Noise Control Of Buildings
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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