Audiology

Hearing Sensitivity

The human ear has an extremely wide perceptual dynamic range. The lower limit of hearing where sound is just detectable is referred to as the threshold of hearing (also known as absolute threshold or absolute sensitivity). The upper limit of hearing where sound begins to become uncomfortably loud is referred to as the threshold of discomfort (or uncomfortable loudness level). In quantitative terms, the difference in level between these two extremes is of the order of 10 million times. The human ear can hear single frequencies of vibration from around 20 to 20,000 hertz (Hz), although the upper limit, in particular, reduces during the natural ageing process.

The general relationship between the dynamic range of hearing and frequency has been well understood for many years. Studies measuring the minimum audible level of hearing have been made with stimuli presented to each ear separately or both ears together (usually via an earphone and loudspeaker, respectively). The results from the two methods are similar, but not identical, and show human hearing to be generally most sensitive between 500 and 10,000 Hz. The typical values obtained in a group of young healthy individuals, at individual frequencies, are used as the baseline reference level to which listeners with a suspected hearing impairment can be compared.

The Pure Tone Audiogram

The most widely used assessment procedure in clinical audiology is known as pure tone audiometry. The listener's hearing threshold level, in decibels (dB), is plotted on a chart, known as a pure tone audiogram, with hearing threshold level plotted on the ordinate as a function of signal frequency on the abscissa. The conventional clinical audiogram plots hearing level with low values (normal hearing) at the top of the chart and raised levels closer to the abscissa. Therefore, raised hearing levels are plotted lower on the pure tone audiogram chart (see Figure 1, next page).

The reference baseline is called audiometric zero and represents the zero decibels hearing level line on the audiogram chart. If, for example, a listener's hearing threshold level for a particular signal is 60 dB, this means that the listener's hearing threshold is 60 dB higher than the median value obtained from a group of normal healthy young persons. Not every healthy young adult has a hearing threshold level that falls on the zero decibels line, however, and the range of normality is generally taken to be ±20 dB of the zero line.

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