Directionality characteristics of the Tympan
Submitted by Benjamin P. Masters and Joshua M. Alexander of Purdue University
Speech intelligibility benefits significantly with improvements in signal-to-noise ratio. To date, directional microphones are the only on-ear technology that has been shown to improve the signal-to-noise ratio, hence speech intelligibility in environments with spatially distributed interferers. So it is natural that Tympan, an open-source research platform for the development of hearing aids, should feature directionality. Tympan is developing hearing-aid style earpieces with front and rear microphones. We hope production completes Summer 2022. The Purdue team used our prototypes in their work.
For the ASA 2021 Design Challenge, Benjamin Masters and Joshua Alexander of Purdue University have measured the performance of the Tympan Rev-E and Tympan prototype earpieces, focussing on the directionality characteristics of the device. Meet KEMAR, the manikin for hearing aid testing and R&D (picture). It has been designed with median human adult dimensions, and the ear simulation matches the acoustic response of the average human ear. KEMAR was fitted with a Tympan and directional earpieces and subjected to a series of experiments in a semi-reverberant sound field.
To demonstrate the functionality of Tympan’s directionality features, the researchers changed algorithms for the rear and front microphone of the left earpiece. In particular, the relative delays and gain levels between the two microphones were manipulated to optimize directionality in the forward-facing and rear-facing directions. This can be expressed by the Articulation Index weighted Directivity Index (AI-DI). The AI-DI applies weightings to particular frequency bands to assign more weight to frequency bands that have the most importance for speech intelligibility. A higher AI-DI corresponds with a better performance.
By setting the sampling rate to equal the travel time of a sound wave across the two microphones, the researchers set out to create a cardioid response pattern (picture). In a free field, this directional response appears as a heart-shaped pattern with a much greater response for sounds arriving from the forward-facing direction (right of the picture) than sounds arriving from the rear-facing direction and sides (left of the picture). The researchers further optimized the directionality of the two microphones by adjusting their relative gain. As a result, they achieved relative AIDI improvements of around 3 to 4 dB on the manikin, consistent with published research on actual hearing aids.
Tympan, as a portable device, allows for experimenting with these concepts in real-world environments. A living test subject can change preferences such as delay time, gain, and sample rate on the fly and adapt the directionality to changing real-world situations. This holds great potential for future improvements of hearing aids. The researchers are interested in exploring future applications such as automatic directional switching, adaptive directivity patterns, binaural signal processing, and beamforming.
About the TYMPAN ASA2021 Design Challenge
During the ASA (Acoustical Society of America) conference in June 2021, Tympan hosted a design challenge: What is possible with the Tympan?
10 exciting new applications were submitted and presented at the following ASA conference: Enhancements of hearing aids, spatial acoustic processing and smart earphones and much more. Stay tuned if you want to learn what is possible and to keep track of future developments.