Masking in Pure Tone Audiometry Lecture PDF

MASKING in Pure Tone Audiometry Wahid Zaman MSc CS Masking Masking is essential in PTA Premise of masking: use of a masking noise in the non-test ear so can focus on the test ear Need to mask when: there are differences between AC thresholds There are differences between AC and...

MASKING in Pure Tone Audiometry Wahid Zaman MSc CS Masking Masking is essential in PTA Premise of masking: use of a masking noise in the non-test ear so can focus on the test ear Need to mask when: there are differences between AC thresholds There are differences between AC and BC thresholds Why is masking necessary? Audiologists must be able to evaluate each ear individually in order to make diagnostic and rehabilitative decisions Cross hearing makes this difficult in certain situations Masking is complex. Only full understanding can lead to correct interpretation of results. Crossover/Cross Hearing Testing the poorer ear can be problematic… Crossover! sound from one ear, if loud enough can be heard in the other ear the cochlea of the better ear is picking up the sound presented to the worse ear How does cross hearing occur? Transcranial transmission If loud enough, sound passes across the skull to the other ear (primarily through bone conduction). Sometimes ‘pericranial’ if you are testing the poorer ear, sound can be transmitted across the skull to the better ear, so the patient will respond falsely (apparent threshold of the worse ear is a “shadow” of the better ear) Masking prevents the better ear from responding Cross Hearing So Masking… Allows us to establish true hearing thresholds in the presence of crossover hearing The addition of noise to the non- test ear so it cannot respond - this allows us to get the true response from the test ear Interaural attenuation Refers to decrease in sound level at non-test ear compared to test ear Aka transcranial transmission loss: the amount of sound transmitted through the skull Amount of IA Air conduction (headphones) 40dB Average is 60dB (40-80dB) – use 40 dB to be on the safe side Bone conduction 0dB (no Loss) Average is 0dB (-5 to 15dB), i.e. goes equally to each cochlea Principles of Masking To elevate the threshold of NTE by a known amount to enable accurate measurement of TE. Achieved by presenting masking noise in NTE and at the same time measuring the hearing threshold of TE with the test signal. Normally a 1:1 relationship between increase in masking noise and elevation of threshold of NTE. Term ‘not-masked’ used when measuring without masking and not ‘unmasked’ When to mask: 3 Rules of Masking/ Indicators of cross hearing 1st rule (AC.) A difference between the non-masked A.C. thresholds in each ear of 40 dB or more at the same frequency. 55dB when using insert phones. 2nd rule (BC.) A.C. threshold is 10dB or more worse than non- masked bone conduction threshold at the same frequency 3rd Rule (AC.) A.C rule but B.C thresholds 40dB or more acute than non-masked A.C. thresholds Rule 1 (mask AC) Rule 2 (mask BC) Rule 3 (mask AC)  Maskers Narrow band signal approx 1/3 octave centred on stimulus tone (“Critical band”), BS EN ISO 389-4 (BSA 2018). Present masking to the non-test ear via the other earphone (a.c.) Present masking to the non-test ear by a headphone or insert phone (b.c.), with bc vibrator placed on TE mastoid.  Effective Masking Level When the noise is not calibrated in effective masking level, the threshold for masking noise (M) shall be measured. Masking noise should be calibrated in terms of effective masking level (EML) according to BS EN ISO 389-4. In the presence of noise at a particular centre frequency and effective masking level, the pure-tone threshold of hearing (dB HL) at that frequency will be raised to that level. For example, a 1000-Hz noise at 50 dB EML presented to an ear will normally raise its hearing threshold for a 1000-Hz pure tone to 50 dB HL.  Masking noise M Amount of masking in NTE?... How to find “M” in non-test ear? (M) shall be measured-This indicates the lowest level of a masking noise that can be detected, measured in dB (relative to an arbitrary zero). Use the same method as used for pure-tone threshold to determine M. Start at M+10 (in non-test ear) and then continue with masking technique (section 8.5 and 8.8 BSA 2018)  Instructions for Masking notes!! Remember the instructions from the previous slides? The subject must not be told to expect to hear the pure tone in the test ear. The very fact that masking noise is required means that it is not known which ear is picking up the signals BSA 2018. Masking procedure – BSA 2018 section 8.8 Plateau seeking method- For both AC and BC testing Re-establish threshold in the test ear without masking noise- necessary for b-c because the occluded not-masked hearing threshold level is required. Introduce masking noise to the non-test ear= Effective masking level. Re-establish the hearing threshold level in the test ear in the presence of masking noise. Record this level as PT threshold at that level of masking.  Procedure contd.. Increase the masking noise by 10 dB. Re-measure the hearing threshold level in the test ear. Take this tone level as the pure-tone threshold at that level of masking. Continue with 10-dB increases in masking noise until: You have at least four measurements, and three successive measurements yield the same tonal threshold. or until the level of the audiometer is reached. or until the subject finds the masking noise uncomfortable.  Masked threshold ‘Plateau’ -When three successive levels of masking yield the same tonal threshold, or one threshold no more than 5 dB different from the other two. The mode of the three hearing threshold levels at plateau is taken as the correct hearing threshold of the test ear. “Plateau” of 3 points within 5dB – masked threshold Best to use a Masking chart Over Masking Important that correct level of masking is used. If masking level is increased to the level of the IA and beyond, it will cross over and mask the test cochlea, giving a incorrect threshold Masking dilemma… Over masking May end up with a result that looks like SNHL, but is actually a CHL WRONG CONCLUSION! Once the masking is intense enough to crossover, and exceed the level of BC in the non-test ear, conclude the procedure with a note that masking plateau could not be ascertained Under masking Not enough masking is introduced into the non test ear to produce the required threshold shift You cannot accurately judge threshold levels if cross hearing is not sufficiently controlled Types of masking Peripheral masking The shifting of a threshold in the test ear by presenting another signal to it, or a signal to the NTE that will cross over and affect the test ear Central masking A shift in the threshold of the test ear below the level where cross-masking would occur. Occurs through the central nervous system Cross masking Audiogram Symbols Consequences of not masking correctly Incorrect diagnosis Incorrect quantification of hearing loss Failure to recognise a dead ear Can have serious consequences Better to mask correctly at one or two frequencies than incorrectly at all frequencies When not to mask… Dead Ear Reading etc PTA (BSA 2018 document) Critical band and masking (Katz, Ch3 Moore) Dead regions British Journal of Audiology. 2000 Aug;34(4):205- 24. Ear and Hearing. 2001 Aug;22(4):268-78.