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HESP 700- Hearing Aids > Lecture 4: Hearing Aid Components > Flashcards

Lecture 4: Hearing Aid Components Flashcards

1
Q

What is the purpose of a hearing aid?

A
  • Amplify
  • Shape frequency response and output
  • Accomplish amplification and shaping with as little distortion as possible
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2
Q

List the components of a hearing aid.

A
  1. Microphone
  2. Amplifier
  3. Receiver
  4. Power Source
  5. Optional Features
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3
Q

Define a transducer

A
  • A transducer converts energy from one form to another

Ex. Microphone, strain gauge (scale), light bulb

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4
Q

List and define desirable transducer characteristics.

A

Linearity

  • Output signal proportional to input signal
  • Predictable, easier to combine

Dynamic Range

  • Range of amplitude response
  • Distortion, clipping

Sensitivity
- Amount of input needed for a given output

Bandwidth
- Range of frequency response

Low noise
- Internal transducer noise does not contaminate the output signal

Repeatability
- Same output for same input, despite temperature, humidity, etc.

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5
Q

Describe microphones

A
  • A transducer
  • Air displacement –> mechanical energy (usually via a diaphragm) –> electrical signal
  • Provides an electrical signal proportional to the acoustic signal
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6
Q

What are the requirements for a hearing aid microphone?

A
  • Small, low power
  • Low noise
  • Large range
  • Large bandwidth
  • Sensitive, but sturdy
  • Not sensitive to temperature, humidity, vibration
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7
Q

List and describe different kinds of microphones.

A

Early 1900’s: Carbon
- Not sensitive to soft sounds

1920’s: Magnetic

  • More sensitive than a carbon microphone
  • You need a powerful magnet to make the coil move
  • It can be difficult to design the diaphragm in the magnetic microphone to move at the frequency range
  • Used in recording studios

1920’s: Condenser

  • An electronic device that are 2 electrically charged plates
  • Usually one of the plates doesn’t bend; the other is the diaphragm
  • Smaller and lighter than the magnetic microphone
  • Doesn’t require as much power
  • Temperature and humidity sensitive
  • Requires a large amount of biasing voltage
  • Voltage proportional to charge and distance
  • Requires biasing voltage

1970s: Electret
- Material manufactured so that it has electrical charge
- Encase the electrical charges within plastic
- One of the plates is made out of electret
- Doesn’t need biasing voltage
- Doesn’t need an amplifier to make the power bigger
- Mic capsule contains the diaphragm and a small amplifier transistor
- Passageway that allows low frequency sounds to hit the diaphragm on both sides
- Filters low frequency sounds out (below 50 Hz)
- Cutting out the low frequencies saves power and computational time
- Good frequency response (flat from 50-6000 Hz)
- Shock resistant
- Less sensitive to temperature and humidity changes
- Often incorporates an internal low-cut filter

2010s: MEMS Silicone Microphone
- Smaller, lower power, low vibration sensitivity
- Better batch consistency
- Lower temperature sensitivity
- Apply a biasing voltage on a smaller scale
- You can get a small charge on the plate with a very small voltage

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8
Q

What are the different kinds of microphone directionality?

A

1) Omnidirectional
- Sensitive to sound pressure from all directions

2) Directional
- Sensitive to sound from all directions, but sound originating from certain directions is cancelled

Directionality can be accomplished in 2 ways:

  • Acoustically with ports (openings)
  • Electronically with delays between multiple mics
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9
Q

What does an amplifier do?

A

Amplify and manipulate the electrical signal from the microphone

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10
Q

What are the 3 stages of amplification?

A

1) Preamplifier
- Adds enough gain to amplify incoming signals above the circuit noise of the amplifier

2) Signal Processing
- Manipulates the signal, enhances, and/or extracts information

3) Output amplifier
- Amplifies the output of the signal; boots signal and drives receiver

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11
Q

Define amplifier gain

A

The ratio of the input and output signals, usually expressed in dB

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12
Q

Define bandwidth

A

The range of frequencies that the amplifier is able to properly amplify

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13
Q

List and describe the amplifier output types.

A

1) Class A
- Uses constant (and high) current drain regardless of the input signal or volume control setting (not efficient)
- Single-ended design b/c there is only a single output drive terminal

2) Class B
- Push/pull, powerful, generally requires larger capacitors, so harder to fit into smaller hearing aids
- 2 output terminals that operate alternately
- Can provide about 2x output signal amplitude of Class A
- Current drain is independent of output signal (determined by input signal) & volume control (no signal=no current flow)

3) Class D
- Incorporated in same metal can as receiver
- The max. output is 10-12 greater than a Class A amplifier that provides the same amount of gain, giving greater headroom (and hence better sound quality)
- Current drain is less than Class A

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14
Q

What are the 2 signal processing types?

A

1) Linear
- Same change in input level gives you same change in output

2) Compression
- Output varies with the function of the input signal
- Works better with the individual’s dynamic range

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15
Q

What is a digital amplifier?

