Probe Microphone Measures Streamlined for Efficient and Accurate Fittings

Author: Angela Flores, Au.D. and Peter Kossek

It has long been established that probe-microphone measures (PMM) provide a repeatable and objective way to measure the appropriateness of hearing instrument fittings. . Performing PMM is not just a matter of matching a validated prescriptive fitting target because professional organizations like the Academy of Doctors of Audiology, the American Speech-Hearing Association (ASHA) or the American Academy of Audiology (AAA) deem it to be part of a best practice clinical protocol. There is an abundance of research to support the use of PMM. For example, one study suggested that matching a prescriptive target translates to an improved self-perceived benefit compared to simply using a manufacturer’s first-fit target without verification using PMM (Abrams et al, 2012). There is additional evidence indicating that the use of PMM is correlated with greater levels of patient satisfaction and a reduced need for follow-up visits (Kochkin, 2011). As hearing instrument sophistication continues to increase, the need to verify has, perhaps, become even more important. To further bolster the need for PMM, several studies indicate hearing instrument manufacturers’ first fit attempts; even if the fitting software is told to match an independently validated prescriptive target, simply don’t accomplish this goal. In fact, according to a recent study (Aazh et al, 2012), one manufacturer failed to match target within +/- 10 dB at one or more frequencies (between 250-4000 Hz) 71% of the time. This study was a repeat of an earlier investigation in which four different manufacturers were evaluated in a similar manner. In that study (Aazh and Moore, 2007), 64% of the fittings failed to meet NAL-NL1 target within +/- 10 dB at one or more frequency between 250-4000 Hz. If our goal as professionals is to optimize audibility for our hearing instrument patients, and we know that independently validated prescriptive targets contribute to this goal, then shouldn’t we verify that we are actually matching the prescribed target?

Probe microphone measures allow for the measurement of sound to be delivered to the ear canal, while the hearing aid is in place (in situ). PMM take into account individual differences and variable effects such as ear canal volume, microphone placement, venting, etc. The hearing instrument responses, seen in manufacturers’ fitting software, are an estimation of the acoustic response in the ear (or in a coupler). The software can predict, but not know, how these individual differences and variables will affect the response of the hearing instruments. This leads to the inconsistency seen when fitting to a prescriptive target. One could argue that if we are missing target gain by 10 dB at one or more frequency, as seen in the Aazh et al study, we aren’t providing sufficient audibility. We can’t know unless we measure the real-ear response of the device.

Yet, despite the research, the use of PMM with every fitting is far from a common practice. According to a 2012 ASHA survey, 57% of audiologists reported that they provide verification of hearing instruments using PMM. Other surveys, (Kirkwood, 2006; Mueller and Picou, 2010), have indicated that less than half of audiologists self-report routinely utilizing PMM as part of the fit- ting process. The conclusion is that approximately half or fewer audiologists routinely conduct PMM, further indicating that the majority of the patients we serve are not being provided “best practices care”. The pertinent question is, “Why aren’t professionals utilizing a well-researched tool that many believe improves patient satisfaction and benefit?”

One of the major obstacles, to the use of PMM, often cited by professionals is the lack of time. This was illustrated by the ReSound Global Audiology group who conducted an online survey in which they asked hearing healthcare professionals how much time was allotted for an initial hearing instrument fitting. Nearly 60% of respondents, from a wide range of clinical settings, reported that they spend an hour or less on initial fit appointments (Koehler, Kulkarni, 2014). Keeping in mind all that must be accomplished during a fitting appointment, eliminating PMM would, upon first consideration, seem like a reasonable way to reduce the time needed to fit a patient. However, many clinicians may be over-estimating the negative impact of conducting PMM on fitting time. In reality, conducting probe microphone measures isn’t time intensive. In fact, at least one study has shown that using them can actually save clinical time by reducing the number of follow-up visits required (Kochkin, 2011).

According to Kochkin (2011), performing verification, with PMM as part of the fitting process, can reduce the number of return visits on average by .75 visits—nearly equivalent to one full follow-up appointment. Let’s do a bit of math to illustrate how that .75 can make a real difference in the clinic. If we assume that one follow-up visit equals 45 minutes, then .75 visits would equal approximately 35 minutes. Now, let us conservatively assume that a clinic averages 10 new hearing instrument patients per month. If each of those 10 patients were to come back just one more time throughout the year and that visit lasted 35 minutes, that’s a total of 70 additional hours of work per year. To put it another way, for those 10 new patients each month, you have spent a total of almost 2 weeks on follow-up that may not have been necessary. That is time that could have been spent seeing new patients or marketing the practice and creating additional revenue. It should be noted that Kochkin (2011) also indicated that performing both verification with PMM and validation, using some type of standardized patient self-report or questionnaire, further reduced the number of return visits.

