LESSON 2 - Understanding the Hearing Test

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 know as the inverse square law). Sound can vary in loudness and pitch.

(Note: The reason there is no sound in outer-space is because outer-space is a vacuum, and has no molecules. Sound can transfer within objects in space but this stops at the edge of the object because there is then no air to then carry the sound further).

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 here - 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 vowels sounds normally, but the circle on the left shows they are missing the quieter consonant sounds, which are needed for clarity of speech. This client will therefore perceive that people are not speaking clearly - a clear sign of early stage hearing loss.

I hope this helps! - Please do add comments or ask questions.

For an explanation of the hearing test procedure click here.

All the best

John

This entry was posted on 04/11/2009 at 02:23 pm and is filed under LESSONS!, Blogroll. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response or trackback from your own site.