midterm 2 Flashcards
how does the microphone transduce an acoustic signal to an electric signal
-acoustic energy pushed and pulls air molecules that create a sound wave
-hits the microphone, diaphragm and all energy creates compression and rarefaction movements
-microphone uses this to transduce the acoustic signal into an electrical current that is identical to that acoustic signal
piezoelectric microphone
had all the components of a carbon microphone however there were no carbon balls
-behind the diaphragm was an electrified crystal
piezoelectric effect
when you compress or distort the electrified crystal, it will produce the +/- voltage needed to create an electrical signal
-as the crystal is altered, it creates voltage
electret condenser microphone
contains a backplate and microphone, the backplate is coated in a material that would allow it to hold its charge as long as it was not damaged
-we do not need to rely on it charging every time
how does the electret condenser microphone work
sound in, pushes the diaphragm, alters the backplate, this back and forth direction creates the electrical backplate activation
-relies on tension of the diaphragm and the backplate staying charged
MEMS microphone
contains a diaphragm, that is a disc, and a backplate that is now free changed with a charge pump
-charge pump recharges the electric field between the diaphragm and backplate to maintains sensitivity
microphone sensitivity
how much output results from a particular input sound pressure is referred to as sensitivity
both electret and MEMS microphones are sensitivive to a collection from around __________________
100 Hz to 15,000 Hz
explain acoustic resistance and how it appears within microphones
a barrier that can physically change the signal and has the ability to smooth the peaks and valleys
-there are small screens placed over the sound entry port that increases the resistance, these need to be cleaned often
anything that causes the ____________ to move will transduce into an electrical signal
diaphragm
acoustic noise
motion of gas molecules within the air that cause small pressure changes of the diaphragm
-occurs externally
-wind noise is a good example
electrical noise
small vibrations from internal circuits that can cause the diaphragm to move
-occurs internally
what is the acceptable intensity of a mics internal noise floor
~25 dB
noise floor occurs within ________ frequencies and _____________ as the frequency increases
low ; decreases
what is the dynamic range for analog HA’s
115 dB SPL
-collects signals up to 115 before input distortion occurs
what is the dynamic range for digital HA’s
96 dB SPL
-collects signals up to 96 before input distortion occurs
front end distortion
occurs when the collected/incoming signal exceeds the microphones dynamic range
-can be planned (acoustical effects) or unplanned (naturally exceeds limits)
why do digital Ha’s have a lower microphone dynamic range
due to the usage of analog to digital converters (ADCs), which uses a 16 bit that supplies a 96 dB dynamic range
-leads to peak clipping, causes distortion due to there being multiple points being stimulated in one area of the basilar membrane
how is front end distortion managed in digital HA
digital HAs have the ability to shift their dynamic range to fit the listening situation
-when you raise the louder end, you will raise the quieter end
signal to noise ratio (SNR)
difference in volume level between the desired signal and the undesired signal
-signal minus the noise
describe the impact of microphone distance on SNR
as the distance increases, the SNR decreases
-you have increased SNR when the microphone is closer
-ideal microphone sensitivity is within 6 feet
what frequency range is associated with undesired background noise
from 1200-1500 Hz
-above this there is no noise that interferes with speech
null
point of maximum attenuation
-microphone turns off
polar plot
show sensitivity and attenuation at different angles, a 2D image that shows microphone sensitivity
bi-directional polar plot
collection from front and back
-attenuation from 90 and 270
-used in only specific situations
carotid polar plot
collecting sounds from the front
-attenuation from the back
-there is a null at 180
-directivity index of 4.8 dB SNR
super carotid polar plot
collecting from front and back
-attenuates at the sides
-null at ± 125
-directivity index of 5.7 dB SNR
hyper carotid polar plot
collecting a lot from front and some in the back
-attenuation from sides
-nulls at ± 110
-directivity index of 6 dB SNR
function of a directional microphone
-there are two microphone ports (a read one and a front port)
-if sound comes from the rear, it will hit the rear microphone before the front microphone which allows sound to go in on opposite side of the diaphragm
-as the sound goes in, it hits the back port, heads to the diaphragm before the front port does which causes out of phase to occur
