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Audioscan Patient Feedback - March 2019

On-Ear Verification of Open Fittings

On-Ear Verification of Open Fittings
Dave Smriga, MA
February 10, 2017

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Learning Objectives

After this course, participants will be able to:

  • Identify and objectively verify an open fitting condition with any patient.
  • Identify open fit candidates using real ear measures.
  • Fit open fit candidates using real ear measures.

Introduction

Welcome to today's webinar entitled, On-Ear Verification of Open Fittings. Today we’ll explore how Audioscan equipment can be used to effectively verify a patient's candidacy for an open fitting, guide the subsequent on-ear verification process, and facilitate the counseling process needed to ensure maximum understanding and effective use of such treatment. Although we'll be depicting the various test procedures included in today's webinar with images created using the new Verifit2 real ear measurement and verification system, many of these same tests can also be performed with the original Verifit system.

As is the case with any educational material that is produced by Audioscan, any recording, presentation, adaptation, publishing or sharing of this material is prohibited without the express written permission of Audioscan.

Open Fittings

At its core, an open fitting can be defined as any style of hearing instrument and its associated plumbing that creates minimal occlusion of the ear canal when worn. In the case of traditional BTE-style instruments, minimal occlusion has historically been pursued with such plumbing appliances as tube fittings, free-field ear molds, or ear molds with large vents. In the case of miniature BTE devices that incorporate a fixed-shape thin tube or receiver-in-canal (RIC) design, minimal occlusion is often pursued with fitting tips that incorporate multiple large vents in their domed design. Minimal occlusion has even been pursued in ITE designs with large IROS vents.

There are some potential benefits for the patient and the professional if amplification can be effectively delivered to a patient's ear using an open-style of fitting. One potential benefit is eliminating the occlusion effect, which is often associated with a more closed-canal fitting that can create own-voice issues.  An open fitting may provide a more natural sound quality by mixing amplified sound with sound that naturally reaches the patient's ear canal. This would be particularly suitable for patients with low-frequency hearing losses less than approximately 40 dB HL, where occlusion can be more of a concern. However, with an open fitting, the increased feedback potential may limit the amount of usable gain.  Additionally, since any low-frequency gain will be vented out before it reaches the eardrum, these types of fittings are generally best suited for high-frequency hearing losses.

Since 2002, thin tube and RIC hearing aid designs have come to dominate the BTE market. The combination of modern digital feedback management technologies and the more appealing look of these miniature and stylish BTE designs have likely played a key role in their growing popularity. The added comfort and ease of adjustment to wearing a device that is afforded by the soft, non-occluding ear tips also contributes to their popularity. As a result, in the first quarter of 2016, for example, the Hearing Industries Association reported that 64.5% of all of the hearing instruments sold in the United States were RIC style products. Another 16.2% of the total fell into the BTE category, which includes thin-tube hearing instrument styles. It is important to note that although RIC and thin tube product designs now dominate the market, it doesn’t mean that a comparable percentage of today's hearing aid fittings are open fittings.

BTE, thin-tube, and RIC products can be fit in an open or an occluded fashion, depending upon the type of ear mold or dome design they utilize. An open fitting is therefore not defined by the style of hearing instrument but by the way any hearing aid design is coupled to the ear. If the coupling doesn't occlude the ear canal, then that fitting is open.

Methods for Confirming an Open Fitting

When we describe the open-fit verification fitting procedure a bit later in this webinar, we'll discuss how real ear measurements can be used to determine patient candidacy for an open fitting. Verification techniques can also help determine whether or not a given hearing instrument and its associated coupling appliance occludes the ear or not. For this, there are a couple of options. The first option involves the use of probe microphone equipment and Audioscan's Speechmap® screen  to compare the real-ear unaided response (REUR) to the real-ear occluded response (REOR). The second option is the Occlusion Effect Test.  I will describe each of these procedures step-by-step.

REUR-REOR Test. Whenever you use a probe microphone, it's important to ensure that the probe tube is correctly inserted into the patient's ear canal. More information on proper probe tube placement can be found in this video.

Once the probe tube has been properly inserted into your patient's ear canal, select the Speechmap test in the On-ear menu of Verifit's main test selection menu (Figure 1).

Figure 1. Select Speechmap in Verifit’s test selection menu.

This choice is available using any Audioscan device. This brings you to the Speechmap screen. Make sure that the ear, left or right, matches the probe microphone you wish to use. Then, press the play button for test one, which is identified in Figure 2 by the red arrow.

Figure 2. REUR: On-ear result obtained when only the probe tube is in the ear canal.

Make sure your stimulus type is a calibrated signal such as Speech Standard (F) or Speech Standard (1), and that the level you have selected is 65 dB. Then, press the record button. This will execute a long-term average of the speech energy present in the ear canal during this measurement. Upon completion of this measurement, the resulting green-shaded area that appears on the SPL Audiogram grid in Figure 2 represents the speech energy present in the ear canal when nothing else is in the ear canal other than the probe tube. In other words, this is a broad-spectrum representation of your patient's real-ear unaided response, or REUR.

