Exam 2: Microphones Flashcards
how does a carbon mic work
sound wave comes in and hits diaphragm
compression of diaphragm pushes carbon balls together creating a positive voltage flow
decompression causes diaphragm to push back out making a negative voltage flow and carbon balls released
the compression and decompression of carbon balls creates a + and - analog electrical signal
what is viscosity
how easy material flows
thickness
high viscosity
stretches aperature of canal
want this for a tight fitting mold
low viscosity
runny
doesn’t change aperature of ear canal
good for earplugs or making deep impressions
what is stress relaxation
ability for ear impression to go back to shape after removal from ear canal
silicone is high stress
what is compression ratio
shrinkage over a 7 day period
low shrinkage is silicone
what is shore value
after cure hardness, stability in the impression box
low shore value is methyl
high is silicone
Describe the importance of stretching the aperture
stretching this area for a well fit earmold
if we do not do this we get an uncomfortable earmold because the skin rubs against the plastic
Define temporal resolution and the auditory processes which support it.
AS ability to detect small time related changes in acoustic stimuli over time
Good auditory temporal resolution processing is needed to understand speech in noise
temporal resolution
gap detection: spoon vs soon, identifies syllables, start or end of words & sentences
phonemic duration: can vs can’t, similar words are differentiated by differences in duration & order
temporal ordering: keep order sounds enter as (boots vs boost)
suprasegmentals: provide us with meaning, patterns of stress, intonation and rhythm
Explain binaural loudness summation. Its suprathreshold benefits and how this benefit supports hearing aid fitting strategies.
improvement in hearing threshold when compared to monaural hearing
+3 at threshold, +6 at 50dB HL, & +9 at 90 dB HL
Results in PT perceiving greater loudness with bilateral devices
Less gain is needed to reach comfortable listening levels
Can fit a PT with less loudness in order for them to hear the words
What does an IP rating indicate? What does a 6/8 IP rating suggest?
what we look at to decide is the device dust and moisture resistance
IP68
Dust light
Protects against long periods of liquid immersion
Describe the cause of the occlusion effect (OE) and associated patient complaints. Describe the degree of LF threshold loss associated with OE concerns
Common complaint is when LF threshold is better than 50dB HL
Voice sounds boomy, hollow, or in a barrel
This is caused by insufficient venting and/or insufficient canal length
Most noticeable w/ vocalizations of ee and oo
Baby jeanie is teeny tiny
Skull transduces LF energy generated by the voice. The bc signal becomes trapped in the canal
Describe two management techniques used to reduce OE
vent size: venting allows LF environmental sounds to travel directly to eardrum without amp
stabalize devie in bony canal: extend end of canal into bony portion to reduce OE vibrations
Describe the impact vent size has on high frequency output
Larger vent = less HF output you get from the HA
When HF output energy increases it can overcome the vent’s inertia and escape the canal through the vent. This creates a feedback loop limiting the gain added to HF signals
Describe the piezoelectric effect and the limits of piezoelectric microphones
what was attached to backplate was a salt crystal and if you create flow of current threw this and twisted it, the twist and bend would create a + and - electrical current flow
*same mic, have salt crystal to replace carbon balls
problem
high temp & humidity would stop the current flow - HA died quickly (>110 deg)
didn’t last long
Be prepared to explain how a microphone transduces an acoustic signal to an analog electric signal
Compare & contrast Electret microphones and MEMS Microphones how do they function? memes vs electric
know differences? why is one better than the other? explain
MEMS: more stable, doesn’t absorb moisture, doesn’t drain bettery due to smaller space bw backplate and diaphragm, stability remains even with harsh environment donditions (-40 to 185 deg)
ECM: less stable due to susceptibility of humidity, moisture, dirt, debri degrading the sensitivity of mic over years, diaphragm can absorb moisture, backplate loses charge at high temps (110), glue breaks down on the diaphragm due to environmental conditions
Define microphone sensitivity
Mic sensitivity (contemporary): ability of the mic to pick up sounds
What input frequency range can a microphone collect?
Sensitive collection from <100 Hz to as high as 15000 Hz
a mic can collect wide range of frequencies than what appears on HA’s final frequency response
true
Differentiate the causes of acoustic noise from the causes of electrical noise.
Anything that causes diaphragm to move will transduce into an electrical signal
Acoustic - gasses & air flowing around us goes into sound port and moves the diaphragm
Electrical - comes from circuits in HA that are after the mic in ha
What is an acceptable intensity of a mics internal noise floor?
Noise floor of best HA mic is ~ 25 dB SPL
Higher noise floor = more noise to PT
what is internal noise floor
This is a measurement of both acoustic and electrical noise
wind noise output is greatest where
in LF (>/= 80dB SPL depending on direction)
reduces wind using acoustic resistance
wind screen over a mic
What is the dynamic range of an analog hearing aid microphone? Of a digital hearing aid microphone?
