Targert 2cc Prescription Software Update
by Larry Revit, hearing scientist
Part I: FONIX Conversion Factors Compared to NAL-R, POGO, and Libby
Some FONIX reps and users have noted that the 2cc-coupler prescriptions obtained with the Target-2cc software may differ somewhat from prescriptions obtained by other methods, even for the same audiogram and the same target insertion gain. For example, a dispenser could enter a particular client's audiogram into the FONIX Target-2cc FOG program and select POGO as the insertion-gain formula to arrive at a 2cc-coupler prescription. But if the dispenser applied the published tables for POGO to the same audiogram, a somewhat different 2cc-coupler prescription would result. Here's why...
Several published prescription methods (e.g., NAL, POGO, Libby 1/3-2/3) have included tables of prescribed insertion gain along with tables of prescribed coupler gain. By comparing the tables for insertion gain and coupler gain for each method, it is possible to extract the correction figures ("CORFIG" values) that are used by each to convert insertion-gain targets to 2cc-coupler prescriptions for the average ear. Although it might be reasonable to expect that all of the methods would use the same average-ear conversion factors, this is not the case.
Fortunately, the differences are mostly small. Figure 1 shows the average-ear CORFIG values used to convert insertion-gain targets into 2cc-coupler prescriptions for in-the-ear (ITE) hearing aids, across four methods. Note that, with the exception of the POGO curve at high frequencies, all the curves agree fairly well. The reason for the high-frequency discrepancy in the POGO curve is not known. The FONIX curve differs slightly from the NAL and Libby curves at low-frequencies, mostly because of how each method deals with earmold venting effects. Part II of this Larry's Corner tells you how to account for earmold venting and slit leaks using the FONIX Target-2cc software.
Hint for Target-2cc users: The FONIX Target-2cc prescription conversions are independent of the insertion-gain formula chosen. In other words, you can use either the NAL-R, POGO, Berger, or 1/3-2/3 Libby formulas, or enter your own target insertion gain, and the FONIX Target-2cc software will convert the prescribed insertion gain to 2cc-coupler gain using its own, independently verified conversion factors (as with the bold curve of Figure 1).
Figure 1
Part II: Target -2cc FOG Corrections for Venting Effects
When Frye Electronics created the Target-2cc program, we recognized that most prescriptions require venting in the earmold or shell. Yet our program does not contain corrections for venting -- for two reasons: 1] For vent diameters up to 2mm, our experience was that the effects on real-ear insertion gain were relatively small. So to leave venting corrections out of the prescription program would not ordinarily have resulted in significant fitting errors. 2] The precise effects of venting on real-ear insertion gain are quite complex, and the available data did not sufficiently address the complexities involved.
Instead of providing potentially inaccurate corrections, we decided to program a way for the user to modify the calculated prescription (using the arrow keys) according to one's own experience or according to better data as it became available. We can now report that well-studied, average vent corrections for 2cc-coupler prescriptions have become available. Until we implement these corrections in software, the user is encouraged to enter the corrections given in Figure 2, as applicable, to the calculated Target-2cc FOG curves.
| Frequency (Hz) | 250 | 500 | 750 | 1k | 1.5k |
| Tight Seal | -- | -- | -- | -- | -- |
| Slit Leak | 2 | 2 | 1 | -- | -- |
| 1 mm | 1* | 2* | 1 | -- | -- |
| 2 mm | 7* | 1* | -- | -- | -- |
| Long Open | 17* | 10* | 4* | 1* | -- |
| Short Open | 26* | 21* | 14* | 10* | 5* |
Figure 2
NOTE: Use starred values only if prescribed insertion gain is greater than 0 dB at that frequency. Otherwise, use no correction. Blanks indicate use no correction. A slit leak is assumed for all vent conditions except "Tight Seal".