3.1 Clinical Masking Principles & Procedures

Clinical Masking Principles & Procedures

Why Masking is Necessary

In clinical audiometry, our goal is to determine the true hearing thresholds of each ear independently. However, sound presented to one ear (the test ear or TE) can sometimes be heard by the opposite ear (the non-test ear or NTE). This phenomenon is known as cross-hearing. If cross-hearing occurs, the patient might respond to a sound that was actually perceived by the non-test ear, leading the clinician to incorrectly assume the test ear has better hearing than it actually does. The resulting inaccurate threshold is often referred to as a shadow curve, because the thresholds of the worse ear simply "shadow" the better ear's thresholds by the amount of energy lost as the sound travels across the skull.

To prevent the non-test ear from participating in the evaluation, audiologists use clinical masking. Masking involves presenting a competing noise to the non-test ear, effectively keeping it "busy" so it cannot detect the test signals. Knowing precisely when to mask, how much masking noise to use, and what type of noise to apply is a fundamental skill for hearing healthcare professionals.

Interaural Attenuation (IA)

The concept of cross-hearing is entirely dependent on Interaural Attenuation (IA). Interaural attenuation is the loss of acoustic energy that occurs as a sound travels from one side of the head to the other. The amount of IA varies significantly depending on the type of transducer used during testing.

When testing via air conduction using standard supra-aural headphones (which sit over the pinna), the sound vibrates the skull when it reaches a certain intensity, crossing over to the opposite cochlea. The accepted IA value for supra-aural headphones is 40 dB. When using insert earphones (which are placed deep into the ear canal), less surface area of the skull is stimulated. This provides a much greater IA, typically accepted as 70 dB. This higher IA is a major clinical advantage because it drastically reduces the need to mask for air conduction testing. For bone conduction, the transducer is placed directly on the mastoid process, vibrating the entire skull simultaneously. Therefore, the interaural attenuation for bone conduction is assumed to be 0 dB. A signal presented to the mastoid of one ear can stimulate both cochleae equally.

Formulas: When to Mask

Clinicians use specific rules to determine when masking is necessary.

Air Conduction Masking Rule: You must mask for air conduction whenever the presentation level (air conduction threshold of the test ear) minus the bone conduction threshold of the non-test ear is greater than or equal to the Interaural Attenuation. Formula: AC(TE) - BC(NTE) >= IA Because we often don't know the bone conduction threshold of the non-test ear before we start, a more conservative rule is often applied: Mask if the difference between the AC of the test ear and the AC of the non-test ear is greater than or equal to IA.

Bone Conduction Masking Rule: Masking for bone conduction is required whenever there is an air-bone gap in the test ear of 15 dB or greater. Formula: AC(TE) - BC(TE) >= 15 dB Since IA for bone conduction is 0 dB, we always assume the unmasked bone conduction threshold reflects the better cochlea. If the unmasked BC threshold is significantly better than the AC threshold of the test ear (an air-bone gap of 15 dB or more), we must mask the non-test ear to isolate the test ear's true cochlear function.

Types of Masking Noise

Not all noise is created equal when it comes to clinical masking. The goal is to use a noise that efficiently masks the test signal without being unnecessarily loud or annoying to the patient. For pure-tone audiometry, narrowband noise is the standard. Narrowband noise is centered on the specific frequency being tested and contains only a narrow band of surrounding frequencies. It is highly efficient at masking pure tones because it focuses all its acoustic energy within the critical band of the test frequency. For speech audiometry, speech noise is used. Speech noise is a broad-spectrum noise that has been filtered to match the long-term average spectrum of human speech (more energy in the low frequencies and less in the high frequencies). This effectively covers the broad frequency range of speech signals.

The Plateau Method (Hood Technique)

The most widely accepted procedure for obtaining masked thresholds is the Plateau Method, originally described by Hood. The goal of this technique is to find the range of masking noise intensities where the non-test ear is effectively masked, but the noise is not so loud that it crosses back over to the test ear (overmasking).

The procedure begins by establishing an initial masking level in the non-test ear, typically 10 dB above its air conduction threshold. The test signal is then presented. If the patient hears it, the masking noise is increased by 5 dB, and the tone is presented again. This step-by-step increase continues. When the test tone threshold remains stable despite three consecutive 5 dB increases in the masking noise (a 15 dB range), the clinician has reached the masking plateau. This plateau indicates effective masking—the non-test ear is completely isolated, and the patient is responding using their true test ear threshold.

If the masking noise is increased too much, it can cross back over the head and elevate the threshold of the test ear. This is called overmasking. Conversely, undermasking occurs when the noise is insufficient to prevent the non-test ear from hearing the tone. A phenomenon known as central masking may also occur, where the introduction of masking noise in the non-test ear causes a slight (typically 5 dB) shift in the threshold of the test ear, not due to acoustics, but due to central nervous system processing. This is a normal physiological response and should not be confused with overmasking.

The Masking Dilemma

In some clinical cases, particularly those involving bilateral severe conductive hearing losses, a masking dilemma arises. This occurs when the initial masking level required to effectively mask the non-test ear is already so loud that it immediately crosses over and overmasks the test ear. In a masking dilemma, the plateau is non-existent (the undermasking and overmasking regions overlap), making it impossible to obtain true masked thresholds using traditional air-conduction masking techniques. This highlights the importance of recognizing the physical limitations of transducers and interaural attenuation.