False 4-kHz Air-Bone Gaps: A Failure of Standards

By Robert H. Margolis, Robert H. Eikelboom, Brian C. J. Moore, and De Wet Swanepoel

This article is a part of the January/February 2019, Volume 31, Number 1, Audiology Today issue.

During the Eisenhower administration, more than half a century ago, the Third International Congress on Acoustics convened in Stuttgart, Germany. It was a wide-ranging conference covering all areas of psychological, physiological, and physical acoustics. The list of authors from the proceedings is a three-column, three-page Who’s Who that includes many of the most eminent auditory scientists of the day. Some of the familiar names on the roster include Georg von Békésy, Nelson Kiang, S.S. Stevens, Juergen Tonndorf, and Eberhard Zwicker.

One paper, presented there by Edith Corliss and her colleagues at the U.S. National Bureau of Standards and ignored by standards committees for decades, demonstrates a simple, elegant relationship between normal bone-conduction thresholds and frequency. The results were described in this way:

“The threshold displacement amplitude decreases with frequency at a slope of 12 dB per octave, thus the magnitude of the acceleration imparted to the head at the threshold of sensation is very nearly independent of frequency” (Corliss, Smith, and Magruder, 1959, p. 54).

Because the Corliss paper has been ignored for the past 59 years by standards committees, we frequently see audiograms like the one in Figure 1, obtained in the course of the Busselton Healthy Aging Study, a survey of residents in the Shire of Busselton, Western Australia (Swanepoel et al, 2013). Here, air-bone gaps of 20 dB were obtained in both ears at 4 kHz without air-bone gaps at other frequencies. Recent studies have documented the occurrence of air-bone gaps at 4 kHz in listeners with sensorineural hearing loss and normal middle-ear function. Many audiologists encounter this frequently in their practice.

In this article, the source of this problem and possible solutions are presented.