Hearing Aid Verification Using Functional Gain – Part II

Note:  This is a continuation of a previous blog: Hearing Aid Verification Using Functional Gain – Part I, which was last week’s blog.

Functional Gain in Hearing Aids

Functional Gain (FG) = Aided Threshold minus the Unaided Threshold (FG = A-U).

1. Unaided thresholds are measured to tones presented through the loudspeaker(s). Various frequencies are tested and the stimuli are most often warble tones to avoid standing waves that are often found when using pure tones.
2. Aided thresholds are made using the same stimuli.
3. The differences between the two represent the functional gain (FG) provided by the hearing aid(s) (Figure 3).

Figure 3. Functional gain diagrammed. The red circles represent the unaided thresholds for the frequencies shown. The yellow circles represent the aided thresholds, representing the amplification improvement provided by the hearing aid(s). The numbers in the yellow block represent the functional gain (FG) for each of the frequencies.

How Does Functional Gain (FG) Compare to Insertion Gain (IG)?

Essentially, FG and IG are the same, except in certain circumstances. Both show how much gain the hearing aid is providing. The plotting, however, is slightly different. FG is plotted from the unaided and aided audiogram, whereas IG is plotted relative to real-ear gain (Figure 4).

Figure 4. Plotting differences between Functional Gain (A) and Real-Ear Gain (B). Theoretically, the values should be the same even though the measurement procedures are different.

FG and Nonlinear Hearing Aids

When making measurements of FG, little or no high-frequency gain is measured, especially when nonlinear hearing aids are being evaluated (Figure 5). The amount of FG depends on the AGC knee and the hearing levels. For example, if the AGC knee is set low (i.e., 45 dB), and the hearing level is greater than 45 dB, the AGC is activated and will reduce the gain of the hearing aid when the loudspeaker signal is made loud enough for the patient to be able to hear it. In Figure 5, the “Hope to get” plots are based on a hearing aid fitting formula – it could be any that one wishes to use. The problem comes into play when attempting to counsel the patient. The visual display is not comforting to explain in that there seems to be little high-frequency amplification. And, unfortunately, unless the AGC knee is raised substantially, or the hearing aid is made linear, no response adjustment to the hearing aid will improve the FG results.

Figure 5. Expected FG is often not achieved, especially at the high frequencies. This is especially true when nonlinear hearing aids are being evaluated because the AGC knee, if low (i.e. 45 dB), and coupled with a high-frequency hearing loss greater than 45 dB, results in the AGC being activated when the stimulus becomes sufficiently loud to be heard. The AGC action results in less hearing aid gain, which is what the FG results show.

FG Disadvantages

1. Testing is performed at a single measurement level only – threshold. This may be sufficient for linear hearing aids, but not for nonlinear aids. With nonlinear hearing aids, increases in the gain to improve the FG actually result in reduced FG because the hearing aid goes into compression sooner. Nonlinear hearing aids require multiple input levels to show the AGC (compression) performance.
2. If the patient has near-normal hearing (usually in the low frequencies), the aided thresholds are often invalid. The reason for this is because ambient noise masks the test signal and results in poorer FG.

FG Advantages – When to Use

1. The use of FG may be the only way to test an active child.
2. FG allows for one to check out the entire hearing aid and hearing mechanism.
3. Audibility can be checked. As mentioned earlier, real-ear measurements can be made on a cadaver, but there is no indication of audibility.
4. Feedback oscillations are not induced. These are obtained often with high gain or deep-fitting hearing aids due to real-ear mic probe placement.
5. Earphone aided FG is possible with deep canal CIC hearing aids that house all components medial to the pinna, and if a closed coupler is used.

Sound Field and Intelligibility

Although not specifically related to FG, variations in the use of sound-field testing can be useful for:

1. Assessment of how much the hearing aids change the ability to understand in specific acoustic conditions, especially in noise
2. Identification of types of speech sounds that are not well perceived, which is useful for evaluating the type of benefit hearing aids provide
3. Demonstrating hearing aid benefit to the patient or family member
4. Demonstrating to the patient or others the importance of visual cues to understanding
5. Determining the status of one versus two hearing aids
6. Determining which ear to fit if only one hearing aid is being considered
7. Predicting the difficulty the patient might have when communicating in some specific environment when wearing hearing aid
8. Measuring directional microphone hearing aids

Direction of Aided Hearing Aid Measurements

Historical measurement methods include (a) Functional Gain and (b) aided word recognition scores. FG was used to determine if hearing aids provided audibility/gain. Word recognition was used to assess a patient’s ability to use hearing aids in communicative situations.

Today, aided hearing aid performance is best made using real-ear probe microphone measurements. This approach provides objective recordings for gain functions; it is stable, objective, and repeatable. Real-ear measurements circumvent word recognition testing which has been challenged because it is neither repeatable nor reliable, even though it can still provide some important information when performed for hearing aid purposes.