How do you use probe microphone measures to assess the integrity of directional microphone hearing aids?
Probe-microphone measurements could be used to construct a fairly valid ''real-ear polar plot,'' which could show frequency-specific attenuation and the effects of head diffraction and head shadow. This would require a fair amount of work, however, and really is overkill for the everyday fitting of hearing aids. Rather, my thought would be that the average clinician simply would want to use probe-mic measures to assure that the directional microphone hearing aid is working appropriately. When this is the goal, running two REAR curves, one at 0° azimuth and another at 180° azimuth should be all you need (easy to do if patient is in swivel chair). Let's say that you usually fit one of two models of directional instruments, either a Sonotone or a Radioear. After testing four or five different models of the Sonotone, which has a cardiod directional pattern, you find that the typical real-ear front-to-back difference (REAR @ 180° subtracted from the REAR @ 0°) for the 1000 to 4000 Hz range is around 20 dB. The Radioear has a hypercardiod pattern, and the typical front-to-back difference for this product is 15 dB. This gives you some clinic norms to work with. Now, if you're testing a patient with his new digital directional Sonotones and you only see a 5 dB front-to-back difference, you know that something is wrong.
Rather than eyeball the differences in the REAR curves, you might want to have the probe-mic equipment plot the frequency-specific front-to-back difference. If so, then try out this test protocol:
- Place the patient in a swivel chair.
- Set the WDRC to ''linear'' (if not, the WDRC processing will work to counteract the effects of the directional microphone, producing results that underestimate directionality in the real world).
- Use an input signal of 65-70 dB SPL. If you're testing a digital hearing aid, turn off the noise reduction feature, or use test signals designed for assessing digital instruments (e.g., ICRA signals).
- Turn your patient around so that you conduct the first run with the loudspeaker of the probe system located behind the patient, at a distance of around 3 feet. Tell your probe system that this run is the REUR.
- Now turn your patient back so that he or she is facing forward and conduct the second run with the loudspeaker located directly in front of the patient. Tell your probe system that this run is the REAR.
- The probe system will automatically subtract the first run from the second. What is then presented on the monitor is the frequency-specific front-to-back ratio of the directional hearing aid.
H. Gustav Mueller, Ph.D. has an audiology consulting practice in Castle Pines Village, Colorado. He is the senior audiology consultant for Siemens Hearing Instruments, Contributing Editor for The Hearing Journal, and teaches audiology classes at Vanderbilt, Central Michigan, Nova Southeastern and Northern Colorado Universities. His publications related to probe-mic measures include:
- Mueller HG, Hawkins DB, Northern JL. Probe Microphone Measurements. Singular Publishing. 1992
- Mueller, HG. Probe-microphone measurements: Yesterday, today, and tomorrow. The Hearing Journal, 1998, 51, 4: 17-22.
- Mueller HG, Hall JW. Audiologists' Desk Reference. Singular Publishing, 1988.
- Mueller, HG. Probe-mic assessment of digital hearing aids? Yes you can! The Hearing Journal, 2001, 54, 1: 10-17.
- Mueller, HG. Probe microphone measurements: Twenty years later. Trends in Amplication, 2001 (in press).