how does a directional mic use in phase signals
in phases signals move the diaphragm and produces an analog electric signal
-arriving at the diaphragm at the same time
how does a directional mic use out of phase signals
out of phase signals stop the diagram from moving, so the analog electric signal isn’t produced
-cancellation occurs and no sound will be produced
-arriving at the diaphragm at opposite times
internal delay
a calibrated electronic delay designed to the rear mic which is subtracted from the front mic ouput
external delay
remains the same, being based on position of the microphone on the external aspect of the HA
what are directional mic limitations
-real world benefit will always be less than in lab
-DI reduces with increased vent size
-directional mics are less effective for brief or moving sound sources
-benefit will be dependent on users pragmatic use
-visual cues always enhance understanding
directional mic roll off
low frequency output is reduced
-low frequency sine waves are wide and more likely to arrive out of phase from each other
-when they are turned on, there is an attenuation of low frequencies
-this is just a limitation of directionals
-if the LF suddenly drops, there is the reduction in volume
what determines the amount of directional mic roll off
spacing between mic ports
-closer the ports, the more LFs are attenuated
when directional electret microphones stop working, this is due to them falling out of ___________
calibration
explain microphone sensitivity and phase characteristics
the front and back mics are carefully calibrated
-these ports must be positioned parallel to the floor to maintain the external time delay
what occurs to directionality when the microphones are not parallel to the floor
the travel time is calibrated and if not parallel, there is a shorter distance and will take less time to travel as well as being out of calibration
microphone drift occurs when ….
there are high temps (reduces sensitivity), there is moisture (damages the tension ring), and cerumen/debris gets into the mic port
induction
electromagnetic signal moves a magnet that is within a copper coil which transduces an electromagnetic signal
explain the details of induction
-tight copper wire temporarily store energy in the form of a magnetic field
-instead of collecting sound through microphone, induction coils pick up electromagnetic sine waves coming from telephones or telecoil loops
-electromagnetic signal pushes/pulls the magnet thats located inside a tightly wound copper coil
-electromagnetic signal is transduced to an analog electrical signal
how does a telecoil loop work
it uses the induction principle to transduce the electromagnetic signal to an analog electrical signal
-the microphone will turn off and picks up the telecoil
-will only be able to hear what is coming through the telecoil
benefits of the telecoil
low cost, does not require an external power source, eliminates feedback with telephone use and improves SNR on the phone or inside of a room loop
what is the relationship between telecoil and position of the coil
the highest signal will be perpendicular with that of the signal that is fully lost
-if full intensity is up and down, then silence will occur from left and right
field effect transistor (EFT) pre amp
designed to take a signal from the microphone and increase the amplitude
-increases amplitude within initial analog electric signal
transistor based amplifiers
plural due to there being many amplifiers within the HA
-adds gain to an input signal increasing the amplitude
-any input before tone control is affected
output amplifier (final amplifier)
ensures that an additional gain is added to output level to ensure it gets transduced to an acoustic signal
-drives the signal to achieve the highest possible ouput
linear amplification
adds the same amount of gain to all input levels, for every input there is the equal amount of gain added
-1:1 ratio
compressor
level detector control circuit that automatically adjusts the amount of gain an amplifier adds to the input signal as it gets louder
-will detect when to start working or change
what are the two types of compressors
output and input compressors
output compression (AGC-o)
activates compressors at the output stage when the TK is set to a high level and amplified signal is louder than the threshold
-after volume control
-before it is sent to the receiver
input compression (AGO-i)
activates compressors at the pre-amplifier when the TK is set to a low input level and the incoming signal is louder than that threshold
-before volume control
-acting on incoming signals to sustain a constant ouput amplitude
what are the 4 components that affect compression
threshold kneepoint, release time attack time and compression rate
threshold kneepoint (TK)
point of activation for the compressor to begin working
