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Audiometric Masking

Table of contents

  1. What is masking?
  2. Interaural attenuation
  3. Masking example
  4. Required equipment
  5. Test procedure
  6. Auto masking
  7. Masking help
  8. References
 

What is masking?

In cases where you detect a symmetrical hearing loss, traditional audiometry without masking is usually enough.

In cases of asymmetrical hearing loss, one cannot be certain that the intended ear is the one detecting the sound.

To prevent this phenomenon causing an erroneous measurement, you can use masking noise to occupy the good ear (non-test ear) while testing the other ear [1-3].

Masking can be applied to air conduction, bone conduction, and speech audiometry.

 

Interaural attenuation

The need to mask the better hearing ear is linked to the interaural attenuation, which equals the amount of attenuation the sound is exposed to on its way through the skull.

Even though the interaural attenuation is very individual and varies with frequency, it can on average be estimated to be a minimum of 40 dB for supra-aural headphones and 50 dB for inserts.

In bone conduction, the interaural attenuation is a minimum of 0 dB, which means that the stimulus may cross over, and this is what one should assume.

 

Masking example

In patients with normal hearing on one ear, but a moderate to severe hearing loss on the other, there is a potential risk of the good ear hearing the tone when trying to test the damaged ear.

That is, the sound vibration may travel through the head and be heard by the opposite good ear when the vibrations of the signal are large enough.

Thus, you would be measuring the thresholds from the wrong ear.

This could be the case in Figure 1, where the right ear (better ear) should be masked while reassessing the left ear (poorer ear).

Audiogram showing a 50 dB HL asymmetry between right and left at the higher frequencies.

Figure 1: Asymmetrical hearing loss; right ear is better than left ear.

Masking is also needed to differentiate between sensorineural, conductive, and mixed hearing losses.

In Figure 1, it is unknown if the loss in the left ear is sensorineural, conductive, or mixed.

The origin will be revealed by obtaining the bone conduction threshold for the left ear while occupying the right ear with masking.

 

Required equipment

(1) Headphones or insert phones

(2) Bone oscillator

 

Test procedure

The procedure is outline below in several steps, some of which are specific to Interacoustics equipment.

 

Step 1

Perform air conduction audiometry unmasked for both ears.

 

Step 2

Perform bone conduction audiometry unmasked for both ears.

 

Step 3

Apply masking if needed:

(1) In case of an air-bone gap of 15 dB or more, you must reassess the bone conduction threshold while applying masking to the non-test ear.

(2) If the difference between the air conduction threshold in the poor ear and the bone conduction threshold in the good ear exceeds an interaural attenuation of 40 dB (50 dB if using insert phones), you will need masking for air conduction audiometry in the poor ear.

 

Step 4

Select NB in channel 2.

 

Step 5

Select which ear to mask and choose the appropriate masking transducer (headphones or insert phones).

This will activate the masking noise.

 

Step 6

Proceed to do a masked threshold search and press ‘Store’ once you have obtained a correct threshold.

The symbol will appear in the audiogram as masked.

There are many ways of applying clinical masking.

Which one to use is your decision.

Regardless of the masking method, use channel 2 to occupy the better ear.

 

Step 7

To display the masking level information on screen, select ‘Mask info’.

 

Example

If we refer to Figure 1, channel 2 should be set to the right ear (non-test ear) using the preferred masking stimulus (usually NB).

Ensure that ‘Rev’ is active to ensure that the masking noise is continuous.

Channel 1 should be set to the left ear (test ear) using the preferred stimulus (usually tone).

The masking frequency will automatically change along with the tone frequency when masking is turned on.

You can set the masking and tone frequencies by using the frequency up/down buttons.

While trying to establish the true threshold of the left ear, the right ear is now distracted with noise.

When storing a threshold while masking, the final masking level is stored in the masking table under the ear that is being tested.

The term ‘effective masking’ in this situation refers to the fact that the narrow band noise level was loud enough to effectively mask a pure tone of the indicated level heard by the masked ear.

 

Auto masking

Interacoustics auto masking is a tool in the Diagnostic Suite to mask with correct masking levels.

When you have enabled auto masking, the system controls channel 2 and sets it to the appropriate intensity level.

Activate auto masking by selecting the icon that shows a mask with the letter A (Figure 2).

Auto masking icon.

Figure 2: Auto masking icon.

Green indicates that masking is correctly applied (Figure 3).

Correct masking, indicated by a green circle.

Figure 3: Correct masking.

Amber indicates that the masking intensity should be louder and that you need to activate the extended frequency range (Figure 4).

Masking noise should be louder, indicated by an amber circle.

Figure 4: Masking noise should be louder.

Purple indicates that masking is needed but non-feasible (Figure 5).

Masking is non-feasible, indicated by a purple circle.

Figure 5: Masking non-feasible.

 

Caveats

(1) Be aware that patients need proper instructions before audiometry with masking is undertaken.

(2) The switching-on-and-off of the masking noise may be uncomfortable to some patients and cause fatigue.

