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6. Correction Factors
Providers who need to switch between insert and supra-aural earphones may
do so with properly calibrated equipment. Previously provided "out-of-the-box"
correction factors, however, have been judged to be imprecise for clinical
work, particularly when used with audiometers that have been calibrated to
account for equipment changes over time. Standardized correction factors
are therefore not provided. If the audiometer is calibrated for either type of
transducer as the primary earphone, measured correction factors for a
secondary earphone can be obtained by measuring the output on the
appropriate coupler and calculating the difference from the appropriate target
value at each frequency.
confidently applied for the secondary earphone. The example below outlines
the steps, and provides an example worksheet to calculate the correction
factors by frequency where the audiometer is calibrated with a supra-aural
earphone, and an insert earphone is used as the secondary transducer. In
this case, the measured SPLs are the 2cc coupler values obtained with an
insert earphone connected to an audiometer calibrated for a supra-aural
earphone, and the Target SPLs are the audiometer HL setting plus the insert
earphone RETSPLs for the particular 2cc coupler used. The process will work
as well in reverse, i.e. audiometer calibrated for an E-A-RTONE
earphone with a supra-aural earphone as the secondary transducer, as long
as the appropriate Target SPLs (TDH values) are employed.
• Calibrate the audiometer to the appropriate standard for the primary
transducer. (TDH-39, 49 or 50 in this example)
• Connect E-A-RTONES
• Measure the output in a 2cc coupler and substitute obtained values
for the Measured SPLs in the example worksheet below.
• Tabulate at each test frequency the difference between actual and
target sound pressure levels.
• Apply the rounded correction factors obtained to the audiometer dial
readings. Record and post for convenience.
• Repeat the procedure whenever the audiometer is calibrated.
Worksheet example for calculation of individual (insert earphone
correction factors for audiometer calibrated with TDH type earphone
Measured SPL
Target SPL*
Difference
Rounded Correction
Audiogram correction factors can then be
®
to the audiometer.
Frequency in Hz
250
500
750
90.5
76.8
74.1
84.0
75.5
72.0
6.5
1.3
2.1
5
0
0
1000
1500
2000
72.0
73.4
77.5
70.0
72.0
73.0
2.0
1.4
4.5
0
0
5
7
3000
4000
6000
79.3
76.5
63.7
73.5
75.5
72.0
5.8
1.0
-8.3
5
0
-10
*Target SPL = audiometer HL
®
insert
8000
58.0
70.0
-12.0
-10
+
RETSPL
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