Audioscan Axiom - Manual (Page 69)

Axiom ® User's Guide Version 1.8  May 2015
Test-box Speechmap coupling: HA-2 (BTE) coupler
Since the RECD here is measured with a foam eartip, an average earmold is added to obtain an HA-2 RECD
with earmold effects. The error involved will be the difference between the actual earmold and the average used.
Alternate method #3:
RECD On-ear measurement: Audioscan RECD transducer + client’s personal earmold
Test-box Speechmap coupling: client’s personal earmold + HA-1 (ITE) coupler
An average earmold will be subtracted from the measured RECD to obtain an HA-1 RECD without earmold
effects. The actual earmold will be introduced during test box simulation, but error will exist equal to the
difference between the earmold measured on the RECD transducer and the average earmold that was subtracted.
RECD facts
Individual RECD values are derived by comparing the SPL produced by a sound source in the closed ear canal
to that produced by the same sound source in a 2cc coupler. It is commonly assumed that the RECD so
measured is a property of the individual ear and is independent of the sound source used to obtain it, making it
useful for converting HL threshold and UCL obtained using insert phones to SPL values and for simulating real
ear measurements from 2cc coupler measures. However, this assumption is true, only if the acoustic impedance
of the sound source is significantly higher than the acoustic impedance of the ear canal or the coupler. This is
the case for RIC, ITE, and ITC hearing aids and is approximately true for insert earphones with the supplied
foam tips. The acoustic impedance of insert earphones drops if the foam tip is replaced by an earmold or the
tubing that forms part of the HA-2 coupler. At some frequencies, for BTE hearing aids with their earhooks and
earmold tubing (or the HA-2 coupler tubing), the acoustic impedance approaches that of an adult ear and falls
below that of a child's ear. Each of these sound sources will produce a different RECD and, if the acoustic
impedance of the sound source differs for the ear canal and 2cc coupler measurements, further RECD
differences will result.
Audioscan uses the HA-2 (BTE) coupler for the transducer calibration, then our RECD transducer and either a
foam eartip or the client's personal earmold for the real ear measurement. When a foam tip is used, we subtract
the two measurements to get an HA-1 RECD. When an earmold is used on the ear measurement we first subtract
an average earmold (referenced to our RECD transducer) to obtain an HA-1 RECD, then we add the same
average earmold (referenced to an ER3A) to obtain an HA-2 RECD with earmold effects retained.
When the entered HL transducer is Insert + foam, we apply an HA-1 foam tip RECD during the HL-to-SPL
transform to correct the thresholds measured in a non-average ear canal. When the HL transducer is Insert +
mold we apply the HA-2 referenced RECD with earmold effects retained to the HL-to-SPL transform.
In Test-box Speechmap (simulated REM) we apply the HA-1 RECD to the coupler measurment for ITE, ITC,
CIC, RIC instruments, and also to BTE instruments when BTE + mold has been selected (and personal earmold
used in the measurement). When the instrument type is set to BTE (HA-2) the HA-2 referenced RECD is
applied.
RECD values and are stored, displayed, and printed referenced to the HA-1 coupler. Imported data from
software versions 3.10.39 and earlier will be HA-2 referenced and will be converted to HA-1 referenced data for
use, storage, and display. Subsequent printing, saving, or export will show the converted HA-1 referenced values
only.
If you use insert ear phones for audiometry OR if you verify using the test box, you should measure RECD
(preferred) or use age-related average RECD.
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