(3) Masking is not recommended in young children, elderly patients, or difficult-to-test patients, because confusion about the application of masking noise may lead to false responses.

(4) You can avoid most errors by measuring the best ear first and completing air conduction on both ears before measuring bone conduction.

(5) Be careful when storing thresholds where masking was non-feasible, where the risk for crossover hearing is high. In these cases, you should store a no-response at the loudest intensity where masking was still possible (by pressing the N key).

 

Terminology

AC

Air conduction test ear

ACc

Air conduction non-test ear

BC

Bone conduction test ear

BCc

Bone conduction non-test ear

IaA

Minimum interaural attenuation

IaAc

Minimum interaural attenuation contra transducer

Dial

Dial setting test ear

Dialc

Dial setting non-test ear (masking level)

Table 1: Masking terminology.

 

Is masking required?

Masking is recommended when the presentation at the test ear can be heard by the non-test ear, or in a formula:

Dial – IaA ≥ lowest of the two ACc/BCc values

 

Is the masking level too low?

The applied masking level is too low when it does not match the intensity at which the test signal is heard in the non-test ear, or in a formula:

Dialc – (ACc – BCc) < Dial – IaA

 

Is the masking level too high?

The applied masking level is too high when it is so loud that it potentially is heard by the test ear, or in a formula:

Air conduction

Dialc – IaAc ≥ Dial – (AC – BC)

Bone conduction

Dialc – IaAc ≥ Dial

 

Is masking impossible?

Masking is not possible when the needed masking level results in ‘over masking’ at the same time:

Air conduction

Dial + (ACc – BCc) – IaA ≥ Dial – (AC – BC) + IaAc

Bone conduction

Dial + (ACc – BCc) – IaA ≥ Dial + IaAc

Nor is masking possible when the needed masking level is higher than the maximum level of the masking transducer:

Dial + (ACc – BCc) – IaA > maximum available Dialc

 

Recommended masking intensity

If masking is required and possible, the recommended masking intensity is the minimum required masking level plus a fixed preferred amount:

Recommended Dialc = Dial – IaA + (ACc – BCc) + preferred extra amount

The recommended masking level is adjusted for values that cannot be reached by the masking transducer due to maximum values.

When you use the auto masking feature, masking intensities are set to the recommended intensity.

 

Masking help

Interacoustics masking help is a tool in the Diagnostic Suite to make it easier to decide on a safe and correct masking intensity.

When masking help is activated, a status light on channel 2 indicates if masking is applied correctly.

Activate masking help by selecting the icon with the mask (Figure 6).

Masking help icon.

Figure 6: Masking help icon.

Grey indicates that the masking help is not active (Figure 7). Green indicates that masking is correctly applied.

Masking help not active, indicated by a grey circle.

Figure 7: Masking help not active.

Optionally, masking help can give a suggested masking level.

The example here suggests 85 dB, but 75 dB is also within the safe masking range (Figure 8).

75 dB masking level is adequate, indicated by a green circle, but the software suggests 85 dB.

Figure 8: 75 dB masking level is adequate, but 85 dB is suggested by the software.

Amber indicates too much or too little masking, easier clarified by the suggested masking level (Figure 9).

75 dB masking level is incorrect, indicated by an amber circle, and the software suggests 85 dB.

Figure 9: Too much or too little masking. Too little in this case, with 85 dB suggested by the software.

Purple indicates that masking is needed but non-feasible (Figure 5).

 

Frequency-specific interaural attenuation

The interaural attenuation values used by the masking help feature are frequency-specific and you can customize them in the setup.

Table 2 shows the default interaural attenuation values (IaA).

Frequency (Hz)

125

250

500

750

1000

1500

2000

3000

4000

6000

8000

IaA headphones (dB)

35

40

40

40

40

40

40

45

50

50

50

IaA inserts (dB)

50

50

50

50

50

50

50

50

50

50

50

IaA bone (dB)

0

0

0

0

0

0

0

0

0

0

0

Table 2: Default interaural attenuation values.

 

Caveats

(1) Experienced audiologists should not use the option of viewing a suggested masking level.

(2) The masking help indicates if masking is performed correctly with the information given at the time of the measurement. You can avoid most errors by measuring the best ear first and completing air conduction on both ears before measuring bone conduction.

(3) Although clinicians tend not to apply masking for bone conduction in instances where the air-bone gap is less than 15 dB in the better ear, the masking help feature may recommend masking (Figure 10).

Air-bone gap is only 10 dB, but the software suggests a masking level of 45 dB.

Figure 10: Air-bone gap is less than 15 dB in the test ear, but the masking help feature suggests a masking level of 45 dB.

 

References

[1] Stach, B.A (1998). Clinical Audiology: An Introduction. Cengage Learning.

[2] J. Katz (2002). Handbook of Clinical Audiology (5th ed.). Lippincott Williams & Wilkins.

[3] British Society of Audiology (2018) Pure tone air and bone conduction threshold audiometry with and without masking. Retrieved March 31, 2021, from https://www.thebsa.org.uk/resources/pure-tone-air-bone-conduction-threshold-audiometry-without-masking/.

June 2016
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