Simplified Aided Ear Customization for Target 2cc FOG Prescriptions
by Larry Revit, hearing scientist
As currently described in the 6500 manual, customizing a Target 2cc FOG response for the AIDED EAR requires a special insert earphone, a special adaptor, unplugging and plugging in these special devices... In other words, it's too much of a hassle for the average dispenser. That's too bad, because the AIDED EAR customization is the single most important thing a dispenser can do to ensure a successful fit the first time. So we've recently checked out a simplified procedure using a regular BTE hearing aid and no special apparatus. Here's the simplified, 2-step procedure:
- Measure and store the 2cc coupler response of a BTE hearing aid.
- This step can be done without the client present.
- Attach an unvented, foam-type, stock earmold to a BTE.
- Using Fun-Tak, attach the aid and earmold to an HA-1, 2cc coupler.
- Set the volume control for a mild gain setting (about 20 dB), and then tape the volume control in that position.
- Set the 6500 for a 70-dB-SPL, Composite Weighted Gain signal (the default setting), and set the Noise Reduction for 16.
- Go to Multi-Curve and copy CRV 1 to REF 3.
Note: If you have CHAP (the Fonix Computer Hearing Aid Program), this coupler curve can be stored permanently and simply transferred to REF 3 whenever you need it.
- Measure and store the Aided Response of the same BTE on the client.
- It is probably best to be in MANUAL PROBE mode. Also, set the Noise Reduction for 16.
- Using the same earmold as for the coupler response, put a convenient reference mark on a probe tube so that you will know the tube extends at least 5mm beyond the tip of the earmold when the earmold and probe tube are inserted in the ear.
- Insert the probe tube and then the earmold, being careful that the probe remains in the desired position. Insert the earmold as closely as possible to the same depth as the client's final hearing aid will be.
- With the aid in place on the ear, place the Reference Mic over the ear, so that the front grill is as close as possible to the mic of the BTE aid.
- Level.
- Turn the aid on (with the volume control in the same taped position as for the coupler measurement).
- Using a 70-dB-SPL Weighted Composite signal (the default probe setting), measure the Aided Response in the normal way.
- Copy the Aided Response to REF 2.
This is all you have to do. When you go to the Target 2cc FOG program, use the MENU to set "AIDED EAR" for "CUSTOM". The curves stored in REFs 2 and 3 will automatically be incorporated into the prescription conversion.
Shown in Figures 1 and 2 are examples of Aided-Ear-versus-2cc corrections (REF 3 minus REF 2) measured on KEMAR by the above sound-field procedure and by the insert-earphone procedure given in the 6500 manual. As you can see, the sound-field and insert-earphone curves match closely across most frequencies. The low-frequency variations are most likely caused by a "slit leak" in the Aided Response measurement. The high-frequency variations may be caused by the effects of receiver impedance, noise, and/or probe-position. These variations are shown to illustrate the "real-world" potential for such errors. This potential for error is the main reason why we have included the ability for the 6500 users to "Tune" the calculated Target 2cc FOG curve, so the perscription curve can be smooth and resonable. Use the "Course-" and "Fine-Tuning" controls as you see fit (described in Section 8.17.3.2).
A damped sound channel is recommended. The simplified-method curve in Figure 1 was done with no damping in the sound channel, whereas the simplified-method curve in Figure 2 was done with the same hearing aid but with a 1000-ohm damper in the earmold tube near the earhook. Since you can expect a cleaner measurement with damping in the sound channel, we recommend using either a damped earhook or a damper in the earmold tubing for these measurements.
The Aided-Ear-versus-2cc measurement is an important part of the custom prescription process. Including or not including the measurement in a prescription can "make or break" a hearing-aid fitting. Now that the procedure has been simplified, we urge Fonix 6500 Quik-Probe II users to take advantage of this important feature of the Target 2cc program.