Written by
Hlolo Ramatsoma
Hlolo Ramatsoma

Hlolo is a clinical, research & support Audiologist at eMoyo. He is involved in many parts of the business, from consulting to R&D to supporting and training customers. He earned his BSc in Audiology from the University of Cape Town and is an experienced clinical audiologist specialized in ototoxicity monitoring, product specialist and training audiologist.

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Masking in Audiology: Rules For Knowing exactly when to mask?

Binaural hearing, or simply put, the ability to hear with both ears helps us to localize sounds, be aware of your surroundings, hear better in background noise (when one has normal hearing), perceive better sound quality, and strain less when listening.

While both ears working together is beneficial, it may cause difficulty when conducting audiometry testing. Recently, I wrote about everything you might not know about masking in audiology. This article covered the science and reasoning behind masking and how to mask.

In this article, we look at the question “When should I mask?” and provide answers including some examples.

Masking is a procedure clinicians use while conducting audiometry to separate ears acoustically. Masking allows for a clinician to get ear specific thresholds, without the non-test ear assisting the test ear to hear - resulting in better thresholds. In audiology, noise is used as a masker by introducing the noise in the non-test ear, while tones (or speech signals) are presented in the test ear.

This ensures that the clinician truly tests the ear that they intend to test.

 

When To Mask?

Masking during Air Conduction

In essence, the question when to mask can be answered by looking at three factors. These factors to be considered included:

  1. Interaural attenuation (IA),
  2. Unmasked air-conduction thresholds in the test ear, and
  3. Bone conduction hearing thresholds in the non-test ear.

When a decision is made about when to mask, air conduction thresholds in the test ear (ACTest Ear) are compared to bone conduction thresholds in the non-test ear (BCNon-Test Ear).

Do note that when crossover hearing occurs, the non-test ear is primarily stimulated via bone conduction mechanism.

The rule for contralateral masking can be simply put in a formula below:

ACTest Ear − BCNon-Test Ear ≥ IA

With this formula, ‘’apparent’’ bone conduction thresholds will be used to determine the need for masking. The term ‘’apparent’’ is used as you may remember that unmasked BC threshold does not convey ear-specific information.

Without masking during BC, it is assumed that responses could be from both ears, or the better hearing ear (which we do not know till we mask). In addition, because it is not possible to measure BC at 8000 Hz, it is necessary to predict the BC threshold at 8 kHz given the other frequency findings.

For example; if there are no air-bone gaps between the other frequencies, one can assume that the BC finding at 8 kHz would’ve been the same as the AC threshold at that frequency.

 

Here's a tip. 

Ask your patient if they are aware if one of their ear may hear better than the other. If this is the case, it may save you a lot of time. Commence testing with the better ear first (as described by the patient), so that when you begin testing in the poorer ear and there is a considerable difference between the ears (IA), a masking noise can be presented to the better hearing ear then-and-there to prevent the patient hearing with that ear.

A lot of Audiologists will obtain air conduction (AC) thresholds for both ears prior to measuring bone conduction thresholds.  In this case, a preliminary decision about the need to mask can be made by comparing AC thresholds of both ears.

 

Have a look at this formula:

ACTest Ear − ACNon-Test Ear ≥ IA

 

It is important to remember, again, that cross hearing for air conducted sounds primarily occurs via BC mechanism. Therefore, it is necessary for one to re-evaluate the need for masking during AC once unmasked BC thresholds have been obtained. As mentioned above, traditionally, AC thresholds are obtained before BC thresholds.

However, a recommended approach is to obtain unmasked BC thresholds before obtaining AC thresholds. Decision regarding the need for masking then can be made using BC thresholds.

 

*For the purpose of this exercise insert earphones will be utilized.

  • Note that with insert earphones, interaural attenuation (IA) is approximately 60 dB. If you are using supra-aural headsets in your clinic, you may apply 40 dB as your IA during this exercise.