From the hearing instrument wearer’s perspective, additional appointments generally aren’t desirable. At a minimum, they can pose an inconvenience and, at worst, they may lead to dissatisfaction, either with the hearing instruments or with the professional or both. We also know, from the EuroTrak study (Hougaard et al, 2012), that hearing instrument satisfaction is higher for those who wear their hearing instruments the whole day (8 or more hours), compared to those who wear them only part of the day (4 hours or less). Of course, there can be many reasons why a wearer might not be satisfied, including physical fit, ability to manipulate the hearing instrument etc. Nonetheless, we could argue that if the hearing instruments are not acoustically comfortable, patients will not wear them. Thus, if we match a prescriptive target, we can be confident the hearing instruments are acoustically comfortable, and the patient will be more likely to wear the instruments.

The concern remains having time to perform verification in a busy clinic. While there is no getting around the fact that conducting PMM does indeed take time, PMM doesn’t have to be time intensive. In fact, with modern PMM equipment, a full verification procedure including unaided, occluded, aided and MPO measures, can take 5-8 minutes or even less, depending on the number of adjustments needed. Modern PC-integrated PMM equipment can not only minimize the time spent, it can also help reduce errors, improve documentation, communicate with Electronic Medical Record (EMR) or Electronic Health Record (EHR) databases, and allow for customization for individual clinic needs.

Advantages of Modern PC-Integrated PMM Equipment
PC-integrated PMM equipment, is driven by software which is generally Noah compatible. This means that audiometric data that was saved within the Noah database will automatically appear within the PMM module, which immediately saves time, as there is no need to enter the audiogram or clear the audiogram from the previous patient. In addition, PC-integrated PMM equipment will measure real ear responses wirelessly and simultaneously in both ears as seen in Figure 1. This too can reduce the time needed to perform measures such as unaided and occluded gain, as you are measuring both ears with one click of the button. Obviously, if you need to make adjustments to aided gain, to match target, this still must be done with each individual ear.

Figure 1. Binaural real-ear measures

Figure 2. OnTop mode allows for the simultaneous viewing of the hearing instrument fitting software and the PMM software

Figure 3. ReSound AutoREM feature as seen in the Aventa fitting software

However, here too the PC-integrated PMM equipment can save you time, depending on its individual features. Rather than having to look from one screen to the next, as was the case with stand-alone PMM equipment, you can now visualize in real-time, the hearing instrument manufacturer’s fitting software, while viewing the gain/output curves—without switching between windows on the screen. As seen in Figure 2 adjustments to gain, compression, etc. can be made while the verification signal is being presented. The result is a real-time display of how the changes you are making to the hearing instruments translates to the in-the-ear response. Again, this saves time and effort as as there is no need to continually shift between screens or pieces of equipment in order to verify changes.

To further speed the verification and fitting process, some PC-integrated PMM systems, like the AURICAL FreeFit system, have integrated a features into their products that will automatically determine and apply gain corrections to meet your desired prescriptive target. This software communicates with PC-integrated PMM systems to automatically fine-tune the fitting to your chosen target.

Figure 4. OnTarget as seen within OTOsuite software

Additional PC-integrated PMM system tools have also been developed to make adjusting hearing instruments to target much simpler, by automatically calculating the difference between the target and the measurement curve. The difference is shown in real-time within a dedicated, easy to visualize graph, making programming adjustments straightforward. These tools can typically be utilized for commonly used prescriptive targets and for most any hearing instrument (Figure 4). These tools are particularly useful for acclimatization management, should you want to approach a target but not completely match it. It is easy to view how far below the target you are across frequencies.

If you are fitting an experienced hearing instrument patient, it is not uncommon for the transition to new hearing aids to be problematic. Some PC-integrated PMM equipment and software offer features that can help ease the acclimatization process, minimizing the need for return visits.

Another commonly stated barrier to the use of PMM is related to how to verify a sophisticated hearing instrument. Which tests are necessary and why?

