When Normal Hearing in Not Really Normal: New Research Provides Insights

by Brian Taylor, Editor-at-Large

Two recent scientific reports underscore the need for exquisite high frequency hearing and the value of measuring it clinically. 

Brian Taylor, AuD

As most clinicians’ attest, there is a substantial group of patients that present to the clinic with complaints of an inability to understand speech in the presence of background who nevertheless have normal audiograms. These audiograms, of course, are typically confined to frequencies between 250 and 8000 Hz. Although ultra-high frequency audiometry has been commercially available for decades, it is not commonly used in most clinics. However, the findings of these two studies suggest it might be worth the investment of time and equipment to measure ultra-high frequency thresholds in some individuals. 

Clinical Value to Be Found in Ultra-High Frequency Assessment?

In the first article, published November 19 in the esteemed Proceedings of the National Academy of Sciences (PNAS) in the United States of America, researchers evaluated the clinical value of assessing pure tone thresholds above 8 KHz. The study investigated the use of the Digits-in-Noise (DIN) test to evaluate hearing thresholds above 8000 Hz.

The researchers, led by Lina Motlagh Zadeh of the University of Cincinnati, used standard and ultra-high frequency audiometry, self-report, and successively higher cutoff frequency filters (2 to 8 kHz) in the DIN test to investigate contributions of higher-frequency hearing to speech-in-noise perception.

According to the researchers, three surprising results were found:

1. A total of 74 of 116 “normally hearing,” mostly younger adults had some hearing loss at frequencies above 8 kHz. This finding suggests that minimal hearing loss above 8 KHz is easily measured, and a preventive early warning to protect hearing.
2. Hearing loss correlated with self-reported difficulty hearing in noise, indicating that validated self-reports of hearing difficulties are an effective method of identifying individuals at-risk for communication difficulties related to ultra-high frequency hearing loss. 
3. Even with the broadest filtered noise (≤8 kHz), DIN hearing thresholds were significantly better (P < 0.0001) than those using broadband noise, thus sound energy above 8 KHz contributes to speech perception in noise.

Collectively, this study suggests it is worth the time measuring hearing thresholds above 8000 Hz, either with ultra-high frequency audiometry or the DIN test, in those individuals who express difficulty understanding speech in noise but have normal hearing in the frequency ranges tested during a routine hearing assessment.

Difficulty Hearing in Noise with ‘Normal Hearing’

In a second article, published November 14 in Nature Scientific Reports, British researchers discussed their rationale for developing a new clinical test to identify individuals with so-called normal hearing that exhibit problems hearing in noise.

Emma Holmes & Timothy D. Griffiths developed a new figure-ground test that require participants to identify a tone pattern from background of random tones.

According to the authors this test separated variability in speech-in-noise perception related to mechanisms for detecting static (same-frequency) patterns and those for tracking patterns that change frequency over time. In addition, elevated hearing thresholds that are widely considered to be ‘normal’ explained significant variance in speech-in-noise perception, independent of figure-ground perception.

Overall, their results demonstrated that successful speech-in-noise perception is related to audiometric thresholds, fundamental grouping of static acoustic patterns, and tracking of acoustic sources that change in frequency.

The authors suggest that speech-in-noise deficits are better assessed by measuring central (grouping) processes alongside audiometric thresholds. The entire study can be found at here.