Audioscan Verifit 2 - Manual (Page 80)

Verifit ® User's Guide Version 4.2 © June 2015
WRECD 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 (1) correcting hearing level (HL) data measured using insert earphones for individual
ear acoustics when the standard foam tip has been used (2) correcting hearing level (HL) data measured
using insert earphones for individual acoustics and the use of a custom earmold in place of a standard foam
tip, and (3) converting measured coupler SPL in Test box mode to estimated occluded ear SPL. 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 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.
The Verifit requires RECD data up to 12.5 kHz to facilitate wideband testing, and as a result RECD has
been renamed to WRECD (Wideband Real Ear to Coupler Difference) and utilizes the 0.4cc wideband
coupler in place of the ANSI 2cc coupler. The Audioscan WRECD measurement protocol uses the
Audioscan WRECD transducer as the sound source for ear canal SPL measurement and the 0.4cc wideband
coupler calibration. The use of the 0.4cc coupler in Test box Speechmap is desirable because it raises the
high frequency SPL of the hearing aid by more than 20 dB, making the rising noise floor of the coupler
microphone less significant. Its use for high frequency RECD measurement is desirable because of
resonances in the HA-1 coupler.
The revisions of ANSI S3.46 (2013) and IEC 61669 (now in process) define RECD as utilizing a high
impedance coupled sound source and the HA-1 coupler. The Audioscan wideband transducer with a foam
tip meets the high impedance requirement but the use of the 0.4cc coupler in the Verifit 2 means that the
resulting WRECD is not the RECD defined in the revised standards.
If you use insert ear phones for audiometry OR if you verify using the test box, you should measure
WRECD (preferred) or use age-related average WRECD.
If you use headphones or sound field for audiometry AND you use on-ear tests for verification, WRECD is
not applied in any way and need not be measured.
Insert phone audiometry with measured WRECD and on-ear verification provides the best fitting accuracy,
with test box verification being slightly less accurate. Insert phone HL data with age-related average
WRECD is next best
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