Masking Comps

When do you mask for air conduction?

When the difference between ears exceeds interaural attenuation

When do you mask for bone conduction?

When bone conduction is 10-15 dB or more away from air conduction threshold (air-bone gap) 

When do you mask for SRT?

When difference of SRT and BCT (NTE) is greater than IA

What noise do you use for WRS and SRT masking?

Speech noise

What is the OE correction factor for bone conduction masking at 250Hz?

20 dB

What is the OE correction factor for bone conduction masking at 500Hz?

15 dB

What is the OE correction factor for bone conduction masking at 1,000Hz?

10 dB

Why are OE correction factors used?

Used in bone conduction to try to recreate natural hearing in the NTE during masking. Naturally, the ear is not plugged up with an earplug, so adding OE factors helps accommodate for the earplug blocking the NTE’s canal.

What would happen if we didn’t use OE correction factors?

The NTE would falsely hear better than it actually does (inaccurate thresholds)

When do you mask for WRS?

When WRS when difference of WRS presentation level and BCT (NTE) is greater than IA

How do you find starting masking threshold (NTE) for air conduction?

Add 10 dB to NTE ACT and then add 10 dB (safety factor)

How do you find starting masking threshold (NTE) for bone conduction?

Add OE correction factor and 10 dB (safety factor) to NTE’s ACT

How do you find starting masking threshold (NTE) for bone conduction?

Add OE factor and 10 dB (safety factor) to NTE’s ACT

How do you find starting presentation level (TE) for bone conduction?

Use BCT of the better ear (the pt can’t hear any better than this)

How do you find starting presentation level (TE) for SRT?

Choose an amount between minimum masking and over masking

How to calculate effective masking level for bone conduction masking?

ACT (NTE) + OE + 5 dB (SF)

How to calculate effective masking level for air conduction masking?

ACT (NTE) - 5 dB (SF)

How to calculate effective masking level for SRT masking?

Estimated SRT (TE) - IA + 5 dB (SF)

How to calculate effective masking level for WRS masking?

PL (TE) - IA + 5 dB (SF)

How do you calculate over masking?

BCT (TE) + IA

How do you calculate minimum masking?

Calculate how much SRT (TE) - BCT (NTE) difference is over IA, then add 15 dB to the crossover amount (e.g. SRT: 60 dB, BCT: 5 dB, difference: 55 dB, crossover amount is 15 dB; therefore minimum masking level is 30 dB)

How do you find starting masking threshold (NTE) for SRT and WRS?

Choose level at least 15 dB above minimum masking but still below over masking

How many times do you need to plateau?

3

What are the three modes of crosshearing?

Transcranial, transcortical, intracranial

What is transcranial crosshearing?

Sound leaks out of headphones/inserts and travels around head

What is interaural attenuation?

When the energy of sound is lost from traveling from TE to NTE. Aka amount of dB lost through crosshearing

What type of conduction is transcranial?

Air conduction

What type of conduction is transcortical?

Air conduction then bone conduction

What type of conduction is intracranial?

Bone conduction

What is transcortical crosshearing?

Sound from TE is loud enough that is vibrates skull bones and travels via bone conduction to NTE

What is intracranial crosshearing?

Bone conduction on TE mastoid stimulates both cochleas simultaneously

Why do we mask?

To make sure our threshold responses are ear-specific, to eliminate crosshearing to NTE during suprathreshold testing

If you over mask, what do you do?

Note over masking threshold with a * and record “likely over masking” in the comments

What is the masking dilemma?

When you have to over mask NTE in order to gain masked thresholds for TE

When is a masking dilemma possible?

Whenever a bilateral CHL of 35 dB or more is present, most notably if the NTE is in the 55-60 dB range

IA of headphones for transcranial and transcortical crosshearing?

40 dB

IA of inserts for transcranial and transcortical crosshearing?

60 dB

IA of bone conduction/intracranial?

0 dB

What happens when you under mask?

You obtain inaccurate thresholds for TE as they will appear better than they actually are. If you fit amplification with these, then you will under fit amplification, obtain poor speech audiometry, and therefore misdiagnose nature of HL

What happens when you over mask?

TE can hear masking noise (typically happens in bone conduction), obtain inaccurate thresholds, may overfit amplification and damage hearing as NIHL as thresholds will appear worse than they are. For BCT masking, you could mistake a CHL for SNHL and prevent or delay medical treatment

What is central masking?

A small shift (about 5 dB) in the threshold of a pure tone when a masking noise is introduced to the opposite ear

Why does central masking occur?

The elevation of threshold is produced by inhibition that is sent down from the auditory centers in the brain