I’m just starting to measure hearing aid directionality in my test box, but I sometimes get confusing results such as no test curve separation between the omni-directional and directional microphone settings. Am I doing something wrong?
With the Verifit and Verifit 2 test box directional mic test, not getting the results you expected may not necessarily mean you are doing something wrong. It may mean that there is something wrong with the directional microphone you are testing.
Here are some common situations that can occur as well as recommended tips for troubleshooting.
No Curve Separation
Say you have oriented the hearing aid(s) properly in the test box, programmed the hearing aid(s) to be in their directional mode, and have completed a directional microphone test where you did not get the curve separation you expected. Perhaps the two curves are overlapped, but you were expecting that they would be separated. Here are some possible reasons for this result:
Test Level Intensity. The level of the test stimulus can play a role in whether an automatic directional microphone system activates or not. If the intensity level that you are using to run the test is not sufficiently high enough to cause the automatic directional microphone to activate, simply increase the intensity level of your stimulus. This may cause the curves to now separate. If this happens you have not only verified that the directional microphone is working, but you have also verified that the hearing instrument’s directional system has a level dependency in its design.
SNR. Another reason for absent curve separation is that your signal-to-noise (SNR) ratio may not be appropriate. When using the Speech-Noise stimulus option, a variety of SNR choices are available within the stimulus menu. For some automatic directional microphone hearing aids, it is possible that the directional microphone function is not active with +12 dB SNR coming into the hearing instrument (speech input 12dB more intense than the jamming noise input), but fully active with 0 dB SNR coming into the hearing instrument. If you do not have curve separation but you have a highly positive SNR defined in your input condition, you may want to decrease the SNR and see if you can get the directional microphone result that you expect by changing that ratio between the speech and noise signals.
Incorrect test placement. Another possible reason for not getting curve separation is that you have placed the hearing instruments incorrectly in the test box. For the Verifit Classic, the most common error is that you inadvertently placed the hearing aid in the test box so that the front microphone is not the one closest to the left speaker. Instead, the front microphone of the hearing aid is closest to the right speaker. If you do that, the two curves may indeed be flipped or reversed.
If you do not have the hearing aid microphone ports oriented properly in the test box (i.e., the angle of the mic ports is not angled up at 45 degrees toward the left speaker in the Verifit, or not parallel to the floorboard of the test box in the Verifit 2), curve separation can be compromised. The correct orientation is necessary to maximize the curve separation result. Check the placement of the hearing instruments to make sure they are properly positioned in the test box.
Directional features are not activated. Another reason for a lack of separation between the two simultaneous measurement curves is that the directional feature of the hearing aid has not been activated in the programming software. You might want to check that, and once you turn it on, then the curve separation should become evident.
Troubleshooting the hearing aid. You also can start to troubleshoot the hearing aid if you do not observe the anticipated directional effect during your measurements. For instance, if one or more of the hearing aid mic ports are plugged with dirt or debris, the curve separation may not be as robust as it was expected to be, or there may be no curve separation at all. You may want to clean the microphone ports and/or change the microphone screen cover on the hearing aid (if applicable) and determine whether or not debris is the cause of your unexpected directional results.
Directional microphone does not work. The final possible cause for a lack of expected curve separation during the directional test is that the directional microphone simply is not working. Having a test like this and using this sort of analysis can help lead you to the conclusion that perhaps a hearing aid / microphone repair is required.
Curves are Reversed
Another difficult-to-interpret measurement situation is where the two output curves are reversed, (the thick ‘front’ curve is below the thin ‘rear’ curve). This could be the result of incorrect (reversed) hearing aid placement inside the Verifit Classic test box. (It is harder to place the hearing aid backwards inside the Verifit 2 test box.) It could also be an indication that the hearing aid microphone itself is wired backwards.
To resolve the issue, check to make sure that the microphone orientation of the hearing aids in the test box is correct. If it is and the curves are reversed, you may want to alert your manufacturer to this anomaly and have the hearing aids replaced or repaired.
Poorer Curve Separation than Initially Benchmarked
When conducting the directional test, perhaps you encounter a result where you observe curve separation, but the curve separation is not as robust or as obvious as when you had previously tested the hearing instruments. Baseline directional mic testing is a good thing to collect and save on any hearing instrument you receive from a manufacturer prior to fitting. Then, when you later test the hearing instrument’s directional microphone capabilities, you will have a reference performance measure to which you can compare.
If the more recent test result shows significantly less curve separation than the earlier benchmark result, it could be an indication of the following:
- HA placement may be incorrect or different – Make sure that the two microphones are parallel to the floorboard of the test box in a Verifit 2 or sitting on the 45-degree angle line pointing at the left speaker in a Verifit Classic.
- Your current coupling method may not be equivalent to the original coupling method used when collecting the benchmark. This possibility may arise particularly if you are testing an ITE hearing instrument or a BTE instrument with the earmold. In either of these cases, putty is involved, and if the putty has not fully sealed the instrument to the coupler, directional mic curve separation may be affected. Remove the instrument from the coupler and re-attach it, being careful to seal any “vent” on both the lateral and medial sides and avoid risk of slit leaks.
- One or more mic ports are compromised by debris, not to the point where it causes the two measurement curves to come together completely, but instead for the two curves to not separate as significantly as they once did. As discussed previously, to the extent possible consider cleaning any mic ports and/or install new microphone screens on the instrument should they appear to be saturated with dirt or debris.
- Microphone drift – As directional microphone hearing instruments get older, there is the potential for asymmetrical drift in the sensitivity of the two omnidirectional microphones. Thus, poor curve separation at some later point in the use life of the hearing aid may be an indication of this microphone drift.
For a more detailed instruction on directional microphone measurements, please refer to the Audioscan course, The Verifit Directional Mic Test: Evaluating Modern Directional Microphone Technologies, that is available in text and recorded formats. For the latest information and resources from Audioscan, please visit www.audioscan.com or the Audioscan Expo Page on AudiologyOnline.