A University teaching faculty member asked me recently if I would write a review about the use of functional gain as a method for hearing aid verification.
My initial reactions were that I thought we were beyond this with real-ear measurements, and then, that this might be as exciting as watching grass grow, not only for me, but for the readers as well. But then, I live in a place where I don’t have a lawnmower and one needs dynamite to make a hole in the ground just to plant anything. So, at least here, watching grass grow is actually a major event. To add to my dilemma, I was informed that many who test hearing do not have an appropriate understanding of this topic and how it relates to hearing aid verification, and could I present information about this in an easily understood manner? So, somewhat reluctantly, here I go.
Functional Gain Involves Sound-Field Measurements
What this means is that measurements of how the hearing aid is performing are made via loudspeakers. In theory, such measurements could conceivable be made under large earphones, but in reality, such measurements are not practical (hearing aid positioning, comfort, feedback, etc.), and as a result, loudspeakers are used to make these measurements.
Sound-field (SF) measurements are NOT the same as free-field (FF) measurements. The latter require an anechoic room for measurement, which is not the same as a sound-treated room, and is what most clinical hearing facilities have.
Is There Value to SF Measurements in the Hearing Aid Selection, Fitting, and Verification Process?
Mainstream verification procedures involve real-ear measurements. Therefore, is there any value to SF testing? And if so, what is required?
Conventional comments state that the value of SF measurements is that the patient is an active participant in the testing process. The patient lets the evaluator know if sound is audible, as opposed to a cadaver (as an example) where insertion gain (IG) is measured using a probe microphone, but is not audible. It can also provide information as to if and how well speech is understood through:
- The use of word recognition scores
- In quiet aided, and in noise aided (using the HINT or QuickSIN)
What Are SF Needs?
1. A calibrated system
Calibration for normal hearing for SF speech differs from earphone normal hearing for speech. Speech uses a calibration tone of 1000 Hz. This is recorded at a level equal to frequent peaks in speech. From this, the calibration tone is set to “0” (considered normal hearing reference) on the VU meter. The SPL for normal hearing of speech is shown in Figure 1.
Figure 1. Calibration SPL values comparing earphone and different SF loudspeaker placement positions to 0 dB HL equivalency. When properly calibrated, average SRTs obtained monaurally under earphones and in the sound field should be essentially equal.
Additionally, results vary depending on where the loudspeaker is placed. For example, if a single loudspeaker is used, it is oriented at 0 degrees azimuth to the forward-facing head so that sound will be presented equally to both ears (Figure 2a). If the loudspeaker were oriented more toward one ear, sound from the loudspeaker will be heard the loudest at that ear (Figure 2b). For functional gain measurements a single speaker positioned as in Figure 2a functions well if overall hearing aid gain improvement is the measurement of interest. It is not uncommon to see two speakers oriented at 45 degrees azimuth to the forward facing head, which allows for more complicated sound-field evaluation. As shown in Figure 1, when this is done, the speakers must each be calibrated to the appropriate levels and placed properly in the test environment.
Figure 2a. Normal speaker orientation for monaural speech audiometry testing - loudspeaker at 0 degree azimuth.
2. Controlled test environment
Because the ears are not covered during the test, an ambient noise controlled environment is required.
a. Patient Placement
Patient positioning in the test environment depends on the measurement to be made. Most procedures recommend a distance of 1 meter from the loudspeaker(s). If using pure-tones as a stimulus, the head must remain stationary to minimize standing waves, and in this case, have the speakers within 1 foot of the listener, and calibrate appropriately for that distance. The angle of the loudspeakers depends on how they are calibrated and on the number of loudspeakers used. If a single loudspeaker is used, it is placed at 0 degrees azimuth. Essentially, this would be a monaural assessment.
Figure 2b. If the loudspeaker is oriented at 0 degree incidence, sound should be presented equally to both ears (2a). However, if it is placed at an incidence closest to one ear, sound will be heard the loudest at that ear (2b).
When two loudspeakers are used, they are placed at 45-degree azimuths for binaural assessment.
b. Ambient Noise
If the ambient noise is below 45 dB(A), the masking effect will be minimal, regardless of the patient’s hearing level. Regardless, use as quiet an environment as possible, realizing also that low-frequency noises are worse.
To be continued…. Next week’s blog will finish this presentation on measurement of functional gain.
