Music to Ears in Hearing Impaired: Signal Processing Advancements in Hearing Amplification Devices

Abstract

Music is not just a complex sound encompassing multiple tones but an amalgamation between the harmonicity of the spectral components and its temporal relationships. Music perception involves complex auditory processing, starting at cochlea wherein fundamental pitch, duration, and loudness are encoded in the tiny hair cells of the inner ear. Derailing of the hair cells causes hearing loss, which in turn affects the spectrotemporal resolution. The offshoot of the hearing loss at the cochlear level translates physiologically into deficits in the extraction and coding of the pitch at the brainstem level, which in turn impairs pitch and temporal perception at the cortical level. Thus, hearing loss adversely affects perception of pitch, temporal, and loudness, all of which compound as difficulty appreciating the normal aspects of music perception. The contemporary solution FOR improving audibility and perception of speech sounds in the hearing-impaired population are the use of hearing aids and cochlear implants. Although these devices are primarily meant to amplify speech sounds, their utility is also advocated for music perception. Digital hearing aids employ sophisticated signal processing techniques to improve the perception of speech sounds. However these techniques do not, in music processing, and are not an alternative to human cochlea. The multichannel amplitude compression, used in hearing aids to improve audible range of loudness levels, can cause distortions in the temporal envelope of sounds resulting in poor quality for music perception. Additionally, fast-acting compression circuitry used in the modern digital hearing aids causes more temporal smearing (compared to slow-acting compression), adversely affecting music perception. The limited input dynamic range and higher crest ratio in AD converters of hearing aids fall short of processing live music. Unlike hearing aids, cochlear implants work on the principle of electrical stimulation. This auditory prosthesis processes the incoming sound and delivers its electrical output directly into the auditory nerve bypassing the cochlea. Though it is more sophisticated and advanced than hearing aids, it was developed to improve speech perception rather than music perception. A cochlear implant uses “N” number of electrode channels situated along the human cochlea to deliver its output. Since the partition along the human cochlea codes various frequencies in sounds (place coding) which cannot be matched by a surgically implanted electrode array, the cochlear implant users experience difficulties with pitch perception. The rate of stimulation used by the cochlear implant may not be higher enough to deliver the higher harmonics of musical sounds. The above discussed physiological limitations of hearing loss and technological limitations of hearing amplification devices on music perception are elaborated in this chapter. The book chapter also features comprehensive discussion on the advancements in signal processing techniques available in hearing amplification devices (hearing aids and cochlear implants) that can address these shortcomings.