Errors of the Acoustic Gain Prescribed Via Real-Ear-to-Coupler Differences (RECD)

Editor’s Note:

This is an interesting concept and was introduced at the April 2011 meeting of the ANSI standard’s working group 80, now working on a revision of the S3.46 real ear standard. In this suggestion, the HA-2 coupler result is not as good as the HA-1, especially when using the same custom earmold that would be used in the fitting. An even better response results from the use of a custom earmold, the HA-1, and when using the hearing aid as the sound generator in place of the ER3A. This would require that the hearing aid chip also be a sound generator than only an amplifier. Slight changes have been made by the editor to modify this from its original PowerPoint presentation.

Wayne Staab

Comments from George Frye, Frye Electronics relative to this method suggestion.

The real ear to coupler difference (RECD) has found its most popular use with the verification of hearing aid performance on children. Children are, shall we say, not very patient, and tend to have short attention spans. It is therefore, sometimes a problem to get a child to sit still while a complete diagnostic verification is performed on the child’s new hearing aid. But it is usually possible to get one good real ear test completed before the eruption.

On the other hand, it is much easier to use a hearing aid analyzer with its sound chamber and coupler to adjust and evaluate the hearing aid. By taking a single real ear measurement on the child to establish the RECD, the hearing aid evaluation can be done in a sound chamber.

Questions arise, however, about the accuracy of the RECD. Acoustic differences between sound generating devices like the ER3A, the HA1 and the HA2 microphone couplers, and the earmold or foam plug used to connect the sound from the hearing aid to the ear and then to the coupler, add possible errors in the generation of the RECD. Oleg Saltykov and Tony Gebert of Siemens have studied the problem and have arrived at methods that can yield quite accurate RECD’s.

They suggest, furthermore, is that their method works so well that it could be applied to all hearing aid fittings, eliminating most of the tedium associated with Real Ear. One patient measurement is all that is needed to gather the acoustic data needed for a very accurate RECD. Then all the adjustment work could be done in the relatively controlled and relaxed environment of the analyzer’s sound chamber.

SALTYKOV AND GEBERT PRESENTATION

 

The goal of this study was to analyze potential errors of RECD methods and to develop recommendations for a standard RECD procedure (ANSI Working Group 80).  RECD measurements were made using different reported procedures for small and large behind-the-ear (BTE) hearing aids using tubing lengths ±5 mm from nominal.  The reported methods are identified as Methods 1, 2, and 3. The 4^th method is the recommendation to be considered.

Definition

Real-Ear-to-Coupler Difference (RECD) is the difference between the sound pressure level in an individual ear and the sound pressure level in a 2-cc coupler measured with the same sound source.

Why are RECDs used?

• To predict the sound pressure level in an individual ear (RESPL) based on the sound pressure level measure in a 2-cc test coupler.
• To correct the hearing threshold levels for the difference in impedances of an individual ear when compared to a “typical” adult’s ear as represented by a 2-cc coupler for hearing aid fitting purposes.

Four Steps Involved in Obtaining a RECD

Measurement Procedures and Results

Method 1:  HA-1 coupler and ER-3A insert earphones with foam tip

This method was shown to produce small initial errors (-2.0 to +0.5 dB) when used with adult ears over the typical hearing aid response range.  The RECD curve is smooth.  This method produces small errors (+0.5 to –1.0 dB) when RECD is applied to the same instrument but with different tubing lengths (±5 mm from nominal) and to a different instrument with the nominal tubing length.

Method 2:  HA-2 coupler and ER-3A insert earphones with a custom earmold

This method produces small errors (-2.0 to +0.5 dB) and a smooth RECD curve when used for adults and if the length of the custom earmold tubing is equal to the length of HA-2 tubing (nominal).

Method 2 produces medium errors (+5 to –5 dB) when the custom earmold tube length varies ±5 mm from the standardized length of the HA-2 tubing.

Method 2 produces small errors (+0.5 to –1.0 dB) when RECD is applied to a different hearing aid with the nominal tubing length.

Method 3:  HA-2 coupler and ER-3A insert earphones with foam tip

Method 3 produces large initial errors (±10 dB) when applied to the same and different hearing aids and the RECD curve is not smooth.

Method 4:  Internal generator to couplers HA-1

In this suggested method, the input signal is generated by the hearing aid rather than from an insert earphone such as the ER-3A.

HA-1 Coupler

This produces no initial errors (0 dB), and the RECD curve is smooth.  It does produce small errors when the RECD is applied to the same hearing aid with different tubing lengths (±5 mm from nominal), and to a different hearing aid having nominal tubing length.

Method 4:  Internal generator to couplers HA-2

HA-2 Coupler

This produces no initial errors (0 dB), and the RECD curve is smooth.  It produces larger errors than with the HA-1 coupler when RECD is applied to the same hearing aid with different tubing lengths (±5 dB from nominal).  It produces small errors (+0.5 to –1.0 dB) when RECD is applied to a different hearing aid with the nominal tubing length.

Note: Custom hearing aids have similar errors to BTE hearing aids (although slightly lower) when measured with an HA-1 coupler and insert earphone.

Conclusion

Method #4 is being suggested as an easy alternative to real-ear measurements (REM) to obtain the RECD.  It is expected to be more robust than those using REM.  A disadvantage is that the procedure has not yet been standardized.