Transcript Masking: - University of Florida

Masking:
 Insuring
that the non-test
cochlea is not helping out.
Making sure that the ear you
want to test is the one that is
responding.
Putting noise into non-test ear via
Air Conduction
 To
"keep it busy."
 For tones use Narrow band noise (1/3
octave).
 For speech use white or pink (speech)
noise.
WHY? Cross Hearing!
 Key
Concept=Interaural attenuation!
The amount of sound required to
bridge across the head and stimulate
the opposite cochlea by BC.
 Unmasked thresholds trace out a
“shadow curve” displaced from the
non-test ear by the size of IA.
INTERAURAL ATTENUATION
 TDH-49
:
approx. 40 to 60 dB.
 Bone Vibrator: approx. 0 to 10 dB.
 Insert Phone: approx. 75-90 dB.
Supra-aural Minimum IA Values:
(Test Ear AC– Non-test BC)
Frequency (Hz)
Min IA:
125
35
250
40
500
40
1000
40
2000
45
4000
50
8000
50
Effective Masking
 Calibration
of decibel level of maskers
 Level to which tone threshold will be
shifted in the presence of the masker.
 e.g., a 45 dB EM narrow band noise
centered at 2000 Hz would raise
someone’s threshold for a 2000 Hz
tone to 45 dB HL **
When to mask Air Conduction?
 AC:
when unmasked threshold in TE is
greater than BC threshold in NTE by
the minimum IA value or more.
When to mask Bone Conduction
 Mask
any significant Air-Bone Gap:
 Katz:
>10 dB
 Many others: >10 dB
 Air-Bone
Gap = ACte -BCte
Masking
 Air
Conduction: Performed w/
earphones in place.
 Bone Conduction: Performed w/ bone
in place and earphone on NTE only!
Threshold in TE (dB HL)
Ranges of Masking
100
90
80
70
60
50
Sufficient
Under
Over
40
30
20
20
30
40
50
60
70
Masker Level (dB EM)
80
90
Undermasking:

condition when noise presented to
the NTE is not sufficient to keep it from
contributing to the response.
Sufficient Masking:

condition when the noise presented
to the NTE is sufficient to keep it from
contributing to the response and is not
intense enough to influence the ability
of the TE to respond.
Overmasking:

condition when the noise presented
to the NTE is intense enough to cross
over to the TE and mask it.
How much Noise?
Where to start/Where to stop?
 Initial
Masking:
 Katz: 30 dB SL EM in non-test ear
 If
no shift in test ear threshold?
 If threshold shifts, then what?
Hood Technique: following
unmasked thresholds.
 leave
tone at unmasked threshold, set
masker to Initial Masking
 present
–
–
tone
if yes, masker up 10 dB & repeat,
if no, then tone up 10 dB & repeat.
Hood Technique (continued)
 when
responses to tone obtained at
same level through several steps of
the masker, you have reached the
"plateau." leaving masker at that level
repeat threshold sweeps. Mark
masked threshold symbol and final
level of masking.
Turner’s Optimized Masking
1. Measure an unmasked AC threshold
for each ear and an unmasked BC
threshold.
2. At each frequency, determine if
masking is required using conventional
criteria.
3. Set the initial masking level equal to
the AC threshold of the test ear (TE)
minus 10 dB and reestablish threshold.
Optimized Masking (cont’d)
4. Determine the dB shift in threshold due to
the masker.
5. Increase the masking level an amount
equal to the threshold shift and reestablish
threshold.
6. If threshold improves or does not shift, the
masker is in the plateau and the actual
threshold has been determined. If the
threshold shifts, repeat steps 5 and 6.
Optimized method
 Valid
measure of threshold
 As simple as the plateau method
 More time efficient than plateau
method
 Can replace the plateau method in
MOST situations
Recommended Protocol
(can be used in all masking situations with supra-aural or insert
phones)
 When testing the poorer ear:
– If the unmasked AC thresholds differ by more than
20 dB, then use the optimized method.
– If the difference in unmasked AC thresholds is 20
dB or less, then there are two options:
Use the plateau method with 5 dB masker steps or
 Initially use the plateau method with 10 dB steps. If no
plateau can be identified, then retest using 5 dB steps.

Recommended Protocol
 When
–
–
–
testing BC thresholds:
Always set the initial masking level at least 30 dB
greater than the unmasked BC threshold.
If the masking method being used specifies a greater
value, use that.
This strategy automatically compensates for occlusion
effect correction factors up to 20 dB.
Recommended Protocol
 When
testing the better ear (seldom
necessary for AC thresholds):
–
Use the plateau method with 5 dB masker steps
Additional notes
 Thresholds
can be reestablished using
the traditional up-down search or the
single-tone procedure. The single-tone
procedure will reduce testing time.
 Try to avoid high masking levels.
–
Occasionally, the optimized method may specify a
large increase in masking level, and that increase may
produce a masker greater than 80 dB HL. When this
occurs, consider using smaller increases in masking
level to identify plateau.
The Occlusion Effect
 Enhancement
of Bone Conduction
produced by closing off the external
ear canal.
 Most pronounced at frequencies below
1000 Hz
 Can be as much as 25 dB at 500 Hz
 Issue for Masking? (Optimized-No)
What to do about the Occlusion
Effect?
 Add
to your starting level at low
frequencies:
 250 Hz -- additional 15 dB
 500 Hz -- additional 15 dB
 1000 Hz -- additional 10 dB
(Goldstein & Newman, 1994)