9 - When and How to Mask AC

Question 1

Why do we mask PT AC?

Answer 1

Because air conducted sound can travel from TE to NTE cochlea when presented via supra-aural or insert earphones

Question 2

Describe the types of hearing loss that may require AC masking for PT audiometry

Answer 2

Unilateral or asymmetric hearing loss (any type)

Symmetrical conductive or mixed hearing loss

Question 3

What amount of interaural attenuation should we assume for an AC stimulus delivered via supra-aural earphones?

Answer 3

40 dB HL

Question 4

What 3 things might affect the amount of IA seen for AC?

Answer 4

• subject variability (skull properties)
• frequency spectrum of the test signal
• earphone transducer type (supra-aural vs inserts)

Question 5

What is the masking rule for PT AC?

Answer 5

AC (TE) - BC (NTE) > or = IA for AC

Supra-aural:
ACTE-BCNTE > or = 40 dB

Inserts:
ACTE-BCNTE > or = 60 dB

Question 6

If BC is not available (such as at 6 and 8 kHz) how do we determine if we need to mask?

Answer 6

AC (TE) - AC (NTE) > or = IA for AC

Question 7

_________ can occur via BC mechanisms for loud sounds presented by AC

Answer 7

Signal Crossover

Question 8

Describe how we mask for PT AC

Answer 8

1. Determine if masking is required using the formula:
   AC(TE) - BC(NTE) > or = IA for AC
2. Calculate Initial Masking Level for AC masking using the formula:
   IML = AC(NTE) + 10dB (SF) + MEMC (assume audiometer is calibrated unless notified otherwise)
3. Use plateau method similar to BC masking