| Definition: | of a sound, in phons, numerically equal to the median sound pressure level in decibels, re 20 μPa of a free progressive wave having a frequency of 1 000 Hz presented to listeners having normal hearing facing the source that in a specified number of trials is judged equally as loud as the unknown sound
NOTE – The manner of presenting the unknown sound, such as from earphones or in a diffuse sound field, must be stated and forms one of the characteristics of the sound.
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