Directional Microphones

Transducers

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

Microphone

A type of transducer that changes acoustic energy (sound wave) to electrical energy (electrical voltage)

Receiver

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

Amplifier

amplifies electric signal

Battery

supplies power

Brief History of Microphones

1) Carbon microphones - (early 1990s) first electrical HA; restricted bandwidth; large distortion

2) Crystal microphones - (mid 1930s) didn’t require an external power source; wider frequency response than previous mics; used piezoelectric salt crystals; vulnerable to heat & moisture

3) Magnetic/ electromagnetic - (mid 1940s) used mainly in body worn aids; size, low impedance and energy efficiency— became common when transistor aids were introduced

4) Ceramic Microphones - (late 1960s) modern version of the crystal mic; more stable and resistant to shock than electromagnetic

5) Electret Condenser Microphone - (1970s) low resistance to mechanical vibration / better resistance to shock; uses a special type of capacitor which has a permanent voltage built into it, and therefore doesn’t require any external power for operation; currently used in the vast majority of HAs!!

Basic (electret) Microphone function

• sounds enter inlet port and reach the diaphragm

• pressure fluctuations within the sound wave cause diaphragm (front plate) to move up and down

• when sound pressure forces the diaphragm towards the electret, closer diaphragm and electret, the greater charge on the diaphragm

(see slides 6&7 for more detail/diagrams)

Future of microphones

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

• currently used in Starkey hearing aids

• extremely small and low vibration sensitivity because of thin diaphragm

• may be produced on same chip as amplifier

• but relatively high internal noise

HA mic frequency response

should ideally have a flat frequency response with a wide dynamic range (should be equally sensitive to all frequencies)

Two General Types of HA Mics

1. Omnidirectional - one sound inlet; signals processed equally regardless of azimuth /equally sensitive to sound coming in from all angles

2. 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 

SNR

Signal-to-Noise Ratio = relationship between the loudness of the desired speech signals and the undesired noise background

As reverberation increases, SNR _________

decreases

What is the SNR:

1) singal 70dB, noise 65 dB

2) singal 70dB, noise 75dB

3) singal 70dB, noise 70dB

1) +5dB SNR

2) -5dB SNR

3) 0dB SNR

Directional Microphones

• sounds coming from the front are given priority compared to sounds arriving form other directions

• directional mics depend on spatial separation of the noise and signal

• greatest increase in speech recognition in the presence of noise is achieved by reducing distance between speaker and mic

Two types of directional mics

1. single directional microphones

2. dual microphones

Single Directional Microphones

• aka pressure difference mic / pressure gradient mic

• two sound entry ports going to ONE microphone

• the diaphragm motion is driven by the difference in pressure on its two sides

• external time delay & internal time delay

directional mics usually provide ______ low-frequency gain than omnidirectional 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 HAs

• 2 perfectly matched omnidirectional mics (mics are equally sensitive)

• Dynamic matching: HA constantly compares the relative sensitivity of the two microphones 

• greater distance, better directionality

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

• a HA with dual microphones can still be put in omnidirectional mode (it would just use one of the mics)!

Multi-Microphone/ Beamforming Arrays

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

• little addition benefit when number of mics is >5

• sometimes referred to as “beamformers”

Polar Plots

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

• describes the directional sensitivity of microphones

Directional Microphone designs 

Cardioid (max attenuation from rear - 180 degrees)

Super-cardioid (sensitivity to sounds in back grows, but sensitivity to sounds on side diminish)

Hypercardiod (max attenuation at 110 and 250 degrees)

Bi-drectional (max attenuation at 90 and 270 degrees) 

Cardioid directional mic

max attenuation from rear - 180 degrees

Super-cardioid directional mic

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

Hypercardiod directional mic

max attenuation at 110 and 250 degrees

Bi-drectional directional mic

max attenuation at 90 and 270 degrees

List two directional hearing aid features

1. automatic switching

2. adaptive polar patterns

Automatic Switching

• directional hearing aid 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 info, the HA will “automatically switch” to the best microphone mode (omni or directional) for that listening environment

• algorithms vary across companies

Adaptive Polar Patterns

• directional hearing aid feature

• can be changed by changing the electronic delay between the two omnidirectional mics

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

• 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

List three 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:

  • 0-1dB omindirectional mic

  • 4-6dB directional mic

  • 12-14dB multiple array mic

• higher DI = better directionality

• every 1 dB improvement in DI increases speech recognition by 7-10%

• DI is a good estimate of how helpful the directional mic will be in difficult listening situations

AI-DI - weighted directivity index 

• articulation index provides measure of the percent of speech energy that is audible 

• AI-DI: directivity index at each frequency is calculated by multiplying the AI weight and then performing a RMS rum of resulting products to equal one number 

• a directional microphone that extends to higher frequencies will have higher AI-DI because high frequencies contribute more to intelligibility than other frequencies 

How does the count-the-dot method of AI calculation work?

count the number of audible dots and then divide by 100

Front-to-back Ratio (FBR)

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

• omnidirectional — front and back frequency responses essentially same (FBR = 0)

• directional — greater separation of front/back signals

  • greater FBR (ratio of front to rear sensitivity)

  • only good for cardioid, with null at 180 degrees

• only gives info about suppressing noise directly behind the person