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Open-canal fittings: Ten take-home tips

Mueller, H. Gustav; Ricketts, Todd A.

doi: 10.1097/01.HJ.0000286216.61469.eb
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H. Gustav Mueller, PhD, is Professor of Audiology at Vanderbilt University, Senior Audiology Consultant to Siemens Hearing Instruments, and Contributing Editor to The Hearing Journal. Readers may contact him at gus.mueller@mindspring.com. Todd A. Ricketts, PhD, is Associate Professor at the Vanderbilt University Bill Wilkerson Center for Otolaryngology and Communication Sciences.

Fitting hearing aids involves innovation and compromise. This is especially true in deciding the tightness of the fit, whether of an earmold or a custom product. Tight-fitting instruments are good for obtaining high levels of gain and output, especially in the lower frequencies, and for minimizing feedback problems. However, they often are uncomfortable, sometimes create an unacceptable occlusion effect, and can give the patient a “plugged up” sensation. If the patient has normal or near-normal hearing in the lower frequencies, it is common practice to move toward a more open fitting to alleviate occlusion and let low frequencies into the ear naturally. In this case, the compromise is reduced gain and output, and a greater chance of unwanted acoustic feedback.

There is little documentation of the first open-canal (OC) fittings, but we know they received considerable attention immediately after the CROS-type hearing aid was introduced.1 It was quickly discovered that the open earpieces (or tubing only) used for the CROS hearing aid also could be used for ipsilateral high-frequency amplification (hence, the term IROS [ipsilateral routing of signals], referring to a large vent).

In the late 1960s, researchers showed that OC fittings could provide useful high-frequency amplification.2–4 In the ensuing decades, there wasn't much excitement about open fittings, though there was some renewed interest during the Libby horn's peak of popularity5 and on other occasions when a product designed specifically for high-frequency hearing loss was introduced.6

The ho-hum attitude toward OC fittings that persisted for more than 30 years was stirred up considerably a few years ago when five key aspects were combined in a single OC product: a small “cute” BTE casing, multichannel compression and gain adjustments, a thin tube channeling the sound from the hearing aid to the ear, a comfortable non-occluding eartip, and, most importantly, effective feedback-reduction algorithms. As reviewed by Mueller,7 these five factors combine to offer a wide range of potential patient benefits, and OC products have rapidly taken over a sizable share of the total hearing aid market.

While the peer-reviewed journals have published little about these modern OC fittings, several articles about this style have appeared in the trade journals, and the OC product is a hot topic at hearing aid workshops and seminars. However, there are still some key issues related to these products that need further clarification, and maybe even a couple areas where misconceptions exist. In this paper, we'll provide some tips on OC fittings, which we hope will clarify more than they confuse.

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ISSUE #1: INSERTION LOSS

The term “insertion loss” is used quite frequently when OCs are discussed, usually making the point that with the open-canal fitting tips used with these products, there is little insertion loss. Presumably that means that the ear canal's resonance is maintained. But what does “insertion loss” really mean?

Over the years, the term has been used in different ways regarding hearing aid measurement. In some cases, it has been used to describe the difference between coupler gain and real-ear (or manikin) gain.8 This is related to the CORFIG (COupler Response for Flat Insertion Gain), which is calculated by subtracting the real-ear coupler difference (RECD) and the microphone location effect (MLE) from the real-ear unaided response (REUR).9,10 In the case of the CORFIG, if real-ear gain for a closed earmold is compared with coupler gain, there indeed will be “insertion loss” in the higher frequencies for some hearing aid models, especially those with a poor microphone location, such as BTEs.

If we use the open ear rather than the coupler as the reference, we also would expect to see a loss of gain for the high frequencies for the closed condition (e.g., it's likely that the benefit of the residual open-canal resonance would outweigh the benefit of the reduced canal volume). We'll show you some of these findings later. While this is all interesting, it isn't the definition of insertion loss that is commonly used with probe-microphone measures.

In the early days of probe-mic measures, it was common to verify hearing aid performance via the in-situ response, which we now refer to as the real-ear aided response, or REAR. It was observed that the in-situ response was greater than insertion gain (what we now call the real-ear insertion gain, or REIG) and that difference was referred to as insertion loss. In other words: REAR minus REIG = insertion loss. The REIG is calculated by subtracting the real-ear unaided response (REUR) from the REAR, and, therefore, insertion loss is simply the REUG (real-ear unaided gain). So what if the REUG is not totally “lost” when the earmold plumbing tip is placed in the ear canal, as it is with most OC fittings? This has prompted some to question if it is still okay to subtract the REUG and use REIG calculations. The quick answer is yes, but this too will be explained more in a later section.

The third use of the term “insertion loss”–the misleading use we're concerned with here–relates to the change in ear canal SPL for a given input when a hearing aid (or earmold) is placed in the ear, with the hearing aid turned off. This is assessed by conducting the real-ear occluded response (REOR) and calculating real-ear occluded gain (REOG), which often is negative.11,12 It is tempting then to subtract the REOG from the REUG and think of this as insertion loss. Because the REUG in the high frequencies is usually around 15–17 dB, and the earplug effect often results in an REOG of −10 dB to −20 dB in the highs, the UG-OG difference can be quite large. This calculation has led some to conclude that in a closed fitting, the amplifier must add as much as 40 dB of gain to compensate.13 Why is this logic wrong? This amount of insertion loss would only apply if the patient never turned on the hearing aid. We'll explain.