Next, place the hearing instrument and its associated coupling appliance, dome or mold, into the patient's ear canal along with the probe tube that's already there. Make sure not to push the probe tube in further than your previous measurement.  Either mute the hearing instrument or turn it off. In Figure 3, the hearing instrument being used is a RIC device with an open dome attached to the receiver that has been placed in the ear canal alongside the probe tube. Using the same stimulus type and level as in test one, press the record button for test 2 to obtain a second long-term average of speech energy in the ear canal under this new test condition. The result of this measurement will appear as a pink-shaded area on your SPL Audiogram graph. This is a representation of the patient's real-ear occluded response, or REOR, when the aid plus coupling device is present in the ear canal and turned off.

Figure 3. REOR (open dome): On-Ear result obtained when the probe tube and a muted open-dome RIC instrument is in the ear canal.

You can use the hide-show buttons or icons associated with each of the tests and display both test results on the SPL graph at the same time, as can be seen in Figure 4. If the two results are virtually identical, as is the case in this example, then we have confirmed that the fitting is indeed open.  In this case, the physical presence of the hearing instrument and open dome has not changed the acoustic properties of the patient's ear canal. 

Figure 4. REUR and REOR superimposed – open fitting: Note that the two measurements are virtually identical, indicating that this is indeed an open fitting.

In Figure 5, the same RIC product was used, but with a single-wall closed dome attached to the receiver instead of an open dome. The REUR was measured as described previously, and as can be seen in this test two result, the long-term average speech energy reaching the probe tip with the hearing aid in place and turned off has been modified.

Figure 5. REOR (closed dome): On-ear result obtained when the probe tube and a muted closed-dome RIC instrument is in the ear canal.

This change is particularly evident when superimposing the REUR and this second REOR on the screen (Figure 6).  The low frequency energy below 1000 Hz is reaching the probe tip virtually unaltered, but the high frequency energy is significantly less intense in the new REOR condition. This, at a minimum, is due to the loss of ear canal resonance created by the presence of a more occluded tip. The result also reflects the occlusion of the ear canal caused by the closed dome, which attenuates some amount of high frequency energy traveling directly down the ear canal to the probe tube's position. This result indicates that this is not an open fitting.

Figure 6. REUR and REOR superimposed – closed fitting. The two measurements are not identical, indicating that the presence of the hearing instrument/plumbing changes the ear acoustics.

In this next example, a double-walled power dome was attached to the RIC device prior to measuring the REOR.  Further reduction of the speech energy reaching the probe tube is evident (Figure 7, pink response), and this further reduction becomes even more obvious when this REOR is superimposed with the REUR (Figure 7, green response). Energy across virtually the entire spectrum is being suppressed at the probe tube tip due to this much more occluded ear canal condition. Once again, this verification test confirms that this is not an open fitting. Such a result is also likely to be obtained when conducting this test using a micro-mold, of course, with the vent plugged.

Figure 7. REUR and REOR superimposed – double-wall power dome. Note the significant difference between the two measures when this magnitude of occlusion is present.

Test results such as these can and do vary.  For example, sometimes the perimeter of a closed dome doesn't seal the ear canal all the way around, creating some venting during this test that would modify the result obtained. Sometimes, the thinness of the dome material easily allows sound energy to pass through it, creating more of an open result than you might expect, even with a reasonable perimeter seal. There have even been some cases observed where an open dome REOR shows clear evidence that it is occluding the ear canal.

Therefore, you cannot assume that a fitting is open or closed simply based on your dome or tip choice; you must determine what is actually happening in the ear canal of your patient.  A verification procedure, such as this REUR-REOR test, can be very informative. The bottom line is this: If there is no difference between the REUR and REOR, then by definition, it is an open fitting. However, even if the difference between these two measures is due to a loss of ear canal resonance in the REOR, by definition it is not an open fitting.

Occlusion Effect Test. Another method for determining whether or not the instrument and coupling you intend to use delivers an open fit condition or not is to use the Occlusion Effect Test, also available with Audioscan instrumentation.

The Occlusion Effect Test is located in the Verifit On-ear menu (Figure 8, left). When this test is selected, it opens up the Occlusion Effect test screen (Figure 8, right).

Figure 8. Selecting the Occlusion Effect Test.

There are three sound level meter scales displayed on this screen. The top scale is associated with sound pressure that reached the probe microphone of the probe assembly being used; the middle scale is associated with the sound pressure that reached the reference microphone of the probe assembly being used; and the lower scale is used to display the difference between the levels measured in the first two scales.

