The Tip of the Probe
by Larry Revit, hearing scientist
Part I: Adjusting the Probe Tube Insertion Depth
This Larry's Corner marks the beginning of a series of tips on making probe microphone measurements and using them in hearing aid fittings. With the advent of hearing aids having extended high-frequency range, an improved technique for measuring the high frequencies in real-ear measurements has become important. This first "Tip of the Probe" will present a new protocol that can improve high-frequency accuracy in routine insertion-gain measurements.
Research on insertion-gain techniques has indicated that as long as the tip of the probe tube is in the same place for both the unaided and the aided measurements, the insertion-gain results should be accurate no matter where in the earcanal the probe tip actually is (e.g., Hawkins and Mueller, 1986). So all the dispenser needs to do is carefully mark the probe tube and be sure the mark is in the same place for both the unaided and the aided measurements...right? Well... there is a possible complication.
Even a very small change in the position of the probe tip can influence a real-ear measurement in a way that has nothing to do with the performance of the hearing aid being tested. The graph to the right (Fig. 1) shows how the sound pressure level (SPL) measured in the earcanal varies with the distance between the probe tip and the eardrum. The horizontal axis is the distance from the eardrum. Each curve shows the SPL at a different frequency as the distance from the eardrum changes. The dip in each curve is caused by sound reflecting off the eardrum. This interference pattern is called a "standing-wave null". Standing-wave nulls cause dips in the frequency response measured in the earcanal at the probe tip. The result is that even slight changes in the position of the probe tip can change the measured earcanal frequency response, especially in the high frequencies.
Figure 1
So what happens when we measure insertion gain? The picture of the ears on the next page (Fig. 2) shows hypothetical positions of the probe tube for unaided and aided measurements. The tick marks below the earcanals, scaled at about 2 mm between each mark, show the relative probe tip positions for each condition. If you look carefully, you can see that the probe tip has moved slightly to the right in the aided case as compared to the unaided case, even though the position of the mark on the tube outside the ear remained the same in both cases. In the aided case, the probe tube was forced to follow the contour of the earpiece; but not in the unaided case. Thus, the distance between the probe tip and the eardrum changed when the earpiece was inserted.
Figure 2
Let's look at some measurements. For the graphs on the right, an ink-mark was placed on the probe tube 30 mm from the tip. Figure 3 shows an unaided (REUR) measurement, in bold, and two aided (REAR) measurements for the same hearing aid. For the aided curve labeled "old way", the position of the ink-mark was kept in the same position for the aided measurement as for the unaided measurement (refer to Fig. 2). Note that the "standing-wave null" in the earcanal response for this aided curve is at a lower frequency than that for the unaided curve. This misalignment of the nulls indicates that the probe tip has indeed moved back from the eardrum for the aided measurement. For the aided curve labeled "new way", the probe tube was advanced 2-3 mm toward the eardrum before the earpiece was inserted. The result was that the standing-wave null for the aided curve occurred at the same frequency as for the unaided curve, indicating that the probe tip was in the same place for both. The effect of advancing the probe tube toward the eardrum before inserting the earpiece was to compensate for the anticipated change in the position of the probe tip once the earpiece was inserted.
Figure 3
In Figure 4, three repeated insertion-gain tests for the same hearing aid (a K-AMP ITE) were done the "old way" (ink-mark in the same position for the unaided and aided tests) and three were done the "new way" (ink-mark advanced before inserting the earpiece). In each graph, the thick, gray curve is the insertion gain as measured with the probe tube right at the eardrum; the thick, gray curve, thus, is taken to represent the "true" insertion gain. For the measurements done the "old way", wide swings in the high-frequency response are apparent. For the measurements done the "new way", only one of the samples shows relatively minor variations from the true insertion gain.
Figure 4
Because of the improvement in accuracy demonstrated by these and other trials, a suggested new insertion-gain protocol is:
- Place a larger-than-usual ink-mark on the probe tube at a position corresponding to the tragus or the tragal notch. As Figure 5 shows, when aligning the mark with the appropriate point on the earpiece, the tip of the probe should extend at least 5-6 mm beyond the sound outlet of the earpiece. The mark should begin at the point of the tragus or tragal notch and extend back (latterally) about 4 mm.
Figure 5
- When inserting the probe tube for the unaided measurement, position the tube so the entire mark is outside the tragus or tragal notch (Fig. 6A).
- Measure the unaided response.
- Before inserting the earpiece for the aided response, advance the probe tube 2-3 mm further into the earcanal so all but the last millimeter or so of the mark rests inside the tragus or tragal notch (Fig. 6B).
- Now carefully insert the earpiece so just a millimeter or so of the mark remains visible (Fig. 6C).
- Measure the aided response.
Figure 6
The results shown in this report are for only one ear. Unfortunately, there is no guarantee that compensating the probe tip position by 2-3 mm is exactly what is needed for every ear/earpiece combination. After all, each concha has a different shape, and so the road traveled by the probe tube into the earcanal can vary across individual cases. But compensating the probe tip position by 2-3 mm will likely yield higher accuracy and repeatability than not compensating at all. If you wish to verify that you have the accuracy you need, try remeasuring with the ink-mark at a different position on the probe tube (preferably deeper, if you have room). You should get repeatable results.
Hearing aids having extended-range frequency reponse are becoming increasingly available. The above protocol can improve the high-frequency accuracy of insertion-gain measurements for these instruments. If you try the new protocol, please write or call to let me know your experiences. As stated in a Larry's Corner of old, "Accuracy is bliss".
References
Hawkins DB and Mueller HG (1986). Some variables affecting the accuracy of probe tube microphone measurements. Hearing Instruments 37(6):8-12,49.
Gilman S and Dirks DD (1986). Acoustics of ear canal measurement of eardrum SPL in simulators. Journal of the Acoustical Society of America 80(3):783-93.