Masking in the Clinic

Crossover occurs when the signal is physically present in the opposite ear, whereas cross-hearing occurs only when it is audible. The distinction is clarified using the following example based on a hypothetical patient: The level of the 1000 Hz tone reaching this person's right (nontest) ear will always be 50 dB less than the amount presented from the left earphone due to IA. Consider these three cases:

dB HL at left earphone

(a) 60 dB

(b) 80 dB

(c) 55 dB

- Interaural Attenuation

-50 dB

-50 dB

-50 dB

= dB HL present at right cochlea

= 10 dB (at threshold)

= 30 dB (20 dB SL)

= 5 dB (5 dB below threshold)

In (a) the tone reaches the right ear at 10 dB HL and is heard because this is the right ear's threshold. In (b) the tone reaches the right ear at 30 dB HL and is heard because this level is 20 dB above the right ear's threshold (20 dB SL). In both of these cases signal crossover resulted in cross-hearing. However, the tone in (c) reaches the right ear at only 5 dB HL which is 5 dB below threshold and is thus inaudible. Here there is crossover because the signal is present in the NTE but there is no cross-hearing because it is below threshold.

The minimum IA value typically suggested to rule out crossover for clinical purposes is 40dB using supra-aural headphones
The minimum IA value typically suggested to rule out crossover for clinical purposes is 50dB using insert earphones
The minimum IA value typically suggested to rule out crossover for clinical purposes is 0dB using bone conduction

The type of masking noise is called narrow-band noise

When to Mask for Air Conduction

Cross-hearing for an air conduction signal occurs via the bone-conduction route and depends on three parameters:

the sound level in dB HL presented to the TE
the amount of IA in dB which determines how much of the signal crosses over
the true bone conduction threshold in dB HL of the NTE, which determines whether this signal is audible in the NTE

Consequently, one must mask for an air-conduction whenever the sound (in dB HL) being presented to the TE is able to reach the bone-conduction thresold (in dB HL) of the NTE.

Using the minimum IA value of 40 dB (headphones) as our criterion, masking for AC is needed whenever the test ear's conduction threshold (ACt) and the nontest ear's bone-conduction threshold (BCn) differ by 40 dB or more. Some audiologists call the spread between ACt and BCn an air-opposite-bone-gap (AOBG) or an air-contralateral-bone-gap (ACBG).

Using this terminology, masking is necessary whenever the following relationship applies: AOBG > or = 40 dB

When to Mask for Bone Conduction

A bone conduction threshold should be retested with masking in the NTE whenever there is an air-bone-gap (ABG) within the test ear that is greater than 10 dB, that is 15 dB or more (due to incremental changes of 5dB) which may be written as: ABG > 10dB

Because testing is done in 5dB steps, this rule also can be stated this way: A bone-conduction threshold should be retested with masking in the NTE whenever the (ABG) within the test ear is 15 dB or more or ABG > or = 15 dB

The Plateau Method

The plateau method is a widely accepted strategy for finding the true masked threshold of the TE however there are three distinct ranges, labeled undermasking, plateau and overmasking

undermasking - the amount of noise is not sufficient to exclude the NTE from the test. Thus, undermasking occurs when there is not enough masking noise, so that the tone is being heard by the non-test ear.
effective masking - threshold for the tone stays the same even though the masking noise is raised. This plateau reveals the test ear's true masked threshold for the tone. The width of the plateau is sometimes called the range of effective masking.
overmasking - occurs when so much noise is presented to the NTE that it crosses over and masks the test ear. It has been proposed that overmasking occurs when the level of the noise in the non-test ear (MNn) is equal to or more than IA plus the bone-conduction threshold of the test ear (BCt) or when MNn > or = (IA = BCt)

The Masking Dilemma

A special case in which overmasking occurs at the initial masking level is aptly called the masking dilemma. This problem occurs when the unmasked audiogram reveals large air-bone gaps in both ears; ABGs are ~55 dB wide. This situation is a delemma because we must retest both ears with masking even though overmasking would occur at the initial masking levels. For example, the IML of a nose needed to mask the right ear at 1000 Hz would be 55 + 5 + 10 = 70 dB HL, but the unmasked bone-conduction threshold is 0 dB HL. Overmasking occurs because presenting the IML to the right ear would cause masking at the left cochlea, which would shift the left ear's air and bone conduction thresholds. In such a case, we do not really know whether the left ear's threshold shifted because of (1) effective masking or (2) overmasking.

Masking for Speech Audiometry

We know that the "when to mask" question involves three factors

The level of the signal being presented to the TE; in speech audiometry this will typically be spondee words when obtaining the sppech reception threshold (SRT)
Interaural attenuation is the second factor. Individual IA values for speech using standard audiometric earphones. Gelfand prefers using the ore conservative value of 40 dB at least under typical conditions.
The bone conduction threshold of the NTE.

Thus, masking should be employed during speech audiometry when the speech level in the test ear (St) minus interaural attenuation equals or exceeds the "best" pure tone bone conduction threshold of the NTE (BCn). For those who like formulas, masking for speech audiometry is indicated when: St - IA > or = BCn. Here, IA may be 40 or 45 dB, although 40 dB provides a more conservative margin of safety.

References

Gelfand, Stanley A. Essentials of Audiology. Thieme, 2016.
DeRuiter, Mark, and Virginia Ramachandran. Basic Audiometry Learning Manual. Plural Publishing Inc., 2017

http://tjaudiology.com/rule-3-explained-some-tips/