LESSON 88

When Is Air-Conduction Masking Needed?

Name: When Is Air-Conduction Masking Needed?
Uploaded: 2026-11-04T03:30:00+03:00
Duration: 10 min 28 s
Description: In air conduction testing, masking is needed when the difference between the air conduction threshold of the test ear and the bone conduction threshold of the non-test ear reaches interaural attenuation. In this lesson, we use sample audiograms to work through the test ear (TE) and non-test ear (NTE), the masking rule and the difference between supra-aural and insert earphones.

In air conduction testing, masking is needed when the difference between the air conduction threshold of the test ear and the bone conduction threshold of the non-test ear reaches interaural attenuation. In this lesson, we use sample audiograms to work through the test ear (TE) and non-test ear (NTE), the masking rule and the difference between supra-aural and insert earphones.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson, we learn how we decide when masking is needed in air conduction testing.
We will apply the rule step by step to sample audiograms.
First, let us define two key terms: the ear whose threshold we are measuring is called the test ear, or TE.
The other ear is called the non-test ear, or NTE.
In masking, the tone goes to the TE, and the masking noise goes to the NTE.
When testing the right ear, the right ear is the TE and the left ear is the NTE; for the left ear, the roles switch.
Whether masking is needed is decided separately for each frequency.
Masking may be needed at one frequency but not at the neighboring one.
The masking rule for air conduction starts with the air conduction threshold of the test ear.
From this threshold, we subtract the bone conduction threshold of the NTE.
If the difference is equal to or greater than the interaural attenuation, or IA, cross-hearing is possible.
In that case, the TE air conduction threshold is measured again with the NTE masked.
Why the bone conduction of the NTE? Because the crossed sound reaches the opposite cochlea by bone conduction and stimulates it directly.
So even if the NTE has a conductive loss, its bone conduction threshold decides whether it picks up the crossed sound.
In the diagram, imagine we deliver a seventy decibel tone to the TE.
As the sound crosses the skull, it is weakened by about forty decibels in this example.
About thirty decibels reach the NTE cochlea; if the NTE bone conduction threshold is thirty or better, the tone is heard there.
A conductive block in the outer or middle ear of the NTE does not stop this path, because sound reaches the cochlea through the skull.
In this case, we deliver noise to the NTE to keep its cochlea busy and prevent cross-hearing.
The IA value used in the rule is chosen according to the earphone used.
For supra-aural earphones, the conservative value is forty decibels; if the difference is forty or more, we mask.
IA is higher with insert earphones; guidelines usually use fifty five decibels as the conservative value.
In this lesson, we will use fifty five decibels for inserts; some protocols use separate values for each frequency.
Because IA for bone conduction is taken as zero, its rule is different and is the topic of the next lesson.
In the first example, the right ear is normal; its air and bone conduction thresholds are in the normal range.
The unmasked air conduction thresholds of the left ear are between fifty and seventy decibels.
When testing the left ear, the left ear is the TE and the right ear is the NTE.
At one thousand hertz, left air conduction is sixty and right bone conduction is ten decibels; the difference is fifty.
Fifty is greater than forty, so with supra-aural earphones, masking is needed at this frequency.
When we do the same calculation at the other frequencies, the difference is above forty at every one.
So with supra-aural earphones, all left air conduction thresholds must be measured again with the right ear masked.
Now imagine testing the same patient with insert earphones and using an IA value of fifty five decibels.
At two hundred fifty and five hundred hertz, the difference is forty five decibels, which is below fifty five.
At one thousand hertz, the difference is fifty decibels, which is also below fifty five.
At two thousand and four thousand hertz, the difference is fifty five decibels; it reaches the limit, so masking is needed.
At eight thousand hertz, bone conduction is not measured, so NTE air conduction is used; the difference is fifty, so no masking.
As you can see, insert earphones clearly reduce the number of frequencies that need masking.
In the second example, the right ear has a sensorineural loss of around seventy decibels, and the right ear is the TE.
The air conduction thresholds of the left ear, the NTE, are between thirty five and forty five decibels.
But left bone conduction is normal, so the left ear has a conductive loss.
Comparing the air conduction thresholds of the two ears at one thousand hertz, the difference is only thirty five; masking seems unneeded.
But by the rule, the correct comparison is with NTE bone conduction: seventy minus ten, which is sixty decibels.
Sixty is well above forty, so the air conduction thresholds of the right ear must be measured with masking.
The crossed sound bypasses the conductive block in the left ear and reaches the left cochlea directly.
This example clearly shows why the comparison is made with bone conduction, not air conduction.
In the clinic, the air conduction thresholds of both ears are usually measured first, followed by bone conduction.
So at first, a provisional decision is made from the difference between the air conduction thresholds of the two ears.
After bone conduction is measured, the rule is checked again using the NTE bone conduction threshold.
This check may reveal new frequencies that need masking, as in the second example.
Because unmasked bone conduction reflects the better of the two cochleae, using this value in the comparison is a conservative approach.
Some guidelines also use the air conduction difference between the ears as a separate rule when bone conduction is not known.
In the third example, both ears have a sensorineural loss of similar degree.
The right and left air conduction thresholds are very close, and bone conduction thresholds match air conduction.
At no frequency does the difference between TE air conduction and NTE bone conduction exceed fifteen decibels.
This difference is far below forty, so no masking is needed for air conduction.
In symmetrical losses with no air-bone gap, the need for masking is usually small.
Let us recall a common mistake.
If the opposite ear has a conductive loss, the air conduction thresholds of the two ears may look close to each other.
In that case, deciding that masking is not needed by looking only at the air conduction difference is misleading.
After measuring bone conduction thresholds, recheck the decision at every frequency.
This quick check lets you catch a faulty audiogram before it goes into the report.
Let us tabulate the decisions at one thousand hertz: in the first example, with supra-aural earphones, the difference is fifty, so we mask.
In the same example with insert earphones, the difference is still fifty, but it is below fifty five, so no masking is needed here.
In the second example, the difference is sixty decibels; even though the air conduction thresholds look close, masking is essential.
In the third example, the difference is ten decibels; no masking is needed.
In every case, the question is the same: after losing IA, can the sound given to the TE be heard in the NTE cochlea?
Air conduction thresholds found with masking are marked with a red triangle on the right and a blue square on the left.
This way, anyone reading the audiogram knows the threshold was confirmed under masking.
The level of the masking noise used is also written on the form or below the audiogram.
Even if the threshold did not change after masking, the symbol showing a masked measurement is used.
We will see what level of noise to use in the lessons on effective masking and the plateau method.
The ear whose threshold we measure is the TE, and the opposite ear that receives the noise is the NTE.
In the air conduction rule, the NTE bone conduction threshold is subtracted from the TE air conduction threshold.
If the difference reaches forty for supra-aural earphones or about fifty five for insert earphones, masking is needed.
After bone conduction is measured, the decision is checked again at every frequency.
Test yourself: Right air conduction is seventy and left bone conduction is fifteen decibels. With supra-aural earphones, what should we do?
The correct answer is B: the left ear is masked while the right ear is tested.
The difference is fifty five decibels, above forty, so the right air conduction threshold is measured again with noise in the left ear.
In the next lesson, we will talk about when masking is needed in bone conduction testing, and about the occlusion effect.
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Prerequisites

LESSON 87 Why We Mask : Interaural Attenuation Explained