Transcript 14 binaural and bilateral considerations in

Figures for Chapter 14
Binaural and bilateral issues
Dillon (2001)
Hearing Aids
Horizontal localization
Far ear
Near ear
Figure 14.1 Variation of the source direction in the horizontal plane.
Source: Dillon (2001): Hearing Aids
Interaural time diff (ms)
Inter-aural time differences
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-180 -120 -60
0
60 120 180
Horizontal angle of incidence (deg)
Sounds from left
Sounds from right
Figure 14.2 Interaural time difference for low-frequency sounds as a
function of direction measured from directly in front. Data are the
average of measurements on people and on a manikin (Kuhn, 1982).
Source: Dillon (2001): Hearing Aids
Inter-aural level differences
Interaural level
difference (dB)
25
90o
20
45o
15
10
5
30
o
0
0.1
1
Frequency (kHz)
10
Figure 14.3 Interaural level difference for three source directions in the
horizontal plane. Data are calculated from Shaw (1974). Interaural level
differences are zero for frontally incident sound.
Source: Dillon (2001): Hearing Aids
Vertical localization
Figure 14.4 Variation of the source direction in the vertical plane.
Source: Dillon (2001): Hearing Aids
Head diffraction (dB)
Head diffraction
30
20
30
60
o
120
o
10
0
-10
-20
100
o
o
o
-120 -60 -30
1000
Frequency (kHz)
10000
Figure 14.5 Head diffraction effects from the undisturbed field to the
eardrum for five source directions in the horizontal plane, with positive
angles representing sound arriving from the side of the ear in question.
Data are from Shaw (1974).
Source: Dillon (2001): Hearing Aids
Effect of head diffraction on SNR
60o
SNR decreased by 9 dB
at 3 kHz, and by 8 dB
averaged across
frequency
30o
SNR increased by 11 dB
at 3 kHz, and by 9 dB
averaged across
frequency
Figure 14.6 Effect of head diffraction on the SNR at each ear,
relative to the SNR in the undisturbed field. The SNR at the
right ear is thus 20 dB better than at the left ear at 3 kHz, and
17 dB better when averaged across frequency.
Source: Dillon (2001): Hearing Aids
Inter-aural time differences
Noise
Signal
+
+
Left
Right
Figure 14.7 Waveforms at the left and right ears when noise
arrives from directly in front and a signal (in this case a pure tone)
arrives from one side.
Source: Dillon (2001): Hearing Aids
Demonstrating binaural advantage
Bilateral Fitting
Figure 14.8 Test
arrangement for
demonstrating bilateral
advantage, showing
the location of the
speech (S) and noise
(N) loudspeakers.
Speakers should be 0.5
m or more from the
patient. For unilateral
fittings to the left ear,
the S and N sources
should be reversed for
both the bilateral and
unilateral tests.
Unilateral Fitting
Source: Dillon (2001): Hearing Aids
Detecting negative binaural interactions
Bilateral Fitting
Unilateral Fitting
Figure 14.9 Test arrangement for
detecting negative binaural
interactions. Speech and noise both
come from the same loudspeaker.
Source: Dillon (2001): Hearing Aids
Binaural cross-over effect
Frequency (Hz)
125
250 500
1k
2k
4k
8k
Hearing threshold (dB HL)
0
20
40
60
80
100
120
Figure 14.10 An audiogram for a person who is likely to benefit
from the hearing aid cross-over effect if a bilateral fitting is provided.
Source: Dillon (2001): Hearing Aids
Poorer ear fitting
Frequency (Hz)
125
250 500
1k
2k
4k
8k
Hearing threshold (dB HL)
0
20
40
60
80
100
120
Figure 14.11 An audiogram where the poorer ear should
be aided if the person chooses to have a unilateral fitting.
Source: Dillon (2001): Hearing Aids
Better ear fitting
Frequency (Hz)
125
250
500
1k
2k
4k
8k
0
Hearing threshold (dB HL)
20
40
60
80
100
120
Figure 14.12 An audiogram where the better ear should
be aided if the person chooses to have a unilateral fitting.
Source: Dillon (2001): Hearing Aids