Some Observations on ‘Normal’ Hearing and Hearing Loss

By Christopher Spankovich

This article is a part of the September/October 2020, Volume 32, Number 5, Audiology Today issue.

What is ‘Normal’ Hearing?

Ask an audiologist what “normal” hearing is and, not surprisingly, you will get a variety of responses (Figure 1). Certainly, normal pure-tone threshold sensitivity does not rule out hearing difficulty or the presence of auditory pathology, including cochlear and auditory neural peripheral or central deficits. Further, a number of non-auditory factors can contribute to a patient’s perceived hearing difficulty (e.g., cognitive capacity, attention, medications, etc.).

While hearing in the real world is much more complex than detecting the presence of brief pure-tone stimuli, hearing difficulty is highly related to pure-tone audiometry. How we define normal in regard to pure-tone hearing is deeply entwined in our differential diagnosis and treatment recommendations and, therefore, has significant implications for people with hearing loss.

The Reference Sound-Pressure Level

The reference sound-pressure level for plotting auditory thresholds is 0.0002 dyne/cm² (also written as 20 ŲPa or 20 microbar). Notably, this reference sound-pressure level is found in most audiology textbooks, but the citation for a study or studies that establish this reference value is more difficult to find. Recent textbooks provide no citation.

Some of our readers, of course, know that the reference level was set by an international agreement as a value that reasonably approximated the threshold of hearing at 1000 Hz in healthy young adults. But who were these healthy,  young adults? Unfortunately, you will find limited description, as the reference was, in reality,  based on numerous studies over many decades.

Early work provided foundations to the ultimately agreed-upon reference sound-pressure level (Toepler and Boltzman, 1870; Rayleigh, 1877; Wien, 1903; Fletcher and Wegel, 1922; Fletcher, 1923). For example, Wien (1903) reportedly provided the first recorded estimate of the amplitude of just-audible sounds using an electromagnetic receiver as a source of sound and optical measurements of its diaphragm movement. The observer listened through a hole in a large screen 30 cm from the receiver.

Fletcher and Wegel (1922) made use of a vacuum-tube oscillator, condenser transmitters, and thermal receivers to examine hearing sensitivity in 93 presumed normal ears (males and females, at least 20 females) and reported in dynes per square centimeter (dynes/cm²). The testing was completed in a room layered with loose felt, sheet iron, and cheese cloth.