2. Inhale….now, exhale
Masking is a very difficult concept to understand
You must understand the science behind masking
Masking requires memorization of interaural attenuation
values and formulae for the amount of masking to use
Many testers are nervous that they are not masking
effectively/appropriately.
When in doubt, ask the patient which ear they are hearing the
beeps in!
Example: If you are masking in the right ear and trying to obtain the
left ear threshold, ask the patient which ear they hear the beeps in.
If they say, “right,” then you know you need to increase the masking
noise in the right ear in order to prevent that ear from hearing the
sound.
3. Why do we mask?
To prevent the non-test ear (NTE) from participating
in the test (cross-hearing)
We want to make sure that when we are testing the
right ear, that we are getting the true threshold of the
test ear (TE) and that the NTE is not helping out
This is necessary because of BONE CONDUCTION!
Our ears are housed in a bony skull, which vibrates
BOTH cochleae when sound is present at a certain level
4. Interaural Attenuation
Interaural attenuation (IA) is the amount of sound
that is attenuated/reduced when crossing from one ear
to the other
The IA varies:
Depending on the frequency
From patient-to-patient (skull thickness)
Depending on the transducer used (phones, inserts,
bone)
5. IA of Air Conduction - Headphones
The minimum IA of headphones is 40 dB HL
If we present 80 dB to the test ear, 40 dB of that sound
is reaching the cochlea of the non-test ear
Image from: wikipedia.org
6. IA of Air Conduction - Inserts
The minimum IA of inserts is 70 dB HL
If we present 80 dB to the test ear, 10 dB of that sound
is reaching the cochlea of the non-test ear
You are less likely to mask for air-conduction testing
when you use inserts
Image adapted from: wikipedia.org
7. IA of Bone Conduction
Image from: wikipedia.org
The minimum IA of bone
conduction is 0 dB HL
If we present 30 dB to the test
ear, 30 dB of that sound is
reaching the cochlea of the
non-test ear
You must mask for bone-
conduction testing in
conductive losses and
asymmetrical losses
8. When to Mask-Air Conduction
Headphones: when there is a 40 dB or more difference
between the air conduction threshold of the test ear and
the bone conduction threshold of the non-test ear.
Inserts: when there is a 70 dB or more difference between
the air conduction threshold of the test ear and the bone
conduction threshold of the non-test ear.
NOTE: Because we begin our audiogram with air
conduction testing, we assume that there is no air-bone gap
and compare air-to-air to determine the need for masking.
Occasionally, you will have to go back and mask some AC
thresholds after you have actual BC thresholds to compare
to.
9. When to Mask - Bone Conduction
When there is a 15dB or more difference between the
air conduction threshold of the test ear and the bone
conduction threshold of the same ear
aka: Mask for BC when there is an air-bone gap
10. Types of Masking Noise
Puretone Testing
The masker is a narrow-band noise (NBN) that is
centered around the test frequency
Speech Testing
The masker is a wide-band noise that consists of sound
energy from 300-3000 Hz
11. How Much Noise?
Well, you need to “cover up” the NTE….so, the level of
the masking noise has to be greater than the patients
AC threshold in the NTE, but not so loud that
overmasking occurs.
12. Undermasking
Not using enough noise in the non-test ear
The obtained threshold is really the response of the
non-test ear
13. Overmasking
Using too much noise
The obtained threshold is exaggerated when the
masker crosses the head and shifts the threshold of the
test ear.
Traditionally, we use the plateau method to ensure
that we have obtained a true threshold and are not
undermasking or overmasking.
14. The Plateau Method
Always start with the noise 10 dB above the AC
threshold of the NTE
With noise in the NTE, present the tone in the test ear
at threshold level
If the patient hears the tone, increase the masking noise in
the NTE by 5 dB
If the patient does not hear the tone, increase the tone in the
TE until the patient responds
After the patient responds to the tone with three
consecutive, 5 dB increases of noise in the NTE, you
have achieved your plateau, and the patient’s masked
threshold can be recorded.
15. Occlusion Effect
During masking for bone conduction, the bone oscillator is
on the TE mastoid, while the NTE is plugged up with a
phone or insert.
In this situation, the occlusion of the plugged up, non-test
ear will actually increase the likelihood that the NTE will
respond to the tone (especially at .25, .5, and 1 kHz).
As a result, the patient’s occlusion effect must be added to
the AC threshold of the non-test ear at the onset of
masking.
It’s recommended to add an extra 10 dB at .25 and .5 kHz and
an extra 5 dB at 1 kHz. So instead of starting your masking at
10 dB above the NTE threshold at .25 kHz, you would start
your masking at 20 dB above the threshold.
16. The Masking Dilemma
In bilateral, conductive hearing losses, the amount of
masking noise required is often sufficient to result in
overmasking and a shifting of the true threshold.
In these cases, it is best to record the unmasked
thresholds and note below the audiogram, “could not
mask-masking dilemma”
17. Pearls of Wisdom
When in doubt, mask
Using insert earphones will greatly reduce the need for
masking due to a higher interaural attenuation
Inserts also reduce the risk of overmasking
Ask the patient to tell you which ear they hear the
beeps in (if they can)