Consider that when a relatively closed fitting is used, the hearing aid is an earplug. In the turned-off measurement, sound must pass through the hearing aid (or earmold) and around the hearing aid, or through the vent (if there is one), to be picked up by the probe mic in the ear canal. But, once the hearing aid is turned on, a new pathway is established through the microphone inlet port and the hearing aid. Hence, the total “earplug effect” is immediately eliminated, no matter how big it was. At this point, it doesn't matter if the REOG, before the hearing aid was turned on, was −30 dB or 0 dB. It is unnecessary to make up for this negative REOG with amplifier gain.

An example of this is shown in Figure 1. The top curve shows this patient's REUR, and the two lower curves represent the REOR for two different earmolds. Earmold #2 was deeper and tighter fitting, which is why this curve is lower in ear canal SPL. If we fall into the trap of subtracting these REORs from the REUR, we would find that in the 2000-Hz range, the UR-OR difference is about 23 dB for earmold #1 and 30 dB for earmold #2. But this doesn't mean this patient needs 7 dB more amplifier gain if he is fitted with earmold #2. In fact, because earmold #2 is a deeper fit, and there is a smaller residual ear canal volume, he actually might need less amplifier gain with this fitting.

Figure 1

Figure 1

So, as you can see, the term “insertion loss” has been used in different ways and misuse can lead to confusion. While subtracting the REOR from the REUR does provide guidance concerning the “openness” of the OC fitting, which probably will help predict the degree of occlusion effect, it is not a very useful measure for fitting the hearing aid. The term “insertion loss” was perhaps handled best in the ANSI Real-Ear Standard S3.46–1997. Of the 50 or so terms defined in this standard, insertion loss was not one of them.

Take-Home Tip #1: REOGs are not a true measure of insertion loss. They simply indicate how tight the fit is (or how vented). The size of the REOG does not systematically influence your REAR measures or REIG calculations.

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ISSUE #2: EARTIP OCCLUSION

As mentioned, one potential benefit of OC fittings is the use of an eartip in the ear canal that reportedly is non-occluding. Manufacturers have designed a variety of eartips for this purpose. But it's important to ask, how non-occluding are these new eartips, and what is the resulting ear canal resonance when they are in place?

To answer these questions, we conducted probe-mic testing on 14 adults, seven males and seven females, ranging in age from 25 to 58. We measured the REUR for the right ear of each participant. Each person was then fitted with an OC product (turned off) using an open eartip that was sized to give each person a “tight” fit. An REOR was then conducted.

The resulting REUG and REOG mean findings are shown in Figure 2. Observe that the average REUG is fairly typical, and not too different from the average values used in common prescriptive methods or as an average-ear reference in probe-microphone equipment. The mean REOG values indicate that, indeed, these eartips were not very occluding, as the resulting gain for the “occluded” ear was not too much different from the open ear. Note that the typical REUG peak at 3000 Hz was reduced by 3 dB, but there now is an REOG peak at 2000 Hz. Although we haven't investigated this systematically, we have observed in OC fittings, as might be expected, that the REAR peak is aligned with the patients REOR peak. In case you're wondering, we have no explanation for why the placement of the eartip in the ear canal moved the resonant peak to a lower frequency.

Figure 2

Figure 2

We obtained these results only using eartips made by a single manufacturer, but we would predict similar results with other eartips. For example, Yanz and Olson, using a different eartip, show essentially the same UG-OG difference (also with a trend toward a lower average peak for the REOG).14 Meanwhile, Kuk et al. showed little or no occlusion with another eartip, and MacKenzie16 found no difference among three different eartips when he used a modified REOR measure (with the patient's own voice as the input). To date, our UG-OG comparisons have not included OC instruments that place the receiver in the canal, but indirect data suggest that the eartip for at least one of these models also is not very occluding.17

If the UG-OG difference is small, it would seem there would be little or no hearing aid occlusion effect. Recent research has shown this to be true. Most recently, MacKenzie compared the occlusion effect for the OC product of three manufacturers.16 In all cases, the occlusion effect was absent or so small that it was not considered annoying.

Eliminating the occlusion effect can make a dramatic difference for some patients, and can spell the difference between hearing aid acceptance and rejection of amplification. An example of this is shown in Figure 3.

Figure 3

Figure 3

This represents the probe-mic measurement of the occlusion effect for a patient experiencing occlusion problems. The patient's own voice, the “eee” vocalization, was used as the input signal, and the hearing aid was turned off during testing. Observe that with his own standard earmold, the ear canal SPL was 25–30 dB higher in the low frequencies compared with the open-ear condition. With the OC eartip, the occlusion effect was nearly eliminated and only a few dB remained around 200 Hz. This improvement is noticeable and significant and nearly always leads to improved patient satisfaction (assuming that the fitting can deliver appropriate gain).

Take-Home Tip #2: Commonly used OC fitting eartips are indeed mostly non-occluding, and significant ear canal resonance remains.

