Sound level meters of 50 years Free

The first quantitative noise measurements appear to have been published by Lemon in 1925, in connection with the then new balloon tires. He adjusted a calibrated buzzer until its sound just masked the sound inside an automobile. Others similarly sought a physical measurement directly related to a sensation such as loudness. The reference sound pressure, in the first American Tentative Standards for Sound Level Meters Z24.3‐1936, was chosen as the threshold of hearing for an acute ear. For more than 30 years, in Germany the instrument whether of the physical or subjective kind) was named a “loudness meter” whose unit of measurement was the phon. The functional diagram of the sound level meter of 50 years ago is much like that of today: a stable microphone, an amplifier, bandpass filters for optional use, an attenuator, a frequency weighting network (much like the A‐), a rectifier, and an indicating instrument. Then and now, the instrument is supposed to respond to the mean‐square, frequency‐weighted sound pressure, with exponential time averaging. The squaring rectifier long gave trouble; today squaring is often accomplished by doubling the logarithm of the sound pressure. By strong tradition in the U.S.A., the goal for the microphone has been frequency‐constant sensitivity on the average of all directions; the goal in Germany has been frequency‐constant sensitivity perpendicular to the diaphragm.