Fast and simple step-by-step PMM procedures for mini-BTE/open fittings

The techniques outlined here are designed to be quick and easy, adding valuable information that allows you to maximize the service provided to patients. The three techniques provide valuable information on the degree of the “openness” of the fitting, a fast calibration check of the REM system, and a quick subjective estimate of the “real-world” loudness that the patient will experience while wearing the hearing aids.

It was only a few years ago that “open-fit” micro-behind-the-ear (BTE) fittings were considered unique. Whether coupled to the ear with a thin tube or a receiver placed in the ear canal, they allowed dispensing professionals to offer occlusion-free devices and options such as directional microphones without compromising cosmetic appeal. The popularity of these devices is demonstrated by the fact that, in the first half (January through June) of 2011, 70% of all hearing aids dispensed were behind-the-ear style,1 a dramatic increase from historical purchase patterns.

Wendy Switalski, AuD

Wendy Switalski, AuD, is president of Audiometric Consultants Inc, Waterford, Mich, an audiometric equipment distributorship serving Michigan and Northern Ohio. She is also the owner of Alliance Hearing Care, an audiology clinic in metro-Detroit, and a consultant for Otometrics, specializing in the development and implementation of fitting systems.

A review of currently available devices from hearing instrument manufacturers shows that a significant portion of these are of the micro-BTE variety. Recently published MarkeTrak VIII data2 indicate that this technology has had an “expansionary” effect; consumers who were previously reticent about hearing aids are now opting to pursue amplification because micro-BTEs are available.

As the use of mini-BTEs grows, the available options and fitting ranges have been expanded. This is welcome news for hearing care practitioners. Both experienced and new hearing aid users often request this style of device, presumably due to its reduced visibility. While initially best suited for those with mild-to-moderate hearing loss, we are now able to offer varied power levels of receivers and can couple the device within the canal using numerous types of domes, both generic and custom. These hearing aids can also be conveniently selected, changed, and replaced within the office as power needs change or repair is required.

Despite the commonly used term “open-fit BTE,” these devices encompass far more variance than the term encompasses. The product’s evolution warrants modification of our processes. The main goal of this type of fitting—maximizing access to speech and environmental sounds despite a compromised auditory system—is not fundamentally different from traditional fittings. However, the wide array of options that micro-BTEs offer should be applied deliberately in order to maximize their effectiveness and provide greater patient benefit and satisfaction. Data from MarkeTrak VIII2 suggests that the positive impact of micro-BTE fittings was not correlated with improved consumer perceptions on benefit or value; neither did it lead to increased levels of positive word-of-mouth advertising. A need for a renewed focus on the professional services we offer in combination with these products deserves attention.

The probe microphone measurement (PMM) techniques described below focuses on three modest process modifications. All measurements were obtained using the Otometrics Aurical Modular Fitting System.

TECHNIQUE #1: How Open Is Your Open?

Purpose: This procedure assesses the true openness of fit, streamlining the programming time spent in both initial fitting and follow-up programming sessions.

Time required: About 30 seconds.

  1. Following otoscopic examination and probe tube placement, measure the Unaided Response to evaluate the patient’s specific ear canal resonance by presenting 5 seconds of pink noise.
  2. Using the manufacturer’s fitting software, place hearing instruments in ears in “OFF” or “MUTE” setting.
  3. Measure the Occluded Response, again by presenting 5 seconds of pink noise.
  4. Compare the curves to determine what degree the ear canal response has been altered by the placement of the domes.


    Figure 1

    Figure 1. Unaided response (black line) is nearly identical to occluded (aided) response, indicating an open fit.

  1. Your fitting is open: In the example shown in Figure 1, the curves are identical, demonstrating that the domes are not creating occlusion in the ear canal.
  2. When this is good: If you have planned for a “typical” open fitting for a patient with normal low frequency thresholds.

    When this is not good: If your amplification goals require low frequency gain or greater power than can be easily offered in the open configuration, this result suggests the need to change to a larger or more occluding “power” dome or consider a custom mold.