Bonus Tip: If the REUG-REOG difference is small, there will be little or no occlusion effect.

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ISSUE #3: SOUND DELIVERY LOCATION

As discussed, and illustrated in Figure 2, there appears to be substantial residual ear canal resonance, even after the OC eartip is placed in the ear. This is a good thing, as we assume this residual resonance will assist in providing the patient with necessary high-frequency gain for aided use as well. But, for hearing use, there is a difference in the location of the sound source.

Recall that the REOR measurement typically is taken with the patient sitting a meter or so from a loudspeaker, which is delivering the measurement signal. When the patient is fitted with an OC hearing aid, the signal is delivered in the ear canal. In some of the deeper fittings, this sound delivery is not too far from the TM. We wondered if the same residual resonant properties observed in the mean REOG data in Figure 2 would still be present when the signal is delivered in the ear canal rather from the sound field.

To examine this, we conducted measures using a Zwislocki coupler in the KEMAR. Specifically, a steady-state noise was delivered into the coupler using an Etymotic ER-2 earphone. This earphone terminates in a tube in a similar fashion to common insert earphones. However, in this case we did not place a foam tip on the end of the tube as is usual. The tip of the tube was then placed at three insertion depths relative to the opening of the Zwislocki coupler (2 mm, 12 mm, 25 mm). The output was then calculated based on digital recordings made through an Etymotic ER-11 microphone coupled to the Zwislocki coupler.

As Figure 4 shows, there was little or no change in the frequency location of the peak or the magnitude of the output when the signal was delivered at the microphone (25 mm; eardrum location) or at the entrance of the ear canal (2 mm). This suggests that REOR data are a good predictor of ear canal characteristics for a signal presented from a hearing aid and delivered in the ear canal itself.

Figure 4

Figure 4

Take-Home Tip #3: The residual ear canal resonance observed in the REOG of an OC fitting is also present when the signal is delivered in the ear canal.

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ISSUE #4: SUMMING AND CANCELLATION EFFECTS

Consider that with an OC fitting, considerable sound enters the ear canal through the direct pathway (i.e., unprocessed by the hearing aid), which is then mixed with the amplified sound. When these two inputs (from the same source) are fairly equal in amplitude, summation and cancellation can occur across frequencies, depending on the phase relationship at the tympanic membrane. That is, for frequencies that are in phase, there will be summing effects (in theory, as much as 6 dB).

This sometimes is reported as a plus for OC fittings. We've heard the comment: “You don't need as much amplifier gain because of summing effects.” However, for frequencies that are out of phase, there can be cancellation effects. It has been suggested that this could be a negative consequence of OC fittings, because of distortions.18

To help explain this, we will use data collected at the University of Iowa Auditory Research Laboratory by Ruth Bentler and Yu-Hsiang Wu (see Figure 5). This figure shows the ear canal SPL for three different conditions: the direct path, the hearing aid output path, and the combined. The input signal was the speech-shaped babble of the CST.

Figure 5

Figure 5

Observe that for the lower frequencies, the only path is the direct path. That is, because of the open fit, there is no hearing aid gain for this region and no summing or cancellation is present. For the higher frequencies, however, the two signals interact, which results in summing and cancellation effects observed as a ripple in the combined signal. This ripple is most evident from about 500 to 1500 Hz, the region where the signals from the two pathways are similar in amplitude, resulting in notches greater than the 6-dB “bumps” observed at other frequencies. It should be noted however, that the “notches” are generally narrower than the bumps, and they may therefore have less perceptual impact on signal level.

Those of you who conduct probe-mic measures may or may not have observed these notches, depending on the sampling rate and smoothing of the system you use. In fact, on some systems, the notches between the bumps are smoothed out, making it look as if there is simply a little more gain.

From a clinical standpoint, it is important to consider the significance of the effects shown in Figure 5 for routine OC fittings. First, it seems unlikely that the summing effects will provide much added benefit. They are largest when the two signals are similar in level, which is when the hearing aid is providing minimal gain, and they never exceed 6 dB. If the hearing aid was programmed to provide minimal gain, then we'd presume the patient needs only minimal gain, and doesn't need the added summation effects.

In addition, perception of the summing effects under these conditions is expected to be at least partially offset by the cancellation effects. When the patient needs 15–20 dB or more of gain, the summing and cancellation effects are minimized, as the hearing aid path signal will have significantly higher amplitude than the direct path.

Take-Home Tip #4: Summing effects do not occur in the absence of cancellation effects and are largest when the hearing aid is providing little or no gain. As a result, summing effects are not expected to play a significant role in achieving desired real-ear gain and output.

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ISSUE #5: “FREE” HIGH-FREQUENCY GAIN

We often hear clinicians say that people fitted with OC products “don't need much high-frequency gain” or read reports that state that with OC products manufacturers can take advantage of the 15–20 dB of natural amplification of the unaided ear.19 Is there really “free gain” in the high frequencies with these products? And if so, how much?

The issue of free gain is related to the insertion loss issue that we discussed earlier. On one hand, with an OC fitting we have the residual ear canal resonance in the 2000- to 4000-Hz range, which will enhance amplifier gain, thus giving an advantage to the OC. On the other hand, with a closed fitting, especially using a deep earmold or deep custom-fitted instrument, we have a reduced residual volume and impedance changes which also increase high-frequency gain, which gives an advantage to the closed condition. Who wins?

