HA components
function of microphone and types of microphones
modern mini electret mic, directional mic, dual mic
how is sensitivity defined
ratio of size of output voltage compared to input pressure
what three things can an amplifier do
2. make current larger w/o affecting voltage
3. make current and voltage larger
describe peak clipping
when amplifiers are near max output and the input signal is increased, the signal will distort by peak clipping which cuts off the top of signals to no longer be a sine wave
compression amplifier
change amplification when input level changes (an amplifier that turns down its own gain as the input from the amplifier increases)
function and types of amplifier
compression amps, analog and digital
makes small electrical signal into a larger one
characteristics of microphone frequency responses (4)
1. determines frequencies of energy that are transduced
2. most often flat
3. can include low cut or acoustic resonance of 5 dB somewhere b/t 4-10 kHz
4. imperfections such as random electrical noise, vibrations, internal feedback, wind noise
function of transistors on integrated circuits
allows analog amplifier to control a current by a smaller current or voltage
what happens when peak clipping occurs
it creates distortion products: frequency components at the output of the signal that were not there at the input
how do frequency responses of BTEs with 3 inches of tubing and that of ITE, ITCs, or RITEs compare?
BTE - high acoustic output at port compared to air in tube, tube open at one end, closed at other, tube has resonances at 1, 3, 5 kHz
ITE, ITC, RITE have resonances at 2.2-4 and 5 kHz
function and types of filters
unsure
directional microphones
suppress noise coming from the back
function of diodes on an integrated circuit
allows current to flow one way, but not the other and sense the size of signals
Harmonic Distortion
degrades quality of speech and other signals, decreases speech intelligibility
when do resonant peaks become a problem for patients? what can be done about them?
provide boosts in sounds, interferes with speech recognition, pts complain when peaks are 6 dB more than smooth curve joining the peaks, may interfere with electroacoustics of the HA, dampers can smooth the peaks out
function and types of receivers
mini loudspeakers
omnidirectional microphone
pick up sounds from 360 degrees
function of capacitors on integrated circuits
used to make filters, can be very small and fit into integrated circuits, but can't exist on their own
amplifiers in analog hearing aids
several amplifiers in a chain with the last amplifier in the sequence using most of battery current
How a telecoil works as an input transducer
small coil of wire that produces a voltage when an alternating magnetic field flows through it, telephones and other devices have a magnetic field surrounding them that has the same waveform as the signal, not all HA have telecoils but most high power BTEs do (old have MTO switch, new have telephone program)
function and types of telecoils
unsure
adaptive directional
switch b/t fixed points based on location of signal, usually speech, and the noise
what are circuit boards
electrical connections printed on them, can be made of fiberglass or plastic, provide connections between discrete components, assist in making of HA
amplifiers in digital hearing aids
use a pre-amplifier prior to the analog-to-digital computer with an output amplifier later in the process for the greatest power following the digital-to-analog converter
How do remote controls work? Four ways they can send signals?
Secondary devices that allow user to change the way the HA functions w/o touching it
infrared involves controls transmitting info, ultrasonic controls emit a very HF sound that is picked up by a HA mic, radio waves use electromagnetic radio wave transmitted by remote and received by a small aerial within HA
magnetic reduction sends the control signals by creating a magnetic field at a frequency above the audible range