Analog mic = 115 dB SPL (can collect input signals up to this before input distortion
Digital mic = ONLY 96 dB SPL (anything > results in input distortion)
what is mic dynamic range
loudest & quietist acoustic input a mic can collect & convert to a usable electric signal
what is front end distortion
Front end distortion occurs when sounds picked up by the mic exceeds its dynamic range
A mic can only take so much intensity before distortion occurs
Complaints are due to loud music or the user’s own voice
Why do musicians prefer analog HA’s?
better replicate original signal
they like things loud and as long as it is below 115 it is picked up and nice and clearly replicated
Distortion in analog happens with input
> /= 115 dB SPL (0-115, anything louder sounds distorted)
Distortion in digital mic happens with input
> /= 96 dB SPL (0-96, anything louder sounds distorted)
why do digital hearing aids have a lower microphone dynamic range
range difference b/w analog & digital HA’s
This is due to analog to digital converter (ADC)
Used to transduce an analog electric signal to a digital signal
16 bit ADC supplies a 0-96 dB dynamic range and louder sounds are peak clipped
\
how is front end distortion managed in digital hearing aids
Mic has dynamic range
Analog could be 0-115
Digital to 96 - starts distorting above it
If we shift the range of mic in loud situations you can collect loud sounds without peak clipping but can’t collect soft
If we shift range in soft situations you can collect soft sounds but not loud ones anymore
Move form 16 bit to 19 bit HA in other digital devices allowing_____ dB dynamic range
108
what is the ideal distance for sound collection
w/in 6 ft
what does +10 dB SNR mean
desired signal is 10 dB louder than the undesired signal
ideal listening
what is 0 dB SNR
desired and undesired signals are equally as loud
what is -10 dB SNR
desired speech is -10dB SOFTER than the undesired signal
undesired is 10 louder than undesired
what is ideal signal to noise ratio is for people with normal hearing
+2 SNR
Describe the impact of microphone distance on SNR
SNR becomes poorer as distance bw mic and desired signal increases
Closer to the mic the louder it is over the noise
Improved SNR
Further it is from the mic harder it is to hear over the noise because noise becomes louder
Reduced SNR
what is PT complaint we encounter in regards to mic distance on SNR
cannot hear spouse talking from another room
mic cannot pick up sound ideally
6 ft is furthest is can reach - shaking hands is the closest you should be with ideal amplification
What is the ideal distance for sound collection?
Ideal mic sensitivity w/in 6 ft
what is SNR
difference in volume bw desired and undesired signal
Which frequency range is associated with undesired background noise?
energy of noise is in lfs and a lot of noise that is louder than speech at 250 up to 1500 Hz
speech signal
all energy for intelligibility is in the highs
Complaint of noise comes from amp signals around 1200-1500 Hz and above this not a lot of noise masks speech above this
what is a null
refers to point of max attenuation
where mic stops collecting sound
0 deg azimuth
right in front
center
90 deg azimuth
to the right if ha is on the right
180 deg azimuth
directly behind
270 deg azimuth
to the left if ha is on the right
what is an omni directional mic
mic that collects sounds equally from all directions
what is a directional mic
mic focuses sound collection towards one azimuth and attenuates sensitivity from others
what is a polar plot
How the mic responds to sound from different directions
shows sensitivity and attenuation (null) at different angles
shows where mic collects sounds and stops collecting sounds at different points
does a null only attenuate horizontally
NO
does so up and down and around
describe cartiod polar plot
mic collects sound from the front but attenuates all sound at the rear
Looks like a heart
Collects max at 0 deg but at 270 and 90 starts to attenuate and at 180 there is a null
describe hyper-cartiod polar plot
a lot of signal collection at the front
Nulls are on the side with come collection at the rear
Max sensitivity at 0 deg, some from behind with nulls at 120 deg and 240 deg
describe super-cartiod polar plot
similar sound collection at front, back is broadened allowing more signal from back
Max sensitivity at 0, more from behind than hyper, still null at the sides
Better reduction of noise
describe bi-directional polar plot
attenuation is happening at 90 and 270 deg with sensitivity at front and back equally
Use this when driving a car to hear what is in front and behind you but not the window noise beside you
Polar pattern is a sphere on paper
omnidirectional
Has equal output regardless of orientation to sound source
omnidirectional
Can pick up a group
omnidirectional
Cannot be aimed at one source to favor one source over another
omnidirectional
Most sensitive to sound sources in the front of mic and least sensitive to sound sources directly behind
cartiod unidirectional mics
a lot of signal collection at the front
Nulls are on the side with some collection at the rear
hyper cartiod
more sensitive to sound sources that are directly behind the mic than hyper
signal collection at the front
super-cartiod
Equally sensitive to sounds from the front and back and least sensitive to the sides
bi directional
no null angle
di = 0dB
omnidirectional
looks like a circle
+/- 90 deg null angle
DI = 4.8dB
bidirectional
dipole
looks like a snowman
180 deg null angle
DI = 4.8 dB
cardioid
looks like a heart
+/- 120 deg null angle
DI = 5.8dB
supercardioid
looks like a mushroom
+/-110 deg null angle
DI = 6.0dB
hypercardioid
looks like our face scrubber
How does a directional mic use in-phase and out-of-phase signals to create a polar plot null
in phase = analog electric signal
out of phase = stops diaphragm movement & doesn’t create an analog electric signal
in phase signals
they move the diaphragm creating an analog electric signal
out of phase signals
they STOP diaphragm movement not creating an analog electric signal
Define directional mic roll-off
Happens when directional mics are turned on
Directional roll off = low frequency output is reduced
why is LF output reduced with directional mic roll off
when directional is on, attenuation occurs naturally including LF output and because they are wide and broad, LF are more likely to arrive out of phase to the mic making a loss of LF output
What causes roll off and how is it managed?
