4 - BC: When to Mask

Question 1

What do “TE” and “NTE” stand for?

Answer 1

• Test Ear (the ear being assessed)

- Non-Test Ear (may receive stimulus intended for the TE)

Question 2

True or False: signal crossover of a BC signal from TE to NTE means the NTE may contribute to a patient’s TE response

Answer 2

True

Question 3

What is Interaural Attenuation (IA)?

Answer 3

The amount of energy (dB) lost during the transmission of sound by bone conduction through the skull to the contralateral cochlea (NTE)

Question 4

What two things regarding signal crossover must we assume are happening during BC?

Answer 4

1) signal crossover is always considered to be present during BC
2) IA is assumed to be 0 dB for BC signals

Question 5

The NTE may contribute to a patient’s response (known as cross-hearing) if the NTE cochlear sensitivity is ______ (worse than/ equal to/ equal to or better than) the TE sensitivity

Answer 5

Equal to or better than

Question 6

How does masking allow us to acquire correct results for the TE?

Answer 6

Masking noise “covers” the test tone reaching the NTE and effectively removes it from participating in the test

Question 7

When is masking required for BC?

Answer 7

When ABG > 10 dB

**Note: Gelfand and ASHA state “when ABG > or = 10 dB”

Question 8

If the right ear (RE) AC - BC = 20 dB, and the left ear (LE) AC - BC = 5 dB, which ear(s) to we PRESENT the masking noise to, and which ear(s) is/are our TE when we repeat the BC test?

Answer 8

ABG > 10 dB for the RE only, so:
TE = Right Ear
NTE = Left Ear = will be presented with masking noise

Question 9

In the formula to determine the IML (Initial Masking Level), what does OE stand for?

Answer 9

Occlusion Effect - only an issue between 250 - 1000 Hz

Question 10

What is the formula to find the Initial Masking Level?

Answer 10

IML = AC (NTE) + 0 (MEM) + 10 (SF) + OE

AC (NTE) = Air Conduction in Non-Test Ear

MEM = Minimum Effective Masking (assume 0 because usually accounted for during audiometer calibration)

SF = Safety Factor

OE = Occlusion Effect 
250 Hz = 20
500 Hz = 15
1000 Hz = 5
*Note: Gelfand states OE should be tested for using audiometric Bing test (occluding NTE with headphone without noise and testing)

Question 11

Describe the procedure of masking (from start of AC test to presenting the first tone during masking)

Answer 11

• measure unmasked AC & BC threshold
• determine if BC masking is required (i.e. which frequencies, if any, show a ABG > 10 dB)
• determine frequency specific IMLs for NTE using formula
• place bone oscillator on TE and earphone in NTE
• turn on NTE narrow-band masking at appropriate IML
• begin masking test

Question 12

Name 1 factor that can influence signal crossover

Answer 12

• placement of bone oscillator
• IA increases as pure tone frequency increases
• individual subject factors
• IA assumption of 0 dB is conservative (may be as great as 15 dB)

Question 13

If a patient shows matching RE AC and masked BC levels at 30 dB HL, and RE unmasked BC levels that match the LE AC levels at 10 dB HL, do we still need to mask for LE BC?

Answer 13

Yes - there could be as much as 15 dB of interaural attenuation between ears. If this is the case, even though the LE AC levels are normal, the BC levels could indicate a conductive hearing loss.