-think of it as an on off switch, it has compression for input and compression for output based on where it is
-the signal for the compressor to alter or begin
attack time (AT)
time it takes for the compressor to activate and apply compression when the level crosses a TK
release time (RT)
time it takes for the compressor to deactivate when the level falls below the TK
fast attack is best suited for ____________, slow attack is best suited for ______________
sudden loud sounds ; conversational speech
fast release time is best for ______________ ; slow release time is best for _______________
brief, intense sounds ; longer intense sounds
pros of fast acting compression
-adapts quickly to new input levels
-sudden loud sounds stay below uncomfortable loudness levels while soft sounds remain audible
-soft speech sounds remain more audible after loud vowel sounds
cons of fast acting compression
-fast ATs applied to soft signals compresses peak amplitudes of the spectral envelope significantly changing its shape
-appearance of speech envelope changes as temporal nuances are reduced
-compressed speech envelope no longer matches an individual auditory memory so more working memory is needed to interpret the temporal characteristics of this new speech signal
-patients with lower cognitive abilities benefit from slower ATs
pros of slow acting compression
-preserves original spectral envelopes shape for easier processing for those with poor working memory severe HL
-promotes naturalness of sound
-decreases listening effort
-preserves interaural differences between ears
cons of slow acting compression
-soft sounds produced immediately after louder sounds may be inaudible because output doesn’t rise fast enough
compression ratio
ratio of gain added to the input when compared to the output
-bigger numbers are more flat lines
-smaller numbers are more closer to linear lines
harmonic distortion
presence of frequency component in a HAs output that were not present in the input signal
output limiting compression (AGC-o)
compressor detects excessively loud amplitudes after signal amplification, activates at loud levels
-compression is applied to a loud output signal to protect from overamplified sounds
output limiting compression limits ______ of HA to avoid loudness discomfort
MPO
output limiting compression reduces __________ but does not eliminate it
distortion
differentiate peak clipping and output limiting compression
with peak clipping, this abruptly changes the signal and makes them square at the top while OLC compresses the signal to fit within the threshold
wide dynamic range compression (WDRC) (AGC-i)
acts to expand the dynamic range where more gain is added to soft signals for audibility and less gain is added to moderate signals to maintain comfort and minimal gain is added to intense sounds so they are perceived as loud without causing discomfort
-improves audibility and restores loudness perception
WDRC ________ the dynamic range by allowing more access to sound before loudness discomfort is reached
expands
WDRC theory
cochlear damage results in sensitivity loss with soft sounds, with little or no loss of sensitivity for intense sounds
-soft input signals require more gain then intense sounds
-frequency response should change with varying input levels
bass increase at low level circuit (BILL)
in loud environments, low frequency output is reduced to alleviate the loudness of the noise and upward spread of masking can occur
-TK around 50 dB
-improves understanding in noise
-low frequency gain is lowered with louder sounds
treble increase at low level circuit (TILL)
when the input is quiet, high frequency gain is boosted
-improves intelligibility of speech by adding more high frequency gain to soft input signals
-high frequency gain decreases with louder sounds
k-amp circuits
analog circuit that makes the signal more clear
-lots of gain to soft sounds, no gain to loud sounds
shifting the TK down (making smaller) ___________ the output of soft sounds
increases
shifting the TK up (making bigger) ____________ the output of soft sounds
decreases
what are some practical reasons to adjust the TK
if the patient can hear and perceive soft sounds as being soft, lowering would be beneficial however if the patient cannot hear those soft sounds they would benefit from shifting the TK up
expansion
rapidly adds gain to very soft input signals until the first TK, which lower output of those signals
-CRs lower than 1:1
-manages intensity of very soft input signals and circuit noise
frequency shaping bands
frequency controls that correlate to a certain range, which are used to add gain to a narrow range of frequencies based on the patients threshold
-will not change the CR
compression shaping channels
allows us to control the different variables of the input and output within our curve
-will change the CR