The proposed streamlined protocol below is intended for the adult population. However, according to AAA guidelines for children, PMM or Real-Ear-to-Coupler Difference (RECD) measures are highly recommended with the hearing instru- ments programmed to match a prescriptive target that has been validated for the pediatric population. For PMM, the minimum measurements required are the response of the hearing instrument to an average level speech input and the maximum power output of the hearing instrument. For fur- ther information on pediatric amplification guidelines, please see the American Academy of Audiology Clinical Practice Guidelines on Pediatric Amplification (June, 2013). It should be noted that pediatric verification measures can also benefit from the use of modern equipment, which can help expedite the fitting process. For example, PC-integrated PMM systems are wireless and make binaural measurements. This can be particularly useful with children, as the absence of wires frees the child to sit and play while the measurements are made, and may reduce resistance to wearing the measurement collar. Also when time is of the essence, binaural measures can be obtained quickly. If RECD measures are the preferred fitting method for the audiologist, the touch of a button on the back of the collar will quickly obtain the unaided responses for both ears. These measurements are then utilized with a complimentary piece of equipment, the Hearing Instrument Test Box or HIT. Since both pieces of equipment are managed through the associated software, the unaided measures are automatically transferred ready to be utilized for the RECD fitting.

For adult fittings, AAA Guidelines for the Audiological Management of Adult Hearing Impairment (2006) recommends either PMM or RECD measures to verify the fitting to an adult validated prescriptive target. The guidelines also recommend the use of actual speech or a speech like signal to verify target. Various measures are recommended, depending on the prescriptive target utilized, but include the real-ear unaided response (REUR), aided responses (REAR) and maximum power output (MPO) measures. The guidelines further recommend the verification of special features such as telecoil. For directional microphones, in-situ measures, in particular, are recommended. Ideally, a professional would want to follow these fitting guidelines without sacrificing time. A streamlined approach to PMM is outlined below and includes the estimated time needed for each measure.

Proposed Minimal Testing Protocol
Equipment Setup
Time = 2 minutes
For proper infection control, new probe tubes should be placed on the equipment for each new patient. You may also want to run a probe tube calibration, which takes just a few seconds as it is accomplished binaurally. You will also want to let the software know which prescriptive fitting method you are choosing (for example NAL-NL), whether you are fitting binaurally or monaurally and whether or not you wish to utilize an open fitting.

Figure 5. Position the wearer approximately one meter from the speaker and facing the speaker directly.

Patient Orientation and Setup
Time = 2 minutes
The wearer should be positioned in front of the loudspeaker at a distance of approximately one meter (36 inches). As measures are obtained binaurally, the wearer should be at 0 degrees azimuth to the speaker (Figure 5). Following proper clinical guidelines, otoscopy should be performed to ensure that there are no contraindications for PMM. Place the PMM collar around the wearer’s shoulders and insert the probe microphones into the ears. The probe tubes should be placed into the ears to the appropriate depth, which is recommended for their age and gender. Most PC-integrated PMM systems provide a ruler on each probe to help guide the insertion depth.

Figure 6. An example of REUR. The right ear response shows proper insertion depth of the real-ear probe tube. The left ear response shows the characteristic dip in the curve below zero above 4000 Hz indicating a shallow probe tube insertion depth.

Real-Ear Unaided Response (REUR)
Time = 5 seconds
Pink Noise

According to statistics, recently reported by the Hearing Industries Association, 58 % of all hearing aid sales in the U.S. are receiver in-the-canal or receiver-in the-ear (RIC/ RITE) style hearing instruments. Many of these devices are intended to provide an open canal fit. However, depending on how the receiver is coupled to the ear, this may not actually be the case. Obtaining an REUR accomplishes a few goals in a matter of seconds. First, it can give you an idea of whether or not you have placed the probe microphones into the ear appropriately. If the insertion depth of the probe is too shal- low, you will see the tell-tale sign of the response curve dip- ping below zero beyond the 4000 Hz range in the REUG view (Figure 6). Having a shallow probe tube insertion depth may result in an underestimation of high-frequency gain. If you see the curve dip above 4000 Hz, simply reposition the tubes slightly deeper into the ear canal. For further information please see the following video: watch?v=95INcoB2mSo.

Figure 7. QuickView allows for quick access to the AURICAL OTOcam 300 video otoscope without leaving the PMM module.

Another tool within comprehensive PC-integrated PMM systems allows audiologists to view the ear canal shape, path and any obstruction to the probe tubes. As seen in Figure 7, within the PMM module this tool can provide ready access to the video otoscope. Without leaving the PMM software you can see the ear canal and even the position of the probe tube. These systems streamline the fitting process by eliminating the need to move to different software applications or to view different screens.

The REUR can also serve as a reference to the Real-Ear-Occluded Response (REOR) and tell you definitively whether or not the fit is truly “open” or not.

Real-Ear-Occluded Response (REOR)
Time = 5 seconds
Pink Noise

The REOR is simply a measure of the hearing instrument in the ear but turned off or muted. This will give you an idea of the “openness” of the fitting. If you see very little change in the occluded response, compared to the REUR, then you have achieved an open canal fit. However, if the natural resonance of the ear is diminished or disappears, the fitting cannot be truly considered an “open canal” fit. If your fitting was never intended to be “open” then this measure could be omitted. If the wearer is complaining “my voice sounds funny,” the REOR can be a useful measure to ascertain as to whether or not “ampclusion” (caused by inappropriate frequency/gain characteristics) is present, or whether or not you need to adjust venting.

