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Measuring the degree of hearing impairment 

Audiologists carry out pure tone audiometry, recording the quietest pure tones from 250-8000 Hz a patient can hear to identify their hearing thresholds as the standard clinical measure of hearing impairment and establish whether hearing aids would be beneficial. Hearing aids help people with hearing impairments by amplifying sounds to make them louder and clearer.

Audiograms

The results are represented by an audiogram: a plot of thresholds in dB HL (hearing level; dB SPL adjusted for our differing sensitivity across frequencies) as a function of the standard pure-tone frequencies.

The audiogram provides a metric of basic hearing ability, helps visualise the impairment, and can also indicate certain types of hearing disorders, such as noise-induced hearing loss. One of the primary uses of the audiogram is to set the amplification of hearing aids to provide audibility for that person based on a formula (i.e., applying gain that counters that person’s hearing loss at each frequency).

Frequencies are averaged out to give an overall hearing threshold score, usually for the ear with better thresholds, which is classified into broad categories, as follows:

Mild hearing loss (20 – 40 dB) 

Moderate hearing loss (41 – 70 dB)

Severe hearing loss (71 – 95 dB)

Profound hearing loss (96+ dB)

In the following audiogram, the person has a binaural symmetrical hearing loss which would be categorised as mild at lower frequencies and severe at higher frequencies. Using the British Society of Audiology guidelines, averaging across five frequencies (250, 500, 1000, 2000, 4000) and taking the better ear, the person would be categorised as having a mild hearing loss (average 33 dB). In general, as in this example, hearing loss tends to be greater (worse) with increasing frequency for older adults and worsens with age at a rate of roughly 1 dB/year.

Figure 1, An example Audiogram.

The values of the audiogram defines how much gain the hearing aid needs to apply, with the calculation typically done by one of a group of "prescription rules", e.g. CAMFIT, NAL-NL2 or DSL.

Note that the scale of an audiogram is in “dB HL” = “dB Hearing Level”. This is not dB SPL; instead, it’s relative to an international standard such that 0-dB is “normal hearing” at every frequency. For background see Why the Audiogram Is Upside-down | The Hearing Review and The Quest for Audiometric Zero | The Hearing Review

Whilst, as the BSA guidelines note, ‘the ability to detect pure tones in a quiet environment is not in itself a reliable indicator of hearing disability and audiometric descriptors alone shall not be used as the measure of difficulties experienced' (p. 27), the descriptors provide a useful summary of an individual’s hearing ability, and research shows generally consistent patterns of deficits (e.g. difficulty hearing speech in noisy environments, difficulty hearing out melodies/harmonies in music) within each category. PTA provides a quantitative measurement of hearing impairment necessary for use in the challenges.

For further reading on hearing-aid compression, the following papers may be of interest: