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.