Microphones Flashcards
What creates a soundwave?
The push and pull of acoustic energy
What is a galvanometer?
An instrument for detecting and measuring small electrical current changes
Measures back and forth changes (pos and neg)
Transduction that occurs in a mic
What is in a digital hearing aid?
A microphone
An A/D converter
A DSP chip
A D/A converter
And a receiver
What is compression?
Positive movement resulting in high air pressure
What is rarefaction?
Negative movement resulting in low air pressure
How is an analog electric signal produced?
The acoustic signal transfers its energy to the mic diaphragm creating compression and rarefaction movement
This movement is used to transduce the acoustic signal into an electric current
Identical to the original
In the diaphragm, is there a battery that adds voltage to the metal backplate?
Yes, and this energy then flows through the carbon balls
What type of signal do compressed carbon balls produce?
Positive
What type of signal do decompressed carbon balls produce?
Negative
What microphone replaced the carbon mics?
Piezoelectric
What is the piezoelectric effect?
If you twist, compress, or distort a thin electrified crystal, it produces the +/- electrical voltage needed to create an analog electric signal
Were piezoelectric mics popular?
No, they were short lived
Had adverse affects to humidity and high temperatures
What are the two current options for mics?
Electret condenser microphone
MEMS microphone
When did the electret mic start being in use?
In the 70s
How does the electret mic work?
The diaphragm is a stretched polyester disc adhered to a tension ring which is suspended over an electret plate (precharged backplate)
Does the backplate of the electret mic hold charge after exposure to an electric field?
Yes
There is no need for an extra power source
What is an electret mic’s sensitivity dependent on?
The tension of the diaphragm, and the charge maintained on the backplate
When was the MEMs mic introduced?
Recently
What is different about a MEMS mic?
The diaphragm is now a free-floating silicone disc
The backplate has a charge pump to control the charge (automatically recharges the electrical field to maintain sensitivity)
What is the stability of the electret mic?
After several years of use, moisture, temp fluctuations, dirt, and debris reduce the sensitivity of the mic
Adhesion on diaphragm degrades
Backplate loses electron charge at high temps
What is the stability of the MEMS mic?
More stable
Silicone doesn’t absorb moisture
Diaphragm is not tension dependent
Distance between diaphragm and backplate is much smaller (more efficient)
Sensitivity maintained with charge pump
Not impacted by harsh environments
What is the sensitivity range for both the electret mic and the MEMS mic?
<100 Hz to as high as 15,000 Hz
Is the microphone collection range wider than the device’s final frequency response?
Yes
Why is there a small screen placed over the sound entry port?
To increase acoustic resistance and smooth frequency response
Provides a physical barrier that changes its properties
Does the small screen over the port eventually get dirty?
Yes, that prevents sound from getting through
Will diaphragms pick up anything that will cause it to move?
Yes, it will transduce an electrical signal
Diaphragm designed to pick up on small movements
What is acoustic noise?
Random motion of gas molecules in the air cause small pressure changes of the diaphragm
What is electrical noise?
Small vibrations from internal circuits causes the diaphragm to move
Inside the hearing aid and after the mic
What is the noise floor a measurement of?
Both acoustic and electrical noise
What is the noise floor of the best hearing aid mic?
About 25 dB SPL
Do we have limits of what levels of internal noise are acceptable?
Yes
Is wind noise a big problem for hearing aids?
Yes, even at low speeds
Is output greater in the lower frequencies for wind noise?
Yes
And they can reach up to 80 dB SPL or greater depending on the wind’s direction
What are wind screens?
Screens that are placed on top of the mic ports to reduce diaphragm turbulence
Can easily pop off
Do mics have a dynamic range?
Yes
From the loudest and quietest input a mic can collect and convert into a usable electric signal
What is front-end distortion?
Distortion that occurs when a collected signal exceeds the mic’s dynamic range
What is the dynamic range of an analog mic?
115 dB SPL
Can collect input up to 115 dB SPL before distorting
What is the dynamic range of a digital mic?
96 dB SPL
What are typical complaints related to front end-distortion about?
Complaints of loud music or user’s own voice
Why is there a difference between analog and digital dynamic ranges?
Because of the analog to digital converter
Used to transduce an analog signal to an electric one
A 16-bit ADC supplies a 0 to 96 dB SPL range
Louder signals are peak clipped
What are some solutions to the reduced dynamic range for digital devices?
Some automatically shift the ADCs dynamic range to accept louder inputs when needed
Some use a 18 to 19-bit ADC allowing for a 108 dB SPL dynamic range
Is mic sensitivity limited by distance?