To conduct the Occlusion Effect Test, insert the probe tube into the ear canal, followed by the hearing aid, which should be muted or turned off to create the REOR condition described previously. Then, click the ‘Start Test’ bar, which will then turn into a ‘Stop Test’ bar. This activity will activate the probe and reference microphones. Then, ask your patient to vocalize the sound “eeeee” at a moderate level vocal effort.  As your patient creates this input condition, the sound pressure that is reaching each of the measurement microphones is simultaneously being recorded, and the difference between these two measurements is displayed on the bottom scale. Once you press on the stop test bar, this measurement data will be captured.

Figure 9 shows the Occlusion Effect Test result obtained when an open-dome RIC device is in the ear and turned off. Notice that because the hearing instrument and open dome did nothing to change the acoustic properties of the ear canal, the SPL reaching the probe mic (pink bar), and the SPL reaching the reference mic (blue bar), are essentially the same. This is because the bone conducted energy of that vocal utterance is easily escaping from this non-occluded ear canal. This results in a very small difference bar on the bottom scale. Since there's little difference, there is no occlusion, and the difference bar is colored green.  A green result indicates the fitting is open.

Figure 9. Occlusion Effect Test: Open dome.

Figure 10 shows the same test using a closed-dome condition. Because of this more closed canal condition, some of the bone-conducted energy of the vocal utterance is unable to escape from the ear canal, which results in more SPL reaching the probe microphone than the reference microphone. The difference in this example is approximately +10 dB, which causes the difference bar to be colored yellow to caution the fitter that there is likely an occlusion effect.  A yellow result indicates that it is not an open fitting.

Figure 10. Occlusion Effect Test: Closed dome.

The next example shows the Occlusion Effect Test results of a fitting using a power dome. In this case, there is a difference measure of +14 dB, and the difference bar is colored red. This indicates that there is indeed an occluded ear canal. A red result means that the fitting is not open.

Figure 11. Occlusion Effect Test: Power dome.

In summary, if the Occlusion Effect Test yields a green difference bar, than you have an open fitting; if the result is either red or yellow, it is not an open fitting. Although the Occlusion Effect Test does not provide the frequency-specific indications of occlusion that the REUR-REOR test provides, it is a relatively quick and effective way to determine the potential presence of occlusion, and thus the potential openness of the fitting.

Open Fit Acoustic Considerations

Now, I’ll discuss the various acoustic considerations that are at play within an open fitting.

Venting

The first acoustic consideration is the venting, and the gain that may be lost via venting in an open ear canal environment. Kuk and Baekgaard (2008) compared the effects of various vent diameters and an open fitting condition on hearing aid output, as compared to a closed fitting. Their data show that traditional ear mold venting results in lower ear canal SPL below approximately 600-750 Hz. In the case of a 3.0mm vent, they found as much as a 20 dB reduction in output at 250 Hz compared to the unvented condition. With an open fitting, output reduction was seen up to 2000 Hz, with approximately 10 dB lower output at 1000 Hz relative to an unvented condition.

Let’s look at venting effects on a Speechmap fitting screen. In Figure 12, an NAL-NL2 target is on display for an audiogram that includes a 45 dB HL threshold at 1000 Hz. The gray-shaded area represents the unaided average speech energy envelope for a normal conversational speech input of 65 dB SPL. The line in the middle of the gray-shaded area is the unaided long term average speech spectrum (LTASS). The purple dots indicate the NAL-NL2 targets for the aided LTASS for this input level. In this case, the prescription calls for 10 dB of gain at 1000 Hz.

Figure 12. NAL-NL2 target for 45 dB HL threshold at 1kHz.

If we attempt to fit this patient with an open fitting, it's likely that at least 10 dB of the gain programmed into the hearing aid at 1000 Hz will be vented out of the ear canal before it has a chance to reach the eardrum or the probe tip. This loss of energy needs to be properly accounted for in the programming of the hearing aid, and/or by adjusting the venting or plumbing characteristics of the device.

This points out the value of the Speechmap fitting screen in open fittings. By comparing unaided LTASS to the target values, we can easily determine if the patient requires low-frequency gain to achieve audibility; or, if direct sound traveling through the vent down the ear canal to the eardrum would be sufficient for targeted audibility to exist at the low-frequency levels. If low-frequency gain is required, we can conclude that this patient is likely not an open-fit candidate.

Since the aided measurement we will later obtain when conducting a Speechmap test with the hearing aid present is an aided output measurement rather than an insertion gain measurement, the result will reflect venting effects on the aided eardrum SPL at each frequency.  Therefore, it will verify whether or not the gain programmed into the hearing aid is actually delivering the desired aided performance in that ear under that vent condition.

Ear Canal Resonance

Open fittings also have a unique impact on ear canal resonance. Mueller and Ricketts (2006) fit five subjects with two mini-BTE devices programmed to NAL-NL1 using the same audiogram. Real ear recordings were made for each of these two coupling conditions: open-fitting, using thin tubing and tip; and closed-fitting, using a half-shell closed mold. They averaged the results for these two conditions. They found that the insertion gain difference at 1000 Hz between the two coupling conditions was approximately 10 dB, consistent with the open-venting effects we saw from Kuk and Baekgaard.

