Hearing Aids Final

A hearing aid must contain this low-pass filter to make sure that signals going into the A-D converter are indeed lower in frequency than half the sampling rate and can be handled by the chip

After sampling has occurred, each of the samples has to be represented by a number (code).
The sampled waveform has now been digitized.
The digitized code values are broken up into bits.
CD player/hearing aids use 16 bit words, which means 65,536 different numbers can be represented.

The modified numbers from the digitized code must be converted into an acoustic signal.
DAC outputs an analog voltage which in turn is fed to a receiver to make the final conversion to sound.

1K, 3K, 5K

Between the mass of air in the tube and the volume of air inside the receiver
4K

Resonance of the receiver and of the tubing
2K

Selectively pick up sounds coming from the direction of choice by changing the sensitivity of the microphones

Answer

Alternative to A/D conversion Sometimes called 1-bit processors Sigma-Delta conversion makes extremely efficient digitization without any significant fear of introducing distortion
Instead of using a multi-bit processor, the signal is compared (referenced) to a high frequency (1 bit) processor. It is compared (sampled) many times to code how the waveform is fall or rising.
The width of pulses correspond to amplitude

Answer

If the phase is different between front & back they won't cancel in directional mics.
low frequencies are canceled more actively than high frequencies

1. Loss of audibility- audibility of formants (oo and ee have similar first formants, so if the second formant is not audible, the person with hearing loss will have a hard time understanding it)
2. Loudness recruitment- Reduced dynamic range -abnormal growth of loudness -compensate for this by using compression
3. Poor frequency resolution -leads to worse signal in noise ratio -spectral contrast is reduced -the tuning is not as sharp (lose clarity) when there is OHC loss (OHCs are most vulnerable to hearing loss)
4. Susceptibility to masking -contributes to clarity of sound

Uses a special capacitor which has a permanent voltage built in during manufacture.
The electrets is a fluorocarbon plastic with a metallic coating (holds the charge)
Doesn’t require any external power (battery) for operation.
ADVANTAGES: small, sensitive (from a distance), wide band frequency response, low internal noise and reliability, insensitivity to mechanical vibrations (like walking)

Sound enters the tube and passes through a small slot in the can
Sounds pressure fills the small volume on one side of the diaphragm.
Sounds pressure in the front volume causes the diaphragm to vibrate
Backplate is coated with an electret that stores a large static charge - There are holes in the backplate that equalizes pressure, or else there will be distortion
Motion of the diaphragm near the charged backplate generates a small voltage signal on the backplate.
Signal voltage travels up a wire from the backplate and connects to an amplifier circuit inside the microphone.
Electrical output signal comes from microphone terminals.

Internal noise originates in the microphone and appears in its electrical output How loud? About 28 dB SPL
CAUSES: electrical noise from circuit and amplifier, or acoustic noise from the flow through small passages (modern HAs have entirely acoustic noise)
In today’s hearing aid microphones, the internal noise originates entirely in the flow through small passages.