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AUDE 5450 - Amplification 1 > Microphones > Flashcards

Microphones Flashcards

1
Q

What creates a soundwave?

A

The push and pull of acoustic energy

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2
Q

What is a galvanometer?

A

An instrument for detecting and measuring small electrical current changes
Measures back and forth changes (pos and neg)
Transduction that occurs in a mic

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3
Q

What is in a digital hearing aid?

A

A microphone
An A/D converter
A DSP chip
A D/A converter
And a receiver

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4
Q

What is compression?

A

Positive movement resulting in high air pressure

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5
Q

What is rarefaction?

A

Negative movement resulting in low air pressure

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6
Q

How is an analog electric signal produced?

A

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

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7
Q

In the diaphragm, is there a battery that adds voltage to the metal backplate?

A

Yes, and this energy then flows through the carbon balls

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8
Q

What type of signal do compressed carbon balls produce?

A

Positive

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9
Q

What type of signal do decompressed carbon balls produce?

A

Negative

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10
Q

What microphone replaced the carbon mics?

A

Piezoelectric

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11
Q

What is the piezoelectric effect?

A

If you twist, compress, or distort a thin electrified crystal, it produces the +/- electrical voltage needed to create an analog electric signal

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12
Q

Were piezoelectric mics popular?

A

No, they were short lived
Had adverse affects to humidity and high temperatures

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13
Q

What are the two current options for mics?

A

Electret condenser microphone
MEMS microphone

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14
Q

When did the electret mic start being in use?

A

In the 70s

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15
Q

How does the electret mic work?

A

The diaphragm is a stretched polyester disc adhered to a tension ring which is suspended over an electret plate (precharged backplate)

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16
Q

Does the backplate of the electret mic hold charge after exposure to an electric field?

A

Yes
There is no need for an extra power source

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17
Q

What is an electret mic’s sensitivity dependent on?

A

The tension of the diaphragm, and the charge maintained on the backplate

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18
Q

When was the MEMs mic introduced?

A

Recently

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19
Q

What is different about a MEMS mic?

A

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)

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20
Q

What is the stability of the electret mic?

A

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

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21
Q

What is the stability of the MEMS mic?

A

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

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22
Q

What is the sensitivity range for both the electret mic and the MEMS mic?

A

<100 Hz to as high as 15,000 Hz

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23
Q

Is the microphone collection range wider than the device’s final frequency response?

A

Yes

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24
Q

Why is there a small screen placed over the sound entry port?

A

To increase acoustic resistance and smooth frequency response
Provides a physical barrier that changes its properties

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25
Q

Does the small screen over the port eventually get dirty?

A

Yes, that prevents sound from getting through

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26
Q

Will diaphragms pick up anything that will cause it to move?

A

Yes, it will transduce an electrical signal
Diaphragm designed to pick up on small movements

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27
Q

What is acoustic noise?

A

Random motion of gas molecules in the air cause small pressure changes of the diaphragm

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28
Q

What is electrical noise?

A

Small vibrations from internal circuits causes the diaphragm to move
Inside the hearing aid and after the mic

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29
Q

What is the noise floor a measurement of?

A

Both acoustic and electrical noise

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30
Q

What is the noise floor of the best hearing aid mic?

A

About 25 dB SPL

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31
Q

Do we have limits of what levels of internal noise are acceptable?

A

Yes

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32
Q

Is wind noise a big problem for hearing aids?

A

Yes, even at low speeds

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33
Q

Is output greater in the lower frequencies for wind noise?

A

Yes
And they can reach up to 80 dB SPL or greater depending on the wind’s direction

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34
Q

What are wind screens?

A

Screens that are placed on top of the mic ports to reduce diaphragm turbulence
Can easily pop off

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35
Q

Do mics have a dynamic range?

A

Yes
From the loudest and quietest input a mic can collect and convert into a usable electric signal

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36
Q

What is front-end distortion?

A

Distortion that occurs when a collected signal exceeds the mic’s dynamic range

37
Q

What is the dynamic range of an analog mic?

A

115 dB SPL
Can collect input up to 115 dB SPL before distorting

38
Q

What is the dynamic range of a digital mic?

A

96 dB SPL

39
Q

What are typical complaints related to front end-distortion about?

A

Complaints of loud music or user’s own voice

40
Q

Why is there a difference between analog and digital dynamic ranges?

A

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

41
Q

What are some solutions to the reduced dynamic range for digital devices?

A

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

42
Q

Is mic sensitivity limited by distance?

A

Yes
The SNR becomes poorer as the distance between the mic and the desired signal increases

43
Q

What distance is ideal mic sensitivity achieved with?

A

6 feet

44
Q

What is SNR?

A

The difference in volume level between the desired signal and the undesired signal

45
Q

Why do people with hearing loss complain that it’s difficult to hear in noise?