LF output is reduced causing a6dB/octave LF roll-off of intensity perceived by PT as reduced volume
fixed with equalization filters that add LF energy to replace the attenuated output signal
what is PT complaint of roll off we encounter
When PT goes into noise, background noise is attenuated but volume also drops too
This is because LF output gives us volume/loudness and when taken away voices get softer
gives us clarity & understanding
HF
gives us volume/loudness and when taken away voices get softer
LF
what is an equilzation filter used to fix roll off
add LF energy to replaced the attenuated output signal
Bass boost in softwares to counteract this natural issue with mics
If PT has normal/near normal LF hearing, do not need to add bass boost but if they have HL in LF >/= 50 then add it
would you add equalization filters if PT has normal/near normal LF hearing
no do not need to add bass boost
would you add qualization filters if PT has HL in LF >/= 50dB
yes add bass boost
what is the amount of roll off based on
spacing of the mic ports
closer mic ports =
more LF attenuation
smaller devices roll off _____ frequencies
MORE
further the mics are =
less LF attenuation
what are directional mic limitations
real world benefit DI of ~2-3 dB
Larger vent size = lower DI
directional mic is less effective with moving sound or brief and sudden sounds
visual cues help understanding
can the PT use it correctly and understand how it works
what is the importance of electret mic matching and calibration
they stop working if they fall out of calibration
sound traveling horizontally shortens time delay and directionality no longer functions
what is the importance of tubing for calibration of electret mic calibration
if the tubing or receiver length is too short or long the HA mic will not sit parallel to the floor anymore losing the external time delay
sensitivity difference > _____ dB decreases mic’s DI
1
front/bac mics have to be level within _____ deg otherwise DI decreases significantly
20
what happens in microphone drift
When something plug the port, the sound coming in from the back doesn’t reach the diaphragm which doesn’t cause an out of phase signal which means no directionality occurs
how does mic drift occur
High temps reducing mic sensitivity
Moisture causes damage to diaphragm tension ring
cerumen/debris clog mic port
Need cleaned/checked every 6 months
Electroacoustic measures are done to verify directional mic performance
why would we choose telecoil over BT
BT is expensive, drains battery life, costly
telecoil costs around $50, cheaper, already in most BTE’s just have to teach PT’s how touse it
benefits of telecoil
inexpensive
no external power source needed because magnetic field generates its own
no telephone feedback because mic is off
improved SNR on phone or in loop room
in a looped environment, what telecoil position is needed
verticle
if using telecoil for phones, what position is needed
horizontal
pick-up room loop signals
vertical telecoil position
pick-up telephone signals
horizontal telecoil position
Describe the principle of induction
Induction principle: takes electromagnetic signals and transduces it into an electrical sine wave
Electromagnetic signals move a magnet with a copper coil
This movement bw copper coil & magnet turns electromagnetic signals to a +/-analog electrical signal without adding an additional power source
MIC is turned off
how does a telecoil work
tight copper wire coil stores energy in form of magnetic field
induction coilds pick up electromagnetic sine waves that come from telephones or telecoil loops
electromagnetic signal pushes & pulls magnet in copper
signal is then transduced to an analog electrical signal
What produces electromagnetic signals?
cell phones
hardwired phones (best signals)
microwaves
flip phones
computers
moving-coil microphones — work via electromagnetic induction
dynamic mic
capacitor microphones — contain a thin, stretched conductive diaphragm positioned close and parallel to a metal back plate
condensor mic
full intensity for verticle coil
verticle
silenced horizontally
full intensity for horizontal coil
horizontal
silenced vertically
what is the purpose of a listening check
it is a subjective eval of a hearing instrument
will only pick up significant device malfunction
what are the LING 6 sounds and why are they used
ah, ee, oo, ss, sh, mm
used to test across a frequency range
how is a HA linear
If there is no change in amplitude from soft to loud ling sounds
how is a ha nonlinear
mplitude is louder for soft and softer for loud ling sounds
how do you determine a directional mic
If you turn HA 360 deg and sound attenuates behind HA
If it doesn’t attenuate from behind = omnidirectional
dual directional mics consist of
2 omnidirectional mics
what are the components of dual directional mics
External delay is the same
Internal delay is replaced with a calibrated electronic delay in an algorithm of the back mic that is subtracted from the front mic output
2 mic sensitivities HAVE TO MATCH (calibrated) at ALL frequencies for effective phase cancellation
true
How do time delays result in a directional mic?
Why do we have internal and external delays?
mild HL
AGC-i
severe HL
AGC-o
