Acoustics and Psychoacoustics Applied - Part 2: Audiometry and psychoacoustic testing

7.2 PURE-TONE AND SPEECH AUDIOMETRY
In this section, a number of acoustic and psychoacoustic principles are applied to the clinical measurement of hearing ability. Hearing ability is described in Chapter 2 and summarized in Figure 2.10 in terms of the frequency and amplitude range typically found. But how can these be measured in practice, particularly in the clinic where such information can provide medical professionals with critical data for the treatment of hearing problems?

The ability to detect sound and the ability to discriminate between sounds are the two aspects of hearing that can be detrimentally affected by age, disease, trauma or noise-induced hearing loss. The clinical tests that are available for the diagnosis of these are:

• Sound detection: pure-tone audiometry.
• Sound discrimination: speech audiometry.

Pure-tone audiometry is used to test a subject's hearing threshold at specific frequencies approximately covering the speech hearing range. These frequencies are spaced in octaves as follows: 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz and 8 kHz. The range of sound levels that are tested usually start 10 dB below the average threshold of hearing and they can rise to 120 dB above it; recall that the average threshold of hearing varies with frequency.

A clinical audiometer is set up to make diagnosis straightforward, and quick and easy to explain to patients. Because the threshold of hearing is a non-uniform curve and therefore not an easy reference to use on an everyday basis in practice, a straight line equating to the average threshold of hearing is used instead to display the results of a hearing test on an audiogram. A dBHL (hearing level) scale is defined for hearing testing, which is the number of dBs above the average threshold of hearing.

Figure 7.19 shows a blank audiogram which plots frequency on the x axis (the octave values between 125 Hz and 8 kHz inclusive as shown above) against dBHL between -10 dBHL and +120 dBHL on the y axis.

FIGURE 7.19 A blank audiogram.

Note that the dBHL scale increases downwards to indicate greater hearing loss (a higher amplitude or greater dBHL value needed for the sound to be detected). The 0 dBHL (threshold of hearing) line is thicker than the other lines to give a visual focus on the average threshold of hearing as a reference against which measurements can be compared.