For about a century, hearing loss has been diagnosed based on the pure-tone audiogram, which describes the pure-tone level needed at a given frequency for the listener to just hear it. The audiogram is a powerful tool that determines the amount of amplification provided in a hearing aid such that the user has access to all sounds. However, it does not tell us very much about how well a hearing-aid user can distinguish between different sounds with hearing aids. For example, they may hear but not understand speech in background noise and might thus need additional support. This research has been conducted in an across-Demant collaborative effort between Eriksholm, Oticon headquarters, and Interacoustics Research Unit.
Funded by the William Demant Foundation.
A test design was selected based on previous studies, which requires the listener to detect certain speech-related characteristics (moving spectral ripples) in an artificial noise. The test was further developed in a number of studies to optimize its predictive power with respect to speech understanding with hearing aids in realistic acoustic scenarios, and to obtain a clinically viable test design.
Example ACT stimulus train with “waves” of carrier noise and moving ripples imposed on them (red squares). The upper plot shows a time-frequency representation whereas the bottom plot shows the corresponding time signal in gray and its envelope in black
The resulting test is named Audible Contrast Threshold (ACT™) test. It is currently being applied in several studies to confirm its predictions of hearing-aid users’ speech understanding difficulties and to investigate users’ preferences for advanced hearing-aid features (e.g., noise reduction).
Time-frequency representation of a speech signal (left) and of the ACT stimulus (right)
The initial studies demonstrated that the test indeed predicts speech understanding ability in hearing-impaired listeners with hearing aids. This relationship was more pronounced for realistic acoustic conditions than for more artificial laboratory-based conditions. Furthermore, the ACT test was shown to be clinically viable in terms of a very short test duration and a high reliability. The connection between ACT and speech understanding has since been confirmed in several large-scale studies.
Speech reception thresholds (SRTs, left panel) and SRT benefit induced by strong noise reduction (right panel), both as a function of performance measured in a precursor version of ACT. In both cases a strong correlation was observed
These studies also connected ACT to the need for more help from advanced features in terms of speech understanding, whereas the connection to user preference is still being investigated. In October 2023, Interacoustics has introduced the ACT test to the market, which will also be integrated with the Demant hearing-aid brands.
Speech reception thresholds (SRTs) as a function of ACT performance, measured in 100+ hearing-impaired participants of a large-scale study. A strong correlation was observed