We examined this in two different ways. The first method was to examine output in a Zwislocki coupler for “closed” versus “open” conditions. A steady-state noise was output into the coupler using an Etymotic ER-2 earphone. The tip of the tube was then placed at an insertion depth of approximately 25 mm so we could examine the output for an “open” condition. Without moving the tube, we then measured output for the “closed” condition by sealing the entrance of the Zwislocki coupler with Fun-Tac. We sealed only the entrance of the coupler because we were interested in avoiding big changes in the coupler volume; this is different from an actual hearing aid fitting where the ear canal volume becomes smaller when a closed fitting is used. The output was again calculated based on digital recordings made through an Etymotic ER-11 microphone coupled to the Zwislocki coupler.

The difference in output between these open and closed conditions was calculated by subtracting the closed response from the open response, and is shown in Figure 6. Not surprisingly, given the resonance frequency of the coupler (which, of course, is intended to mimic the average ear), up to 10 dB more output was achieved for the open condition in the high frequencies, with a peak in the 3000-Hz range. This advantage is consistent with the well-established ear canal resonance pattern (minus the pinna and concha resonances) described in detail by Shaw.20 Equally unsurprising, up to 30 dB less output was achieved in the open condition for frequencies under approximately 2000 Hz, which is consistent with well-established venting effects.

Figure 6

Figure 6

We also believed it was important to examine open versus closed fitting for real ears. For this, we used two popular mini-BTE OC products, and had half-shell closed earmolds (medium canal depth) custom-made for five subjects (three males and two females) using the same tubing and connector as used for the OC fit. An audiogram ranging from 20 dB at 250 and 500 Hz down to 70 dB at 4000 Hz was used for all participants.

Using the modulated speech-shaped composite noise signal of the Fonix 7000 probe-mic system, we fitted both products to the NAL-NL1 targets for a 65-dB input for the open condition, or as close to target as possible without feedback. The reference microphone was disabled for all testing (more on this when we get to Issue #8). REAR measures were conducted and REIGs were calculated using average REUGs. The closed earmolds were then placed on the instruments, and the testing was repeated. The hearing aids were not fitted to target for the closed condition, but simply tested at the same settings as were used for the OC fit.

Figure 7 (panels a and b) shows the average results for the two different products for the two different conditions. As expected, considerably more gain in the low frequencies was obtained for the closed condition. The unusual pattern of the REIG values for the closed condition is a good reminder of the amount of amplifier gain that is needed to obtain just a few dB of real-ear gain in the low frequencies for an OC fitting. This was especially noticeable for Hearing Aid A.

Figure 7

Figure 7

The minimal low-frequency gain for Hearing Aid B apparently is due to manufacturer design, as gain for this region was programmed at “max” (important to know if one were ever to use this product with a closed earmold). Also, due to some feedback problems, Hearing Aid B had slightly less average REIG—not a concern, however, as we were only interested in relative differences.

Our question related to free gain, however, is best answered, or at least visualized, by calculating a difference curve (open REIG versus closed REIG) for both products, and this is shown in Figure 8. Note that for the frequencies of 1000 Hz and above, the results for the two different products were essentially identical—an expected finding, but one that adds to the reliability of the measures.

Figure 8

Figure 8

Note that in the 1500- to 4000-Hz region, there is increased gain for the OC fit, with a mean peak difference of about 5 dB. Figure 8 is somewhat misleading, however, as the OC advantage occurred for different frequencies for the different subjects, which is obscured in the mean data—four of the five subjects had an OC advantage of 8 to 10 dB at some frequency in the range of 2000 to 4000 Hz.

While we are conducting more of these measures on a larger and more diverse sample, these finding are similar to what was observed in the coupler (see Figure 6). Although we only have limited data to support this next statement, we suspect that the OC advantage (“free gain”) will be the greatest for individuals with large ear canals and large REOGs, as they would lose the most and gain the least when their ear canal is closed.

Take-Home Tip #5: If the hearing aid settings remain constant, there probably will be “free” high-frequency gain for OC fittings, compared with a closed fitting. It's reasonable to expect an advantage of 5 to 10 dB around the region of the REAR peak.

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ISSUE #6: MAXIMUM HIGH-FREQUENCY GAIN

In the preceding section, we examined the boost in gain that can be obtained for an OC fitting over a closed fitting when all hearing aid parameters remain constant. A different question would be, what type of fitting provides the most high-frequency gain following hearing aid gain adjustments? As reported by Mueller, industry leaders tend to rate the OC product slightly higher than other available products for providing greater high-frequency gain, and also improved speech intelligibility (presumably because of the high-frequency gain).7 The same impression appears to be shared by a sizable group of dispensers.21 But is it true?

We haven't conducted probe-mic testing with this topic in mind, but there are a few obvious variables that we can discuss. First, the answer probably will depend somewhat on the maximum gain of the instrument. As shown in Figure 8, there is an OC advantage in the high frequencies for lower gain settings. If the maximum gain of the instrument is also quite low and max gain can be reached in the open fitting, then, as shown in Figure 8, it's probable that the open fitting wins. If the instrument's maximum gain is high, then the closed condition would be more apt to win.

