Masking in Pure Tone Audiometry

Masking in Pure Tone Audiometry

• Masking is crucial in pure tone audiometry (PTA)
• Masking involves using noise in the non-test ear to focus on the test ear.
• Masking is needed when there are differences in air conduction (AC) thresholds between ears.
• Differences between AC and bone conduction (BC) thresholds also necessitate masking.

Why Masking is Necessary

• Audiologists need to evaluate each ear independently for accurate diagnosis and rehabilitation.
• Cross hearing makes individual ear assessments challenging in certain situations.
• Masking is complex; thorough understanding is vital for correct result interpretation.

Crossover/Cross Hearing

• Testing the poorer ear can be problematic due to crossover.
• Sound from one ear can be heard in the other ear, especially if loud enough.
• Sound from the better ear's cochlea can be picked up by the worse ear.

How Crossover Occurs

• Sound can travel across the skull (transcranial transmission), largely through bone conduction.
• Pericranial transmission occurs sometimes.
• The sound reaching the better ear can create a "shadow" of the worse ear, leading to inaccurate measurements.
• Masking prevents the better ear from responding, allowing accurate assessment of the worse ear.

Interaural Attenuation (IA)

• IA refers to the decrease in sound level at the non-test ear compared to the test ear.
• IA is also known as transcranial transmission loss, representing the amount of sound passing through the skull.

Amount of IA

• Air conduction (headphones) IA is 40 dB (average 60 dB, use 40 dB as a safety measure).
• Bone conduction (no loss) IA is 0 dB (average 0 dB).

Principles of Masking

• Masking elevates the threshold of the non-test ear (NTE) to accurately measure the test ear (TE).
• Masking noise is presented to the NTE while measuring the TE threshold with a test signal.
• There's a 1:1 relationship between increasing masking noise and elevating the NTE threshold.
• The term "not-masked" is used when measuring without masking, distinct from "unmasked".

When to Mask

• Rule 1 (AC): A difference of 40dB or more (55 dB with insert phones) in AC thresholds between ears at the same frequency indicates masking.
• Rule 2 (BC): If AC threshold is 10dB or higher than the non-masked BC threshold at the same frequency, masking is required.
• Rule 3 (AC): If BC threshold is more than 40dB higher than the non-masked AC threshold, masking is needed.

Maskers

• Narrowband signal (approximately 1/3 octave) centered on the stimulus tone is often used.
• Maskers are presented to the non-test ear via earphones or insert phones (in air conduction or bone conduction settings respectively).

Effective Masking Level (EML)

• EML is measured when masking noise isn't calibrated to an EML.
• Masking noise should use EML (according to BS EN ISO 389-4), and the levels directly correlate with the pure-tone hearing threshold.
• A 1000 Hz noise at 50 dB EML will raise the hearing threshold of a 1000 Hz pure tone to 50 dB HL.

Masking Noise (M)

• The lowest masking noise level (in dB) detectable in the NTE is considered the masking noise M. (Arbitrary zero is not detailed)
• Masking procedure starts around M +10 in the non-test ear (then gradually increased).

Masking Procedure (BSA 2018 section 8.8)

• Plateau-seeking method is used for both air and bone conduction.
• Re-establish the threshold in the test ear without masking.
• Introduce masking noise to the non-test ear to determine effective masking level.
• Re-establish the hearing threshold level (PT threshold) in the test ear with the present masking.

Procedure (Continued)

• Increase masking noise level by 10 dB.
• Re-measure the hearing threshold level in the test ear and use it as the pure tone threshold at that level.
• Keep increasing until at least four measurements show identical or very similar results. (Plateau).
• Stop when the audiometer reaches its highest point or the subject finds the noise uncomfortable.

Masked Threshold

• A plateau of 3 points( within 5dB) signifies the masked threshold.
• The modal threshold of the three values on the plateau is the masked threshold.

Over Masking

• Using too high of a masking level will mask the test ear also (incorrect result).
• In this case, conclude the assessment with a note that a masking plateau couldn’t be established; the measurement isn't accurate.

Under Masking

• Using too little masking level is problematic as it isn't controlled sufficiently- won't produce the required shift in the threshold.
• Inaccurate measurement(s).

Types of Masking

• Peripheral masking affects the test ear due to signals presented to either the test ear directly, or the other ear.
• Central masking shifts the test ear threshold below the crossover point, affecting the central nervous system.
• Cross masking involves signals masking the non test ear.

Audiogram Symbols

• Audiogram symbols are used to visually represent results of audiological assessment (different levels of masking).

Consequences of Incorrect Masking

• Diagnoses and quantification of hearing loss can be incorrect.
• A "dead ear" might be missed.
• Serious and concerning consequences are possible.

When Not to Mask

• This part of the notes discusses when masking should not be used.

Dead Ear

• A "dead ear" is when an ear has no measurable sound response.

Reading (etc.)

• This part provides a list of supporting materials for further study.

This quiz explores the importance of masking in pure tone audiometry (PTA), essential for accurate hearing assessments. Understand the role of masking in evaluating air and bone conduction thresholds, and learn about crossover and its impact on testing. Ideal for audiology students and professionals looking to deepen their grasp of auditory testing techniques.