In this case a customer had a patient with an ear that failed DPOAE in-the-ear calibration due to channel separation. The channel separation was presumed to be caused twisty ear canal as well as being on a steep part of the function due to standing-wave null around 3 kHz. The residual ear canal volume was also probably quite large. These two issues (separated channels and a standing wave null) cause the difference between the two channel functions (around 4 kHz) to be quite large and that is presumably what is triggering the warning. Now in frequency regions where the stimulus level is rapidly changing, any minor probe movement can cause quite a big change in actual stimulus levels. It's still safe to test because the DPOAE test checks to see if the stimulus levels are on target. But to improve the reliability of the measurement, I would recommend trying a deeper fit so as to push the standing wave null to a higher frequency and to flatten out the calibration function. One way to do this is to use a smaller ear tip. This may or may not work as it might not create a sufficient seal, but it's worth a try. Otherwise, compress the foam tip more and try to push it in a bit deeper. Reviewing our "how to prepare a foam tip for testing" tutorial could also be useful. But if none of this helps, ignoring the warning and continuing with testing is fine. But it's always good to aim for a flatter calibration function if possible.
Customer: I think the take-home message is that if I get the message, I should try pushing the eartip a little deeper. So, what do you do if you can't get a good fit despite attempts to reposition the eartip? Can you still run the DPOAE? Will the DPOAE be affected?
Ultimately, if you can’t get a good calibration you forge ahead anyway – HearID should warn you again during the DPOAE measurement if it the stimulus levels are too far off. You will also get better with practice. The DPOAE could be affected in that test-retest variability could be higher. How big a problem this is depends on your purpose. If you’re doing a longitudinal/prospective study with repeated measurements, higher variability masks changes in DPOAE caused by pathology or treatments, so you want to do all you can to keep variability low. The other problem you could get is that the actual stimulus levels achieved during the test could vary more from the target levels. HearID will warn you if the levels get too far from target (usually if the probe has moved or slipped out of the ear). Subtle changes in the two channels could affect the relative difference between L1 and L2. e.g., if you set it to 65/55 and there is a 3 dB tolerance on the levels, one level could change to 68 and the other to 52 which could change the DPOAE amplitude. But I’ve not noticed this to be a particularly common problem. If your study needs very specific DP L1/L2 levels then you’ll want to keep a close eye on the achieved stimulus levels (which are plotted on screen). For scientific research, rather than patient testing, I usually post hoc screen my DPOAE results to ensure measurements not meeting quality control criteria are excluded.
Hints to help with probe fits include: pulling the pinna up to straighten the canal, wiping the ear canal entrance with a cotton bud if the ear is oily (stops probe from slipping) or has debris (decreases wax partially occluding the tip end), ensuring the foam tip is smoothly compressed and the tip is domed (check out the tutorials under the Help menu), changing the orientation of the probe head (ie rotating it round a bit so that the two channels are positioned slightly differently), and ensuring the cable isn’t dragging or pulling on the probe head. Sometimes I use another foam tip in the outer ear to help balance and stabilize the probe head. Sometimes the same ear will be easy to fit one day and tricky the next. I’ve found sometimes just getting a different tester to try fitting the probe can make a difference. Every ear is different.
One of the keys to success is understanding what the calibration plot is showing you as that will help you know what adjustments to make. Though in general, deeper is better!
Update: avoiding standing wave nulls is now possible with the OtoStat 2.1 system using forward-pressure-level calibration - currently available for research users.