therefore expressed in decibels. Unit: dB. loudness: that attribute of auditory sensation in terms of which sounds may be ordered on a scale extending from soft to loud. Unit: sone. masking: the process by which the threshold of hearing for one sound is raised by the presence of another sound. maximum fast sound pressure level: the maximum sound pressure level recorded using fast exponential time weighting. noise criteria (NC): a single number criteria for the HVAC or mechanical noise level in a room derived from measured octave band data. The octave bands are weighted to de-emphasize low frequencies because the human ear is least sensitive to these frequencies. This metric is not valid for outdoor measurements. noise induced permanent threshold shift (NIPTS): the permanent hearing loss resulting from noise exposure. noise isolation class (NIC): a single number rating derived from measured values of noise reduction between two enclosed spaces that are connected by one or more paths. This rating is not adjusted or normalized to a standard reverberation time. noise reduction (NR): the difference in sound pressure level between any two points along the path of sound propagation, e.g. the difference in level between the interior and exterior of a building where the sound level inside is due only to exterior noise. octave: the frequency interval between two tones whose frequency ratio is 2. omnidirectional microphone: a microphone whose response is independent of the direction of the incident sound wave. Pascal (Pa): a unit of pressure. 1 Pascal = 1 Newton per square meter ( 1 N / m 2 ). permanent threshold shift (PTS): a permanent increase in the threshold of hearing at a given frequency.
point source: a source that radiates sound as if from a single point. receiver: a person (or persons) or equipment which is affected by sound.
refraction: (1) the phenomenon by which the direction of propagation of a sound wave is changed as a result of a spatial variation is the speed of sound. (2) The angular change in direction of a sound wave as it passes obliquely from one medium to another having different sound speed. reverberation time (RT 60 ): of an enclosure, for a sound of a given frequency or frequency band, the time that is required for the sound pressure level in the enclosure to decrease by 60 dB after the source has stopped. Unit: second. room criteria (RC, RC Mark II): an octave band metric for evaluating HVAC noise inside a room. RC is a two dimensional metric consisting of a curve number that is the arithmetic average
44 of 58 Spendiarian & Willis Acoustics & Noise Control LLC
12/15/2019
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