Yes
The SNR becomes poorer as the distance between the mic and the desired signal increases
What distance is ideal mic sensitivity achieved with?
6 feet
What is SNR?
The difference in volume level between the desired signal and the undesired signal
Why do people with hearing loss complain that it’s difficult to hear in noise?
Loss of frequency resolution
Loss of temporal resolution
Spatial hearing
What SNR do normal hearing listeners require?
2 dB SNR
Allows you to hear about 50% of what was said
When were directional microphones introduced into single mic hearing aids?
1969
What is omnidirectional?
Microphone collects sound equally from all directions
What are directional mics?
A mic that focuses sound collection towards one azimuth while attenuating sensitivity from other azimuths
What is the null?
The point of max attenuation
What is a polar plot?
It shows the sensitivity and attenuation at different angles
Shows mic sensitivity as a 2-dimensional image
Are there two microphone ports?
Yes
Sound arrives to the mic at different times
One goes to one side of the diaphragm and the other goes to the other
What kind of delay does the distance between the two mic ports create?
External delay
What kind of delay does internal acoustic resistance inside the mic port create?
Calibrated internal delay
What happens if the acoustic signal arrives to the diaphragm at the same time and are in-phase?
They create an electrical signal
What happens if the acoustic signal arrives to the diaphragm at the same time and are out-of-phase?
Diaphragm doesn’t move and no signal is created
Does current technology use dual directional mics?
Yes
What are dual directional mics?
Consist of 2 omnidirectional mics
External time delay remains the same
Internal time delay replaced by a calibrated electronic delay (rear mic subtracted from the front mic output)
Do the 2 mics in dual directional mics need to be calibrated to each other at all frequencies?
Yes, they need this to have effective phase cancellation
What is the directionality index?
The amount of attenuation provided by different polar patterns
What is the null direction for an omnidirectional polar plot?
None
What is the null direction for a cartiod polar plot?
180 degrees
What is the null direction for a supercartiod polar plot?
+/- 125 degrees
What is the null direction for a hypercartiod polar plot?
+/- 110 degrees
What is the directionality index for an omnidirectional polar plot?
0 dB SNR
What is the directionality index for a cartiod polar plot?
4.8 dB SNR
What is the directionality index for a supercartiod polar plot?
5.7 dB SNR
What is the directionality index for a hypercartiod polar plot?
6 dB SNR
Does head shadow degrade the directional mic performance by changing the polar plot?
Yes, the real-world directionality index for a mic is 2-3 dB SNR
But it’s enough for patients
Is the directionality index reduced with increased vent size?
Yes
There are more direct sounds coming into the canal
Are directional mics less effective for brief or moving sound sources?
Yes
The mics have a hard time keeping up
Will the benefit of directional mics be dependent on the user’s pragmatic use of the device?
Yes, if the patient does not understand how the device and directionality should work, they will have a harder time
Do visual cues always enhance understanding?
Yes, people tend to hear better with auditory cues
What is directional roll off?
Low frequency output is reduced when directional mics are turned on
Why does directional roll off occur?
Low frequencies are wide and broad, so they are more likely to arrive to the diaphragm out-of-phase
What does directional roll off sound like to patients?
Perceived as a reduction in volume
What is a solution for low frequency roll off?
Equalization filters that add low frequency energy to replace attenuated output signal
What is the amount of directional roll off determined by?
The distance between the mic ports
The closer the mics are, the more low frequency attenuation
Do directional electret mics stop working when they fall out of calibration?
Yes
A sensitivity difference of more than 1 dB will significantly decrease the DI
Front and back mics must be level within 20 degrees, otherwise the DI decreases
What happens when mics fall out of calibration over time?
Microphone drift
Sensitivity shifts
When does drift occur?
When high temps reduce sensitivity
When moisture damages the diaphragms tension ring
When cerumen and debris get into the mic port
How does a telecoil work?
Uses the induction principle to transduce an electromagnetic signal to an analog electric signal
What is the induction principle?
Movement between a copper coil and magnet transduce electromagnetic signals to a +/- analog electrical signal without adding an additional power source
Does an electromagnetic signal coming from a tele-loop push and pull the magnet that is located inside the copper coil?
Yes
This transduces an analog electric signal
When was it discovered that telephone handsets leaked magnetic energy?
1947
What 2 problems of using hearing aids on the phone did telecoils fix?
Poor SNR
Feedback
What are some benefits to telecoils?
Inexpensive
Do not require an external power source
Eliminates feedback with telephone use because mics are turned off
Improves SNR on phone or inside of room
What kind of signals do vertical loops pick up?
Looped room signals
What kind of signals do horizontal loops pick up?
Telephone signals