That takes us to the second factor, the effectiveness of the product's adaptive feedback-cancellation system. The more effective the feedback-cancellation algorithm, the greater the probability that maximum gain in the highs will be obtained for the OC fitting. Finally, the open-closed difference will depend somewhat on the patient's residual ear canal resonance (with eartip in place).

While it's certainly possible to achieve more high-frequency gain with an open than a closed fit, we would consider it risky to generalize this to all fittings. A good example is seen in Figure 9, which shows one of the five participants from the study we reviewed earlier, who for some reason was particularly prone to having feedback problems. In this case we had the patient move around, talk, make exaggerated jaw movements, etc., until we were sure that we were just below feedback for the OC fitting. This, of course, results in a lower gain setting than what can be obtained if the patient simply sits quietly, which was the condition for the data collected for Figure 7.

Figure 9

Figure 9

Note the REAR that we obtained for the OC fitting (Figure 9). The unusual peak in output in the 4000-Hz range was the hearing aid going into feedback at the time of our measurement, showing that, indeed, we were at maximum gain for the open condition. Notice that it was captured in the REAR, but was not on long enough to be reflected in the OC REIG calculations below. For this patient, as shown in the REAR curve and the REIG calculation, clearly the greatest high-frequency gain was obtained with the closed fitting, as we minimized the feedback problem, which allowed us to push the REIG to the maximum high-frequency gain of this low-power instrument.

From a clinical standpoint, we are not suggesting that the best solution is necessarily a closed earmold. The point is, however, that we didn't select the OC because it provided the best high-frequency gain. Rather, it provided the best compromise between desired high-frequency gain, cosmetics, and the reduction of the occlusion effect.

Take-Home Tip #6: While it is possible to achieve more high-frequency gain with OC products on individual patients, we believe that in many cases, because of feedback issues, maximum high-frequency gain will be obtained with a closed fitting.

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ISSUE #7: PRESCRIPTIVE FITTINGS

Evidence-based practice reviews and published fitting guidelines consistently recommend use-validated prescriptive fitting methods when hearing aids are selected and verified.22,23 Following a validated prescriptive method may be even more important for OC fittings, because as illustrated by Bentler et al., the manufacturer's “first fit” may be sadly inadequate.24

Some of you may be familiar with this statement from the gain verification section of the recent American Academy of Audiology Hearing Aid Fitting Guidelines: “…some prescriptive formulas for open fittings may be inappropriate as there is no need to correct for the insertion loss created by including an earmold or hearing aid shell in the fitting process.”23

We are puzzled by this statement. It could be that the authors were referring to the proprietary fitting methods of some manufacturers. Some, however, have interpreted the statement to mean it's inappropriate to use validated fitting methods, such as the NAL-NL1 or the DSL 5.0 with OC fittings. Indirectly, this notion is supported by many manufacturers who have developed new gain and output fitting targets for open instruments. What's wrong with the fitting methods we use for all other styles of hearing aids?

If one considers that the verification of the prescriptive fitting targets is based on the amplified (or unamplified) signal delivered to the eardrum, then it seems it shouldn't matter how the signal got there. For example, let's say we're using the NAL-NL1 method, and for a given patient with a 70-dB loss at 3000 Hz, our gain target is 23 dB (for REIG verification) and our output target is 105 dB in ear-canal-SPL (for REAR verification). These values remain the same whether we're fitting a CIC, a body aid, or a 20-year-old used Crystal Ear. Moreover, these desired targets are not related to insertion loss, as suggested in the AAA guidelines statement.

Now of course, the prescriptive 2-cc coupler values needed to obtain these desired real-ear values do change as a function of the “openness” of the fit, primarily for the lower frequencies (these corrections have always been part of prescriptive fitting methods such as the NAL-NL1). This, however, isn't related to verifying the prescriptive fitting, which is conducted using real-ear, not coupler measures.

To summarize, with OC fittings, your prescriptive fitting targets are the same as always, you just achieve them in a slightly different way. This thinking appears to be in agreement with NAL prescriptive guru Harvey Dillon25 and at least one manufacturer of probe-mic equipment.26

We recognize that the openness of the fit may change the “quality” of the speech signal, but this typically concerns the lower frequencies, where direct sound exceeds amplified sound (see Figure 5). If a person is a candidate for an OC fitting, then he has no hearing loss in the lows and no gain is prescribed by the fitting method. So again, it doesn't seem that this would alter the fitting rationale, which is geared toward audibility and intelligibility.

Take-Home Tip #7: It's okay to continue using the popular validated prescriptive fitting methods when you fit OC products.

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ISSUE #8: PROBE-MIC VERIFICATION

For 20 years or so, probe-mic measures have been considered the gold standard for hearing aid verification. Using these methods is generally considered preferred clinical practice and has been recommended in all published fitting guidelines since 1990. Despite rumors to the contrary, this verification method is as appropriate for OC fittings as for any other style of hearing aid. Without using probe-mic, it would be very difficult to know that you are using the prescriptive methods we just discussed in the preceding section.