A
  • Analog to digital conversion
  • Represented as ones and zeros (like the computer)
  • Processed using a digital signal processor and then converted digital-to-analog

Why the double conversion?

  • Requires less components to complete different filtering techniques
  • Allows multiple programs, more complex processing, in the same package and the device is the same for everyone
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16
Q

What are some factors influencing digital signal processing?

A

1) Processing power
2) Sampling rate
3) # of bits
4) Processing delay

17
Q

What is a receiver?

A
  • A transducer that converts the (amplified) electrical signal into an acoustic one
  • A reverse operation from the microphone
  • Operate electromagnetically
18
Q

Describe the operation of the moving coil receiver.

A

See diagram on study guide.

19
Q

What is the power of the hearing aid?

A
  • Batteries
  • A chemical reaction provides electrical energy to drive the hearing aid components
  • Hearing aids use single cells that supply 1.2 to 1.5 volts
  • Cells are designed to die quickly
20
Q

What are battery classifications?

A

Material

  • Mercury: environmentally hazardous
  • Silver oxide: expensive, but high power
  • Zinc-Air: commonly used, long shelf life

Voltage
- 1.2 to 1.5 volts

Capacity: amount of energy stored

  • Expressed in millampere hours (mAh)
  • Larger cell stores more energy

Battery life = battery capacity (mAh)/current drain (mA)

21
Q

Describe rechargeable batteries.

A
  • Different battery chemistries have different energy densities
  • Most rechargeable batteries have lower energy densities than Zinc-air, so they don’t last as long
  • Lithium-ion batteries have high density and light weight, but operate at 3.7 Volts, requiring a complete redesign of the hearing aid circuitry
22
Q

What is volume control?

A
  • VCW is a variable resistor that controls current flow through an amplifier
  • As volume is increased, resistance is decreased

Taper characteristics
- Amount of gain as a function of the % of the VCW rotation

  • Many VCWs are nonlinear, which means that when the VCW is half-on, does not mean the gain if half-on
  • Distortion increases when VCW set to nearly full-on
  • A well fit hearing aid will have about 10 dB of reserve gain

VCW Locations
BTE = upper backside
ITE = wheel on the faceplate
CIC = screwdriver setting

23
Q

What is tone control?

A
  • Tone control changes the frequency response of the hearing aid

LC= low-frequency control
- Manipulates the amount of gain provided in the low-frequencies (patients may describe the sound as hollow or barrel like)

HC= high-frequency control
- Manipulates the amount of gain provided in the high frequencies (harsh and tinny)

24
Q

List the other hearing aid controls.

A
  • Gain control
  • Output control
  • Resonant frequency control
  • Compression knee point
  • Compression ratio
  • Crossover frequency
25
Q

Describe the telecoil.

A
  • Alternate input transducer
  • Coil built into aid picks up energy from a magnetic field and converts it to electrical energy bypassing the microphone
  • Magnetic field varies with the acoustic signal

Telecoil uses

  • Position of the telecoil in the hearing aid affects its sensitivity and effectiveness
  • Horizontal
  • Vertical
  • Angled
26
Q

Describe an omnidirectional microphone.

A
  • Pressure changes cause the diaphragm to vibrate
  • Equally sensitive to sounds coming from any direction
  • The sounds behind the person and the sounds in front are perceived at the same intensity
  • Can pick up competing background noise
  • Does not improve signal to noise ratio
  • If both the noise and signal are picked up at the microphone, then the noise and the signal is sent through the process acoustic signal
27
Q

Describe a directional microphone.

A
  • Directional microphone can improve speech understanding in noise
  • For every 1 dB improvement in SNR, you get an 8-10% better understanding of speech
28
Q

Describe a 2-port microphone.

A
  • Sounds enter the rear and front port simultaneously
  • Yields bidirectional polar pattern
  • By adding a small mechanical screen in the rear port, then sound can be slowed down arriving through the rear port
  • Allows whole sound waves to arrive at the diaphragm at the same time, the sound originating from the back is cancelled out
29
Q

Describe a multi-mic directional system.

A
  • Rear mic takes the place of the rear port; front mic takes the place of the front port
  • Electronic circuit takes the place of the mechanical screen
  • Electronic delay
30
Q

What determines the polar pattern of a directional system?

A
  • The ratio of the internal electronic time delay to the external time delay
  • This ratio is known as BETA
  • The delay is dependent on the position of the microphones
31
Q

What is the directivity index (DI)?

A
  • The difference between microphone sensitivity to sounds directly in front of the listener compared to sounds that come from all other directions (for a specific frequency
32
Q

What is the AI-DI?

A
  • The articulation index weighted directivity index
  • It places more emphasis on the frequencies that are critical fro understanding speech
  • We want to focus on 1000 to 2000 Hz range
  • 20% at 500 Hz, 23% at 1000 Hz, 33% at 2000 Hz, and 24% at 4000 Hz

HESP 700- Hearing Aids (9 decks)

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