CSD 673 - HA I - microphones

use for test #2

transducers

devices that transform energy from one form of energy to anouther form of energy

• microphone

• receiver

microphone

transduce acoustic energy to electrical energy

receiver

converts electrical (output of amplifier) to acoustic energy to the ear

amplifier

amplifies electric signal

battery

supplies power

microphone

a type of transducer that changes acoustical energy (sound wave) into electrical energy (electrical voltage)

brief history of microphones

1. Carbon microphones (early 1900s)

2. Crystal microphones (mid 1930s)

3. Magnetic/electromagnetic (mid 1940s)

4. Ceramic Microphones (late 1960s)

5. Electret Condenser microphone (1970s)

6. Silicon Microphones/ Micro-Electronic-Mechanical Systems (MEMS)

Carbon microphones

(early 1900s) 1st electrical HA, restricted bandwidth, large distortion

Crystal microphones

(mid 1930s) 

• didn’t require external power source

• wider frequency response than previous microphones

• used piezoelectric salt crystals; vulnerable to heat & moisture

Magnetic/electromagnetic

• (mid 1940s)

• used mainly in body worn aids

• size, low impedance and energy efficiency — became common when transistor aids were introduces

Ceramic microphones

(late 1960s)

• modern version of the crystal mic (Piezoelectric)

• more stable and resistant to shock than electromagnetic

Electret Condenser Microphone

(1970s)

• low resistance to mechanical vibratino/better resistance to shock

• uses a type of capacitor which has a permanent voltage built into it

  • doesn’t require any external power for operation

  • good ones usually include a pre-amplifier (still requires power)

• currently used in the VAST MAJORITY of HAs

Today’s smallest electret HA occupies only .02 cm³ — a reduction in volume of __ times compared with microphones 50 years ago

150

In a microphone, sounds enter __ port and reach the diaphragm

inlet

Pressure fluctuations within the sound wave cause diaphragm (aka front plate) to move ___

up and down

air path

small air space separates diaphragm from a rigid metal plate (back plate)

back plate

coated with thin teflon material called electret

electret

material has a permanent change

When sound pressure forces the diaphragm towards the electret, the closer diaphragm and electret, the ___ charge on the diaphragm

greater

Think of the front and back plates as forming a ___.

capacitor

Capacitor

• an electronic component that has reactance (rather than resistance) because it stores energy in the form of an electrostatic field

• used to convert acoustical energy into electrical energy

The front plate is made of very light material and acts as the ___

diaphragm

The diaphragm vibrates when struck by sound waves, moving the font and back plates ___ to each other

closer

When plates are closer together, capacitance ___ and a charge current occurs.

increases

When the plates are further apart, capacitance ___ and a discharge current occurs.

decreases

Capacitance

amount of charge the capacitor can store per voltage

Capacitance and reactance are ___ related

inversely

Silicon Microphones/ Micro-Electronic-Mechanical Systems (MEMS)

still vastly under development, but currently used in Starkey hearing aids

Advantages of Silicon Microphones/ Micro-Electronic-Mechanical Systems (MEMS)

• Extremely small and low vibration sensitivity because diaphragm is so thin

• HA mic may be produced on same chip as HA amplifier

• significantly decreases HA component size

Disadvantage of Silicon Microphones/ Micro-Electronic-Mechanical Systems (MEMS)

relatively high internal noise

Hearing Aid Microphone Frequency Response

•Should ideally have a flat frequency response with a wide dynamic range

•Should be equally sensitive to all frequencies across its frequency range

Two General Types of HA microphones

Omnidirectional & Directional

omnidirectional

• One sound inlet

• Signals processed equally regardless of azimuth/ equally sensitive to sound coming in from all angles

• Azimuth/angle

• Equally sensitive to every angle

Directional

• Two sound inlets

• More sensitive to sound coming from one direction than from another direction

• Ideally more sensitive to sound coming from the front than to sound coming from other directions

• Want 0 degrees (front)

Signal-to-Noise Ratio (SNR)

Relationship between the loudness of the desired speech signals and the undesired noise background

As reverberation increases, SNR ___

decreases

signal 70 dB, noise 65 dB, = __ dB SNR

+5

signal  70 dB, noise 75 dB, = ___ dB SNR

-5

signal 70 dB, noise 70 dB, = ___ dB SNR

0

directional microphones

Sounds coming from the front are given priority compared to sounds arriving from other directions.

Directional mics depend on

spatial separation of the noise and signal.