There is one issue concerning probe-mic verification, however, that must be revisited. We say “revisited,” because it was debated back in the 1980s when different probe-mic systems were introduced.27 The issue is equalization (calibration) of the input signal. If this procedure employs a reference microphone in the vicinity of the ear being tested, as it usually does, it is termed the “modified pressure method.”

One variation of the modified pressure method measures and adjusts the sound field in real time as the probe-mic measurement is being made. As defined in ANSI Standard S3.46–1997, this is the “modified pressure method with concurrent equalization.” Another option is to base the sound field equalization on data obtained from a prior measurement. The ANSI standard refers to this as the “modified pressure method with stored equalization.” Note that the stored equalization method is not the same as the “substitution method,” in which the field is equalized with the patient absent (see Mueller for review11).

Early on in clinical practice, it was quickly learned that the concurrent equalization method provided the most reliable results, as patients often do not sit as still as the KEMAR. Probe-microphone systems were designed using this equalization method as the default (and sometimes the only method). There can be a problem, however, with concurrent equalization when sound leaks out of the ear, as is the case with OC fittings. This isn't a new problem, and it was addressed in the ANSI standard. But the problem has become more prevalent and significant because of the popularity of OC fittings and the effective feedback algorithms in today's products, which allow for more open-ear gain.

What we are referring to is the “outflow” of amplified sound from the open ear canal into the reference microphone. If the reference microphone is relatively close to the ear canal, it's possible for the intensity of the signal leaking out of the ear to exceed the signal delivered from the loudspeaker. This will cause the control loop of the system to reduce the input, which in turn will reduce the measured ear canal SPL. The magnitude of the effect will depend on how close the reference microphone is to the opening of the ear canal, the similarity between the distance of the reference microphone and the hearing aid microphone from the point of leakage, and the gain of the instrument (and probably also is related to the peak of the residual resonance). The bottom line is that you may conclude that a hearing aid has less gain or output than it really has.

We've read that this is a problem,28,29 but wanted to test it for ourselves. We conducted testing using two different systems, starting with the Audioscan Verifit (software version 2.4). Software has recently been introduced for this equipment for testing OC fittings; however, this software wasn't available when we conducted our testing. However, we used two ways to work around the problem without the new software.

One method, available in the speech mapping mode, is to mute the hearing aid after the calibration chirp occurs, but just before the recorded speech signal begins. The system will then use the stored equalization. Another method is to use the CROS test mode, and then use the reference microphone on the opposite ear. In using stored equalization, it's important that the patient not move his head, and when the CROS method is employed, the listener must face directly toward the loudspeaker (to prevent any head diffraction or shadow effects).

We tested using both methods and compared our results with the standard concurrent equalization method (for this system the reference microphone is located about an inch below the opening of the canal). We obtained essentially the same difference values for both methods, and what is shown in Figure 10 (a and b) is for the CROS mode method. As expected, the difference increased as hearing aid gain increased. For this reason, we continued to increase hearing aid gain in 2-dB steps until we found the region where differences occurred, and then continued to increase gain until feedback was reached.

Figure 10

Figure 10

As seen in Figure 10, when measured REIG was in the range of 20–25 dB, there was no difference between the standard concurrent and contralateral CROS equalization methods (panel a). Above that level, we started to observe a difference, and an approximate difference of 5 dB (panel b) was noted when the REIG was in the range of 25–30 dB (max gain before feedback for this fitting).

Using the same hearing instrument and patient, we then conducted similar testing with the Frye Electronics Fonix 7000. In this case, we simply turned the reference mic “on” and “off” for the two conditions. The reference microphone was located about 1.5 inches above the ear canal. Again, we started observing a separation in the output between the two conditions when gain of about 20 dB was reached. Figure 11 illustrates the difference observed when we reached the just-below-feedback setting.

Figure 11

Figure 11

Again, the “mistake” is around 5 dB, a difference similar to what has been reported by others,28,29 although there are reports suggesting that differences of 10 dB or more can be present when the hearing aid is pushed to maximum gain and the reference microphone is located at the hearing aid microphone.30,31

As these examples show, stored equalization should, if possible, be used whenever OC fittings are tested. In addition to the equipment already mentioned, we know that this equalization method is possible with the GN Otometrics Aurical systems using the openREM mode (2.50 software), with the Siemens Unity using the ICRA input signals, and with the MedRx Avant REM Speech system using ICRA signals or live speech mapping. We suggest checking with the manufacturer for details.

If you have an older system that has only concurrent equalization, there really isn't a work-around. But it shouldn't be a major problem. For starters, if the reference mic is below the ear and is adjustable, farther away is better than closer. It also appears that until the REIG exceeds 20 dB, you probably won't make a serious mistake. Above 20 dB of REIG, you could assume that “true gain” is probably 5 dB or so greater than measured, or maybe larger when the feedback system is providing maximum reduction. That should give you a reasonable prediction. Even in this worse case scenario, you're still much better off than the folks who don't conduct real-ear testing at all.

Take-Home Tip #8: Use stored equalization rather than concurrent equalization when conducting probe-mic verification for OC fittings.