However, in addition to this finding, they also showed evidence of additional “gain” in the open condition for the frequency region typically associated with ear canal resonance.  In other words, because the ear canal is open during the aided measurement, there is no loss of ear canal resonance in the aided condition.  On an insertion gain scale where the REUR is subtracted from the real ear aided response (REAR) to get the real ear insertion gain (REIG), this contribution of “unlost” ear canal resonance is reflected as additional gain on the open REIG result. 

There is actually no difference in the amount of gain provided by the hearing aid in this frequency region between these two test conditions.  Rather, this difference is due to the contribution of the unlost ear canal resonance on the open REAR. 

As mentioned before, from a speechmapping verification perspective, when an REAR measurement is employed, this actual aided output would be reflected in the aided speech energy result, and would automatically be included in the overall audibility calculations.

Acoustic Pathways

In an open fitting, sound can reach the eardrum via a couple of pathways. There is the direct pathway, where sounds reach the eardrum directly and are thus unprocessed by the hearing aid, and the indirect pathway, where sounds reach the eardrum via the output signal produced by the amplification from the hearing aid itself. These two sources can combine with one another, and in some cases, they can interact with one another depending upon their relative levels.

Advantage of REAR Measurement

When you measure the REAR, as is the case when conducting a Speechmap test, the measurement displays the combined input condition. It measures and displays the aided SPL at the probe tip, regardless of how that SPL was delivered. Therefore, it's giving you exactly the information you're interested in when doing this type of aided measure.  

Open Fit Verification Considerations

In an open fitting, objective verification is restricted to the on-ear modality only.  In other hearing instrument fittings, a real-ear to coupler difference (RECD) measurement can identify the acoustic differences between a coupler and a patient's ear canal, thus allowing speechmapping to be conducted in the test box.  The test box measurements can display the aided measurements obtained there as if they had been conducted on the patient's ear.  This process has historically been described as simulated real ear measurement, or S-REM.  However, open ear acoustics / venting cannot be accurately simulated in a coupler-based test box measurement, and the RECD isn't designed to capture open-ear effects. So, in an open fitting situation, all verification measurements must be conducted on-ear using a probe microphone.

In addition, when conducting on-ear verification of an open fitting, the traditional “concurrent” calibration (equalization) method should not be used. When the calibration pulse is presented to the patient's ear from the sound field speaker, its energy not only reaches the reference microphone, but the hearing aid microphone as well. If the hearing aid is on and amplifying, the calibration pulse will be amplified, and some of the amplified energy may vent from the open ear canal and reach the reference microphone. Thus, the SPL measured at the reference microphone may not be only the SPL coming from the speaker (as it should be), but may also contain some of the amplified sound from the hearing aid that flows out of the open ear canal.  If this happens, the subsequent speech signal presented from the real-ear loudspeaker may be adjusted incorrectly, i.e. lowered, and thus not deliver the appropriate signal level and spectrum to the hearing aid for test purposes.

“Stored” Equalization

To avoid this potential error in signal equalization, it is necessary to use a “stored” equalization process rather than a “concurrent” equalization process. The stored equalization process is automatically initiated when an on-ear speechmap test is attempted with the Speechmap instrument menu selection set to Open. The system instructs you to mute the open hearing instrument in the patient's ear along with the probe mic, prior to pressing an Equalization button. Then, the calibration pulse will be presented without being immediately followed by the speech. Since the hearing instrument is not on, there's no leak potential of amplified sound from the ear canal to the reference microphone. Once an accurate sound field equalization is obtained, you are instructed to unmute the hearing instrument before continuing the test. At that point, the speech signal would be presented at the appropriate level and spectrum as determined by the stored equalization procedure. Note that the equalization procedure only calibrates the sound field for the location where the calibration was performed. Therefore, it's important that the patient remains in the same location when the subsequent speechmap measurements are done. Otherwise, the calibration procedure will need to be conducted again to accurately capture the new sound field location.  This will be reviewed shortly as we get into the verification examples.

Mueller and Ricketts (2006) documented the potential impact of this calibration artifact due to leakage. Since the concurrent equalization method causes the speaker level (i.e., input signal to the hearing aid) to be reduced, the effect is a lowering of the resulting REAR. If the REAR appears lower due to a compromised calibration process, the fitter may attempt to raise the gain in order to achieve prescriptive targets.  Muller and Ricketts showed that the effects of this inadvertent leak of hearing aid amplified sound from the ear canal to the reference mic during the equalization calibration can incorrectly lower the level of the subsequent speech signal presentation for frequencies above 1000 Hz by approximately 5 dB for devices having 25 dB of high frequency gain.

Open Fit Verification Procedure

Identifying Patient Candidacy

Now let's walk through an open fit verification procedure. We'll start by revisiting the patient candidacy procedure mentioned earlier.

