04/11/2009 by John Lloyd.

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

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13/08/2009 by John Lloyd.

Lesson 1

What is a ‘digital’ hearing aid?

Firstly let me explain what ‘analogue’ is:

An analogue signal uses a physical medium to convey information. So for sound, fluctuations in air pressure strike the diaphragm of a microphone which then causes corresponding fluctuations in the voltage (or current) of the connected electrical circuit. The voltage (or current) is said to be an “analog” of the sound. These voltage fluctuations are passed through an amplifier to give greater voltage which is passed through a speaker and thus the sound is made louder. Old analogue hearing aids invariably just made sounds louder, and had a volume control so the wearer could regulate the volume of the hearing aid to suit the volume of the environment they were in. i.e. They would turn it up to hear soft / moderate sounds which they struggle to hear, and turn it down in loud environments because upper loudness thresholds are typically normal (or even hyper-sensitive).

To produce a ‘digital’ signal the originating sound pressure strikes a microphone and the resulting analogue signal instead of being passed through an amplifier, is ‘digitised’ by being passed through an electronic device called an analogue-to-digital convertor, which builds up a stream of sequential numbers (binary code) which can then be stored, manipulated and then reproduced according to specific and exact requirements. This requires a micro-chip as in a computer.

To clarify:

The word digital comes from the same source as the word ‘digit’ and ‘digitus’ (the Latin word for finger), as fingers are used for discrete counting.

‘Digital’ is simply a form of processing a signal, and for this process we use binary code.

Binary code is the numerical representation of a signal using streams of only two digits; 1 and 0; where 1 = pulse present and/or high, and 0 = pulse absent and/or low.

Computer programmes then read this stream of binary code to analyse and manipulate the data to give us predetermined results.

An analogue hearing aid takes in an analogue signal (sound wave), amplifies it, and sends it out via a speaker in exactly the same form, but louder.

A digital hearing aid takes in an analogue signal (sound wave), coverts it to digital signal (binary code), modifies the code/signal, and then sends the modified signal out in analogue form via a speaker.

Computers are required to read this binary code. Digital Hearing Aids are effectively mini-computers possessing micro-chips, some of which can undertake 120million calculations per second – what we call processing speed.

The advantages of this digital processing over an analogue signal is that we can:

Give the exact amount of amplification at each frequency to suit a persons particular hearing loss (i.e. high frequency loss).
Reduce the amplification of background sounds, i.e. steady state background noise such as air conditioning, traffic, aircraft noise etc.
Use directional microphones to help raise the speech to noise ratio (we cannot completely remove sounds, as sounds do bounce off walls!)
Control feedback / unwanted whistling to a degree
Automatic volume control – as sounds get louder the amplification automatically reduces
Use frequency shifting technology to restore lost regions of sound by transposing or compressing them into audible regions of sound
Reduce echo reverberant sounds and wind noise.
Have autopilot settings so the hearing aid analyses the environment the wearer is in and changes the above settings to suit each persons preferences.

Digital technology is advancing at unbelievable rates. Microchips double in power and speed and computer memory doubles in capacity every 12-18 months – this is called the ‘cycle of innovation’.

Cameras had 5 megapixels capacity last year but 10 megapixels this year; laptops had 120gigabytes of storage last year and 250Gb this year; hearing aids have 120million calculations per second this year, so what next year ….….?

One day feedback will be an issue of the past, background noise will be much easier to manage, and Bluetooth will open up a world of hearing solutions.

We must remember though, a person with a hearing loss has damaged hearing organs, and no matter how good a hearing aid becomes, the hearing organs are still damaged. We are not replacing the hearing, we are utilising technology to maximise the potential of the residual hearing.

I do hope this helps. Please add your comments or ask any questions.

(To follow soon: Lesson 2 - Hearing Aid Channels)

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