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ISSUE #9: REIG VS. REAR

When prescriptive fitting targets are used, verification can be conducted with either the REIG or the REAR (or both). Because earlier prescriptive methods (around 1980) were based on functional gain measures, REIG verification, which is similar to functional gain, has been the popular choice. In recent years, however, more dispensers are switching to the REAR verification approach, partly because of the teaching of the DSL group, and also because of the trend toward using both recorded and live speech signals as an input when measuring the REAR (referred to as “live speech mapping,” “recorded speech,” “speech map” and a host of other terms).

We have heard from several sources that for OC fittings the REIG is not a valid verification tool, and that it is necessary to use the REAR. The basis of this logic is uncertain, except that the REAR for an OC product with little or no gain in the highs looks a little better than the REIG. Consider that if there is an invalid REIG calculation, it would have to be the REAR that is responsible, as it's not possible to calculate the REIG without the REAR data (and the REUG doesn't know or care what style of hearing aid is going to be fitted!).

The confusion could simply be due to the fact that REAR use and OC fitting were emerging about the same time, and people have made a casual connection. Or it could be that the REIG calculation doesn't make sense when the ear canal resonance isn't lost. And it truly doesn't make sense, but that doesn't make the concept wrong.

Here's a simple example to make our point. Let's say a patient's NAL-NL1 REIG target is 30 dB for a 50-dB-SPL input for 3000 Hz. In one case, he is fitted with a closed earmold and in the second case with an OC. And, we'll say that with the open tip placed in the ear, the residual ear canal resonance is 15 dB at 3000 Hz, the same as his REUG at this frequency (which would make the measured REUR 65 dB SPL at this frequency for a 50-dB-SPL input).

To obtain the 30-dB target fit for the closed fitting for a 50-dB input, we will need an REAR of 95 dB (REAR minus REUR = REIG). Most of this will be amplifier gain, since with a closed fitting there is no residual resonance to help out. For the open fitting we still need an REAR of 95 dB, although we will need less amplifier gain to achieve it because of the residual resonance. This is more or less what we displayed back in Figure 8. Therefore, it's still okay with an OC fitting to subtract the 15-dB REUG from the REAG to calculate the REIG, as what we are interested in is the gain that the patient obtained. It doesn't matter if the patient still has his natural “gain” with the eartip in place.

The point is, while you might see some small differences in REIG versus REAR verification, these differences are not larger when the OC style is fitted. To illustrate, we have a patient who was first fitted with the manufacturer's default settings. The REIG (using pink noise) and REAR (speech mapping using a shaped real-speech signal) for a 65-dB-SPL input signal are shown in Figure 12a and b. Notice that there is a troublesome lack of gain above 2000 Hz, regardless of what measure is used. After some adjustments, we obtained the REIG and REAR results shown in Figure 13a and b. We now have a good match to target, regardless of what measure is used. Assuming that your input signals are chosen carefully, a good REIG fit should also be a good REAR fit, and vice versa.

Figure 12

Figure 12

Figure 13

Figure 13

Take-Home Tip #9: While we usually recommend verification using the REAR, both REAR and REIG verification methods are valid with OC fittings

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ISSUE #10: SPECIAL FEATURES

One special feature that many manufacturers advocate for use with OC fittings is directional technology. While we are still examining this issue, we can draw some preliminary conclusions. First, no positive directivity will be achieved in frequency regions where there is no gain and venting allows audible sounds to enter from the environment. Therefore, with OC fittings, a directional advantage is expected only in the higher frequencies; in general, it's as good as with a closed fitting for this frequency region.

Figure 14 illustrates this with a simple front-to-back comparison. Observe, however, that no directional benefit is observed for the lower frequencies. Our measurements to date suggest that the overall directional advantage for OC products may be limited to approximately half of what you might obtain from a hearing aid fitted with good directivity across the entire frequency range, gain in the low frequencies, and employing an occluded fitting.

Figure 14

Figure 14

There is an additional issue that may further limit directional benefit in some OC fittings. Specifically, the mini-BTE cases of open-fit instruments are quite small and sometimes are placed further behind the pinna to improve cosmetics. Unfortunately, we know that the microphone opening orientation can reduce directivity and the expected directional benefit.32 If the plane through the microphone port openings is horizontal (pointed at the sound source), directivity is maximized for most instruments. But if the directional system is “pointed” at the ceiling (or sky), directivity can be reduced.

Previous work suggests this is especially problematic in BTE instruments when they are tucked tightly behind the ear, because the sound shadow from the pinna on the microphone ports can have a significant negative impact on directivity. Therefore, cosmetics, the specific instrument being fitted, and the need for improved performance in noise must all be weighed when considering the potential for directional benefit in OC fittings.

Another special feature typically employed in OC fittings is digital noise reduction (DNR). As with directional technology, the impact of DNR will be reduced in OC fittings for many of the same reasons. If it was the low-frequency noise reduction that provided the “wow effect” for a closed fitting, a given patient might not notice this effect much with an OC product. Figure 15 shows the effects of DNR for “Max” versus “Off” for a typical OC instrument. Observe that, as with the directivity measures, there is little or no DNR effect in the lower frequencies.

Figure 15

Figure 15

Take-Home Tip #10: Directional and DNR advantages are present in OC fittings, but not to the extent available in a closed fitting.

