LESSON 87

Why We Mask : Interaural Attenuation Explained

Name: Why We Mask : Interaural Attenuation Explained
Uploaded: 2026-11-04T03:30:00+03:00
Duration: 11 min 46 s
Description: Sound delivered to the test ear can cross the skull to the opposite cochlea and lead to false thresholds. In this lesson, we use sample audiograms to look at cross-hearing, the shadow curve, interaural attenuation (IA) values and the advantages of insert earphones.

Sound delivered to the test ear can cross the skull to the opposite cochlea and lead to false thresholds. In this lesson, we use sample audiograms to look at cross-hearing, the shadow curve, interaural attenuation (IA) values and the advantages of insert earphones.

Watch on YouTube

Follow along

Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson, we learn why clinical masking is needed and what interaural attenuation means.
We will also learn to recognize the shadow curve and see why insert earphones have an advantage.
In pure tone audiometry, we want to find the threshold of each ear separately.
But when a loud enough sound is delivered to the test ear, the earphone also sets the skull vibrating.
This vibration reaches the cochlea on the opposite side by bone conduction, and that cochlea may detect the sound.
This is called cross-hearing.
The patient reports hearing the sound, but actually heard it with the opposite ear, not with the ear being tested.
So the threshold we record is not the true threshold of the test ear.
If hearing in the two ears is similar, this problem rarely arises; the real risk comes when there is a large difference between the ears.
The diagram shows the test ear and the opposite ear.
Sound delivered to the test ear is weakened to some extent as it crosses the skull to the other side.
The sound reaching the opposite cochlea is lower in intensity than the original sound.
This amount of weakening is called interaural attenuation, or IA.
For example, if IA is forty decibels, a seventy decibel sound in the test ear reaches the opposite cochlea at about thirty decibels.
If the bone conduction threshold of the opposite ear is thirty decibels or better, this sound can be heard there.
To prevent this, we deliver noise to the opposite ear; this procedure is called clinical masking.
If cross-hearing goes unnoticed, the loss in the poorer ear looks milder than it really is.
For example, an ear with no hearing at all can be recorded as if it had a moderate loss.
Unmasked bone conduction responses can make a sensorineural loss look like a conductive loss.
These errors can lead to wrong referrals, unsuitable hearing aid decisions and unnecessary investigations.
Because severe one-sided losses may need further investigation, an accurate threshold in that ear matters even more.
In short, masking is the basic step that guarantees each threshold on the audiogram really belongs to the ear it is shown for.
Now let us consider an example: in this example, the right ear hears normally.
The left ear has no hearing at all; the left cochlea does not respond to any stimulus.
When we test the left ear without masking, we still get responses between fifty five and seventy decibels.
These responses do not belong to the left ear; the sound crossed the skull and was heard by the right cochlea.
The curve formed by these false thresholds is called the shadow curve; it follows the good ear's curve about IA lower.
The unmasked bone conduction thresholds of the left ear come out almost the same as the right ear.
This picture can be misread as a conductive loss in the left ear, although the left ear has no hearing at all.
Masking is done precisely to prevent serious errors of interpretation like this.
In the same patient, let us deliver a suitable level of masking noise to the right ear and test the left ear again.
Because the right cochlea is now busy with the noise, cross-hearing no longer occurs.
In the left ear, there is no response even at the audiometer's maximum output; the down arrows show no response.
Masked air conduction thresholds of the left ear are marked with a blue square.
The audiogram now correctly shows that there is no response in the left ear at the limits of the equipment.
When masking is used, different symbols are used on the audiogram.
The masked air conduction threshold of the right ear is shown with a red triangle, and that of the left ear with a blue square.
Masked bone conduction thresholds are marked with an opening square bracket for the right ear and a closing one for the left.
In this example, the left ear is normal, and the right ear has a sensorineural loss with thresholds confirmed by masking.
Anyone reading the audiogram can tell from these symbols right away that masking was used.
So if masking was used, the correct symbol must be chosen, and the masking level used must also be recorded.
The IA value varies from person to person, by frequency and by the transducer used.
For supra-aural earphones, which sit on the auricle, IA is taken to be at least about forty decibels.
