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TRG-TRC007-EN 71
absorbed sound Sound energy that strikes a material and is converted from
sound energy to heat energy within the material.
absorption coefficient The ratio of the sound energy absorbed by the material
to the total sound energy incident upon the surface of that material.
AMCA Air Movement and Control Association International (www.amca.org)
ARI Air-Conditioning & Refrigeration Institute (www.ari.org)
ASHRAE American Society of Heating, Refrigerating and Air Conditioning
Engineers (www.ashrae.org)
attenuation The reduction in the sound level as it travels along the path from
a source to the receiver.
A-weighting A single number used to describe sound. It uses weighting
factors, by octave band, to approximate human response to sound in the range
where no hearing protection is needed. It is most appropriately used for low-
volume (or quiet) sound levels and is expressed as dBA.
broadband sound Sound energy that occurs at many frequencies, usually
covering the entire audible range.
center frequency Single frequency used to identify an octave band. It is
calculated by taking the square root of the product of the lowest and highest
frequencies in the octave band.
decibel (dB) A dimensionless ratio of two quantities that is used to describe
both sound power and sound pressure. It is defined as ten times the logarithm
to the base ten (log
10
) of the measured quantity divided by the reference
quantity.
dynamic insertion loss The sound insertion loss of a duct silencer with air
flowing through it.
free field A homogeneous, isotropic medium, free from boundaries. An
example of a free field over a reflecting plane would be a large open area void
of obstructions, like a parking lot or meadow.
free-field method A common method for testing HVAC equipment that is too
large to be tested in a reverberant room, such as water chillers and cooling
towers. The equipment is placed on a hard surface on a large parking lot
outdoors to approximate the sound conditions in a free field above a reflecting
plane. The sound pressure waves travel evenly in a hemispherical pattern away
from the equipment. Sound-power levels are determined by measuring sound-
pressure levels on an imaginary hemispherical surface surrounding the
equipment.
frequency The number of cycles, or oscillations, per second of a wave in
periodic motion. Expressed in hertz.
Glossary