing material that can be transmitted through the structural framework and re-transmitted from the opposite side. Staggered studs are often employed to break this internal connection. Also using heavy face sheets to lower the response frequency can be effective in raising the transmission loss of a partition.
Sounds from sources within an occupied space can be made less intrusive by covering reflective surfaces with sound absorbing materials. This has the effect of lowering the reverberation time of the space so that the sounds decay rapidly, as would be experienced in an outdoor environment. An important secondary effect is the improvement in speech intelligibility so that conversations can be conducted at a lower level. An extreme example of this effect was the installation of acoustic ceilings in a prison dining hall which resulted in a reduction of 30 decibels in the measured noise levels.