Masking in audiometry is the clinical procedure of introducing a calibrated noise to the non-test ear (NTE) to prevent it from detecting the sound intended for the test ear (TE). This process ensures the true hearing threshold of the ear being evaluated is accurately determined, yielding a reliable result for diagnosis. Without masking, a patient with hearing loss in one ear might unknowingly respond to the test tone via their better ear, leading to an incorrect assessment of the poorer ear’s function.
Understanding Cross-Hearing and Interaural Attenuation
Masking becomes necessary when the sound presented to the test ear is loud enough to be transmitted across the head to the non-test ear, a phenomenon known as cross-hearing. When the intensity of the tone in the test ear reaches a high level, the sound energy vibrates the skull and stimulates the cochlea of the opposite ear through bone conduction.
The amount of energy lost as the sound travels from the test ear to the non-test ear is called Interaural Attenuation (IA). This value represents the sound level difference required for the tone to be loud enough in one ear to be heard in the other. The IA value depends on the type of earphone used during the testing procedure.
For standard supra-aural headphones, the minimum IA value used in clinical practice is 40 dB. Insert earphones, which are placed directly into the ear canal, offer greater separation between the ears, providing a higher IA value typically assumed to be 60 dB.
The Air Conduction Masking Decision Rule
The decision to apply masking for air conduction is based on a calculated risk that the test tone is crossing over to the non-test ear. Masking is required whenever the intensity of the test tone, after accounting for the IA, is equal to or greater than the non-test ear’s bone conduction threshold (\(BC_{NTE}\)).
The rule is mathematically expressed as: Masking is needed if the Air Conduction Threshold of the Test Ear (\(AC_{TE}\)) minus the Interaural Attenuation (IA) is greater than or equal to \(BC_{NTE}\). For example, using supra-aural headphones (IA of 40 dB), if the test ear’s AC threshold is 60 dB HL and the non-test ear’s \(BC_{NTE}\) is 5 dB HL, the difference is 55 dB.
Since 55 dB is greater than the 40 dB IA, the 60 dB tone presented to the test ear loses 40 dB, arriving at the non-test ear’s cochlea at 20 dB HL. Because the non-test ear’s threshold is only 5 dB HL, the 20 dB crossing tone is audible, confirming the need for masking.
Calculating the Initial Masking Level
Once the decision to mask has been made, the next step is to determine the starting intensity of the noise, called the Initial Masking Level (IML). The IML must be loud enough to effectively cover the test tone crossing over to the non-test ear, thereby elevating the non-test ear’s threshold.
The IML is calculated using the non-test ear’s established Air Conduction (AC) threshold. A common formula is to set the IML equal to the non-test ear’s AC threshold plus a safety factor, usually 10 dB. This 10 dB cushion ensures the noise is presented as an Effective Masking (EM) level, guaranteeing it is heard by the non-test ear.
The masking noise, which is a narrow-band noise centered around the test frequency, is delivered to the non-test ear at this starting level. For example, if the non-test ear’s AC threshold is 15 dB HL, the IML would be set to 25 dB EM.
Utilizing the Plateau Method to Verify Thresholds
The true, masked threshold of the test ear is verified using a systematic procedure known as the plateau method. This method involves presenting the test tone to the test ear while simultaneously increasing the masking noise in the non-test ear in small, fixed steps. The goal is to find a range of masking levels where the test ear’s threshold remains stable.
The process begins in a state of under-masking, where the noise is too low, and the non-test ear may still contribute to the response. As the noise level increases, the measured threshold in the test ear will also increase until the non-test ear is completely silenced.
The “plateau” is established as the range of at least 15 to 20 dB where the test ear’s threshold does not change despite further increases in the masking noise. Finding this plateau confirms the non-test ear is fully occupied by the noise, and the patient is responding solely based on the tone heard by the test ear.
If the masking noise is increased beyond the plateau, it can enter a state of over-masking, where the noise crosses back from the non-test ear to the test ear. This excessive noise artificially elevates the measured threshold of the test ear, making the result inaccurate.