A

Loss of frequency resolution
Loss of temporal resolution
Spatial hearing

46
Q

What SNR do normal hearing listeners require?

A

2 dB SNR
Allows you to hear about 50% of what was said

47
Q

When were directional microphones introduced into single mic hearing aids?

A

1969

48
Q

What is omnidirectional?

A

Microphone collects sound equally from all directions

49
Q

What are directional mics?

A

A mic that focuses sound collection towards one azimuth while attenuating sensitivity from other azimuths

50
Q

What is the null?

A

The point of max attenuation

51
Q

What is a polar plot?

A

It shows the sensitivity and attenuation at different angles
Shows mic sensitivity as a 2-dimensional image

52
Q

Are there two microphone ports?

A

Yes
Sound arrives to the mic at different times
One goes to one side of the diaphragm and the other goes to the other

53
Q

What kind of delay does the distance between the two mic ports create?

A

External delay

54
Q

What kind of delay does internal acoustic resistance inside the mic port create?

A

Calibrated internal delay

55
Q

What happens if the acoustic signal arrives to the diaphragm at the same time and are in-phase?

A

They create an electrical signal

56
Q

What happens if the acoustic signal arrives to the diaphragm at the same time and are out-of-phase?

A

Diaphragm doesn’t move and no signal is created

57
Q

Does current technology use dual directional mics?

A

Yes

58
Q

What are dual directional mics?

A

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)

59
Q

Do the 2 mics in dual directional mics need to be calibrated to each other at all frequencies?

A

Yes, they need this to have effective phase cancellation

60
Q

What is the directionality index?

A

The amount of attenuation provided by different polar patterns

61
Q

What is the null direction for an omnidirectional polar plot?

A

None

62
Q

What is the null direction for a cartiod polar plot?

A

180 degrees

63
Q

What is the null direction for a supercartiod polar plot?

A

+/- 125 degrees

64
Q

What is the null direction for a hypercartiod polar plot?

A

+/- 110 degrees

65
Q

What is the directionality index for an omnidirectional polar plot?

A

0 dB SNR

66
Q

What is the directionality index for a cartiod polar plot?

A

4.8 dB SNR

67
Q

What is the directionality index for a supercartiod polar plot?

A

5.7 dB SNR

68
Q

What is the directionality index for a hypercartiod polar plot?

A

6 dB SNR

69
Q

Does head shadow degrade the directional mic performance by changing the polar plot?

A

Yes, the real-world directionality index for a mic is 2-3 dB SNR
But it’s enough for patients

70
Q

Is the directionality index reduced with increased vent size?

A

Yes
There are more direct sounds coming into the canal

71
Q

Are directional mics less effective for brief or moving sound sources?

A

Yes
The mics have a hard time keeping up

72
Q

Will the benefit of directional mics be dependent on the user’s pragmatic use of the device?

A

Yes, if the patient does not understand how the device and directionality should work, they will have a harder time

73
Q

Do visual cues always enhance understanding?

A

Yes, people tend to hear better with auditory cues

74
Q

What is directional roll off?

A

Low frequency output is reduced when directional mics are turned on

75
Q

Why does directional roll off occur?

A

Low frequencies are wide and broad, so they are more likely to arrive to the diaphragm out-of-phase

76
Q

What does directional roll off sound like to patients?

A

Perceived as a reduction in volume

77
Q

What is a solution for low frequency roll off?

A

Equalization filters that add low frequency energy to replace attenuated output signal

78
Q

What is the amount of directional roll off determined by?

A

The distance between the mic ports
The closer the mics are, the more low frequency attenuation

79
Q

Do directional electret mics stop working when they fall out of calibration?

A

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

80
Q

What happens when mics fall out of calibration over time?

A

Microphone drift
Sensitivity shifts

81
Q

When does drift occur?

A

When high temps reduce sensitivity
When moisture damages the diaphragms tension ring
When cerumen and debris get into the mic port

82
Q

How does a telecoil work?

A

Uses the induction principle to transduce an electromagnetic signal to an analog electric signal

83
Q

What is the induction principle?

A

Movement between a copper coil and magnet transduce electromagnetic signals to a +/- analog electrical signal without adding an additional power source

84
Q

Does an electromagnetic signal coming from a tele-loop push and pull the magnet that is located inside the copper coil?

A

Yes
This transduces an analog electric signal

85
Q

When was it discovered that telephone handsets leaked magnetic energy?

A

1947

86
Q

What 2 problems of using hearing aids on the phone did telecoils fix?

A

Poor SNR
Feedback

87
Q

What are some benefits to telecoils?

A

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

88
Q

What kind of signals do vertical loops pick up?

A

Looped room signals

89
Q

What kind of signals do horizontal loops pick up?

A

Telephone signals

AUDE 5450 - Amplification 1 (14 decks)

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