Abstract
For the vast majority of people, hearing aids assist in the hearing of speech. For some, music is an “after thought.” The issue of music as an input to a hearing aid is not new. Clinical audiologists and other hearing health care professionals have had long histories of client complaints that they can hear speech fine, but music sounds awful. What is it about music that is causing hearing aids to be less than stellar?
The theme of this issue of Trends in Amplification is music. Compared with speech, music can be rather intense, highly intermittent, and it may possess some interesting acoustical differences that cause hearing aid circuitry to operate in a less-than-optimal domain. Unlike speech that is complex but well defined, music can be highly variable—a violin may be speech like, whereas percussion is clearly not. The long-term average speech spectrum of all the languages of the world is well defined and is restricted to a 30- to 35-dB wide dynamic range. This is directly related to the characteristics of the human vocal tract—we simply cannot generate linguistically meaningful sounds that are in excess of 90 dB Sound Pressure Level (SPL). Music can have a low-frequency emphasis, high-frequency emphasis, narrow bandwidth, or broad bandwidth. Music can also be an impulsive or a steady-state signal. Violins are speech-like while trumpets and clarinets are entirely different. Speech has integer multiples of the fundamental frequency (because the vocal chords function as a half wavelength resonator) as do violins. In contrast, trumpets have odd numbered harmonics of the fundamental (because brass instruments and the lower register of clarinets function as quarter wavelength resonators).
In short, music can be speech like, or music can be very different than speech. So how are we able to design a hearing aid, or how can an audiologist program a hearing aid clinically, to optimize it for music, whatever the input spectrum is? These are not easy questions and the field of music and hearing aids is still very much in its infancy.
Part of the solution lies in the limitations of modern digital hearing aids. In many ways, hearing aids of the late 1990s and early 2000s were better for transducing high-fidelity music than the modern digital hearing aids of today. “Old style” analog hearing aids did not have many weak links in their design chain. One particular strength of analog hearing aid technology is that it does not require an analog-to-digital (A/D) converter. It is this A/D converter that distorts many of the more intense components of music. Once the music is distorted, no amount of software programming or reprogramming will improve things. In most cases, a hearing aid’s difficulty in transducing high-fidelity music is a technical component-related issue rather than an issue of not being able to program a hearing aid sufficiently for music.
Many of the technical innovations in the hearing aid industry are designed to circumvent the problems of a poorly configured A/D converter. It may be more than adequate for speech—even loud speech—but, in part due to the 16-bit architecture of modern hearing aids, most forms of music are transduced along with a significant amount of distortion. No adjustments that occur later in the circuitry will ameliorate the sound quality.
There are a number of clinical strategies that have been successfully used in the field (Chasin, 2012). Hearing aid wearers can reduce the volume of the car radio or home stereo, and if necessary, turn up the volume control of the hearing aid to compensate. This is essentially like ducking under a low hanging bridge or doorway. A poorly configured A/D converter design is very much like a low hanging bridge. Another clinical strategy is to place several layers of tape over the hearing aid microphones, thereby “fooling” the A/D converter into thinking that the input is less than it actually is. Again, the hearing aid wearer may or may not need to turn up the volume in order to compensate for the lower input level. And finally, if the music is quite intense, many hard of hearing people may only require several decibels of amplification (for inputs of 90 to 100 dB SPL). Simply removing the hearing aid may also be an excellent strategy when listening to, or playing, loud music.
Some of the technical innovations that have improved the fidelity of amplified music are discussed in the various contributions to this issue. These innovations range from less-sensitive microphones for the intense lower frequency components of music, to a shifting of the 96 dB dynamic range of an A/D converter into the range that has been optimized for music. A review of some research on the effects of bandwidth and amplification settings on the perceived sound quality of music can also be found in this issue.
Other topics featured in this issue revolve around the ingenious uses of assistive listening devices in conjunction with personal hearing aids while performing in live situations and factors that exist for brass players who are wearing hearing protection. Solutions that work well for one person are not necessarily those that would be optimal for another. Solutions can be as varied as the number of hard of hearing people that would like to listen to, or play, music.