The real value is higher in most people, but the lowest value is used to stay on the safe side.
For insert earphones, placed in the ear canal, IA is higher; when inserted correctly, the conservative value is about fifty five decibels.
With insert earphones, IA depends on frequency: at low frequencies it is often above seventy decibels, and at high frequencies it is lower.
For the bone vibrator, IA is taken to be about zero decibels; the whole skull vibrates and both cochleae are stimulated.
So we cannot know for sure which ear an unmasked bone conduction threshold belongs to.
IA values were measured in people with total one-sided loss, by comparing unmasked thresholds with the thresholds of the good ear.
These measurements show wide differences between people, even with the same earphone.
With supra-aural earphones, IA can vary from about forty to eighty decibels, depending on the person and the frequency.
How the earphone sits on the head and how firmly it presses also affect the result.
With insert earphones, how deeply the foam tip is inserted into the ear canal is the deciding factor.
Because we cannot know the patient's own IA, in the clinic we always use the lowest, most conservative value.
We roughly estimate the level reaching the opposite cochlea by subtracting the IA value from the level presented to the test ear.
Because the crossed sound stimulates the opposite cochlea by bone conduction, we compare it with that ear's bone conduction threshold.
If the crossed sound is equal to or above that bone conduction threshold, cross-hearing is possible and masking is needed.
For example, with an IA of forty decibels, a seventy decibel test sound reaches the opposite cochlea at about thirty decibels.
In the next lesson, we will work through how to make this decision for air conduction, using sample audiograms.
For air conduction, masking is needed when the test ear threshold minus the opposite ear's bone conduction threshold reaches IA.
For bone conduction, because IA is taken as zero, masking is needed when a meaningful air-bone gap is seen in the test ear.
In speech tests, masking is also used when the presentation level is high enough to cross to the opposite ear.
We will look at these three situations one by one, with sample audiograms, in the coming lessons.
With supra-aural earphones, the low IA means masking is needed often.
Especially in one-sided or asymmetrical losses, masking may be needed at almost every frequency.
The pressure of supra-aural earphones can also make the ear canal collapse in some people, giving falsely high thresholds.
Covering the opposite ear with this earphone during bone conduction testing also creates a stronger occlusion effect.
Because IA is higher with insert earphones, the situations that need masking become fewer.
The masking dilemma seen in bilateral conductive loss also occurs less often with insert earphones.
Foam tips are single use, which is an extra advantage for hygiene.
However, insert earphones may not be usable in conditions such as ear canal discharge or atresia.
In masking, noise is always delivered to the non-test ear, that is, the opposite ear.
The noise temporarily raises the threshold of the opposite ear, so the crossed sound cannot be heard there.
In pure tone testing, narrowband noise centered on the test frequency is used.
Narrowband noise gives the same masking effect with a lower overall loudness.
In speech tests, speech noise, which resembles the speech spectrum, is preferred.
The patient is told clearly to ignore the noise and to respond only when they hear the tone.
How much noise is needed is covered in the lessons on effective masking and the plateau method.
Here is a practical tip from the clinic.
In a one-sided loss, the unmasked thresholds of the poorer ear may run parallel to the curve of the better ear.
If the difference is also around forty to sixty decibels, this is a sign of a shadow curve.
Do not report such an audiogram as the final result without masking.
If there are also unmasked bone conduction responses in the same ear, remember that a false air-bone gap may appear.
An intense test sound can cross the skull to the opposite cochlea; this is called cross-hearing.
The loss the sound undergoes while crossing is called interaural attenuation, or IA.
The conservative IA value is about forty decibels for supra-aural earphones and about fifty five decibels for insert earphones.
For bone conduction, IA is taken as about zero, so unmasked bone conduction does not tell us which ear it belongs to.
Test yourself: For the bone vibrator, about how many decibels is interaural attenuation taken to be?
The correct answer is A: about zero decibels.
The bone vibrator sets the whole skull vibrating, and both cochleae are stimulated at almost the same level.
In the next lesson, we will use sample audiograms to learn when masking is needed in air conduction testing.
Subscribe to the channel so you never miss a new lesson. See you soon!

Keywords

Click a keyword to search all lessons for that term.

Why We Mask : Interaural Attenuation Explained

Keywords

Prerequisites