 

Case 1

Consider the audiogram below. Because IA for BC sound is considered ~0dB, unmasked BC thresholds can be obtained using either mastoid, or by placing the bone vibrator on the patient’s forehead.

 

no air bone gaps audiogram

 Courtesy of audstudent.com

Because the audiogram in this case has both unmasked BC and AC thresholds, we will use this formula: ACTest Ear − BCNon-Test Ear ≥ IA to determine the need for masking. In addition, because there are no air-bone gaps between 500 Hz to 4000 Hz, we can assume that the ‘BC threshold’ at 8000 Hz would be similar to the AC threshold.

 

 

250 Hz

500 Hz

1000 Hz

2000 Hz

4000 Hz

8000 Hz

Right Ear (Test Ear)

65

70

65

75

95

NR

BC Non-Test Ear

5

15

20

30

40

55

 

65-5≥60?

70-15≥60?

65-20≥60?

75-30≥60?

95-40≥60?

NR-55≥60?

Should I mask?

Yes

No

No

No

No

No?

 

This is a very interesting case. At 8000 Hz, there is no response for AC. Masking in the opposite ear will not change the no response. To understand this, remember that contralateral masking is ONLY required whenever there is the possibility that the test signal can be perceived in the non-test ear, ALLOWING the non-test ear to respond instead and help the test ear.

Clearly, in this case, the non-test ear did not/does not help the right ear (test ear).

 

 

250 Hz

500 Hz

1000 Hz

2000 Hz

4000 Hz

8000 Hz

Left Ear (Test Ear)

30

25

25

35

45

55

BC Non-Test Ear

5

15

20

30

40

55

 

30-5≥60?

25-15≥60?

25-20≥60?

35-30≥60?

45-40≥60?

55-55≥60?

Should I mask?

No

No

No

No

No

No?

No contralateral masking will be required when testing the left ear.

 

Case 2

Consider the audiogram below. In this example, no BC thresholds have been obtained. 

air conduction thresholds

 Courtesy of audstudent.com

 

One can make preliminary decision regarding weather to mask or not using the AC thresholds.

 

Use this formula:

ACTest Ear − ACNon-Test Ear ≥ IA

 

250 Hz

500 Hz

1000 Hz

2000 Hz

4000 Hz

8000 Hz

Right Ear (Test Ear)

75

85

85

90

90

95

Left Ear Non-Test Ear

15

10

15

20

30

20

 

75-15≥60?

85-10≥60?

85-15≥60?

90-20≥60?

90-30≥60?

95-20≥60?

Should I mask?

Yes

Yes

Yes

Yes

Yes

Yes

  

In this case of asymmetrical hearing loss, using the formula above, it is clear that contralateral masking is required to ensure that accurate thresholds are obtained in the right ear (test ear).

 

 

250 Hz

500 Hz

1000 Hz

2000 Hz

4000 Hz

8000 Hz

Left Ear (Test Ear)

15

10

15

20

30

20

Right Ear Non-Test Ear

75

85

85

90

90

95

 

15-75≥60?

10-85≥60?

15-85≥60?

20-90≥60?

30-90≥60?

20-95≥60?

Should I mask?

No

No

No

No

No

No

 

From the look of the audiogram alone, it is clear that the left ear is not being assisted by the right ear to hear better. The calculation above shows that contralateral masking is not required when testing the left ear in this case.

 

Masking During Bone Conduction

With BC testing, IA should always be considered to be 0 dB. If one wants ear specific information is needed, one always has to mask the opposite ear. This is true for both mastoid and/or forehead bone vibrator placement.

The other factor to consider when asking yourself whether to mask or not is whether unmasked BC threshold/s suggest the presence of a conductive component - this is necessary for the clinician to determine the type of hearing loss the patient has.

Given the above statements, contralateral masking during BC is not always required, we will see these in a few cases below.