    Figure 2

    Figure 2. Occluded (aided) response is significantly different (lower) than unaided response, indicating a closed/occluded fit.

  3. Your fitting is closed/occluded: In a closed/occluded fitting (Figure 2), the ear canal is effectively sealed off. This is indicated by the loss of ear canal resonance. In fact, not only is the loss of canal resonance apparent, but attenuation of the original input signal after it enters the ear canal is also seen (also known as insertion loss).
  4. When this is good: This is appropriate for fittings that require more power. The adequate seal is important for preventing leakage of sound directed into the ear. This minimizes the opportunity for feedback occurrences, and simultaneously maximizes available gain. Despite the increasingly sophisticated and effective feedback cancellation systems in today’s hearing instruments, the most effective “feedback management” encompasses both physical fitting characteristics (optimized dome/ear canal match) in addition to digital instrument capabilities.

    When this is not good: This finding may be problematic when you have recommended an open fitting for a patient with narrow ear canals, and you started with the smallest dome available.

    Figure 3

    Figure 3. Occluded (aided) response is similar to but not completely matching unaided response, indicating partially open/occluded fitting.

  5. Your fitting is partly open/occluded: Sometimes, the results indicate that ear canal resonance has been altered by placement of the dome but not enough to create full occlusion (Figure 3). The effect can range from slight to more significant. What to do in this case, as with the other results, depends on what your goals are.
  6. In cases where an open fitting is the goal, you can try a less occluding dome. However, this may not be necessary, especially if the effect is slight. You may still be clear to proceed with device programming using probe microphone measurements for verification because the probes are already in place once you have measured these responses. In these cases, make note of the finding to streamline potential “own voice” issues during the initial fitting or a later follow-up. Additional information follows below about this application.

    In cases where a closed/occluded fitting is the goal, a power dome, larger-size dome, or custom mold may be necessary to provide the necessary gain and output. If you are unable to more effectively close the ear canal, be aware that your control over delivering amplification in the low frequencies can be limited. In addition, any low frequency gain the patient is receiving may not correspond with the manufacturer’s fitting software indication. Relying on your probe microphone measurements will be vital to gauge the true amount of gain being delivered.

    An added bonus to evaluating the “openness of the fit”: Comparing the unaided and occluded responses does not provide a direct measure of the “occlusion effect” that is often seen in fittings with limited venting (ie, custom products or traditional earmolds). However, the openness of fit can guide you towards the most-appropriate option for resolution—or at least an acceptable level of improvement—when patients report issues with the perception of their own voice.

    The adaptation that occurs following consistent hearing aid use will often allow many side-effects, including those related to their own voice, to resolve on their own. However, in cases where resolution does not take place, or when more rapid patient acceptance is needed, applying the information acquired using the above technique is invaluable in determining the best approach to take.

    In a fitting that shows any level of occlusion, changing to a truly open configuration by selecting a smaller or more vented dome (if not contra-indicated by power needs) can have a dramatic and sudden positive effect. Conversely, in a fitting that is confirmed to be open, own-voice issues may require minor frequency response adjustments in the hearing instruments.

    Knowing which tool to reach for first—the dome kit or the programming cables —provides you with the right solution for each individual patient. Additionally, the opportunity to quickly resolve an issue like this has an added benefit. Recent data indicates a high percentage of patients achieve “above-average success” with hearing aids that were fit in 1 to 2 visits versus the 4 to 6 visits reported by patients with “below-average success.”3

TECHNIQUE #2: OpenREM Calibration Procedure

Purpose: Ensures that sound presented by the hearing aids in a patient’s ear canals is not detected by the reference microphones, which are resting outside the ears.

Time required: About 20 seconds.

