15_Directional Mic Technologies

Question 1

What’s the difference between external delay and internal delay (regarding directional microphones)?

Answer 1

External delay = d/c
where “c” is the speed of sound
- it is the propagation delay due to the physical separation b/w front and rear mics
- typically small distance (10mm -> delay of a few microseconds)

Internal delay - the delay applied by the HA to the signal transduced by the rear mic

Question 2

What happens if the internal delay is equal to the external delay, and you subtract one from the other?

Answer 2

You get a residual output of zero

Question 3

In order to have directionality, you need at least __ microphone ports

Answer 3

2

Question 4

What are the 2 ways to set internal delay?

Answer 4

Acoustically, with a physical element placed in the sound inlet of the rear mic

Electronically, with 2 omnidirectional mic cartridges, and an electronic delay of the rear mic

• has the advantage of programmable change in delay value, which is useful for adaptive directionality
• used c/ most modern HAs

Question 5

External delay relies on the ______ of arrival

Answer 5

Angle

Question 6

What is the equation for propagation delay?

Answer 6

te = d/c * cos(theta)

Question 7

If the internal delay is set to the propagation delay, then sounds coming from that angle will be _______

Answer 7

Attenuated

Question 8

In a cardioid polar plot, which sounds are attenuated?

Answer 8

The sounds from the back

Question 9

What are the four directional patterns of polar plots?

Answer 9

Cardioid
bidirectional
hypercardioid
supercardioid

Question 10

What is a directivity index (DI)?

Answer 10

The ratio of frontal power sensitivity to average power sensitivity to sounds from all other directions
- can include azimuth and elevation (3D) or just azimuth (2D)

Question 11

Which directional pattern is closer to a bidirectional configuration and which is closer to a cardioid configuration:
hyper-cardioid
super-cardioid

Answer 11

Super-cardioid is closer to cardioid

Hyper-cardioid is closer to bidirectional

Question 12

Which directional pattern has the highest 3D DI value?

Answer 12

Hyper-cardioid

Question 13

Which directional pattern has the highest 2D DI value?

Answer 13

Super-cardioid

Question 14

What happens to polar plots once the hearing aid is placed on a person, as opposed to being measured in free field?

Answer 14

The polar plot is no longer as clean

There is a reduction in DI as a result of positioning

Question 15

Does frequency have an effect on DI values?

Answer 15

Yes

Question 16

Which frequencies have a higher DI value: highs or lows?

Answer 16

Highs

Question 17

Which instruments have higher DI values:
BTEs or ITEs
(either omni or directional)

Answer 17

ITEs

Question 18

By weighting the DI values at different frequencies, we can compute a single index of directional performance, called ______

Answer 18

AI-weighted DI

Question 19

For the AI-weighted DI, which frequency is weighted the highest?

Answer 19

2000 Hz

Question 20

In general, 1 dB of improvement in AI-DI equates to ____% improvement in sentence-based speech intelligibility

Answer 20

10%

- SNR improvement of roughly 60% of AI-DI for speech recognition in noise

Question 21

What might cause an amplitude and phase mismatch between microphones? How can we help prevent this

Answer 21

Aging (causes “drift”)
Debris

Regular clearing of the microphone ports

Question 22

What is adaptive directionality?

Answer 22

The process where the directional pattern is automatically varied and the pattern that provides the lowest output intensity is selected (usually only considers the rear hemisphere)

Varying the internal delay and picking the polar plot that gives the best performance for that situation

• adaptation time varies from few ms to almost 20 s.
• internal delay parameter can be made frequency specific (multiband or multichannel directional processing)

Question 23

Which 3 things are necessary in order for adaptive directional mics to remove noise more effectively than fixed directional mics?

Answer 23

• nearby noise source that dominates over other noise sources
• dominant noise maintains its direction long enough for the adaptive algorithm to lock on
• dominant noise source is not already aligned with the null in the directivity pattern of the fixed directional mic

Question 24

What factor most limits performance of adaptive-delay directional mics?

Answer 24

Reverberation

• The noise power reaches mic evenly from all directions.
• Algorithm selects the pattern that most attenuates the noise (hyper-cardioid b/c it has the highest 3D DI)
• Performance will be identical to fixed hyper-cardioid directional mic
• SNR will, at most, improve to equal the DI of that pattern

Question 25

What is the difference b/w adaptive directionality and automatic switching?

Answer 25

Adaptive Directionality:

• automatically adapt the polar plot such that overall output is minimized
• often independently adapted in multiple frequency regions

Automatic Switching:
- toggling b/w omnidirectional and directional modes depending on the situation

Question 26

What amount of directional advantage was measured in a difficult but realistic environment? Describe the environment

Answer 26

~2 - 4 dB (measured in both adults and children)

• listener facing and close to talker
• poor SNR (10+)
• competing moderate noise

Question 27

In what situation (regarding direction and proximity of noise and source) would a directional mic not be effective?

Answer 27

Source and noise outside of critical distance

Question 28

Name 3 situations (regarding direction and proximity of noise and source) in which directional mics are effective

Answer 28

**Source always in front of listener

Noise and source within critical distance, but noise behind listener

Noise within critical distance and behind listener, source outside critical distance

Noise outside critical distance, source within

Question 29

Name 3 limitations of directional mics

Answer 29

• increased internal noise
• increased impact of wind noise
• reduced performance in environments with greater reverberation
• impact on sound localization cues (cue-preserving bilateral directional mic processing is a potential solution)

Question 30

Why is wind noise more of a problem with directional mics?

Answer 30

Wind results in turbulent inputs that are picked up by front and rear mics, but since there is no timing relationship, it is like picking up 2 unrelated sources, and delaying and subtracting them actually increases the overall output, especially in low frequencies

Question 31

Which mode do most HAs usually switch to to limit the effects of wind noise?

Answer 31

Omnidirectional