Hearing Aid Components Flashcards
Microphones
Microphones are called transducers because they convert one form of energy into another.
They convert sound energy into electrical energy.
The relationship between the input SPL and the output voltage is known as the sensitivity of the microphone
They come in different sensitivities and sizes but are basically the same
The ELECTRET microphone
Since the 1980s
Soundwaves enter through inlet port, hit thin metal diaphragm, causing it to move up and down. When the diaphragm is forced closer to the ELECTRET backplate there is a flow of electrical charge. This charge leaves the backplate and flows through the internal circuit and onto the amplifier.
Therefore sound energy is converted into an electric current.
The back plate is coated with a Teflon coating called Electret. It has a permanent electric charge. It has an excess of electrons on one side and a shortage on the other. This causes opposite electric charges on the diaphragm and backplate.
As the diaphragm moves to and from the backplate there is a change in charge and causes current
Silicone is more desirable than ELECTRET because cheaper to manufacture and can be automated but does have more internal noise
Problems with microphones
Limited Frequency Reponse
- sampling rate is now a limiting factor in high frequency response
Don’t Have a Flat Frequency Response
Break Down
- When exposed to sweat/wax etc. - particularly bad in CICs
Generate Random Electrical Noise
Sensitive to Vibrations
Placement can lead to Feedback
Adversely effected by Wind Noise
Directional Microphone
Only proven way to increase SNR in a way that consistently improves speech intelligibility
More sensitive to sounds arriving from certain directions than others
Two designs
- One mic with an opening leading to opposite sides of the diaphragm (first order). Acoustic damper in rear port creates an internal delay. Sound from the back arrives at the rear port first creating an external delay - Two seperate omnidirectional mics whose outputs are digitally summed/subtracted (2nd or higher order)
Both designs institute a delay on the rear mic signal
Two mics are more flexible, can vary the delay
Higher order directional microphones
dual microphones or beam formers
May have delays producing fixed polar plots or variable delays producing polar plots that change dependent on the HAs guess about the direction of noise (adaptive)
Benefit of DM decreases with reverberation - why?
SNR benefits of DMs decrease as environment becomes more reverberant because the sound comes from multiple directions
Name and Describe the Methods of Describing the Effectiveness of DMs
Directivity Index
- Difference between aid output for diffuse presentation of sound and output with sound presented directly from the front only
- the effect of directivity on speech intelligibility can be estimated by averaging DI across frequencies - because some frequencies contribute more to intelligibility than others it is desirable to weight the DI, do this using AI - every 1dB in AI-DI results in around 10% better speech intelligibility - AI-DI of omni = -1-0 - AI-DI of directional = 2-5dB
SNR 50
- Behavioural method of describing DM effect, point at which the client scores 50% on a speech test with signal from front and noise from rear
Microphone location and directivity
The microphones must be in the horizontal plane.
With DM ITE’s draw a horizontal line whilst the mould is in the ear to assist the manufacturer.
Fitting of directional BTEs: fitting the mould to the ear hook, so that the aid sits properly on the pinna and is not sloping forward or backward.
Otherwise the main directional focus will be point up towards the ceiling or down towards the floor.
Amplifier
Make a small electrical signal into a larger electrical signal
Can change the voltage, current, or both.
Total Harmonic Distortion
Amplifier specifications may include a rating of THD
When peak clipping occurs, the output is no longer a sine wave and contains components at frequencies not in the input signal.
These additional components are called distortion products
When the input is a sine wave, the distortion products occur at frequencies that are harmonics (i.e. integer multiples) of the input frequency and the process is called harmonic distortion
The power of all the distortion products is summed and expressed relative to the power of the wanted output signal component
THD levels of less than 1% are usually considered inaudible
Compression Amplifiers
Turns gain down as input increases
Important for reduced dynamic range and recruitment
Benefits of Digital Technology
Less internal noise
Ability to do complex operations in small intergrated circuits that use very little power
Greater precision when modifying the signal
Extremely flexible
Allows for many programs and paired comparisons to be made
Less battery power
Processor updates technically possible
5 main types of filters
High Pass
- more gain to HFs
Low Pass
- more gain to LFs
Band Pass
Band Stop
- uncommon in HAs
Arbitrary shaped filters
What are bands and channels?
Bands are frequency regions over which gain changes can be made
Channels are frequency regions over which signal processing changes can be made independently
Receiver
Converts the electrical signal back into an acoustic signal
The receiver acts through magnetic forces, so that when a current flows through the armature it causes it to become magnetised. It is then attracted or repelled by the permanent magnets and this causes them to move up and down which in turn moves the diaphragm up and down and sound is created
