Functional Gain and Insertion Gain
by Larry Revit, hearing scientist
When are they equal; when are they not?
Functional gain is the difference between behavioral soundfield thresholds under aided and unaided conditions. Insertion gain is the difference between the sound pressure level in the earcanal under aided and unaided conditions. Several studies have indicated that, on average, functional gain and insertion gain are indeed equivalint measures. Yet many users of probe-tube systems find significant differences between functional gain and insertion gain when the two are measured with the same hearing aid on the same individual. Why? Two reasons. 1] The equivalence of the two measures holds true only for linear hearing aid operation. 2] Even though the two measures may be equivalent on average, individual cases may vary considerably for many cases.
Let's look at case #1 first. The problem with anything other than linear hearing aid operation is this: Normally, the signal levels used for the "aided" parts of the two measures are quite different. The "aided" part of functional gain is the aided soundfield threshold. This measurement deals with relatively low signal levels -- at the (aided) threshold of hearing. In contrast, the "aided" part of insertion gain is the real-ear aided response (or REAR). This measurement deals with signal levels that may be considerably above the client's hearing threshold. So what happens if we're measuring functional and insertion gain on a hearing aid having automatic gain control (AGC)? The signal levels are different for the two measures, so the gain of the hearing aid (controlled by AGC) can be different for the two measures -- thus, a disparity between the two measures.
Now let's look at several other sources of individual variability. a] Soundfield requirements: For soundfield threshold measures, as used for functional gain, the test environment must be designed according to stringent requirements; the bandwidths of the signals used must be carefully controlled, and the soundfield must be painstakingly calibrated. If these conditions are not met, considerable error is possible in soundfield threshold measurements. b] Extraneous noise: If there is any audible extraneous noise present during an aided threshold will be masked by the extraneous noise. Thus the functional gain measurement will be influenced by the effective masking level of the extraneous noise. c] Slit leaks (this one affects insertion gain): If there is a change in earmold venting caused by leakage between the probe tube and the earmold, the low-frequency gain registered by the REAR might be lower than what it would be with no probe tube in place -- thus, an error. And in addition to these sources of differences, we have the usual variability associated with thresholds and soundfield measurements: loudspeaker location, head movements, probe-tube position, audiometer step-size, etc.
If the hearing aid you are working with is operating linearly under all test conditions, and if you use a carefully controlled soundfield for thresholds, and if environmental and hearing aid noise do not mask the aided soundfield thresholds, and if no significant extra venting is caused by the probe tube,... then both insertion gain and functional gain will be equal, within the normal constraints of measurement error. Otherwise, there will be differences.
So which is the better measure? Certainly, each has important advantages. Functional gain involves the entire auditory system, including the client's behavioral response. Insertion gain is much faster to do, it measures many more frequencies, calibration is easy to control, accuracy is better, and the results can tell you more about what happens under normal conversational conditions. The choice is yours. But be aware of the normal differences.