Absolute Noise Level of Microphones—a Definition Proposed for Discussion

Abstract

With high quality microphones, fluctuation noise becomes a problem. Analyzing the situation shows that the usual conception of the term “signal-to-noise-ratio” becomes ambiguous here; thus a gap appears, requiring the introduction of a definition. This should be based on the properties of the ear in order to characterize noise performance as perceived. A corresponding definition aiming at a nonambiguous noise rating is introduced, explained, and applied to different types of microphones—dynamic, condenser, and piezoelectric (sound cell type). Merits and weaknesses of the definition, and of possible modifications, and relations to a recently proposed definition of microphone efficiency are discussed. The new quantity termed “absolute noise level” or “absolute deafness” is defined as the loudness level of the noise heard at the acoustical end of an imaginary transmission system which, connected to the microphone terminals, would faithfully reproduce any sound signal present at the microphone, when the ambient sound fields at both microphone and listener locations are zero. This noise level represents the “hearing loss” introduced by listening via microphone-transmission-system instead of directly. The numerical value of absolute deafness (in Phon) is obtained from the microphone response and resistance curves, by a process based on Fletcher-Munson's work.