Estimating the Real-Ear Saturation Response

by Larry Revit, hearing scientist

The following procedure lets you estimate what the real-ear saturation response (RESR) of a hearing aid will be without subjecting the client to a high-level, real-ear frequency sweep. The procedure is a modification of the method described by Sullivan (Hearing Instruments, Vol 38, 10/87). (Italicized items refer to using the FONIX 6500, software versions 2.50 and higher.)

First you find the difference between the response of the hearing aid in the coupler and that in the wearer's ear, using a moderate signal level. This is called the "real-ear-to-coupler difference". Then, to estimate what the saturation response will be in the ear, you apply the real-ear-to-coupler difference to the SSPL-90 response measured in the coupler.

  1. Have the client adjust the volume control (while wearing the instrument) to the normal use setting -- or, choose a low-gain setting. Secure the volume control in this position with tape or putty. This same setting will be used for all measurements, except in step #5.
  2. Using a moderate signal level, measure the Aided Response of the hearing aid in the client's ear. Copy this curve into REF 2 of Multi-curve.
  3. Using the same signal and level as for the real-ear case, measure the response of the hearing aid on the scc coupler in the test box. Copy this curve into REF 3.
  4. Subtract the Aided Response (REF 2) from the coupler response (REF 3). The difference curve (automatically in REF 9) is the real-ear-to-coupler difference (RECD). Copy the difference curve from REF 9 to REF 4 for later use.
  5. Now remove the tape from the volume control and set it for full on. Measure the response of the aid on the coupler in the test box, but with a pure-tone signal at 90 dB SPL.
  6. Subtract the RECD curve of step #4 (REF 4) from the result of step #5 (CRV 1). This new difference curve (in REF9) is the estimated real-ear saturation response (ERESR).
  7. Adjust the maximum power output of the hearing aid and re-test, until the desired estimated real-ear levels are achieved.

Use this procedure to ser the hearing aid in the sound chamber, before measuring the actual real-ear saturation response. Another way to set the hearing aid in the sound chamber before real-ear testing is to use the Target 2cc prescription software. But you will need frequency-specific loudness discomfort data to make a Target 2cc SSPL-90 curve. By using either the estimated RESR method or the Target 2cc method, you can be reasonably sure you won't cause excessive loudness in the client's ear.

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