Measurement of Hearing Quiz 5 - Masking

What is masking?

-The process by which the detection threshold of one sound (the signal) is raised by the presence of another sound (the masker)

• masking may occur unintentionally

  • environmental noise, air conditioning, other people talking…

  • or intentionally: clinical masking is intentional

masking setup for AC thresholds

-In clinical masking, a signal is presented to one ear and a masker is presented in the opposite ear

-the masker is always presented via air conduction to the non-test ear

• you can use insert earphones or supra-aural headphones

masking setup for BC thresholds

-In clinical masking, a signal is presented to one ear and a masker is presented in the opposite ear

-the masker is always presented via air conduction to the non-test ear

• you can use insert earphones or supra-aural headphones

-Present the masker via AC even when the signal is presented via BC

• headphone placement can be tricky here

Why use masking

-masking prevents cross hearing due to crossover from confounding your test results

-if cross hearing occurs, without masking, you may not be testing the ear you think you are

crossover

-when a signal to one ear travels through the skull via bone conduction (BC) and stimulates the opposite cochlea

• it could also travel around the skull via air conduction (AC)

How much crossover (in dB) occurs?

-amount of crossover via BC depends on the interaural attenuation of the signal:

• this varies with skull density, transducer, and the stimulus

What is IA? (interaural attenuation)

-The amount of sound energy in dB lost as an AC or BC signal presented to one ear travels through the skull, via bone-conduction, to the contralateral ear

-IA differs for AC and BC transducers

• and for AC transducer type

• and with freq and stimulus type

Variables affecting IA

-the person

• skull size, thickness, bone density, etc. affect IA

  • Range of IA’s of ~30 dB across subjects

-The freq of test signal

• less IA for lower freq with supra-aural phones but more with insert phones

-The type of earphone/transducer (can be big factor)

Minimum IA for inserts

~50 dB (shallow) ~55 dB (deep)

Minimum IA for supra-aural

~40 dB

Minimum IA for bone oscillator

~0 dB

when do you get max IA?

-with deeply seated inserts

• extra 5-40 dB

IA for mastoid BC

-Avg. BC IA varies (0-10 dB) with frequency

• <500 Hz IA

  • ~3-5 dB

• 500-1800 Hz

  • ~0 dB

• 3000-5000 Hz

  • ~10 dB

• 5000 Hz

  • ~4 dB

-But 20-30 dB range!

minimum IA assumptions

-Conservative estimates, it is likely that most people’s IA is substantially greater!

-Knowing an individual’s IA can help you make masking decisions

• particularly helpful when it is > the assumed minimum!

How would we know someone’s IA is > minimum IA?

-Compare unmasked threshold differences to assumed minimum IA

The minimum IA is assumed to be the larger of the following:

1. The assumed minimum IA for the transducer

   1. 40 or 50 dB for headphones and inserts

2. The difference between the AC (TE) threshold and the AC (NTE) threshold

3. The difference between the AC (TE) threshold and the BC (TE) threshold -i.e. ABG in the TE

4. The difference between the AC (TE) threshold and the BC (NTE) threshold

When do you need to mask AC thresholds conceptually?

-Whenever there is a chance that the signal to the test ear (TE) is actually being detected by the non-test ear (NTE)

-The likelihood of this happening depends on the

• air conduction thresholds (signal level) in the TE

• Interaural attenuation of the AC transducer

• BONE conduction thresholds in the NTE

When do you need to mask AC thresholds quantitatively?

-Mask if the difference between the AC threshold (signal level) in the TE and the BC threshold in the NTE > the minimum IA of the transducer

• Mask if: AC (TE) - BC (NTE) > min IA

-If we don’t know the BC threshold in the NTE, no masking until unmasked AC and BC thresholds are obtained. Alternatively, clinicians may assume initially that the BC (NTE) threshold = AC (NTE) threshold

-

When do you need to mask BC thresholds conceptually?

-same as AC testing- mask whenever NTE may be responsible for signal detection

-Importantly- mask whenever you need to confirm/refute the presence of a conductive component in the test ear

BC testing

-Always assume unmasked thresholds reflect the better ear-not the mastoid placement ear!

-Start BC testing on the poorer hearing ear

• i.e. masker goes into the ear with the better AC thresholds as it is easier to effectively mask the better hearing ear

-Assume min IA for BC testing = 0 dB

• but remember mean BC IA varies with freq- 0 to 10 dB

  • up to 30-40 dB in individuals

When do you need to mask BC thresholds quantitatively?

-Mask if difference between AC and BC thresholds in TE is > 15 dB

• mask if: AC (TE) - BC (TE) > 15 dB (mask if ABG (TE) > 15 dB

  • compare test ear AC & BC thresholds b/c need to confirm/refute the presence of a conductive component

• ASHA (2005) rx to mask if ABG>10 dB

  • conservative but fine approach

• Always mask if ABG (TE) is systematically > 10 dB

  • systematic differences are not random variability

What masker levels should you use?

-enough the “effectively mask” the NTE

• I.e. enough to raise threshold in NTE so signal from the TE is not detected

EM & EMLs

-masking noises are calibrated in EML (dB SPL) but audiometer dial values reflect EM in dB

• the amount of EM is = to the expected shift in pure tone threshold from 0 dB HL if the masking noise was presented in the same ear as the tone

  • e.g. a 60 dB HL pure tone would be just audible at threshold when presented in a 60 dB EM noise (60 dB masker dial value on the audiometer)

-So to mask the signal (make it inaudible in the NTE) you must add at least 5 dB more EM

• e.g. at least 65 dB EM to effectively mask a 60 dB HL tone