Why Perform Real-ear Testing?
The reason for hearing aid real-ear measurement is to determine the actual amplification of the hearing aid as worn on the head of the user.
- As is known (and reviewed in the 7/6/2011 blog), coupler measurements produce varying results, depending on how they are made.
- Real-ear results are not the same as measured using hearing aid artificial ears (couplers).
- Additionally, real ear results are not often the same as the response curve that shows up on the hearing aid programming screen.
Figure 1 is a graphical representation expressing this problem.
Real ear data may be obtained in varying ways. Among the most popular are:
- Direct measurements made on a person. These are made using a mic-probe system with the probe near the tympanic membrane (Figure 2). These measurements are the most accurate and provide individual data. These account for “ear gain or loss” due to coupling, and during the testing, allow for measurable fitting adjustments.
- Accurately designed manikin with ear simulators. This would be KEMAR (Knowles Electronic Manikin for Acoustical Research) using a Zwislocki Coupler of 1.2 cc volume).
- Approximately, with corrections made to 2cc measurements. These are called RECDs (Real Ear to Coupler Differences).
- Ear simulators (OES – Occluded Ear Simulator. These measurements are made using a Zwislocki (or similar) coupler with no KEMAR manikin. These are generalized data.
Traditional mic probe approaches are to lay the probe alongside the earpiece, or to insert the probe through the vent (Figure 2). Deep-fitting hearing aids can present problems to many clinicians because the longer and deeper the fit, the more difficult it is to position the mic probe appropriately along the side for accurate and repeatable measurements. And, many deep-fitting instruments do not include a vent so that the probe tube can be placed through the vent. Some of the problems experienced include:
- Probe often does not protrude beyond the tip
- Probe is bunched up
- Probe tip is often pushed into the canal wall, and thus closes the probe
- Probe is compressed against the canal wall to the point where accurate and repeatable measurements cannot be made
- Probe tip is not in proper position
Mic Probe Procedures for Closed Dome Deep-Fitting Hearing Aids
The following discussion will present a recommended procedure and some results when making real-ear measurements for deep canal fitting hearing aids, specifically those that use a solid dome for closed fittings. A power speaker, rather than a mini speaker, was used for these illustrations. However, the same procedure can be used for mini speakers as well.
Use a standard custom-molded hearing aid wax loop (Figure 3). The metal loop should be fairly strong (not easily bent). It is used to string the mic probe tube through a hearing aid ultra soft dome.
Push the wax loop through the inverted soft tip. Fold the mushroom (dome) back to allow visualization. Then push the wax loop tip through the mushroom as shown in Figure 4.
Thread the probe tube through the wax loop (Figure 5). Push the probe tube through with sufficient length so that it will not pull out of the wax loop when pulled back through the wax loop opening.
Return the soft tip to its original shape and carefully pull the mic probe tube through the soft tip with the wax loop (Figure 6). Be careful to not enlarge the hole more than required for the probe. Note: the probe is doubled when pulled back through the hold in the soft tip.
With the probe tube pulled through the hole in the soft tip, allow the wax loop to slide from the tube (Figure 7).
Pull the probe tube to the desired extension length (Figure 8). With a deep-fitting instrument, this should be about 1 mm beyond the tip rather than the 5mm recommended for traditional hearing aid depth insertions. The reason for this is because the residual volume between the tip of the inserted hearing device and the tympanic membrane is now much smaller, so extending the tip 5mm beyond the hearing aid insert tip would put the tube against the tympanic membrane. The procedure identified here places the probe where it is needed, allows for easy insertion, and holds the mic probe tube in the same position for consistent measurements even with repeated insertion and withdrawals of the hearing aid. In other words, the “guesswork” in placing the probe has been eliminated. Figure 9 illustrates the probe length beyond the end of the hearing aid tip.
Remove the mic probe from the soft tip following the measurement. The same soft tip can be used because the hole closes and does not create feedback.
Mic Probe Extension Comparisons
Figure 10 shows the results of different mic probe extensions during real-ear measurements. Curve #5 (dark blue) was derived from using a standard mic probe placement with a deep-fit instrument. The usual procedure of placing the mic probe into the ear and then fitting the deep-fit instrument often results in the probe tip not extending beyond the tip because of the turns in the ear canal and of not knowing just where the end of the mic probe is with respect to the tip end of the hearing instrument.
Curves 4 (red) and 3 (lighter blue) show the real-ear results using the proposed method – curve 4 with a 4mm protrusion and curve 3 with the mic probe flush with the end of the soft tip. Curve 3 provides the greatest high-frequency amplification. It is speculated that it most closely represents the true real-ear response because the probe tip is least likely to be affected by insertion.