Masking is required in the non-test ear when there is an ‘apparent’ air-bone gap. The word ‘apparent’ here is used as this status may be changed when masking is introduced.

 

What is an air-bone gap?

An air-bone gap is apparent when AC and BC thresholds do not match each other because AC yielded poorer results as compared to BC thresholds (Therefore, Air-Bone Gap = ACTest Ear - Unmasked BC).

 

The rule for contralateral masking can be simply put in a formula below:

Air-Bone Gap Test Ear ≥ 15 dB

 

Katz and colleagues (2014) suggest a potential air-bone gap to be ≥15 dB. The reasoning behind this is that one has to account for the differences that are inherent in BC measurements. There are individuals without a conductive hearing loss who have a greater variability between their AC and BC threshold, therefore a gap of ±10 dB should be expected even in subjects without a conductive hearing loss.

A criterion of 10 dB, as suggested by ASHA, is too strict when considering when to mask during BC. With that being said, some may argue that a 15 dB criterion is too lenient and may result in under-diagnoses.

In conclusion, a clinician should be able to use a criterion of their choice with caution, and interpret BC results in parallel to other audiological tests. For the purpose of this exercise, the 15 dB criterion will be used.

Let us look at the cases below on when one might need to mask.

 

Case 1

Consider the audiogram below. Because IA for BC sound is considered ~0dB, unmasked BC thresholds for this audiogram are not ear specific. In essence, the BC response could be that of the left ear or the right ear.

 

audiogram

 Courtesy of audstudent.com

Using this formula: Air-Bone GapTest Ear ≥ 15 dB, it is clear that contralateral masking will be required when measuring BC thresholds for both ears. The formula suggests air-bone gaps ranging between 50 to 60 dB across the frequencies.

Looking closely to the audiogram, one cannot deduce the type of hearing loss in either ear. BC thresholds may be as good as the unmasked BC thresholds in each ear (we know for a fact that this is true for one ear - we do not know which though), OR BC thresholds in one ear may be as poor as the AC thresholds in that ear. To be certain of the type of hearing loss, masking is necessary in both ears.

 


 

Case 2

Looking at the audiogram below, we can see that contralateral masking is not required for either ear. ‘But we need ear-specific information’ - although this statement is correct, obtaining masked BC thresholds would not provide any additional clinical diagnostic information.

 

sensorineural audiogram
 

We can see above that there are no potential air-bone gaps of 15 dB or greater in either ear. We can conclude here, that the hearing loss in both ears is a sensorineural.

Technically, the BC obtained here are the best they can be - masking would not improve them.

 


 

Case 3

 

when to mask

 

When comparing unmasked BC thresholds and the AC thresholds in the right ear, one can see that there is no suggestion of a air-bone gap, therefore, we can conclude that those are the true BC thresholds of the right ear. Comparisons of the unmasked BC thresholds and the AC thresholds of the left ear suggest air-bone gaps ranging from 20 to 35 dB. Therefore, masking is necessary.

 

Looking at this audiogram closely, specifically for the left ear,

  1. the results of the unmasked BC thresholds may reflect hearing sensitivity of the right ear (better ear),
  2. Or BC thresholds in the left ear may be as good as the unmasked BC thresholds,
  3. Or the BC thresholds may be as poor as the AC thresholds in the left ear. Because we do not know, we need to mask the right ear to get left ear BC thresholds.

 


 

In conclusion

When answering the question “when to mask”, both insert earphones and supra-aural headsets use the same formula. The only difference here is that the IA of insert earphones is larger than that of supra-aural headsets. IA is the reduction of acoustic energy between ears.

Therefore, the need to mask during AC is greatly reduced when using insert earphones for audiometers like the KUDUwave. 

 

Now it's your turn…

During air-conduction, which frequencies would you mask here, using insert earphones (IA= 60 dB) as your transducers?

 

Leave your answer in the comment section.

 

test audiogram
 
 
 

 

Topics: Masking, Featured

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