Masking Flashcards
Why do we need to do masking?
Sound played into one ear May pass through the skull via bone conduction and be heard in the opposite ear - this may compromise the results and effect the correct gain and/or influence HA/ear piece selection negatively
What is TTL?
Transcranial transmission loss - some sound will be lost as it travels across the head (loses energy) soo will be quieter in the opposite ear. The reduction is Known as TTL (or inter aural attenuation)
Cross hearing May occur when the difference in the threshold’s of the two ears are greater than the TTL - what is the minimum TTL for each transducer?
Headphones 40dB
Earphone inserts. 55dB
Bone conduction. 0 dB
What actually is the masking noise?
It is white noise, narrow band noise always delivered by AC.
What is the EML? How was it determined?
Effective masking level - determined by presenting the signal and masker to the same ear. It is the lowest level of noise required to mask a signal to 50% possibility of detection for a large group of normal hearing adults.
What is the initial masking level?
M+10 - first presentation is at the AC threshold for the BETTER ear at that frequency plus 10dB
What is rule 1?
Masking is required at ANY frequency where the difference between the left and right ear (not masked) AC threshold is :
40dB or more with headphones
55dB or more with inserts
What is rule 2?
Masking is required at ANY frequency where the BC threshold (not masked) is BETTER than the AC threshold of EITHER ear by 10dB or more.
Worst ear (AC) is test ear
Better ear to be masked - non test ear
What is rule 3?
Where rule 1 does not apply but the BC threshold (not masked) of one ear is more acute by:
40dB or more with headphones
55dB or more with inserts
Than the AC threshold (not masked) of the OTHER ear.
Why do we need rule 3?
It is necessary because an AC frequency that does not require making under rule 1, May need to be masked if BC results indicate test ear May have a conductive element.
What is the most important factor in cross hearing?
Sensitivity of the NON test cochlea. If BC. Results indicate a conductive element - May need to mask
What is the procedure for masking?
Re-establish hearing threshold in test ear without masking to remind subject what to listen for.
Introduce masking to test ear - presented at AC threshold of the BETTER ear.
Wait few seconds in case subject responds to masking noise
Conduct normal threshold test
Increase making by 10dB
Conduct normal threshold test
Repeat until a minimum of 4 measurements completed (includes initial starting point) AND
3 successive levels yield same result.
( 1 result can be within 5dB of the other 2 - other 2 result will be taken)
At what levels do we proceed with caution when masking?
Masking noise greater than 80dB EML
Tones greater than 100dBHL
What May stop the masking test?
Patient finds masking noise uncomfortable
Maximum level of audiometer reached
What are the 3 possible outcomes of masking?
Stays same as original BC - CONDUCTIVE loss
Drops anywhere between BC result and worst AC result - MIXED loss
Drops to worst AC result - SN loss
