Transcript
TYPES
ROADWAY AIRCRAFT RAILROAD
sound energy emanating from motor
vehicles
3% heavy trucks
97% automobiles
locomotive engines: large diesel-electric
engines
rail cars: wheel-rail interaction
•interaction of the high-velocity exhaust gasses with the relatively still atmosphere through which the aircraft passes •5 to 8 diameters behind the exhaust nozzle
RAILWAY
Rail car retarders can produce a high frequency, high level screech that can reach peak levels of 120 dB at a distance of 100 feet, which translates to levels as high as 138, or 140 dB at the railroad worker’s ear
•Ground effects occur when the sound grazes at a shallow angle over acoustically soft materials such as thick grass, plowed ground, fresh snow, or in theaters, padded opera chairs. •Grazing attenuation.
•acoustic wave interacts with an absorbing surface at a shallow angle of incidence
GROUND EFFECTS
•Grazing attenuation. •reflected wave combines with the original wave and may produce an increase or decrease in overall signal amplitude
•Subdivides the ground into three regions: source and receiver regions, no more than 30 times the respective receiver heights from the source or receiver; and a middle region that includes the remainder of the intervening ground
GROUND EFFECTS
BARRIER: most common way to control exterior noise
point source barriers practical barrier constraints
barrier materials
POINT SOURCE BARRIERS
• If a receiver, located in the bright zone where the attenuation is zero, is lowered toward the shadow zone the attenuation does not jump instantaneously from zero to 5 dB.
• transition zone: sound waves are scattered from the top of the barrier and combine out of phase with the direct path waves, resulting in some attenuation.
PRACTICAL BARRIER CONSTRAINTS
• There are practical limitations to barrier theory. • If barriers are not long, the sound can travel around them • Reflections from nearby buildings can produce flanking paths where the
sound travels around a barrier • Wind: blown from the source to the receiver and the distance is
sufficiently long, the barrier effectiveness is much reduced due to the downward bending of the sound waves
PRACTICAL BARRIER CONSTRAINTS
• There are practical limitations to barrier theory. • If barriers are not long, the sound can travel around them • Reflections from nearby buildings can produce flanking paths where the
sound travels around a barrier • Wind: blown from the source to the receiver and the distance is
sufficiently long, the barrier effectiveness is much reduced due to the downward bending of the sound waves
CONSIDERATIONS
• Nonporous – they must block the passage of air through them
• Sufficient mass – sound traveling through the barrier is significantly less
than the sound diffracting over or around the barrier – barriers: total surface mass density of at least 20 kg/sq m
(4 lbs/sq ft). • Weather resistant
– properly designed to withstand wind and other structural loads appropriate for the location
MATERIALS
• Precast Concrete Panels – treated to look like wood or brick and supported by I
beam columns – commercially available
MATERIALS
• Corrugated Sheet Metal Panels – sometimes are used to construct noise barriers – commercial barriers with both solid and sound-
absorbing perforated skins
MATERIALS
• Fiberglass – can be incorporated behind the perforated panels to
reduce barrier reflections – encased in a plastic bag to protect it from the
weather
•No openings between the barrier and the ground. •openings allow the sound to pass under the barrier and can reduce its effectiveness
NOISE BARRIERS
•Trees, shrubs, and other foliage are not effective •porous •do not meet the mass requirement.
NOISE BARRIERS
•Rows of trees, heavy grass, and dense foliage can provide some excess ground attenuation
•psychological sense of privacy •a sound barrier to make it more aesthetically acceptable.
NOISE BARRIERS
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