Real-Ear Measures for Open-Fit Devices

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Answer

The considerations needed for making real-ear measurements with open-fit devices include the following:

1. You need to do them. Similar to all other hearing aids, it is impossible to predict the actual insertion gain from simulated insertion gain unless you are only fitting KEMAR clinically.
   
   
2. Because most open-fit devices use a stock "dome" tip in combination with a "slim" tube, it is more challenging to achieve stable placement of the probe-tube in the ear canal, and can lead to measurement variability. Also, for this reason, ensure that the probe-tube microphone is not bent or pressed up against the ear canal wall.
   
   
3. Contrary to popular belief, all "open" fit devices are not "acoustically transparent". That is, the differences between REOR (real ear occluded response) and REUR (real-ear unaided response) may in some cases be substantial, and will vary depending on vent size and insertion depth of the "slim" tube. In turn, this will have a direct bearing on sound quality and the amount of gain needed to achieve speech audibility, particularly in the high frequencies. Measuring and comparing REOR to REUR is a useful tool for this purpose.
   
   
4. Although feedback phase inversion has dramatically reduced acoustic feedback, it does not eliminate it completely. The large vent sizes used with "open" fit devices also provides a feedback pathway for sound that will ultimately cause the hearing aid to oscillate. If the hearing aid is equipped with a volume control, it is useful to determine how much "headroom" is preserved by turning the volume up to the point just below feedback. Also, although for most it is more a laboratory exercise than clinical tool, it may be possible to measure how much aided stable gain is provided by the feedback canceller by making measurements with the feedback reduction feature turned on and off.
   
   
5. When pushing the fitting limits of "open" fit devices with large vents, it may be necessary to disengage the reference microphone, which will be influenced by sound coming back OUT through the vent. Use a real-ear measurement system that enables you to turn the reference microphone off in these cases to ensure accurate measurements.
   
   
6. Because the majority of "open" fit devices use nonlinear gain, be sure to use multiple input levels to measure performance. The best way to ensure audibility for soft, moderate, and loud levels is to use the "SPL-o-gram" as defined originally by Norm Erber in the 1970s. Most of the real-ear systems today enable the use of "live" or recorded speech at different input levels (55. 70, and 85 dB SPL are commonly used) to ensure that soft speech is soft, moderate is comfortable, and loud speech and other sounds are not uncomfortable. These same principles apply for "open" hearing aid fittings.
   
   
7. If the device is equipped with directional microphones, you can measure them the same as you would any other hearing aids. Keep in mind that the magnitude of benefit will not be as great as for "closed" earmolds, but you should see a difference for most instruments when the patient is facing the loudspeaker in omni and when s/he is rotated to 180 degrees in directional mode.