The Most Important Real Ear Measurement You May Ever Make
by Larry Revit, hearing scientist
The real-ear-to-coupler difference (RECD) provides a wealth of information applicable to hearing aid fittings.
In early 1990, the advent of "Quik-Probe II" marked a milestone in hearing-aid fitting technology. For the first time, FONIX users could completely customize the ordering of a hearing aid using real-ear measurements. The Quik-Probe II customization procedure includes an individual measure of how the aided response for a given ear will differ from the coupler response. This measurement, called "RECD" (for "real-ear-to-coupler difference"), accounts for variations in earcanal size and impedance. All FONIX real-ear systems can be used to perform this measurement using the insertion-gain feature:
You first measure the coupler response of a hearing aid or insert earphone. To accomplish this, a probe microphone is threaded into an HA-1 coupler as shown in Figure 1(A). Call this measurement the "unaided response". Now measure the response of the same aid or earphone in the ear [as shown in Figure 1 (B)], using the same signal and settings; call this measurement the "aided response". The difference between these two measurements (automatically displayed as the "insertion gain") is the RECD.
Figure 1 (A)
Figure 2 (B)
Figure 2
To investigate the need for applying the RECD measurement routinely in clinical hearing aid fittings, Frye Electronics sponsored a clinical study of the RECD at Northwestern University1. The results showed that when an ear differed from the average adult normal value for any of the standard middle-ear impedance measures, the RECD also differed from average. The consequence is that a hearing aid prescribed using average RECD conversion factors will not perform as expected on ears having non-average RECDs. Conversely, if a custom RECD measurement is used in the prescription, the hearing aid will likely perform as expected.
For the clinical population in the Northwestern study, about half of the subjects had RECDs that were different from average. Some of these cases are shown in Figure2, along with the average (KEMAR) RECD curve shown in bold. (Ignore the data above 4 kHz, because of measurement artifact.) Some examples: Subject 1, who had the greatest deviation from average, has a chronically perforated eardrum. A hearing aid for this individual has to have a large amount of extra low-frequency gain output to compensate for the abnormal RECD. It's no wonder that this subject was, for a long time, considered a "problem fitting". Subject 5's earcanal has an abnormally large equivalent volume. This could be either because of a physically large earcanal or because of a flaccid eardrum. Extra gain and output across all frequencies is called for with this ear. Subject 7's ear has elevated static immittance (indicating a stiff eardrum). This ear requires lower overall gain and output to accomplish the desired real-ear result. It's easy to see that not only adults with non-average earcanals, but also children in general, could benefit greatly from custom RECD measurements applied to their hearing aid prescriptions.
Important new uses of the RECD - prediction of the real-ear saturation response, and simplified real-ear SPL audiometery.
Largely through the work of Richard Seewald and colleagues at the University of Western Ontario, additional uses of the RECD have come to light. If one knows the precise difference between and aid's response in the ear versus in the coupler (the RECD), then one can do most of the fitting adjustments using the coupler in the test box, leaving the real-ear measurement only for final verification. This is especially applicable with children, who won't sit still for anything!
Perhaps the most important application of this idea is to use the RECD with the 2cc-coupler SSPL-90 to predict the real-ear saturation response (RESR). The attraction of this approach is that the subject need not experience high signal levels in the earcanal. Early pioneering of this procedure was reported by Sullivan2 and later adapted for the FONIX 6500, presented in the Spring '92 edition (#13) of Larry's Corner. All the fitter needs to do is add the RECD to the SSPL-90. The result is the estimated RESR. A worksheet provided with this publication can assist with the estimated RESR procedure.
A second new application of the RECD is to estimate the real-ear SPLs corresponding to audiometeric data. The procedure takes advantage of the fact that audiometeric data. The procedure takes advantage of the fact that audiometeric insert earphones are calibrated on a 2cc coupler. This means audiometeric thresholds, most comfortable levels, and uncomfortable levels can be expressed in terms of 2cc coupler SPL. As with the estimated RESR procedure, the RECD is added to the 2cc-coupler values to estimate the real-ear audiometeric values. To the extent that supra-aural earphones give the same audiometeric results as insert earphones, supra-aural earphone data can be used in this method. But insert earphone audiometery will give the most accurate results. A worksheet to assist with this method is also provided with this publication.
Given all the potential uses, the RECD may just turn out to be the most important real-ear measurement you'll ever make.
REFERENCES
Friket-Pasa S, Revit LJ: Individualized correction factors in the preselection of hearing aids. J Speech Hear Res1992; 35:384-400. (Reprints available from Frye Electronics.)
Sullivan RF: Aided SSPL 90 response in the real ear: A safe estimate. Hear Instrum 1987; 38(10),p. 36.