When using directional mics, the patient should position themself so that signal is in ___ and noise behind or off to the side

front

In directional mics, the greatest increase in speech recognition in the presence of noise is achieved by

reducing distance between speaker and microphone

Placing the microphone close to a talker’s mouth, the __ signal is louder at the microphone than are the background noises

speech 

Two types of directional microphones

single directional microphones & dual microphones

external time delay

time it tkes for sounds to get from one inlet port to the other

internal. time delay

rear port contains an acoustic damper, which adds resistance and provides a time delay equal to the external delay of the same sound traveling from the rear to the front port

result of internal time delay

sounds coming from the rear direction hit the front port later than the rear port and signal from the back become out of phase at the microphone and cancel

In single directional microphones, __ sound entry ports going to ONE microphone

two

In single directional microphones, the diaphragm motion is driven by the ___ in pressure on its two sides

difference

Directional mics usually provide ___ low-frequency gain than omindirectional mics due to greater similarity in the amplitude and phase at the 2 mic ports for the low-frequency waveforms

less

dual microphones

most commonly used in today’s hearing aids

• Employs two omnidirectional mics for improved directionality

• BTEs – allow maximum distance between 2 ports to be ~15mm,

 ITE is reduced to 4-10 mm.

• Greater distance, better directionality

• Output from 2nd mic is electronically delayed and subtracted from 1st mic output.

In dual microphones, the two omnidirectional mics must have equal ___

sensitivity

dynamic matching

hearing aid constantly compares the relative sensitivity of the two microphones (dual microphones)

A hearing aid with dual microphones can still be put in omnidirectional moder, if a hearing aid is set to this mode, how many microphones is it using? 

1 out of the 2

Multi-Microphone/Beamforming Arrays

combines outputs of 2 or more directional mics or more than 2 omnidirectional mics

• research shows little additional benefit is achieves when the number of mics exceeds 5

  • Patients can be fit binaurally

polar plots

plot microphone output as a function of the angle of sound incidence

• describes the directional sensitivity of microphones

cardioid microphone design

max attenuation from rear (180 degrees)

super-cardioid

sensitivity to sounds in back grows, but sensitivity to sounds on side diminish

hypercardioid

max attenuation at 110 degrees and 250 degrees

bi-directional

max attenuation at 90 and 270 degrees

directional hearing aid features

automatic switching & adaptive polar patterns

automatic switching (directional HA feature)

-HA will detect the level of the spectrum (speech, noise or music) and the direction (front, back side) of the sounds

-based on this information, the hearing aid will “automatically switch” to the best microphone mode (omni or directional) for that listening environment.

- algorithms vary across HA companies

adaptive polar patterns (directiona HA feature)

Polar patterns can be changed by changing the electronic delay between the two omnidirectional microphones.

-HA samples all polar patterns and then locks on the one that results in the best output or maximum attenuation of noise.

-Also, can track a moving noise source from behind by keeping the null of the polar pattern located on the noise source.

problems with microphones

• break down if exposed to perspiration and other chemical agents

• random electrical noise — all electronic components generate small amounts of noise

• sometimes audible in quiet environments

• sensitivity to vibration of mic generates a voltage reflecting the magnitude and frequency of the vibration (e.g., Clothing next to body, running on hard surface, wind noise)

factors that limit effectiveness of directional microphones 

• Venting/open fittings negatively affects directivity

  • open fittings and hearing aids with large vents only maintain directionality in the high frequencies

• Microphone ports need to be aligned on the horizontal plane

• Directional microphones reduce low frequency output

  • need to increase amplification of low frequencies to compensate

Measures of Directionality

1. Directivity Index (DI)

2. Articulation Index — Directivity Index (AI-DI)

3. Front to back ratio (FBR)

Directivity index (DI)

One number in dB that represents ratio of the mic output for signals from the front (0 degrees) to sounds originating from all directions

DI Ranges

 0-1 dB (or less than 0) → Omnidirectional

4-6 dB → Directional microphone

12-14 dB → Multiple array microphone

higher DI =

better directionality

every 1 dB improvment in DI increases speech recognition by

7-10%

Articulation Index provides measure of the percent of speech energy that is __

audible

Articulation Index has different weights for contribution of each frequency band to the overall intelligibility of speech (e.g. AI applies a weight of .0010 for 250 Hz, .0024 at 1K, .0038 at 2K) and is Measured on a __ scale

0-1

AI-DI

Directivity Index at each frequency is calculated by multiplying the AI weight and then performing a RMS sum of resulting products to equal one number

A directional microphone that extends to __ frequencies will have higher AI – DI because high frequencies contribute more to intelligibility than other frequencies

higher

Front-to-back Ratio (FBR)

difference between the frequency response of the mic with the signal presented from the front and rear

What is the FBR of omnidirectional?

0

Directional microphones have a greater separation of front/back signals, which means it will have what kind of FBR?

greater

a greater FBR is only good for what kind of microphone?

cardioid, with a null at 180 degrees

FBR only give info about suppressing noise directly __ the person

behind