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:

Frequency (Hz) Min IA:

125 35

250 40

500 40

1000 40

2000 45

4000 50

8000 50

(Test Ear AC– Non-test BC)

Effective Masking

Calibration of decibel level of maskersLevel 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 = ACte -BCte

Masking

Air Conduction: Performed w/ earphones in place.

Bone Conduction: Performed w/ bone in place and earphone on NTE only!

Ranges of Masking

20

30

40

50

60

70

80

90

100

20 30 40 50 60 70 80 90

Masker Level (dB EM)

Thr

esho

ld in

TE

(dB

HL

)

Under Sufficient Over

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 thresholdAs simple as the plateau methodMore time efficient than plateau

methodCan 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 orInitially 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 HzIssue for Masking? (Optimized-No)

What to do about the Occlusion Effect?

Add to your starting level at low frequencies:

250 Hz -- additional 15 dB500 Hz -- additional 15 dB1000 Hz -- additional 10 dB

(Goldstein & Newman, 1994)