Directional Microphones: Efficacy

1
Directional Microphones Efficacy Effectiveness

• New Zealand Audiological Society
• 33rd Annual Convention
• July 3, 2009
• Taupo, NZ

2
Directional Microphone

• Single Capsule
• Two ports

3
Directional Microphone

• Single Capsule
• Two ports
• Switch between one port and two ports

4
Electric Delay Network
Electronic internal time delay in pre-amplifier ?
Time delay (?) is an adjustable parameter
5
Directional Microphone

• This is referred to as a microphone array due to
  its linear formation of more than two microphones

6
Polar Response Pattern
Free field characteristics of different types of
microphones (Knowles TB 21)
Omnidirectional Cardioid Hypercardioid
Supercardioid
7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10
(No Transcript)
11
(No Transcript)
12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
(No Transcript)
16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
(No Transcript)
20
(No Transcript)
21
(No Transcript)
22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
(No Transcript)
26
(No Transcript)
27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
(No Transcript)
31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
34
(No Transcript)
35
(No Transcript)
36
(No Transcript)
37
(No Transcript)
38
(No Transcript)
39
(No Transcript)
40
(No Transcript)
41
(No Transcript)
42
(No Transcript)
43
From that polar we can derive an index of
directivity (DI)
44
Or...for three-dimensional measures
45
Or...for three-dimensional measures

• Yipes

46
What kind of DIs are possible?

• Theoretically, up to to 6 dB (max) for 1st order
  mics
• A few more dB for higher order mics
• Free field measures, less
• Head mounted measures, even less!
• Know what you are reading.

47
Theoretical DIs

• First-Order Cardioid Theoretical DI
  4.788135 dB
• First-Order Hyper-Cardioid Theoretical DI
  6.04866 dB
• First-Order Super-Cardioid Theoretical DI
  5.741963 dB

48
Theoretical DIs

• Second-Order Cardioid
• Theoretical DI 8.792686598 dB
• Second-Order Hyper-Cardioid
• Theoretical DI 8.803253 dB
• Second-Order Super-Cardioid
• Theoretical DI 9.077517 dB

49
FF (BTE)
KEMAR (BTE)
Theoretical
Cardioid
Hypercardioid
Supercardioid
50
FF (ITE)
KEMAR (ITE)
Theoretical
Cardioid
Hypercardioid
Supercardioid
51
Evidence that

• DIs obtained from polar patterns measured with
  hearing aid placed on the side of KEMARs head
  relate pretty well to SNR improvement of listener
  (at least in low or non-reverberant
  environments)
• Higher-order mics (made up of more than two
  ports) have higher DI values
• ANSI standard now for measurement by
  manufacturers (but with voluntary compliance
  clause)

52
(No Transcript)
53
(No Transcript)
54
(No Transcript)
55
(No Transcript)
56
Null steering

• Achieved (basically) by comparing outputs from
  multiple mic ports, and settling with the
  electrical delay that results in the least power
  sum from the ports (that would be where the null
  is centered on the primary noise).

57
(No Transcript)
58
Side-band gain reduction

• Clinician chooses polar and sets to Directional
  Focus
• No moving nulls

My term, not theirs!
59
SIs Innova fixed mode at 1ooo Hz
60
Sonic adaptive mode at 1000 Hz note only the
rear reduces (gain reduction, not steering)
61
(No Transcript)
62
Examples of current microphone schemes

• Split directionality (Frequency-specific)
• Channel-less directionality
• Additional gain reduction

63
(No Transcript)
64
(No Transcript)
65
(No Transcript)
66
(No Transcript)
67
(No Transcript)
68
(No Transcript)
69
Quick tutorial, cont.

• Ways to implement directionality in the hearing
  aid case
• Fixed polar pattern
• Program different polar patterns in different
  memories
• Automatic directional mode
• Adaptive directional mode
• Null steering
• Side-angle gain reduction

70
Data?

• Plenty of efficacy data for early designs
  depending upon
• Baseline used
• Speaker arrangement
• Noise type
• Etc

71
Walden, Surr, Cord, 2003
72
Wu Bentler, 2008
73
Why is that?

• Possible (data-based) explanations
• Directivity (DI) too small to have impact
• Reverberation eliminates the advantages of the
  nulls
• Real-world is not that neatly arranged (noise in
  back, e.g.)
• Visual cues already maximize benefit
• Old people have different communication
• DIR mode is too noisy

74
Why is that?

• Other possible implicators
• The classification schemes arent that good
  (i.e., switch at the right time)
• The adaptive schemes cannot manage multiple noise
  sources
• Time constants for switching are too slow
• Important information also comes from off-axis
  locations

75
What about kids?

• Little data to support (or not) directional mic
  use in the pediatric population
• 35 audiologists fit DIR on kids (Rigsby et al,
  2007)
• 4.7 dB DIR benefit (Gravel et al, 1999)
• 5.5-8 dB DIR benefit (Kuk et al, 1999) but
• Only 0O/180o arrangement
• No blinding (important for the selfreport
  only)
• Still doesnt deal with concern of WHO does the
  switching and WHEN and WHERE

76

• Stiles, Bentler, McGregor (2008)
• Do directional microphones hamper incidental word
  learning or speech perception from off-axis
  sources?
• Novel word learning task developed
• PBK words used for speech recognition task

77
Stiles et al, 2008 Word recognition re PBK
78
Stiles et al, 2008 Novel Word Learning
79
Bottom line

• Even though directional mics are not perfect for
  noise reduction, they are the one feature that we
  know has the capability of improving the
  signal-to-noise ratio for the listener.
• Though there is little logic for fitting them on
  infants, there is little danger (and potential
  benefit) in fitting them on children.