The study of the acoustical energy in human speech and voice communication has historically been restricted to the frequency range below ~5 kHz because that is the range the typical conventional hearing aid is capable of transmitting. Evolving research has begun to evaluate the benefits of incorporating higher frequencies (>~5 kHz) with the results suggesting that a full spectrum of sound may deliver more natural sound and improved speech understanding in noise.
In 2003, Moore and Tan1 evaluated the perceived naturalness of music and speech based on the frequency bandwidth of the signal. Normal hearing subjects were exposed to speech and music stimuli that were band-limited at various high and low cut-off frequencies.
The data yielded these observations:
Recent data by Levy and colleagues2 highlights another potential benefit of high frequencies: improved speech understanding in certain complex listening situations. This study evaluated the benefit of extending the audible frequency bandwidth out to 10 kHz for both normal and hearing impaired subjects.
Using the Hearing in Speech Test in both an asymmetric and diffuse-noise set-up (Figure 2), Levy et al. found that extending the bandwidth from 4 kHz to 10 kHz significantly improved the participants’ speech reception in the presence of competing talkers.
These results demonstrate that the addition of high-frequency information above 4 kHz can improve speech understanding in certain situations with spatially separated sound sources, a characteristic of many difficult listening situations. This shows a potential benefit of broadening the frequency range of amplification delivered to patients with hearing impairment.