The patient who is an appropriate open-fit candidate needs little to no gain at or below 1000 Hz. If you're not sure, there is a quick way to find out that only requires the patient’s audiogram, a target rule, and an REUR.  Here are the steps:

  • Select Speechmap from the on-ear test selection menu
  • Enter the audiogram into your verification system
  • Select a fitting target (such as NAL-NL2)
  • Place the probe tube only in the patient’s ear(s)
  • Run and record the REUR for 65 dB speech
  • Compare the displayed LTASS to the displayed target

Let’s look at a few examples.

Example 1: Open Fit Candidacy. In the first example of a candidacy procedure, we’ve entered the HL audiogram shown in Figure 13. 

Figure 13. Patient’s audiogram used for open fit candidacy procedure.

The HL audiogram yields the SPL audiogram (red line) with NAL-NL2 targets (pink dots) shown in Figure 14. With just the probe tube in the patient's ear canal, we ran and recorded the REUR for 65 dB speech, which is shown as the pink shaded area. The LTASS is the thick pink line in the middle of the pink shaded area.  Where the LTASS is on the NAL-NL2 target dots, no gain is required. The unaided condition is already producing adequate SPL at those frequencies based on the NAL-NL2 targets. In this case, since the LTASS is on target through 1000 Hz in this unaided measurement, this is an ideal candidate for an open fitting.

Figure 14.  Determining open fit candidacy.  In this case, no gain is needed through 1000 Hz to achieve targets, so this patient would be a candidate for an open fitting.

 

Example 2. The patient in this next example has an audiogram with some hearing loss in the low and mid frequencies (Figure 15).

Figure 15. Audiogram for Example 2.

This HL audiogram produced the SPL audiogram shown on the Speechmap screen in Figure 16.  NAL-NL2 was also used in this example.  When running the REUR for 65 dB speech, we can clearly see that there is gain needed in the lower frequencies for the LTASS (thick pink line) to reach the NAL-NL2 targets (pink dots). In fact, there is gain needed in the mid frequencies and higher frequencies as well to achieve targets. So, this result indicates that this is not an open fit candidate.

Figure 16. Determining open fit candidacy.  In this case, gain is required in the low, mid and high frequencies to achieve targets, so this patient would not be a candidate for an open fitting.

This candidacy procedure is something that could be done as part of the initial diagnostic visit, and could be used to verify whether your choice of an open fitting is appropriate for the patient or not. You can do this prior to ordering hearing instruments or selecting the coupling type, and prior to counseling the patient about hearing aid selection.

Verifying Aided Performance On Ear

When you have an open fit candidate, the steps outlined below describe how to verify aided performance using real ear measurement with Speechmapping.

Select the Speechmap choice in the On-ear section of the Verifit test selection menu.  This choice is available in all Audioscan devices.

To enter your patient's audiogram, click the “Audiometry” button associated with the right or left ear. In this example, we’ll be conducting an open-fit verification of the left ear for a patient with the HL audiogram shown in Figure 17.

Figure 17.  Verifying aided performance on-ear: Entering the audiogram.

When this HL audiogram is entered, you will next see the SPL version of this audiogram on the Speechmap screen (Figure 18). Follow these five steps to conduct verification.

Step 1: Select Open in the instrument menu.

Step 2: Insert the probe tube into the patient’s ear canal per typical procedures and place the hearing instrument on your patient's ear, and mute the hearing instrument or turn it off. Since you're preparing to fit the hearing instrument, you will likely have it already connected to the programming software, so you can simply mute the hearing instrument via the software control. If this is not the case, simply turn the hearing instrument off.

Figure 18. Verifying aided performance on-ear: Speechmap screen.

Step 3. Select Play in Test 1, and then store the equalization when prompted.  After you click the Play button in Test 1, you will see the Equalize poster shown in Figure 19. This poster will only appear if Open was selected in the Instrument menu. With the patient sitting in front of the sound field speaker with the probe tube in their ear canal and with the muted hearing aid on their ear, click the Equalize bar. This will cause the calibration pulse to be presented from the sound field speaker and the reference mic will record the SPL levels in each of the one-third octaves across the broadband spectrum of this calibration pulse. These levels will be compared to the levels needed to deliver the desired speech signal to this test point and will be used to adjust the subsequent speech signal so that the desired levels reach this test point.

Figure 19. Equalize poster.

The disadvantage of a stored equalization is that the equipment cannot modify stimulus level or spectrum on an ongoing basis to account for any changes in the sound field around the patient. If the patient moves, or other changes in the immediate area around the ear happen, the equalization process must be repeated. When the open instrument type is selected, all running speechmap tests will feature an EQ button, which allows the user to interrupt the test, temporarily mute the hearing instrument, and store a new equalization. After this is complete, the test will continue as before. Please note that this EQ button is only visible while the test is running.