Real-Ear-Aided Response or Gain (REAR or REAG)
Time for measure at average input = 14 seconds
Time to run 3 input levels (50, 65 and 85 dB SPL) = 42 seconds
Speech Signal such as ISTS or Rainbow Passage

The REAR or REAG measures the response of the hearing instrument in the ear with the hearing instrument turned on. The REAR/REAG is the curve you will utilize to try and match your chosen prescriptive target. Depending on your fitting goals, you may not want to match target perfectly. The REAR/REAG provides the frequency response of the hearing instrument relative to target and the audibility you are providing the wearer across frequencies.

At a minimum, it is recommended that you run the REAR/REAG at an average speech level (65 dB SPL). If you wish to see how compression is affecting the gain of the hearing instrument, then running additional curves at 50 and 85 dB SPL input levels are helpful.

Adjustments as Needed with Simultaneous Mode
Approximately 2-3 minutes
As mentioned above, adjustments to the hearing instrument response can be made in real-time while viewing the real-ear response via systems that include simultaneous viewing mode. Two screens in one mean no more looking to different pieces of equipment, providing a more efficient fitting process.

Maximum Power Output (MPO)
Time = 5 seconds
Swept Tones

The MPO measure is critical to determine the loudness comfort level of the hearing instrument(s) as well as to ensure that there is adequate “head room” in the instrument. If the output of the hearing instrument is too high, the wearer will complain that the hearing instruments are too loud and may turn down the volume. While this may solve the problem for high level inputs, it has the unwanted effect of also reducing gain for soft and average sounds. If the MPO isn’t set correctly, it’s likely that the patient will return for follow-up. At worst, they may return the hearing instruments and, historically, this has been the most common reason for returns. However, if the MPO is set too low, then the wearer may complain that the hearing instruments sound muffled or even that speech sounds distorted. So, it’s important to get the MPO setting right. Within the PMM module, a frequency sweep will occur at 85 dB SPL and measure the output of the hearing instrument(s). Again, with PC-integrated PMM systems, this is a binaural measurement, so the loud signal only needs to be presented once in the aided condition. If the curve doesn’t exceed the wearer’s uncomfortable loudness levels, then no adjustments are required. The output target will be calculated by the fitting formula based upon the patient’s individual loudness discomfort levels (LDLs) if entered into Noah or upon averages if no LDLs are available. If the output does exceed their level of comfort, changes in the hearing instrument fitting software should be made, such as reducing gain for loud sounds or lowering the output compression threshold.

Total time = approximately 5-8 minutes depending on programming adjustments required It should be noted that the PMM software can be set up to work as your clinic requires. That is, you can set the parameters of the testing such as signal type, level, etc. and save these preferences as a “User Test” within the software. Each clinician in your office can either do this according to their fitting requirements, or your clinic may use a standardized protocol. Regardless, PC-integrated PMM system software can be customized to your needs. This further streamlines your fitting process as there’s no need to continually load settings to complete the testing.
Verification of hearing instrument fittings using PMM is considered by our profession to be a standard practice care; however, clinical demands on time may lead us to forego this critical part of the fitting process. With the advancement of PC-integrated PMM equipment, verification using PMM can be completed in a very short amount of time and customized to the needs of the individual practitioner. The addition of innovative software tools further expedites the process. With today's PC-integrated PMM systems, there is no reason not to verify your fittings. If your patients knew the importance of verification using PMM, would you risk forgoing PMM routinely?    
Angela Flores received her Master’s degree in Audiology from the University of South Alabama and completed her CFY at Mayo Clinic. She received her Au.D. from Salus University in 2009. She worked clinically with Mayo Clinic and the University of Florida Pediatric Program for over 8 years. More recently, she has worked with Siemens Hearing Instruments in varying roles including Director of Education and Training, Senior Manager of Education and as a Training and Educational Specialist. Her current role is that of an independent audiology consultant. She has lectured all over the U.S. as well as internationally on varying topics in the Hearing Healthcare industry.

Peter Kossek is Senior Product Manager for AURICAL® – a complete, modular fitting solution from Otometrics that encompasses audiometry, PMM, HIT, video otoscopy and an HI programming interface in one system. Audiologists and other hearing care professionals can use the modules independently or combine them as needed to realize greater efficiency gains while boosting the quality of counseling and care.
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