  1. With probe tubes still in place and the hearing aids in the patient’s ears, select the “MUTE” or “OFF” setting in the manufacturer’s fitting software. Select the “Use OpenREM Calibration” option in your PMM system.
  2. A quick calibration stimulus will run, automatically measuring and setting the sound between the loudspeaker and the reference microphones, and then storing that level. The reference microphones are then disabled during the rest of the fitting process.
  3. After turning the hearing aids back on in the manufacturer’s fitting software, proceed with your fitting to set appropriate gain levels and frequency response.

Discussion: This is an important step when performing PMM for open fittings, as it ensures that sound presented by the hearing aids in a patient’s ear canals is not detected by the reference microphones resting outside the ears.

When performing traditional PMM, the reference microphones monitor the stimulus level generated from the loudspeaker and then apply modifications as needed to ensure that the sound always remains at the intended level. Calibrating the system for open fittings applies this feature only once and relies on the stored calibration for all measurements until the OpenREM calibration is repeated or switched off.

While most patients remain relatively still during fitting and programming, if they happen to move (eg, sitting back in their chair), this process can quickly be repeated to recalibrate the level of sound between the loudspeaker and the reference microphone.

TECHNIQUE #3: It’s a Loud World

Purpose: This provides a method to evaluate and set hearing aid output while providing a real-time reminder for patients about how loud the real world actually is—even without hearing aids. It is especially useful for open or partly-open fittings.

Time required: About 2-3 minutes.

Figure 4

Figure 4. Performing a visual check of the response, particularly in the high frequencies, and comparing the measured output with the target helps ensure patient comfort in the presence of loud (eg, 85 dBSPL) sounds.

  1. With probe tubes still in place, explain to the patient that you will now be setting the “ceiling” of their hearing aids. Explain that you will be using a signal that they may find to be louder than they would prefer, but is representative of a level that they will encounter in the real world.
  2. With the hearing aids still in the ears, explain that the patient will first hear the sound without the hearing aids on, so they have a reference for the signal in the unaided condition. Place the hearing aids in the “OFF” or “MUTE” setting in the manufacturer’s fitting software.
  3. Run a sweep tone MPO signal at 85 dBSPL. Briefly ask the patient (and their significant other) about their perception of the loudness of the signal. They will generally acknowledge that it was loud. This provides you with an opportunity to reinforce the reality of loudness for both hearing-impaired and normal-hearing individuals.
  4. Completing a visual check of the response in the high frequency area is helpful (Figure 4). It is not necessary to store the tracing, since it is only being used for comparative purposes.
  5. “UNMUTE” the hearing aids in the manufacturer’s fitting software, then run the same 85 dBSPL sweep tone signal again.
  6. Compare the measured output response to the output target, which was generated either through measurement of the patient’s specific loudness discomfort levels or as provided by the fitting prescription. Additionally, ask the patient to compare the overall loudness of the signal to the signal when their hearing aids were muted.
  7. Adjust output levels in the manufacturer’s fitting software as needed. Ensure that the real-ear output does not exceed the targets. Generally, once output is set appropriately to the fitting targets, patients will report that the signal is loud but is not significantly different in the aided condition.

Despite the fact that they are no longer considered novel, open-fit/micro-BTE fittings continue to present opportunities to move individuals with hearing impairment (even those who are reluctant) toward the amplification and help they need. Ensuring that your fitting processes are keeping up with the technology and design of these increasingly sophisticated devices allows you to provide your patients with greater levels of benefit and satisfaction.

  1. Strom KE. Hearing aid sales holding steady. Hearing Review. 2011;18(10):6.
  2. Kochkin S. MarkeTrak VIII: Mini-BTE’s tap new market, users more satisfied. Hear Jour. 2011;64(3):17-18,20,22,24.
  3. Kochkin S. MarkeTrak VIII: Reducing patient visits through verification and validation. Hearing Review. 2011;18(6):10-12.

Correspondence can be addressed to HR or Wendy Switalski, AuD, at .

Citation for this article:

Switalski W. Three Probe Microphone Measurement Techniques to Enhance Open Fittings Hearing Review. 2011;18(11): 40-49.