For most hearing-impaired children, the early and appropriate selection and use of amplification is the single most important habilitative tool available to us. (Seewald & Ross, 1988)
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The Real-Ear-to-Coupler Difference

What is an RECD?

The real-ear-to-coupler difference (RECD), as used by the DSL Method, is defined as the difference in dB across frequencies between the SPL measured in the real-ear and in a 2cc coupler, produced by a transducer generating the same input signal. RECD values vary as a function of age and frequency.

Why measure the RECD?

The RECD is an important measurement because it allows us to account for differences between couplers used in hearing aid work and real-ears. One approach to accounting for the difference between ears and 2cc couplers is to apply a set of average RECD transformation values from samples of adult listeners. Unfortunately, in young children, whose ears are quite small relative to adults, the average RECD values reported for adults are of limited use. Research has shown that even age-appropriate average RECD values collected from a sample of children are not as accurate for use as individual measures (Bagatto et al, 2005).

The RECD allows the audiologist to accurately convert assessment information collected with insert phones from dB HL to dB SPL (ear canal level) for use within the hearing aid selection and fitting process. The RECD also allows the audiologist to know the difference between the output in the real-ear and the output on a 2cc coupler that is used in the hearing aid fitting and verification process.

How is the RECD measured?

Different real-ear systems will have varying procedures for measuring the RECD. The general RECD measurement procedure consists of two measurements: (1) a 2cc coupler-based measurement and (2) a real-ear measurement. The 2cc coupler measurement is made on the same coupler traditionally used for hearing aid fitting and verification. A transducer from the real-ear system delivers a signal into the 2cc coupler and the system defines the SPL of the signal as a function of frequency. The same signal is then delivered from the same transducer into the individual's ear via a custom earmold or foam or immittance tip. The real-ear system again measures the level of the signal (this time in the individual's ear) as a function of frequency. The difference between the level of the signal on the ear and the 2cc coupler is considered the real-ear-to-coupler difference.

Age-appropriate real-ear-to-coupler difference (RECD) values

The RECD is a clinically useful measurement, and may be feasibly and reliably obtained in the pediatric and adult populations in the majority of cases (Sinclair et al., 1996; Tharpe, Sladen, Huta and McKinley, 2001; Munro and Davis, 2003). The DSL method has always provided age-appropriate average RECD values in software implementations for cases where clinicians have not been able to directly obtain the measurement (Seewald et al., 1997; Seewald et al., 1993). For DSL v5 the age-appropriate average RECD values have been updated to include: (1) frequency-specific predictions by age for eartip coupling; and (2) frequency-specific predictions by age for earmold coupling (Bagatto et al, 2005; Bagatto, Scollie, Seewald, Moodie and Hoover, 2002). The 95 percent confidence intervals for predictions of RECDs for eartip coupling and earmold RECD predictions were examined to determine the accuracy of prediction (Bagatto et al., 2005). Depending on the frequency of interest, an eartip RECD can be predicted to fall within a range of ±5.6 dB (at 500 Hz) at best and ±10.9 dB (at 6000 Hz) at worst for children 24 months of age and younger. Predictions of earmold RECDs can span a range of accuracy from ±6.7 dB (at 2000 Hz) to ±12.4 dB (6000 Hz) for children 36 months of age and younger.

Figure 1
Figure 1 illustrates the measured RECD values in dB as a function of age for one frequency for both coupling procedures. Although more desirable than using adult-based RECD average values when fitting amplification to infant and young children, these results indicate that age-appropriate predictions should not replace a more precise individualized RECD measurement.

Description of a modified RECD measurement procedure for use with infants

Procedures for measuring the RECD in the pediatric population have been published (Moodie et al, 1994). Other publications have provided recommended probe-tube insertion depth guidelines (Tharpe et al, 2001). The typical RECD measurement method described in most studies involves inserting the probe-tube and tip separately. This may not be practical in the infant population due to very small ear canals and the position of the infant during the measurement. Bagatto, Seewald, Scollie and Tharpe (2006) described details and study results for a new technique for obtaining accurate RECD measurements on an infant's ear.

figure 2
Briefly, the strategy involved simultaneous insertion of the probe-tube and the tip into the ear canal (see Figure 2). Study results indicated that extending the probe-tube approximately two to four millimeters (mm) beyond the tip resulted in appropriate insertion depth, as well as reliable and valid RECD values for infants between the ages of two to six months. A suitable insertion depth for the probe-tube was determined to be approximately 11mm from the entrance to the ear canal.


RECD measurement procedures recommended by manufacturers (Coming Soon)

Audioscan Verifit and RM500 SL
Interacoustics Affinity
Frye Electronics Fonix7000 and Frye FP34
Siemens Unity

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