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CLOSING COMMENTS

This is an exciting time in the hearing aid dispensing world, and OC fittings seem to be leading the charge. As with any new product, we learn as we go, so we thought it was time to toss out a few things that we've been thinking about. As we continue researching this product, we'll no doubt have more tips to add, and hopefully we won't have to retract any of the ten we just discussed.

As we said in the beginning, fitting hearing aids is all about compromise. But from our test results and experience, we believe that when the pros and cons of different hearing aid styles are carefully weighed for each patient, the OC product will be the winner in many cases.

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REFERENCES

1. Harford E, Barry J: A rehabilitative approach to the problem of unilateral hearing impairment: Contralateral routing of signals (CROS). J Sp Hear Dis 1965;30:121–138.
2. Dodds E, Harford E: Modified earpieces and CROS for high-frequency hearing losses. J Sp Hear Res 1968; 1:204–218.
3. Green D: Non-occluding ear molds with CROS and IROS hearing aids. Arch Otolaryngol 1969;89:96–102.
4. Dodds E, Harford E: Follow-up report on modified earpieces and CROS for high-frequency hearing losses. J Sp Hear Res 1970;13:41–43.
5. Mueller HG, Schwartz D, Surr R: The use of the exponential acoustic horn in an open mold configuration. Hear Instr 1981;32(10):22–24.
6. Lee L, Humes L, Wilde G: Evaluating performance with high-frequency emphasis amplification. JAAA 1993; 4(2):91–97.
7. Mueller HG: Open is in. Hear J 2006;59(11):11–14
8. Erickson F, Van Tasell D: Maximum real-ear gain of in-the-ear hearing aids. J Sp Hear Res 1991;34:351–359.
9. Killion M, Monser E: CORFIG: Coupler response for flat insertion gain. In Studebaker G, Hochberg I, eds., Acoustical Factors Affecting Hearing Aid Performance. Baltimore: University Park Press, 1980: 147–168.
10. Killion M, Revit L: CORFIG and GIFROC: Real ear to coupler and back. In Studebaker G, Hochberg I, eds., Acoustical Factors Affecting Hearing Aid Performance, 2nd ed. Boston: Allyn and Bacon, 1993: 65–85.
11. Mueller HG: Terminology and procedures. In Mueller H, Hawkins D, Northern J, Probe Microphone Measurements. San Diego: Singular Publishing Group: 41–66, 1992.
12. Sullivan R: An acoustic coupling-based classification system for hearing aid fittings. Hear Instr 1985; 36(9):25–28.
13. Martin RL: Rules for successful open fittings. Hear J 2006;59(9):54–58.
14. Yanz J, Olson L: Open-ear fittings: An entry into hearing care for mild losses. Hear Rev 2006;13(2): 48–52.
15. Kuk F, Keenan D, Ludvigsen C: Efficacy of an open-fitting hearing aid. Hear Rev 2005;12(2):26–32.
    16. MacKenzie DL: Open-canal fittings and the hearing aid occlusion effect. Hear J 2006;59(11):50–56.
    17. Otto W: Evaluation of an open canal hearing aid by experienced users Hear J 2005;58(8):26–32.
    18. Kuk F, Keenan D: Fitting tips: How do vents affect hearing aid performance? Hear Rev 2006;13(2):34–42.
    19. Martin RL: Why open fittings are the next big thing Hear J 2005;58(5):50–52.
    20. Shaw E: Transformation of sound pressure from the free field to the eardrum in the horizontal plane. J Acoust Soc Am 1974;56:1848–1861.
    21. Johnson EE: Segmenting dispensers: Factors in selecting open-canal fittings. Hear J 2006;59(11):58–64.
    22. Mueller HG: Fitting hearing aids to adults using prescriptive methods: An evidence-based review of effectiveness. JAAA 2005;16(7):448–460.
    23. American Academy of Audiology: Guidelines for the Audiologic Management of Adult Hearing Impairment, 2006. Online at
    24. Bentler R, Wu Y-H, Jeon J: Effectiveness of directional technology in open-canal hearing instruments. Hear J 2006;59(11):40–47.
    25. Dillon H: What's new from NAL in hearing aid prescriptions? Hear J 2006;59(10):10–16.
    26. Verifit User Guide Version 2.8. Dorchester, Ontario: Audioscan, 2006.
    27. Madsen P: Insertion gain optimization. Hear Instr 1986;37(1):28–32.
    28. Jensen O, Olsen S, Lantz J: Real-ear measurements and feedback cancellation—the road to openREM. GN Otometrics Application Note 2005 (March).
    29. How to perform real-ear testing on open-fit hearing instruments in two steps. Hear Rev 2006; 13(7):51.
    30. Olsen S, Hernvig L: Objective evaluation of maximum stable gain level and the performance of digital feedback suppression in hearing aids. Presentation at the 21st Danavox Symposium, Kolding, Denmark, 2005.
    31. Lantz J, Jensen O, Olsen S, Haastrup A: Special considerations regarding real-ear measurement verification in open, non-occluding hearing instruments. Proceedings, International Congress of Hearing Aid Acousticians, Nuremburg, Germany, 2005.
    32. Ricketts TA: Directivity quantification in hearing aids: Fitting and measurement effects. Ear Hear 2000;21:45–58.
    © 2006 Lippincott Williams & Wilkins, Inc.