A2. General Acoustics
Sound Pressure Level (SPL) Sound is small, rapidly varying perturbations of atmospheric pressure with respect to the slowly changing ambient pressure. The ambient pressure is measured with a barometer while the small acoustic perturbations are measured with a microphone. The unit of sound pressure is the Pascal (Pa). However, due to the wide range of acoustic amplitudes that can be heard by the human ear, sound pressure is normally expressed on a logarithmic scale having units of decibels (dB). Sound pressure expressed this way is known as the sound pressure level (SPL) and has the following relation to sound pressure. SPL = 20 log 10 p p ref (A2.1)
Here p is the sound pressure in Pascals. p ref is a reference pressure, the threshold of hearing at 1000 Hertz (Hz), 20 x 10 -6 Pa.
A-Weighting The above formulation of SPL is a purely physical quantity. Due to the nonlinear and frequency dependent characteristics of the human ear it does not always correlate well with the perception of loudness. To improve the correlation for noise assessment purposes, a frequency weighting is often applied called A-weighting. The A-weighting function is based on listening tests in which human subjects adjusted tones throughout a range of frequencies to have equal loudness compared to a tone having an SPL of 40 dB at 1000 Hz. 2.1 shows equal loudness contours as measured by Fletcher and Munson.
47 of 58 Spendiarian & Willis Acoustics & Noise Control LLC
12/15/2019
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