Hearing Aid Frequency Response Characteristics for Music

Hearing Aid Frequency

Does a music program have a different frequency response than a speech program?

The long-term average speech spectrum (LTASS) has served as the basis for a number of hearing aid prescriptive formulae such as National Acoustic Labs (NAL-NL2) and the desired sensation level (DSL) approaches. However, there is no long-term average music spectrum (LTAMS). But maybe there should be.

While many musical instruments are similar to the speech spectrum—more low-frequency energy and less higher frequency energy—some, such as percussion instruments, can be quite different. Also, whereas speech is low-frequency harmonic energy (vowels, nasals, liquids) and high-frequency broad band sibilant energy (e.g., [s, l]), music possesses important and significant low-frequency harmonic energy AND high-frequency harmonic energy. And to complicate things, the exact amplitude of the higher frequency harmonics defines the musical instrument.

Different Instruments, Different Spectra

Figure 1 shows two spectra: 1a. one of the flute and 1b. one of the violin. Despite having identical frequency harmonics for any given musical note, the differences are in the amplitude of the higher frequency harmonics—something we do not typically see with speech. In short, while the spectra energy of music and speech can be quite similar, there are significant frequency and amplitude differences.

This is exacerbated by the fact that even within music, there are significant differences between the long-term average music spectrum for different types. Moore(1) compared and contrasted classical music and jazz music and found that “on average,” classical music had significantly more mid-and high-frequency harmonic energy than jazz. Couple that with the fact that classical music is made up of simultaneous sources playing together, jazz having fewer, and speech, in most circumstances, only one.

Hearing Aid Frequency Response Shape and Sound Level

So, how does the hearing aid frequency response differ between a “music” program and a “speech” program? Part of the answer lies in the overall sound level of music as compared with that of normal conversational speech, and part of this lies in Part 2 regarding the changes in the shape of the frequency response.

Figure 2 shows prescribed gain at 1000 Hz for a range of sensorineural hearing losses up to a moderate (60 dB HL) level for a range of input levels, for input levels of not only 65 dB SPL and 80 dB SPL (which are commonly found with medium and loud speech), but also 95 dB SPL, which is a more realistic level associated with many forms of music.

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