10 Questions
What is the definition of masking according to the American National Standards Institute (ANSI) in 2004?
The process by which the threshold of hearing for one sound is raised by the presence of another (masking) sound, and the amount by which the threshold of hearing for one sound is raised by the presence of another (masking) sound expressed in decibels.
What is the unit of measurement for effective masking level?
dB EM (decibels effective masking level)
What is the minimum interaural attenuation (IAA) level for A/C with supra-aural earphones?
40 dB
When is masking needed for AC testing?
When the A/C threshold of the test ear exceeds the B/C or A/C threshold of the non-test ear by 40 dB or more.
What is the minimum IAA level for B/C testing?
0 dB
When is masking needed for pure tone BC testing?
When the BC threshold differs from the AC threshold of the same ear by more than 10 dB.
What is the criterion for masking in SRT testing?
When the SRT of the test ear exceeds the SRT or the B/C of the non-test ear by 45 dB or more.
What is the criterion for masking in WRS testing?
When the PL in the test ear exceeds the SRT or the best B/C threshold in any of the speech frequencies (500, 1000, or 2000 Hz) of the non-test ear by 35 dB or more.
What is the purpose of masking in audiological measurements?
To prevent the acoustic signal from crossing over to the non-test ear and affecting the test results.
What is the relationship between effective masking and the spectrum of the masking noise?
Effective masking is based on the spectrum of the masking noise.
Study Notes
Occlusion Effect
- Improvement in low frequency BC thresholds due to occlusion of the ear
- Experienced by individuals with sensorineural hearing loss or normal hearing
- Not experienced by individuals with conductive hearing loss
- Occurs at 1000 Hz and lower
- Recommended correction factors:
- 250 Hz: 20 dB
- 500 Hz: 15 dB
- 1000 Hz: 5 dB
Masking
- Definition: Raising the threshold of hearing for one sound by the presence of another (masking) sound
- Effective masking: ability of a masking noise to mask a signal of known frequency and intensity
- Effective masking level: 45 dB for 1000 Hz PT and 50 dB for speech signal
- Types of masking noises:
- BBN (Broad Band Noise) or White Noise (2-6/8 kHz)
- Narrow Band Noise
- Speech Noise (300-3000 Hz)
Masking Dilemma
- Occurs when it's impossible to mask the non-test ear (NTE) effectively without exceeding the maximum permissible masking level
- Leads to cross-masking and overestimation of hearing loss in the test ear
- Can occur in AC and BC when bilateral conductive hearing loss is present (≥ 35 dB)
Central Masking
- Threshold shift in the test ear resulting from masking the non-test ear
- Mechanism: Inhibitory response in the CNS
- Average: 5 dB
Masking for Speech Recognition Threshold
- Calculate Starting Level: subtract 35 dB from the presentation level of the unmasked speech recognition threshold and add the air-bone gap of the non-test ear
- Introduce Masking: use subtraction procedure to determine the masking level, accounting for the minimum interaural attenuation
- Determine Outcome: the masked threshold may not shift, shift by more than 5 dB with a plateau, or shift without a plateau
Masking for Word Recognition Score
- Calculate Starting Level: subtract 25 dB from the presentation level in the test ear and add the average air-bone gap of the non-test ear
- Introduce Masking: use subtraction procedure to determine the masking level, accounting for the minimum interaural attenuation
- Determine Outcome: the presentation level of the test stimuli remains constant during word recognition testing
Clinical Masking
- Needed for AC when A/C threshold of the test ear exceeds B/C or A/C threshold of the non-test ear by 40 dB or more
- Needed for pure tone BC testing when the BC threshold differs from AC threshold of the same ear by more than 10 dB
- Needed in SRT testing when the SRT of the test ear exceeds the SRT or the B/C of the non-test ear by 45 dB or more
- Needed in WRS testing when the PL in the test ear exceeds the SRT or the best B/C threshold in any of the speech frequencies of the non-test ear by 35 dB or more
The Occlusion Effect is the improvement in low frequency bone conduction thresholds due to occlusion of the ear. It is commonly observed in individuals with sensorineural hearing loss or normal hearing, but not in those with conductive hearing loss.
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