15_Directional Mic Technologies Flashcards
What’s the difference between external delay and internal delay (regarding directional microphones)?
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
What happens if the internal delay is equal to the external delay, and you subtract one from the other?
You get a residual output of zero
In order to have directionality, you need at least __ microphone ports
2
What are the 2 ways to set internal delay?
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
External delay relies on the ______ of arrival
Angle
What is the equation for propagation delay?
te = d/c * cos(theta)
If the internal delay is set to the propagation delay, then sounds coming from that angle will be _______
Attenuated
In a cardioid polar plot, which sounds are attenuated?
The sounds from the back
What are the four directional patterns of polar plots?
Cardioid
bidirectional
hypercardioid
supercardioid
What is a directivity index (DI)?
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)
Which directional pattern is closer to a bidirectional configuration and which is closer to a cardioid configuration:
hyper-cardioid
super-cardioid
Super-cardioid is closer to cardioid
Hyper-cardioid is closer to bidirectional
Which directional pattern has the highest 3D DI value?
Hyper-cardioid
Which directional pattern has the highest 2D DI value?
Super-cardioid
What happens to polar plots once the hearing aid is placed on a person, as opposed to being measured in free field?
The polar plot is no longer as clean
There is a reduction in DI as a result of positioning
Does frequency have an effect on DI values?
Yes
Which frequencies have a higher DI value: highs or lows?
Highs
Which instruments have higher DI values:
BTEs or ITEs
(either omni or directional)
ITEs
By weighting the DI values at different frequencies, we can compute a single index of directional performance, called ______
AI-weighted DI
For the AI-weighted DI, which frequency is weighted the highest?
2000 Hz
In general, 1 dB of improvement in AI-DI equates to ____% improvement in sentence-based speech intelligibility
10%
- SNR improvement of roughly 60% of AI-DI for speech recognition in noise
What might cause an amplitude and phase mismatch between microphones? How can we help prevent this
Aging (causes “drift”)
Debris
Regular clearing of the microphone ports
What is adaptive directionality?
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)
Which 3 things are necessary in order for adaptive directional mics to remove noise more effectively than fixed directional mics?
- 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
What factor most limits performance of adaptive-delay directional mics?
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
What is the difference b/w adaptive directionality and automatic switching?
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
What amount of directional advantage was measured in a difficult but realistic environment? Describe the environment
~2 - 4 dB (measured in both adults and children)
- listener facing and close to talker
- poor SNR (10+)
- competing moderate noise
In what situation (regarding direction and proximity of noise and source) would a directional mic not be effective?
Source and noise outside of critical distance
Name 3 situations (regarding direction and proximity of noise and source) in which directional mics are effective
**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
Name 3 limitations of directional mics
- 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)
Why is wind noise more of a problem with directional mics?
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
Which mode do most HAs usually switch to to limit the effects of wind noise?
Omnidirectional
