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HA Components & Circuitry (5)
Terms in this set (47)
basic HA components
basic HA components
converts acoustic energy to electrical energy
enhances the weak electrical signal and increases the voltage of it; provides gain
takes the enhanced electrical signal and converts it back into an acoustic signal
name the parts of the HA
e. amplifier or circuit
f. volume control
what does M-T-O stand for
microphone, telectoil, off
supplies power to the amplifier
converts magnetic energy to electrical energy
allows user to increase/decrease the over-all level of the sound after the amplifier
volume (gain) control
T/F: many newer HAs have manual gain control
converts electrical energy into acoustic energy
delivers sound from the receiver to the earmold; may be filtered to further adjust the signal to match the HL
sound hook (tonal tube)
2 types of microphones
2 types of digital circuits
2 types of batteries
2 types of audio streaming
what is the most important component of a HA? why?
the mic; the quality of the sound being delivered to the HA wearer depends on the quality of the sound at the input of the aid
what is the evolution of microphones? (4)
-carbon mics (1900s)
-crystal mics (1930s)
-ceramic mics (1960s)
why is the electret microphone still used? (a lot of these)
it is resistant to chemical changes; more durable (resistant to shock damage), less sensitive to temperature differences than magnetic mics, greater freq response, flatter with a slight low freq cut, smaller diaphragm mass results in better sensitivity, more power, magnetic field not involved so mic and receiver can be mounted closer together, efficient, less costly, smaller size, lower internal noise than past microphones (carbon, magnetic piezolectric)
what is the newest type of mic that may replace the electret
how does an electret microphone work?
-An inlet port allows an acoustic signal to enter.
-Signal causes movement of diaphragm (a thin plate with a metallized surface that moves relative to the back-plate).
-The back plate has holes on it and it is coated with electret, which has an electric charge.
-When sound pressure waves move the diaphragm relative to the backplate, the electrical force between the charges varies and the sound wave is transduced into electrical voltage.
a microphone that provides lower power consumption, good sensitivity, high SNR at about half the current consumption, has a lower mass of diaphragm which makes it less susceptible to vibration; & also the materials used are less impacted by environmental conditions
MEMS microphone (mico-electronic-mechanical-system)
what are 5 potential microphone problems
-sensitivity to vibration
what are 2 consequences of microphone vibrations
-vibrations can be amplified to result in annoying sounds heard by the wearer
-receivers create vibrations as well as sounds- these can be amplified and pass through the receiver into the ears
typically due to poor construction or damage; mic or receiver out of place or displaced
creates pressure fluctuations (turbulence) which are converted to sound by mic- noise dominated by lows and mid freqs
what is the purpose of directional mics?
to increase the SNR so there is easier listening in noise
-microphone that has 1 inlet port or equal between ports; picks up sound equally well in all directions
-what are these good for?
-good for quiet environments and children
microphone that has 2 inlet ports; suppresses noise coming from a certain direction while maintaining good sensitivity to sound arriving from another
with directional mics, where is hearing best?
from 45 degrees while on head
difference between 1st and 2nd order mic arrays
-1st order: 2 ports leading to same mic or 2 separate mics; signal from 1 mic/port is subtracted (mechanically or digitally) from the other mic/port
-2nd order: 3+ mics that make it possible to have "super directionality" but this is more theoretical than clinical
with 1st order mic array, one mic with 2 ports can cause a ____ delay
with 1st order mic array, two mics can cause an ____ delay
used to characterize the sensitivity of a directional mic to sounds from different azimuths
time taken for sounds arriving from front/rear to get from one inlet port to the other in a directional mic
external delay/mic spacing
use of an acoustic damper or resistor in the rear port or electrical delay of output from rear microphone in a directional mic
what happens to the polar plot as you increase mic/port spacing?
you increase sensitivity to sounds from behind and reduce sensitivity at sides (moves from cardiod to supercardiod to hypercardiod)
what happens to the polar plot as we increase internal time delay?
less sensitivity to back, more from rear- greater cancellation of rear energy (back to cardiod)
what happens to the polar plot when 2 omni-directional mics are matched in gain and phase gradually become mismatched?
increased mismatch between mics reduces directionality
frequency response of electret mic
frequency response of electret mic
what is the frequency response of the electret mic
essentially a flat frequency response with intentional roll off in the lows in order to reduce sensitivity to low frequency sounds that often surround us
resonance occurring between the air in the inlet port of the microphone and the volume of air next to the front of the diaphragm that results in a peak in the frequency response around 4-5 kHz
explain why when looking at the frequency response of the electret mic there is a low frequency roll-off
-A LF wavelength can be greater in length than the physical distance between two mic/ports on a HA
-The DM becomes minimally sensitive for the LFs because there is little net movement of the diaphragm; the wave forms are not detected
-Subsequently: LFs are underamplified, sound quality described as tinny
low frequency roll off is less of a problem for open/closed fits. why?
open fits; because LFs are arriving acoustically anyway
2 ways to compensate for low frequency roll off
-adding in LFs or employing split directionality
-involves matching the LF response in DIR mode to that in OMNI mode
-what is the problem with this?
-higher internal noise when too much compensation is employed to overcome the LF roll-off
-results in increasing the amplification for internal mic noise
OMNI mic has an internal noise of ___ dB SPL; DIR mic, with dual mic set up is ____ dB higher
28 dB SPL; 3 dB
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