Definition :
Sound waves in air result from a
physical disturbance of air molecules, such as when a truck drives by a
building or when guitar strings are plucked. Sound waves combine and reach a
listener via numerous direct and indirect pathways.
The listener's inner ear
contains organs that vibrate in response to these molecular disturbances,
converting the vibrations into changing electrical potentials that are sensed
by the brain, allowing hearing to occur.
Acoustical analysis involves not
only the sound source but also the listener and everything in between on the
path of the sound. The perception of the receiver can be influenced by the
treatment of either the path or the source. Some source sound is desirable, for
example a lecturer's voice, and some source sound is undesirable, such as the
sound output from an idling truck outside a window. Undesirable sound is
usually called noise.
Unless
it is a pure tone, a sound wave is typically made up of vibrations at different
frequencies. Like the impact of a stone in a lake, ripples in the water are
created that are analogous to sound in the air. The frequency is basically the
number of waves that pass a single point in one second, moving at the speed of
sound in air. One wave per second is a frequency of one hertz (Hz). A frequency
of 1,000 hertz is a kilohertz
When designing new library
buildings or correcting deficiencies of existing library spaces, materials and
constructions are selected to control noise and other unwanted sound. The human
ear does not perceive all frequencies of sound to the same degree, however,
being less sensitive to lower frequency sound pressures than to middle or
higher frequency sound pressures. People tend subjectively to measure their
perception of the loudness of sounds based more on the SPL of these middle and
higher frequency sounds. Design criteria and sound measurement devices are
therefore weighted toward these upper frequencies in order to reflect the
subjective perception of people in the space.
The term dBA, or A weighted
decibel, is often used to describe noise levels in spaces because this type of
decibel measurement averaged over the range of frequencies within the range of
hearing correlates well with people's subjective perception of the loudness of
the noise. Sound level meters, which average the SPL across frequencies,
usually have a setting for A-weighting, so that measured noise levels correlate
to the human perception of the differences in noise level.
The
NC Rating is an acoustic design criterion for the target level of background
noise in a room. This criterion is based on the fact that human hearing is less
sensitive to lower frequencies than to higher frequencies, so that a specific
criterion for the SPL of background noise in a space varies with the frequency of
the noise spectrum. Figure 2.3 shows the Noise Criteria (NC) used in acoustic
design.
The loudest frequency region of the background noise sets the NC-curve
that applies to the space To meet the criteria of NC-25, for
example, the measured loudness of all frequencies must fall at or below the
NC-25 curve. A
new building program should list the acoustic criteria for each space. These
criteria will usually include an NC Rating requirement, which depends on the
appropriate level of background noise to the tasks and activities in the space.
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