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Amplification I > Lecture 9 > Flashcards

Lecture 9 Flashcards

1
Q

Explain the frequency response of a hearing aid microphone?

A
  • Hearing aid electret microphones have a wide frequency response- very suited for speech and environmental signals
  • Miniaturization of this microphones are well-suited to hearing aid forms
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2
Q

What are hearing aids sensitive to?

A
  • Moisture and humidity
  • Highly important to keep the microphones clear of debris- debris can block the mic port, or damage the internal components
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3
Q

Do microphones create noise?

A

Microphones create noise themselves— usually between 25-30 dB SPL (which may or may not be audible to the patient)

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4
Q

Microphones convert ____ energy into ____ energy

A

Acoustic, electrical

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5
Q

Besides debris, what else do microphones have to be protected from?

A

Microphones of hearing aids are electret microphones and must be protected in the devices themselves and from electromagnetic interference

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6
Q

What are the two types of microphones?

A
  1. Omni directional microphones
  2. Directional microphones
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7
Q

Do custom hearing aids have one or two microphone ports?

A

Small custom hearing aids are omni directional (one port), and the larger custom hearing aids are directional (two ports)

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8
Q

What type of debris gets in the microphone of a custom hearing aid?

A

Skin

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9
Q

Do BTEs/RICs have one or two microphones?

A

Almost all BTEs and RICs are directional microphones because they have the space to have two microphone ports

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10
Q

Where are the microphones located on BTE/RIC hearing aids?

A
  • Front facing directional mic
  • Back facing directional mic
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11
Q

What type of debris gets in the microphone of a BTE/RIC hearing aid?

A

Dust/lint

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12
Q

Explain the functionality of omi-directional microphones

A
  • One port
  • Sound hits one side of the diaphragm
  • Responsive to all sounds regardless of their direction
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13
Q

Explain the functionality of directional microphones

A
  • Two ports (must have physical space between port openings)
  • Sound will hit both side of the diaphragm
  • More responsive to sound originating from a certain direction (usually be the front-most microphone on the hearing aid user)
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14
Q

What is the range of spacing between directional microphones?

A

Spacing between 4-12mm

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15
Q

What are the three possible microphone arrangements?

A
  1. One omnidirectional (no directionality)
    • Capture sounds equally from all directions
  2. One directional (2 ports)
    • Hearing aid is always in a fixed directional sensitivity (fixed directionality)
  3. Two omni microphones
    • Microphones communicate with one another to look at phase and time delays
    • Note: microphones have to be perfectly matched in function
    • Have fixed directionality
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16
Q

Omni vs. directional frequency response

A

Directional microphones have less low frequency sensitivity compared to omnidirectional microphones

17
Q

Explain mismatched microphones

A
  • When two omni directional microphones, working together to learn directionality the following can happen:
    1. Performance of each microphone may not be identical
    2. Two microphones will have different sensitivity or frequency response
  • This effects the directionality function of the aid
18
Q

What can cause mismatched microphones?

A
  • Dirt, debris, and physical damage can cause mismatch
  • Some hearing aids will monitor the mismatch and adjust gain to compensate
19
Q

Candidacy considerations of directional microphones

A
  • The overriding function and purpose of directional microphone technology is to improve sound from one direction and attenuate sound from another
  • WHY? To provide better speech understanding in noise by improving the signal to noise ratios (SNR) and create a comfortable listening environment for users
  • Patients who have challenges in noise (i.e. show impairment in performance of speech in noise testing) benefit from this technology
    • DMs can improve the signal-to-noise ratio by approximately 3dB when noise is to the rear of the listener
  • Patient should sit with their back to the noise
20
Q

Directionality is most effective when the patient is within ____ feet of sound source

A

10

21
Q

What 3 characteristics make up a polar plot?

A
  1. Azimuth
  2. Null (the most amount of attenuation)
  3. Areas of focus
22
Q

What are 5 different polar plots (polar directivity patterns)?

A
  1. Omnidirectional
  2. Bi-directional
  3. Cardioid
  4. Super/Hyper Cardioid
  5. Anti Cardioid
23
Q

What is an anti cardioid polar plot and why might someone have this?

A

Null is forward facing and most sound will be picked up from the back and sides of the patient (someone being pushed in a wheel chair, a patient driving)

24
Q

What are polar directivity patterns for?

A
  • Representation to visualize the amount of attenuation provided by the microphone in a 180 degree plot, generally in the horizontal plane
  • Azimuth is represented between 0 and 180 degrees
  • There can be frequency specific differences in polar plats, often shown in 500, 1000, 2000, and 4000 Hz (can be specific for speech)
25
Q

What 3 ways do you measure directional microphone performance?

A
  • Front to back ratio (FBR)
  • Directivity index (DI)
  • Articulation weighted directivity index (AI-DI)
26
Q

What is the front-to-back ratio?

A
  • Difference in what is coming to the front and back speaker in a directional mode
    In an omnidirectional system those lines would be almost equal
  • Difference in the frequency response between 0 and 180 degrees
  • This can be done in test box or with patient wearing devices
27
Q

What is the directivity index (DI)?

A
  • How sensitivity is a microphone? What is the difference in sensitivity of sounds arriving in the front relative to other directions?
  • The Directivity Index (DI) is a value (in dB) obtained from the polar plot, and can be obtained from individual frequencies
  • Difference in the frequency response between 0 and 180 degrees
28
Q

An omnidirectional microphone would have a DI of ____

A

0

29
Q

Directional microphones have DIs commonly between ____ dB

A

2-6

30
Q

The higher the DI, the more sensitive the hearing aid is to the ____

A

Front

31
Q

What is the articulation weighted directivity index (AI-DI)?

A
  • An articulation - weighted directivity index (AI-DI) looks at the directivity of the microphones in relation to the sounds specifically important for speech (i.e. more weight is given for 2000 Hz, less to 500 Hz)
  • Depends on a lot of things (directionality index and how the person wears their HAs in everyday life)
32
Q

What is automatic and adaptive directionality?

A
  • The switching between omni and directional functioning (patient user control vs. automatic change from hearing aid itself)
  • Generally, omni is best suitable in quiet listening situations and directional in noisy situations
  • Adaptive polar plots and can change quickly to respond to environmental changes
  • Hearing aids select the polar plot it believes to be most appropriate for a specific listening situation
  • How quickly the change is depends on the speed of the processor
33
Q

Why might someone want a beam focus microphone?

A

Beam focus is best when there is high level ambient noise and an individual is trying to have a conversation with someone right in front of them

34
Q

6 Considerations - when is directionality not possible, feasible, or desired?

A
  • Hearing aid form: is directional microphone technology a possibility?
  • Wind noise: turbulence on microphones can be problematic
  • Sounds from direction other than the front are desired?
  • Patient restrictions: either physical or cognitive
  • Venting effects: reduces the effect of directionality
  • Reverberation
35
Q

Why does venting reduce the effect of directionality?

A

The more venting we have, the less effective directional mics are (sound is already coming in through the canal to the auditory system in a natural way)

Amplification I (14 decks)

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