Step 4. Run Test 1: Calibrated soft speech.  After storing the equalization, unmute the hearing aid and select 55 dB speech input. Using one of the calibrated input stimulus options, adjust the gain of the hearing instrument until the green thick line aligns with the target dots shown in Figure 20. Then hit the record button to secure the long term average aided result. In this example, the gain has been adjusted so that the thick green line is meeting the target for soft speech.

Figure 20. Test 1: Calibrated soft speech.

Step 5: Run Test 2: Calibrated average speech. Next, click on the Play button for Test 2, and change the stimulus level to 65 dB using the same calibrated stimulus that you used in Test 1. You can immediately hit the record button of Test 2 to secure the long term average shown in Figure 21. With the gain settings programmed into the instrument during Test 1, you now want to ensure that the thick pink line of Test 2 is also hitting the target dots for this input level. If not, you would need to adjust the hearing aid gain settings specific to this input condition.

Figure 21. Test 2: Calibrated average speech.

In this example, the hide-show buttons or icons associated with Test 2 have been used to display both the aided and unaided speech energy bananas for 65 dB speech. A comparison of these two curves clearly depicts what the hearing instrument has done to change the audibility of normal conversational speech. A lot more high frequency energy is above threshold in the aided condition compared to the unaided condition. In addition, there is clear evidence that the aided energy in the lower frequencies is the same as unaided energy.  This indicates that the hearing aid is not delivering gain in the lower frequencies, as expected in an open-fit condition. The final element to consider on this Speechmap screen is the unaided versus aided speech intelligibility index (SII) cores for this input level. You can see those scores represented on the right hand side of the Speechmap screen in Figure 21. In the aided condition, the SII is 69; in the unaided condition, the SII is 45.

Step 6: Run Test 3: MPO.  The third test is to run the MPO (maximum power output) sweep, which when conducted on-ear, consists of a single sweep of short-duration pure tone beeps, one-third octave apart, presented at 85 dB SPL. Through adjustment of the MPO within the hearing aid's fitting software, make sure that the blue line  on the Speechmap screen is close to, but does not exceed, the UCL which is indicated as asterisks (Figure 22). If you select the DSL fitting rule, it provides target dots for MPO.  In that case, make sure that the blue line approximates the targets without exceeding them. 

Figure 22. Test 3: MPO.

Optional Steps: Run Loud Speech in Test 4 and adjust the gain/frequency response of the device to approximate the targets generated for this input level as was done for other speech inputs. If desired, you could also run an REUR in Test 4. As an option, if you run long term average 65 dB speech with only the probe tube in the ear canal with no hearing aid present, you'll get the patient's REUR. The REUR is the measure we obtained when we were determining patient candidacy for an open fitting. In Figure 23, the unaided speech energy (gold) can be used instead of the predicted unaided speech (gray shaded area) to represent the unaided ear canal condition for that ear. In this case, the REUR looks a lot like the gray shaded area, with the only exception being the ear canal resonance peaks in the actual REUR that represent the input signal.

 

Figure 23. Optional: Run REUR in Test 4.

Comparing this unaided eardrum SPL response to the aided eardrum SPL response obtained in test two for the same input stimulus and level condition may offer an even more understandable demonstration of what the hearing aid has done to improve speech audibility and therefore speech intelligibility (Figure 24).

Figure 24. Actual unaided (gold) and aided (pink) normal conversational speech in ear canal.

For more information about how speechmapping results can be obtained and used as a fitting and counseling tool, please refer to the Audioscan course, Speechmapping As A Fitting and Counseling Tool, on AudiologyOnline.

 Directional Mic Test

Patients who are treated with an open fitting can benefit from additional signal processing options often considered in hearing instrument selection, including directional microphone and noise reduction technology. The function of these features can still be verified both on ear and in the test box, even for open fittings.  However, there are some considerations worth noting.

The Directional Mic test within the Verifit collects two broad-spectrum frequency response curves simultaneously. These curves represent the output measured at the measurement microphone from two simultaneously-presented input stimuli. One stimulus comes from zero degree orientation relative to the patient's head, and one comes from some orientation behind the patient, generally 180 degrees, or approximately 130 degrees, which approximates the null of a typical hypercardioid directional microphone.

In On-ear mode, this test condition is accomplished through the use of an auxiliary speaker mounted on a tripod, that plugs into the rear speaker port on the back of the Verifit unit. This speaker is placed behind the patient while the patient is facing the display unit sound field speaker of the Verifit. With the probe microphone and hearing instrument positioned in the patient's ear, the measurement microphone for this test is the probe mic. The set up for this test is shown in Figure 25.

Figure 25.  On Ear Directional Mic Test.

