Lecture 2.1 - Masking Flashcards
Describe what cross hearing is and how it occurs*
Cross-hearing occurs when the ear that we are not testing detects the pure tone stimulus presented to the opposite ear.
This is a result of the bones in the skull vibrating and the energy being transferred to the opposite cochlear.
This can occur both in air conduction and bone conduction.
It makes it difficult to determine which ear is responding to the stimulus.
Describe what inter-aural attenuation is and how it occurs*
Inter-aural attenuation is the converse (mirror image) to trans cranial transmission (cross-hearing).
It is the loss of sound energy as sound waves pass from one ear over to the other (sometimes known as transcranial transmission loss).
Sound waves subside as their energy is dissipated on their journey through the bones in the skull. The energy is changed to heat.
When the sound level is high enough, it overcomes interaural attenuation (not all the energy is lost) and can be cross heard.
When does cross-hearing become a risk?*
Cross hearing can become a risk when completing bone conduction as there is low inter-aural attenuation.
The risk for cross hearing during air conduction is lower as there is a higher inter-aural attenuation. However, it does still occur.
What is masking?*
When we present a signal to the non-test ear to prevent it from cross-hearing a signal presented to the test ear.
What are the principles of masking?*
What are the rules of masking and their application in practice?*
- Mask during air conduction testing when:
Test ear AC threshold > non test ear threshold by:
≥ 50/60 dB HL (Inserts)
≥ 40 dB HL (supra aural) - And/or
Test ear AC threshold > non-test ear BC threshold by:
≥ 50/60 dB HL (Inserts)
≥ 40 dB HL (Supra aural) - Mask during bone conduction testing when there is an air-bone ga on the test ear ≥15 dB and an AC threshold of ≥20 dB HL
What is the mechanism of cross-hearing?
Primarily through bone conduction.
Even though cross-hearing can occur through air conduction, the sound/acoustic energy can still penetrate the bones of the skull and be conducted to the other ear.
What risk do the various transducers have of overcoming inter-aural attenuation?
The greater the surface area of the transducer, the more acoustic energy which can penetrate.
Bone conduction headphones: low inter-aural attenuation
Circum/supra aural headphones: moderate inter-aural attenuation
Inserts: high inter-aural attenuation
High inter-aural attenuation means a cross-hearing is less likely.
How do we mitigate the effects of cross-hearing?
By using masking
True/false: masking prevents cross-hearing from occurring
False. Cross-hearing still occurs, it is just that the masking prevents the client from hearing it.
What is the principle mechanism of masking?
Energetic masking
What is the ideal pure tone masker?
1/3 octave wide narrow band noise
What is a critical band?
How do you define a masking plateau?
A +20 dB change in the masker with no increases in pure tone threshold
When masking, what do you do when the patient stops responding to the pure tone stimulus?
Increase puretone level to test ear
In masking, what do you do when the patient continues to respond?
Increase the masker level to the non-test ear
When positioning the masking transducer, what do you need to keep in mind?
Do not occlude the test ear. We do not want to create the occlusion effect.
i.e., do not have in insert in the test ear and place the supra aural headphones off the ear
What is cross masking?
If the masking level becomes too high, it can have enough energy to reach the test ear. This starts to mask the test ear and raise the hearing threshold.
To prevent over-masking, you stop once you reach a 20 dB HL plateau.
What is the masking dilemma and what situation is at high risk for this?
- The masking dilemma is where we see over masking before we reach the plateau
- The situation most at risk for this is when there is a large air-bone gap in the non-test ear or bilaterally.
- This forces you to increase the level of the masking to be heard over the conductive loss, thereby risking cross-masking.
- This can be avoided by using insert earphones (which decreases the need for masking due to their high inter-aural attenuation) or to accept a “short plateau” of 15 dB.
What is central masking?
