Here we will try to help you understand your hearing test results.

First, a quick understanding of what ‘sound’ is.

Sound is a pressure wave and is the result of molecules impacting upon each other, caused for example by objects hitting each other (such as a hammer onto an anvil) or from objects vibrating (such as our vocal chords). Sound therefore requires a molecular medium to pass through, and the sound is heard until the energy fades out (this fade through air is halved every doubling of distance and is known as the inverse square law). Sound can vary in loudness and pitch.

Sound loudness is measured by the decibel scale, defined as the decibel sound pressure level (dBSPL)

When we test hearing we use ‘puretone’ sounds which are frequency specific and contain no harmonics. They are by definition ‘pure’.

Many decades ago hearing tests were carried out on several hundred young adults with anatomically healthy ears by measuring the faintest level of puretone signal each could hear, measured in dBSPL. This then gave an average minimum threshold of hearing for people with good hearing in which to measure other people against. However, the natural ear actually hears different frequencies at vastly different pressure levels – this can be shown by ‘Equal Loudness Curves’ (aka Phon curves) shown here:

Note 1: This graph is based on subjective comparison of frequencies based around 1kHz. So whilst 3kHz is less than zero, it just means we can hear 3kHz better than 1kHz (due to the natural amplification characteristic of the shape of the ear) – we cannot possibly hear better than Zero dBSPL (zero sound!). The graph shows that we can hear 3kHz at a much lower sound pressure level than say 20Hz.

Note 2: As shown in the graph above, young adults with healthy ears can typically hear between 20Hz (very low pitch) to 20,000Hz (very high pitch), but as we grow older this range narrows, especially from the high frequency end. Hearing tests are typically limited between 250Hz-8000Hz being that this is the range of speech.

It would be quite confusing to use a graph like this to base every other test upon, so a flat graph called an Audiogram was produced (shown below). This shows the threshold of sound to be ‘zero decibel hearing level’ (dBHL) – defined as the average minimum level of sound which an 18 year old with anatomically healthy ears can hear – and this average perfect is what we compare ALL hearing tests to (note it is possible to hear better than 0dBHL, as it is an average figure). The graph also shows the average upper threshold of hearing where sounds are considered uncomfortable (but not painful) – which is on average 120dBHL. (Note: the graph is always upside down to represent a loss).

Hearing test results can vary wildly, but most people suffer from high frequency loss caused by hair cell damage in the cochlear (the inner ear / nerve centre).

A typical test result is shown below, on an Audiogram which also shows where typical speech sounds occur.

This audiogram shows a client with good low frequency hearing and mild to moderate high frequency hearing loss.

The circle on the left shows that the client is hearing louder resonating speech sounds quite normally (b’s, d’s m’s, o’s etc), but the circle on the right shows they are missing the quieter sharper sounds (s’s, k’s, t’s, etc) and these parts are crucial for clarity of speech. A common complaint with age / nerve related hearing loss is that people just mumble – they also complain they cannot hear when there are many people speaking and that background noise is terrible.

In Lesson 4 we explain why people struggle to hear in noise – even when wearing their hearing aids.

In addition to the above it is important to check a patients upper loudness levels, also called uncomfortable loudness levels, before fitting hearing aids. This is to check they are not suffering from ‘hyperacusis‘  – a condition where people are very sensitive to loud sounds. If a patient has reduced loudness levels of any level it is worth building this into the set-up of the hearing aids so that we are not over amplifying loud level sounds.

In this diagram we can see some bracket symbols along the 95dBHL line – these represent the upper levels of this patient.

The average normal upper loudness level for a healthy young adult is 120dBHL – so the dynamic range of hearing of a young adult, on average, is 0dBHL to 120dBHL across the frequency range.

But, for this patient we can see the dynamic range of hearing varies across the frequency range and at 2kHz is only 45dBHL-95dBHL – less than half what it should be and greatly reduced from both ends. It is imperative to program hearing aids with both readings so that the automatic volume adjustment works to the correct dynamic range of hearing. In Lesson 3 we see how hearing aid channels allows us to do this across the frequency range.