What Is Masking?

Masking is a procedure used during hearing testing in which a steady noise is presented to one ear while the other ear is being tested. The purpose is to raise the hearing threshold of the non-test ear so that it cannot respond to a tone that is intended for the test ear. Without masking, a pure tone may be heard by both ears, and the resulting audiogram may not represent the ear the clinician is trying to measure.

Auditory masking occurs whenever one sound makes another sound harder to hear. In clinical audiometry, this principle is applied deliberately to prevent the better-hearing ear from answering for the poorer ear. This step is essential for identifying true ear-specific thresholds.

Why Masking Is Necessary: Cross Hearing

The skull is not a barrier to sound. A tone delivered by air-conduction headphones creates vibrations that travel through the bones of the skull to the cochlea on both sides. When a sufficiently loud signal reaches the opposite ear, the listener may respond to that crossed sound rather than to the sound in the ear under test. This is called cross hearing.

The amount of sound lost as it travels from one ear to the other is known as interaural attenuation. For supra-aural headphones, interaural attenuation is often around 40 to 70 decibels, depending on frequency and fit. Insert earphones usually produce greater interaural attenuation because they place the sound closer to the eardrum, but even with insert earphones, a very intense tone may cross to the opposite side.

Bone-conduction testing is even more vulnerable. A bone oscillator vibrates the entire skull, so both cochleae receive nearly equal stimulation. Interaural attenuation for bone conduction can be as low as 0 to 15 decibels. Because of this, an unmasked bone-conduction response almost always represents the better-functioning cochlea, not necessarily the one being tested. To measure cochlear function in a damaged ear, the opposite ear must be masked.

When To Mask in Pure-Tone Audiometry

In air-conduction testing, masking is needed whenever the level of the test tone is high enough that it could be heard in the opposite ear. The clinical decision compares the threshold obtained in the test ear with the best hearing sensitivity in the non-test ear. If the difference between ears is large, or if the test signal would be audible to the non-test cochlea after subtracting interaural attenuation, masking is indicated.

The precise rule varies with the type of earphone, the frequency, and the audiometer calibration. A common clinical shortcut is to mask air-conduction thresholds when the difference between the two ears is approximately 40 decibels or more with supra-aural earphones. Insert earphones, which offer greater attenuation, may allow larger differences before masking becomes necessary. The clinician must also consider the threshold in the non-test ear before deciding to mask.

Bone Conduction Usually Requires Masking

Bone-conduction thresholds help determine whether a hearing loss is sensorineural or conductive. Yet because the skull vibrates as a whole, a bone-conduction response rarely originates from only the ear under the vibrator. Masking the non-test ear with noise makes the response more specific to the cochlea on the test side.

When testing bone conduction, audiologists routinely mask the non-test ear whenever there is a risk that the unmasked response could come from the opposite ear. This is especially important when there is an air-bone gap in one ear. If the opposite ear is normal, that normal ear may easily detect the bone-conducted tone, creating a false picture of the damaged ear’s cochlear function.

Types of Masking Noise

The type of noise used in hearing testing is chosen to match the test signal. For pure-tone audiometry, narrowband noise is the usual choice. A narrow band of noise centered on the test frequency masks that frequency efficiently while using a relatively low level of noise. Broadband noise, such as white noise, spreads energy across many frequencies and is less efficient for pure-tone testing.

For speech testing, speech noise is commonly used. Speech noise is shaped so that it resembles the average spectrum of speech and therefore provides effective masking during word recognition tests. The goal is to prevent the non-test ear from contributing to the speech recognition score while keeping the noise level reasonable and comfortable.

The Plateau Method

The most reliable way to determine the correct masking level is the plateau method. In this technique, the clinician measures the threshold of the test ear at one level of masking noise, then increases the masking level and re-measures the threshold. The process is repeated in small increments, often 5 or 10 decibels at a time.

As long as the threshold in the test ear remains unchanged while the masking level increases, the result is considered valid. The range of masking levels over which the threshold stays the same is called the plateau. The true threshold is taken from the middle of that plateau.

If the test-ear threshold shifts upward with every increase in masking, the noise has probably crossed over and is influencing the test ear. If the threshold does not shift at all until a very high masking level and then changes suddenly, the earlier responses may have come from the non-test ear. The plateau method helps distinguish these situations.

Under-Masking, Over-Masking, and Central Masking

Under-masking occurs when too little noise is used, so the non-test ear is still able to respond. This leaves a shadow curve in the audiogram, meaning the threshold measured is actually the hearing level of the opposite ear.

Over-masking occurs when too much noise is presented. The masking sound itself crosses the skull and reaches the cochlea of the test ear, raising its threshold. Over-masking can create the appearance of a hearing loss that is worse than the real one. This is a particular concern during bone-conduction testing, since the noise and the test signal both travel through the skull with little loss.

Clinicians also consider central masking, a small elevation in threshold that happens when noise is present in one ear, even if it never crosses acoustically to the test ear. This effect is usually only about 5 to 10 decibels. Central masking is a neural phenomenon rather than a direct acoustic crossover, and it must be kept in mind when interpreting small shifts in masked thresholds.

Masking in Speech Hearing Testing

Masking is not limited to pure-tone audiometry. Speech recognition tests can also be affected by cross hearing. If the speech signal is loud enough to reach the opposite ear, the non-test ear may assist with word recognition, leading to a falsely good score. In that situation, speech noise is delivered to the non-test ear so that the word recognition score reflects the ear being tested.

Masking is also relevant when measuring uncomfortable loudness levels and in some forms of electrophysiologic testing. Any test that claims to describe a single ear must be able to show that the opposite ear was excluded from the result.

Key Terms at a Glance

• Auditory masking: the elevation of one sound’s threshold by another sound.
• Cross hearing: perception of a test tone by the ear it was not intended for.
• Interaural attenuation: the reduction in sound level as it travels from one ear to the other.
• Air conduction: hearing through the outer, middle, and inner ear using headphones or insert earphones.
• Bone conduction: hearing through vibration of the skull directly to the cochlea.
• Ear-specific threshold: a hearing threshold known to come from one particular ear.
• Narrowband noise: a band of noise centered on a pure-tone test frequency.
• Plateau method: a masking procedure that uses stepwise increases in noise level to verify the true threshold.
• Under-masking: too little noise, allowing the non-test ear to respond.
• Over-masking: too much noise, causing the masker to affect the test ear.
• Central masking: a small threshold shift in the test ear caused by noise in the opposite ear through neural pathways.

Masking is a fundamental part of accurate hearing testing. It protects the integrity of the audiogram by ensuring that each result truly comes from the ear under evaluation. Without proper masking, responses from a healthy opposite ear can hide a genuine hearing loss or create a false one. With careful noise control, the hearing professional can measure thresholds that are valid, repeatable, and clinically meaningful.