When executing an On Ear Directional Mic Test with an open-fitted hearing instrument, you'll likely obtain a result similar to the one shown in Figure 26.  The thick curve represents the output measured at the probe tip for the front input signal, labeled F1. The thin curve represents the output measured at the probe tip for the rear input signal, labeled R1. In an open-fitting condition, the direct sound pathway (i.e., sounds coming directly from the ear canal), will likely dominate the lower-frequency region.  As a result, any directional benefit provided by the hearing aid in that frequency region will not be visible, and you will see the two curves overlapping. However, in the higher frequencies, where the indirect sound pathway (i.e., aided energy) dominates the REAR, the directional effect can and should be evident by a separation of the curves.  In this example, curve separation is visible in the 2000 – 8000 Hz region.

Figure 26. On Ear Directional Mic Test result.

This result aligns with the aided Speechmap result we previously collected in Test 2. In that measurement, amplification reached the probe microphone above 2000 Hz, and here, the directional microphone effect is identified in that same frequency region (Figure 27).  Therefore, the fitter can anticipate that the directional microphone feature may offer some advantages for this patient in competing signal and noise environments. If curve separation was not found in this frequency region, there would likely be no directional microphone advantage experienced by the patient.

Figure 27. Aided on ear REAR for speech, compared to On Ear Directional Mic Test.

Test Box Directional Mic Test

If you do not currently have an auxiliary speaker, useful information about the directional microphone's potential utility in an open fitting can be gleaned from the Test Box Directional Mic Test. When employing a Verifit2 test box, the directional microphone function of both the right and left ear hearing aid can be evaluated simultaneously. This is facilitated by the unique binaural coupler system that is part of the Verifit2 test box design. In Figure 28, the binaural coupler assembly has been configured to test two RIC hearing instruments, which are coupled to the wideband .4cc coupler using the TRIC (thin tube RIC) adapters that take the place of putty. The hearing instruments are held vertically in the test box using hearing aid stabilizer attachments.

Figure 28. Test Box Directional Mic Test set up.

When conducting this test, it is important to correctly orient the hearing aids.  The front and rear mic ports of the directional mic system should be aligned parallel (horizontal) to the floor board of the test box, so that the microphone orientation is perpendicular to and facing the test box speaker (Figure 29). With this orientation, the front test box speaker becomes the front speaker for the directional mic test. Inside the lid of the Verifit2 test box are two additional rear speakers that act as secondary, or jamming, speakers for directional mic testing. When the lid of the box is closed, these speakers end up aligning at an angle of incidence to each hearing aid that approximates the null angle of the common hypercardioid polar pattern often used in directional microphone designs (Figure 30).

Figure 29. Test Box Directional Mic Test: RIC.

Figure 30.  Recommended positioning – Verifit2.

Figure 31 is a Test Box Directional Mic Test result obtained in the Verifit2 test box for the same hearing aid programmed the same way, using the same input stimulus type and level.  Unlike the on ear result, this result was obtained in an occluded coupler condition, where there is no direct signal pathway SPL or vent effects offsetting the low frequency components of this result.  Since the horizontal positioning of the directional mic ports is more controllable in this environment, and because pinna and head effects are non-existent, it is common to see a more robust curve separation than may be present in an associated on ear directional measurement.

Figure 31. Test Box Directional Mic Test result.

Using the previously acquired Speechmap REAR for Test 2, we can identify the frequency region where amplification is reaching the eardrum (Figure 32).  If curve separation exists in that same frequency region when examining the Test Box Directional Mic Test result, you can anticipate that the directional mic feature may provide some benefit for the patient.

Figure 32. Aided On Ear REAR (left) for speech compared to Test Box Directional Mic Test (right).

For more information, view the Audioscan course, The Verifit Directional Mic Test: Evaluation Modern Directional Microphone Technologies, on AudiologyOnline.

Noise Reduction Test

Noise reduction technology is another signal processing feature that may be used in open fittings. Many of today's digital noise reduction algorithms are designed to detect the amplitude modulation patterns of the input environment in each of the hearing instrument’s bands. If these modulation patterns do not indicate that speech is present, band-specific gain reductions are activated automatically, over time, to reduce the audibility and therefore the annoyance of the noise that is present.

On Ear Noise Reduction Test

Figure 33 is an example of a noise reduction feature test result obtained on-ear in an open fitting environment. The input stimulus is a recording of air conditioner noise at 70 dB presentation level. There are two frequency response curves on the screen. The thick curve represents the output of the hearing aid that was automatically captured and stored after the noise stimulus was present for about two seconds. This was the output of the hearing aid before the noise reduction feature had had a chance to activate. The thin curve represents the output of the hearing aid after a sufficient period of time had elapsed for the noise reduction feature to fully engage. The difference between these two curves identifies the magnitude and spectrum of the hearing aid's noise reduction feature as it would be delivered in the patient's ear in an open fitting.

Figure 33. On Ear Noise Reduction Test result.

As was the case with an On Ear Directional Mic Test, the direct pathway SPL from the environment tends to dominate the low frequency area of the response curve, because of both vent effects and the lower gain programmed in this region.  Thus, any noise reduction activity in this region is obscured. However, in the higher frequencies, where the REAR gain was observed during the fitting procedure, this test shows clear evidence of reduced output once the noise reduction feature has been fully activated.

