Sometimes when I'm doing speech mapping, I have too much gain in the lower-to-mid frequencies. I then lower gain to the minimum in the hearing aid fitting software, yet little or nothing happens with the real ear aided response (REAR). Why?
This is something that you will commonly see for an open fitting when the patient has normal hearing in the low-frequencies (e.g., meaning you don’t need or want gain for this region). It’s a little difficult to trouble-shoot, unless you use a seldom used but important probe mic measure, the Real Ear Occluded Response (REOR).
You probably haven’t thought about the REOR for a while, so here are a few things to know:
- The REOR is an REAR with the hearing aid muted or turned off.
- The REOR tells you the impact that the fitting tip, earmold or custom hearing aid has on the sound reaching the ear drum. This is a combination of the potential loss of the ear canal resonance and concha/pinna effects, and the attenuation caused by the occlusion of the ear canal (e.g., "ear plug" effect).
- If the ear is open, the effect will be minimal (e.g., ear canal resonance/pinna effects mostly maintained, no ear plug effect). If the fitting is closed, the effect can be 30 dB or greater (e.g., loss of resonance, pinna effects, and ear plug effect).
If the ear canal is open you may see that the REOR looks like the REUR (real ear unoccluded response), but if the ear is plugged at all, you will see an immediate reduction in the ear canal resonance, and with more closed fittings, the REOR falls below the input (a negative REOG). As a reminder, the REOR is not a test of the occlusion effect, as that is unrelated to the sound reaching the ear canal via bone-conduction.
With that background, let's now look at a case where conducting the REOR can help you find the answer to a puzzling outcome. Figure 1 shows the REAR for an open fitting where the audiogram had normal thresholds in the low frequencies. The dashed line in the figure is the target, and the green line is the output of the hearing aid. You can see there is a pretty good match to target, except in the 1500 Hz range, where it appears that there is too much gain and an overshooting of the target. In this case, we went into the fitting software to lower the gain in this region. We decreased the gain until there was no more gain to take out, but the REAR curve did not change. So, what's going on? You might think that the hearing aid software isn't working, or that some strange resonance is present?
We can see what's really going on by running an REOR. We muted the hearing aid and conducted an REOR, which is the red line in Figure 2. Now we can see that the reason that we couldn't get rid of the gain at 1500 Hz is because it is there even when the hearing aid is muted; in other words, what we're looking at is the sound that's going directly to the eardrum because of the open fitting. And of course, it makes sense that we couldn't get rid of it by re-programming the hearing aid because it is not coming from the hearing aid. Had we not conducted an REOR, I'm not sure we would have known that for certain. Lesson learned: When you have a strange result in your speech mapping, mute the hearing aid, run an REOR, and you might find an explanation. Given that you already have the patient set up for probe-mic measures, it only adds an extra 10 seconds or so to your test time.
Figure 2. The REOR (red line) shows that the gain at 1500 Hz is coming from the signal entering the ear canal due to the open fitting, and not coming from the hearing aid.