PP1603 - High-Frequency Air-Bone Gaps in Listeners with Normal Hearing and Sensorineural Hearing Loss

Abstract
False air-bone gaps at frequencies above 2000 Hz are consistently observed in normal-hearing listeners and listeners with sensorineural hearing loss (SNHL). These gaps have been attributed to incorrect Reference Equivalent Threshold Force Levels (RETFLs) in American and international audiometer standards. This multi-center study was conducted to determine average bone-conduction thresholds in participants with normal hearing and air-bone gaps in participants with SNHL. The findings show that air-bone gaps at frequencies above 2000 Hz averaged 8 dB for participants with normal hearing and 17 dB for participants with SNHL. Based on these findings, changes in the audiometer standards are recommended to eliminate false air-bone gaps.

Summary
Objective
Clinicians have consistently observed air-bone gaps at frequencies above 2000 Hz when using audiometers calibrated according to current American and international audiometer standards. These gaps are believed to result from incorrect RETFLs prescribed in the standards. This study aimed to establish normal bone-conduction thresholds to inform changes in the standards, ensuring that bone-conduction sensitivity is accurately reflected and that false air-bone gaps are eliminated.

Methods
Data were collected at three sites – Arizona State University, Cincinnati Children’s Hospital Medical Center, and the University of South Florida. Twenty normal-hearing listeners and ten listeners with sensorineural hearing loss (SNHL) were tested at each site. Air-conduction thresholds were tested in both ears with circumaural earphones (Radioear DD450) at standard octave and interoctave frequencies (250 – 8000 Hz). Bone-conduction thresholds were tested in one ear at the same frequencies.

Results
Listeners with normal hearing. Mean air-conduction thresholds for all frequencies range from 2-6 dB HL (mean = 4 dB HL). Mean bone-conduction thresholds range from 2-6 dB HL for frequencies 250 - 2000 Hz (mean = 4 dB HL). At frequencies 3000 – 8000 Hz mean bone-conduction thresholds range from 0 to -8 dB HL (mean = -5 dB HL). Average air-bone gaps range from -3 to 0 dB HL at 250 – 2000 Hz (mean = 0 dB HL) and 5 – 12 dB at 3000 – 8000 Hz (mean = 8 dB). The mean 4-kHz air-bone gap is 9 dB.

Listeners with SNHL. Mean air-conduction thresholds range from 18 to 55 dB at 250 – 8000 Hz. The mean air-bone gap at 250 Hz is 10 dB, resulting from an occlusion effect produced by the circumaural earphone at that frequency. Mean air-bone gaps at 500 – 2000 Hz range from –4 to 0 dB (mean = -1 dB). Mean air-bone gaps at 3000 – 8000 Hz range from 7 to 30 dB (mean = 17 dB). The mean 4-kHz air-bone is 16 dB. The 4-kHz air-bone gap increases with the magnitude of the air-conduction hearing loss from about 11 dB for a 30 dB air-conduction threshold to 25 dB for a 70 dB air-conduction threshold.
Conclusions
Current bone-conduction calibration standards result in false air-bone gaps at frequencies above 2000 Hz in both normal-hearing listeners and listeners with SNHL. These gaps are larger in listeners with SNHL and increase with the severity of hearing loss. The smaller gaps in normal-hearing listeners likely result from elevated bone-conduction thresholds caused by ambient and physiological noise masking.

To eliminate erroneous air-bone gaps that do not indicate conductive hearing loss, the standard bone-conduction reference equivalent threshold force levels should be reduced by 7, 14, 21, and 29 dB at 3000, 4000, 6000, and 8000 Hz, respectively.