MASKINGMASKING
BASIC PRINCIPLESBASIC PRINCIPLES
CLINICAL APPROACHESCLINICAL APPROACHES
Masking = Preventing Crossover
• Given enough intensity any transducer can stimulate the opposite cochlea
• Enough intensity to cross over = _________.
• Therefore some unmasked thresholds will be suspect and require masking.
IA in Supra-aural Earphones
• The range for headphones is 40 - 80 dB
• Depends on:
– frequency,
– headphone placement
– size of the patient’s head
• To be safe, use 40 dB as the IA for headphones
• i.e., when HL exceeds opposite ear thresholds by 40 or more, you may need to mask.
IA for Insert Earphones
• Varies with:
– frequency
– depth of placement
– patient
• IA is greater for inserts than headphones
• Once again being safe we can use 60 dB as IA for inserts.
Interaural Attenuation for InsertsFrom:Instructions for the use of E-A-RTONE ® insert earphones
Proper insertion depth is necessary to achieve
maximum ambient noise attenuation and
interaural attenuation.
The graph depicts interaural attenuation as
a function of insertion depth.
IA for Bone Conduction
• 0 dB
• However, we aren’t interested in clinically insignificant air-bone gaps
(Air-Bone Gap = 15 dB or more)
• Mask any unexplained air-bone gaps
When you DO need to mask:
• Unilateral Conductive HL
• Bilateral Conductive HL
• Unilateral SNHL (differing by more than IA)
(GENERALLY)
When you don’t need to mask
• Hearing WNL bilaterally
• Bilateral symmetrical SNHL
(GENERALLY)
NE
ED
TO
MA
SK
?N
EE
D T
O M
AS
K?
NE
ED
TO
MA
SK
?N
EE
D T
O M
AS
K?
Masking Principles
• Remember: Crossover is always via Bone Conduction!!!
• With masking, a 5 dB shift in threshold may occur due to central masking.
• When you introduce masking to an ear, the BC and AC thresholds will be affected the same amount.
• Masking must be above the threshold of the masked ear to have an effect on that ear.
What kind of Noise?
• For Pure Tones?
• For Speech?
• Narrow Band Noise
• Pink or Speech Noise– or white noise
dB EM
• Calibration of decibel level of maskers
• Level to which tone threshold will be shifted in the presence of the masker.
e.g., a 45 dB EM narrow band noise centered at 2000 Hz would raise someone’s threshold for a
2000 Hz tone to 45 dB HL **
Ranges of Masking
20
30
40
50
60
70
80
90
100
20 30 40 50 60 70 80 90
Masker Level (dB EM)
Thr
esho
ld in
TE
(dB
HL
)
Under Sufficient Over
Masking
Normal
Mild
Moderate
Mod-
Severe
Severe
Profound
0
10
20
30
40
50
60
70
80
90
100
110
250 500 1K 2K 3K 4K 6K 8K
Am
plit
ude
in d
B H
LFrequency in KHz
X X X X X
Where is unmasked BC for AD at all frequencies?
Normal
Mild
Moderate
Mod-
Severe
Severe
Profound
0
10
20
30
40
50
60
70
80
90
100
110
250 500 1K 2K 3K 4K 6K 8K
Am
plit
ude
in d
B H
LFrequency in KHz
X X X X X
Unmasked Bone
^ ^ ^ ^ ^
Normal
Mild
Moderate
Mod-
Severe
Severe
Profound
0
10
20
30
40
50
60
70
80
90
100
110
250 500 1K 2K 3K 4K 6K 8K
Am
plit
ude
in d
B H
LFrequency in KHz
X X X X X
Headphones maximum NB masking before crossover: 40 dB
Normal
Mild
Moderate
Mod-
Severe
Severe
Profound
0
10
20
30
40
50
60
70
80
90
100
110
250 500 1K 2K 3K 4K 6K 8K
Am
plit
ude
in d
B H
LFrequency in KHz
X X X X X
Inserts maximum NB masking before crossover: 60 dB
Feel like you’re doing
something wrong?...
It’s OK to
ask!