500, 1000, 2000, and 4000 Hz (four band method) or the corresponding average for the octave bands centered at 500, 1000, and 2000 Hz (three band method). If other octave bands are used they must be specified. Unit: dB. speech privacy index (SPI): The SPI is essentially the opposite of the speech intelligibility index and is defined as 1 - SII and usually represented as a percentage. An SPI above 80% is considered normal privacy while an SPI above 95% would meet the requirements of confidential privacy. speech transmission index (STI): an index for rating the intelligibility of speech that takes both noise and reverberation into account. temporary threshold shift (TTS): a temporary increase in the threshold of hearing at a given frequency. threshold of hearing: for a given listener, the minimum sound pressure level of a specified sound that is capable of evoking an auditory sensation. The sound reaching the ears from other sources is assumed negligible. transducer: a device designed to receive an input signal of a given kind and to furnish an output signal of a different kind in such a manner that the desired characteristics of the input signal appear in the output signal. For example, a microphone takes an acoustic pressure as an input and produces an electrical voltage as an output that is direct proportion to the instantaneous acoustic pressure amplitude. Other common examples in noise measurement would be a loudspeaker, accelerometer, or laser Doppler vibrometer (LDV). transmission loss: the reduction in sound level from one side of a partition to the other. wavelength: the distance a sound wave travels in the time it takes to complete one cycle. weighting: see frequency weighting
46 of 58 Spendiarian & Willis Acoustics & Noise Control LLC
12/15/2019
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