Using the previously acquired Speechmap REAR for Test 2, we can identify the frequency region where amplification is reaching the eardrum.  If curve separation exists in that same frequency region when examining the On Ear Noise Reduction Test result, the patient should experience some reduction in the loudness of that noise when it is at that level, in that frequency region in their environment.  This is shown in Figure 34.

Figure 34.  Aided On Ear REAR for speech (left) compared to On Ear Noise Reduction Test (right).

Test Box Noise Reduction Test

Noise reduction testing can be done using the test box as well. Results are shown in Figure 35. With the Verifit2, the noise reduction functionality of both the right and left hearing aid can be assessed at the same time in the test box. Since this is a closed coupler test, the results are not representative of the result one would expect to receive in an open fitting environment in the lower frequencies.  In the test box condition there is no vent, and little to no contamination from ambient SPL. 

Figure 35. Test Box Noise Reduction Test result.

Using the previously acquired Speechmap REAR, we can identify frequency regions where amplification is reaching the eardrum (Figure 36).  Curve separation indicates the potential advantage of the noise reduction feature.

Figure 36.  Aided on ear REAR for speech compared to Test Box Noise Reduction Test.

Summary

In summary, there are key considerations one must recognize when verifying open fittings.

First, we must be clear on what we mean when we talk about an open fitting. An open fitting exists when any hearing aid, regardless of its design, is coupled to the ear in a way that does not occlude the ear canal.  

We described two methods that can be used to determine if an open condition is present:

  • A simple REUR-REOR test conducted in Speechmap can clearly identify if an open condition exists.
  • The Audioscan Occlusion Effect Test can be used to confirm the existence of an open condition.

We described several acoustic properties associated with an open fitting that makes them unique, especially from a verification perspective:

  • The vent effects of an open fitting can extend all the way up to 2000 Hz, and as much as 10 dB of gain can be vented away at 1000 Hz in an open fitting.
  • Ear canal resonance is not removed in an open fitting, and therefore does not need to be replaced with gain before aided improvement is measurable.
  • Aided eardrum SPL in an open fit condition is a combination of both the natural sound reaching the eardrum through the open ear canal and the amplified sound reaching the eardrum through the hearing aid.

We pointed out that when a true open fit condition exists, the verification procedure must be adjusted accordingly:

  • Pre-fitting the hearing aid by conducting Speechmap in the test box, is not an option, because an open ear canal condition cannot be simulated in the test box.
  • A stored equalization procedure instead of the standard concurrent equalization method must be used.
  • All fitting/fine tuning steps will need to be completed On-Ear

We demonstrated that measuring the REAR is a better reflection of the acoustic conditions present in an open-aided condition than the REIG. This makes it a more useful probe microphone measurement than the REIG for both verifying open fit candidacy and displaying the combined acoustic signal results present at the eardrum in an aided open-fit condition.

We presented the methodology for testing both directionality and noise reduction in open fittings using on ear measures as well as test box measures.  In combination with on-ear Speechmap results, these data can help clinicians anticipate the potential usefulness of these features in an open-fitting environment.

If you have questions or would like further information on any of this material presented in this course, please contact Audioscan at info@audioscan.com.  

References

Kuk, F., & Baekgaard, L. (2008, March). Hearing aid selection and BTEs: Choosing among various open ear and receiver-in-canal options. Hearing Review.

Mueller, H.G., & Ricketts, T.A. (2006). Open-canal fittings: Ten take-home tips. Hearing Journal, 59(11), 24,26,28–32,34,36–39. doi: 10.1097/01.HJ.0000286216.61469.eb

Citation

Smriga, D. (2017, February). On ear verification of open fittings. AudiologyOnline, Article 19326. Retrieved from http://www.audiologyonline.com.

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dave smriga

Dave Smriga, MA

Senior Audiology Consultant

David J. Smriga is senior audiology consultant for Audioscan, a division of Etymonic Design, Inc., a major manufacturer of hearing instrument verification and fitting equipment.  Mr. Smriga received his master’s degree in audiology from Northern Illinois University in 1976.  During his subsequent career, Mr. Smriga has held positions in both clinical and research audiology at the Health Sciences Center in Winnipeg and the University of Manitoba Medical School, as well as senior management positions in sales, marketing, product management and public relations for some of the industry’s leading hearing instrument manufacturing firms.  Mr. Smriga has conducted over 600 lectures in North America and in Europe, and has authored over fifty publications ranging in topic from inter-operative brainstem monitoring to counselor selling.  For the last fourteen years, Mr. Smriga has been Audioscan’s chief lecturer on the use of real-ear measurement technology and audibility-based fitting strategies.  He is well known for his unique way of presenting complex information in a clear, logical and understandable way.  



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