Final answer:
To determine when to mask for Speech Recognition Threshold, masking is needed when there is a significant interaural attenuation or discrepancy between ears. The initial masking level is set 10 dB above the non-test ear's bone conduction threshold or the air conduction threshold if bone conduction is unknown.
Step-by-step explanation:
The formula to determine when to mask for SRT (Speech Recognition Threshold) is based on the concept of interaural attenuation. Masking for SRT is typically required when there is an air-bone gap of 40 dB or more, or when the test ear's air conduction threshold is better than the non-test ear's bone conduction threshold by 40 dB or more. However, the specific criteria can vary based on the equipment used and patient factors. The initial masking level is usually 10 dB above the non-test ear's bone conduction threshold, or if unknown, above its air conduction threshold, provided the bone conduction threshold is not suspected to be significantly better.
For instance, if a patient's left ear SRT is 30 dB HL and their right ear bone conduction threshold is 10 dB HL, you would begin masking the non-test ear (right ear) at 20 dB HL (10 dB above the right ear's bone conduction threshold).