Audiology is the study of hearing, balance, and related disorders. Diagnostic audiometry measures a person’s hearing sensitivity and identifies the quietest sounds they can perceive, known as thresholds. Determining accurate thresholds for each ear individually is crucial for diagnosis and treatment planning. Masking is a core procedure used during these tests to ensure the reliability and validity of the measurements. It involves introducing controlled noise to one ear to prevent it from participating in the test, thereby isolating the ear being tested.
The Problem Masking Solves
Masking is necessary because the human head is not a perfect acoustic barrier, which can lead to cross-hearing. Cross-hearing occurs when a sound presented to the test ear is loud enough to travel through the skull bones. The sound is then perceived by the opposite ear, the non-test ear. This transmission means the measured threshold may reflect the non-test ear’s hearing ability, providing a falsely better result for the ear being tested.
The amount of sound energy lost as it travels from the test ear to the non-test ear is termed Interaural Attenuation (IA). For air conduction testing, IA is typically around 40 decibels (dB) for standard headphones and 50 dB for insert earphones. This means the sound must be at least 40 or 50 dB louder in the test ear than the non-test ear’s hearing level before cross-hearing is likely.
However, when sound is presented directly to the skull via a bone vibrator, the IA is essentially 0 dB. The skull vibrates as a unit, stimulating the cochleae of both sides almost equally. Therefore, a bone conduction test result obtained without masking reflects the hearing sensitivity of the better-hearing cochlea. Masking is a requirement to obtain ear-specific thresholds, particularly when there is a significant difference in hearing between the two ears or when bone conduction is measured.
The Noise Used in Masking
Masking uses a carefully calibrated noise signal presented into the non-test ear. This noise makes the non-test ear temporarily unable to perceive any test signal that crosses over. The noise must cover the test tone without being so loud that it crosses back and interferes with the measurement in the test ear. The type of noise used matches the acoustic properties of the signal being tested.
For pure-tone audiometry, which uses single-frequency tones, the ideal masking signal is Narrowband Noise (NBN). NBN is a segment of white noise centered around the same frequency as the test tone (e.g., 1000 Hz NBN for a 1000 Hz test tone). This focused noise is more efficient than a broad spectrum noise because it only contains the frequencies needed to mask the test tone, minimizing the required intensity. The audiometer is calibrated so the NBN dial reads in “Effective Masking Level,” indicating the threshold shift the noise will produce in the non-test ear.
When testing the ability to hear and understand speech, the masking signal used is Speech Noise. Speech noise is a broader band of sound that mimics the average long-term spectrum of human speech, typically falling between 300 and 3000 Hz. Using speech noise ensures that all frequencies relevant to speech perception are masked in the non-test ear, preventing any crossed-over speech signal from being perceived. The selection of the masking signal ensures the noise is only loud enough to occupy the non-test ear, providing accurate isolation.
Applying Masking in Hearing Tests
The decision to apply masking is based on comparing the sound level presented to the test ear and the hearing sensitivity of the non-test ear. For air conduction testing, masking is needed when the test tone’s intensity exceeds the non-test ear’s bone conduction threshold by the minimum Interaural Attenuation (IA) value (40 or 50 dB). If this difference is met, the sound is strong enough to vibrate the skull and stimulate the non-test ear’s cochlea, requiring masking noise.
For bone conduction testing, masking is needed when there is a significant difference between the air conduction and bone conduction thresholds in the test ear, known as an air-bone gap. Since the IA for bone conduction is near 0 dB, any bone conduction measurement is assumed to stimulate the cochleae of both ears. Masking the non-test ear confirms that the bone conduction threshold is specific to the ear being tested.
The procedure for finding the correct amount of masking noise uses the Plateau Method. This method involves presenting the test tone to the test ear while simultaneously introducing masking noise to the non-test ear. The audiologist starts with a low initial masking level and then increases the noise intensity in small steps, typically 5 dB increments.
With each increase in masking noise, the audiologist re-establishes the threshold in the test ear. Initially, the test ear’s threshold will shift, or appear worse, as the masking noise covers the crossed-over sound; this is the under-masking region. As the noise level increases, a plateau is reached. This plateau is defined by three consecutive increases in masking noise that do not cause the test ear’s threshold to shift, representing the safe and effective range of masking.
If the masking noise increases too much beyond the plateau, it enters the over-masking region. Here, the noise is loud enough to cross back to the test ear and artificially elevate that ear’s threshold. The goal is to find the lowest intensity within the plateau, the minimum effective masking level. This ensures the non-test ear is occupied without the risk of over-masking, resulting in the most accurate and ear-specific measurement.