What is Clinical Masking In Hearing Tests?: Clinical masking allows accurate assessment for each ear separately by withdrawing or preventing the non-test ear or (NTE) from participating in hearing while testing the other ear (test ear/ TE).
Clinical masking must be used as needed/as possible in the following situations:
- Air conduction pure tone testing
- Bone conduction pure tone testing
- Speech Audiometry
Types of Masking Noise
Type of used noise in masking depends on signal being masked i.e. pure tone to be masked pure tone like signal and speech should be masked by speech like signal.
- If the signal is a wide spectrum, like speech and clicks, the masker should be wide spectrum too
- Broad band noise (white noise)
- Pink noise
- Speech-shaped noise
- Multi talker babble
- WBN is not a good choice to mask pure tone as it is inefficient and unnecessarily loud, therefore, critical bands noise optimally to be used i.e. Narrow Band Noise(NBN).
Masking Rules: When to Mask?
First rule: (BC masking rule)
- In masking, the order to the PTA testing is reversed and it is decided whether BC masking is needed or not.
- BC masking is attempted first because need for AC masking depends on the true BC thresholds.
- Generally accepted masking rule for BC is presence of an air-bone gap
- (ABG) within the tested ear is more than 10 dB
ABG (TE) >10 dB
Second Rule: (AC masking rule)
- AC masking is needed at any frequency whenever real BC threshold of NTE is more acute than AC threshold of TE by equal to or greater than IA (40 dB for supra-aural earphones or 70 dB for insert earphones
- AC (TE) –BC (NTE) = IA ( 40 or 70 dB)
- AC(TE)-IA ≥ BC(NTE)
- AC (TE) –BC (NTE) is known as air-opposite bone-gap (AOBG) or air contra-lateral bone gap(ACBG)
Third rule:(AC rule)
- Masking needed at any frequency whenever the difference between the Right and left ear AC thresholds is equal or greater than IA ( 40 dB or 70 dB)
- AC (TE) –AC (NTE) = IA40 or 70 dB)
⇒ Introduction to Audiology – Frederick N.Martin (Book)
⇒ Essentials of Audiology – Stanley A. Gelfand, PhD (Book)