Hearing aid software predictive gain values: How accurate are they?

There are several ways to assess the electroacoustic characteristics of hearing aids. The most commonly used techniques have been the 2-cc coupler, functional gain, and real-ear probe-microphone measurements. In each of these methods, the actual hearing aid that the person will use is assessed either on that individual, in the cases of functional gain and probe-microphone measurements, or in a standard coupler, the HA-1 or HA-2 2-cc coupler. Now, with the advent of digitally programmable and digital signal processing hearing aids, the clinician is offered a variety of choices for how to view hearing aid performance in the software provided by hearing aid manufacturers. On the programming screen, one can choose to display how the hearing aid is programmed in a number of ways, depending on the manufacturer. Options found in the NOAH programming modules include: (1) 2-cc gain, (2) 2-cc output, (3) 2-cc SPL, (4) 2-cc SPLoGram, (5) insertion gain, (6) simulated insertion gain, (7) KEMAR gain, (8) amplified long-term speech spectrum, (9) ear simulator gain, (10) realear SPLoGram, (11) simulated real-ear SPL, and (12) real-ear aided gain. None of these on-screen representations have been made with the actual hearing aid; rather, they represent simulations based on average and expected performance values. SIMULATED AND MEASURED VALUES DIFFER Since the audiologist sees these simulated values while programming and adjusting hearing aids, he or she may be tempted to believe that the computer screen values are for the specific hearing aid being programmed and for the specific patient being fitted with the device. Unfortunately, this is not the case. The 2-cc coupler gain values are what the manufacturer specifies as the standard for that model, not for the particular device being programmed. Given tolerances of components such as the microphone and receiver, the values in a given hearing aid could vary by as much as plus or minus 5 dB at some frequencies. To come up with the simulated insertion gain figures shown on the programming screen, the manufacturer has taken the specified average 2-cc coupler values for that model and the current settings and transformed the coupler values with what the company believes is an appropriate Coupler Output for Flat Insertion Gain (CORFIG). The 2-cc coupler gain modified by the CORFIG yields the predicted insertion gain. For the most accurate prediction of insertion gain, the CORFIG should be specific to the hearing aid style (behindthe-ear, in-the-ear, in-the-canal, or completely-in-the-canal) because of the different microphone locations associated with each style. The CORFIG should also be specific to the earmold configuration (long or short canal, bore diameter if a BTE, and vent size) being used with the patient. Even when these critical factors of microphone location and earmold venting are taken into account, one must still expect sizable errors on some patients due to the variations that can occur across individuals in residual ear canal volume and middle ear impedance. Moreover, there are no universally accepted CORFIGs. Many have been reported and they can be substantially different from one