|Justyn F.D. Pisa, AuD, is a research audiologist in the Clinical Product Research Group at Starkey Laboratories Inc, Eden Prairie, Minn. Correspondence can be addressed to HR or Justyn Pisa at .|
The hearing industry industry is meticulous in collecting facts about patient demographics, hearing aid sales, and market trends. It should be no surprise to learn that 51% of all hearing aids sold in the United States are now behind-the-ear (BTE) instruments.1
According to the 2009 Hearing Industries Association (HIA) first quarter sales report, 29% of those BTEs are dispensed in the form of smaller devices with the receiver in the canal, using a size 10 or 312 battery and featuring a thin tube or cable that delivers sound to the open ear canal.2 These receiver-in-the-canal (RIC) instruments have rapidly grown in popularity due to the fact that they reduce occlusion-related issues—prevalent fittings for the mild-to-moderate hearing-impaired population—while offering a feature-rich and cosmetically appealing option for both new and experienced users.3 With approximately 31 million people with hearing loss in the United States, it is understandable that development of these products continues to focus on extending their application to other populations beyond just those with mild-to-moderate hearing impairment.4
People with severe-to-profound hearing loss often demonstrate a reduced dynamic range accompanied by decreased frequency resolution and highly variable speech recognition. Both of these factors necessitate greater levels of gain and output from a hearing device.5,6 The requirement of higher gain for maximum speech understanding has traditionally meant that this population is relegated to wearing large BTEs powered by 675 batteries, full shell silicone earmolds, and an omnidirectional microphone mode.7 While individual variability is certainly a factor, those with severe-to-profound hearing loss typically prefer lower compression ratios (1.1:1 to 2:1) than might be prescribed using traditional fitting formulas.8 It is also well established that these individuals receive increased objective and subjective benefit from directional microphones.9 What is not known, however, is whether the specific gain, output, and compression needs of those with severe-to-profound hearing loss can be met using a smaller and less obtrusive RIC device.
Considering individuals with severe-to-profound hearing loss in the United States comprise approximately 1% to 2% of the population and make up 10% to 12% of the total number of patients seeking hearing aid-related services, researchers at Starkey Laboratories Inc questioned whether a RIC hearing aid could be customized for use for patients with severe-to-profound hearing impairment.10,11 Historically, this has been a challenging group of patients to fit with power BTEs because of their critical and discerning needs for satisfactory amplification to suit their hearing losses.
An Absolute Option
|FIGURE 1. The Absolute Power RIC with custom receiver.|
Given the potential benefits of high-gain/high-output devices with advanced digital signal processing (DSP), the Absolute Power RIC was created as an option for patients with severe-to-profound hearing loss. This device is a 16-channel hearing instrument based on the S Series platform featuring Drive Architecture DSP and comes equipped with a 131/71 matrix powered by a 312 battery. It also offers adaptive feedback cancellation, environmental classification, noise reduction, and dynamic directionality.
The custom receivers developed for use with this product are manufactured as a deep-canal fitting with a removal nylon thread handle and limited venting. The receiver is attached to the device via a thin cable that runs along the outer contour of the ear and feeds into the body of the processing unit (Figure 1).
The full-on gain curve and OSPL90 response are shown in Figure 2.12 Expected battery life for this product is approximately 8 days. With a broad fitting range, the Absolute Power RIC is designed for individuals who demonstrate audiometric thresholds greater than 70 dBHL as a potential alternative to the larger BTE styles traditionally worn by this population.
|FIGURE 2. The full-on gain curve (in grey) with OSPL90 response (in blue) for the Absolute Power RIC with a 131/71 matrix.|
An evidence-based approach is at the core of every design decision implemented at Starkey Laboratories Inc. As such, an internal clinical trial was conducted to determine whether the Absolute Power RIC might be a viable option for hearing aid users with severe-to-profound hearing impairment. Specifically, the clinical trial attempted to answer three essential questions related to this new product and its target population:
- Will the RIC, using a 312 battery and custom receiver, provide the same or greater gain/output without feedback as compared to the hearing-impaired subjects’ own BTE instruments (typically using a larger 675 battery and full-shell silicone earmold) as determined via real-ear probe-microphone measurements?
- Will advanced features, such as Dynamic Directionality, offer improved speech understanding in noise for this population as measured via speech recognition in noise (ie, HINT)?
- How will hearing-impaired subjects rate the sound quality and performance of the Absolute Power RIC compared to their own hearing aids as used in multiple, daily real-world environments?
Clinical Trial Methods
Subjects. An internal clinical trial occurred over a 6- to 8-week period and included five adults who were all experienced users of traditional power BTE instruments. The average hearing loss across subjects was bilaterally symmetrical, slightly sloping to the higher frequencies, severe-to-profound in degree of loss, and sensorineural in nature (Figure 3). Subjects (one female, four males) ranged in age from 37 to 77 years with a mean age of 55.6. Subjects were fitted bilaterally with the Absolute Power RIC using a 131/71 receiver housed in a custom canal type earmold.
|FIGURE 3. Average hearing loss (with 1 standard deviation) for Absolute Power RIC subjects.|
Procedures. Starkey’s proprietary fitting formula, eSTAT, was used for the first-fit of each test device. The eSTAT fitting formula offers wide dynamic range compression (WDRC) with relatively low compression ratios. The average compression ratios per channel for all subjects are summarized in Table 1 for the initial first-fit compared to final-fit (ie, preferred user settings) for each subject following 6 weeks of wear time. The data were collected during laboratory sessions and field trials that included several objective and subjective outcome measures.
|TABLE 1. Average compression ratios (1 standard deviation = ±0.12 for first-fit and ±0.15 for final-fit) as prescribed by eSTAT per channel for both the initial first-fit and the final-fit (preferred gain settings) after 4 weeks of wear time and three fine-tuning sessions.|
|FIGURE 4. Examples of custom receivers produced for the internal clinical trial of the Absolute Power RIC. Receiver “A” (shown on left) was rejected and not used for data collection due to poor retention within the ear canal. Receiver “B” (shown on right) was remade for the same ear; note the longer canal portion, lack of venting, and added material for improved retention.|
The initial fitting session focused on ensuring that, for each subject, the custom receiver fit comfortably with appropriate retention to reduce the potential for acoustic feedback. The preliminary custom receivers built for subjects (Figure 4a) were found to be inadequate for retention and feedback prevention due to short canal length and venting. The difference between custom receivers “A” (rejected) and “B” (approved) demonstrates that longer canal depth, reduced venting, and increased material at the entrance of the ear canal were crucial components to a proper fit. Another critical element for a successful fitting was to ensure that properly sized receiver cable was selected prior to receiver production. This permitted the cable to lie discreetly against the ear and allowed the device to sit properly and comfortably behind the pinna. Once adequate custom receivers were manufactured using the appropriate cable lengths, the initial fitting sessions were completed (Figure 5).
|FIGURE 5. Sample images of the Absolute Power RIC fittings. A fully occluding custom receiver with the appropriate cable length is critical to provide a comfortable, feedback-free RIC/BTE fitting.|
eSTAT fitting formula. Real-ear probe-microphone measurements were obtained on all the fittings to evaluate the frequency response of the initial prescriptive fit (ie, first-fit) to eSTAT compared to the preferred settings of each set of subjects’ own BTE instruments (Figure 6). Measurements were completed using the Audioscan Verifit system and the recommended male voice (carrot passage). The mean real-ear aided response (REAR) of the Absolute Power RIC generated outputs that were the same or greater than subjects’ own power BTE devices. On average, the RIC first-fit to eSTAT demonstrated an increase in output of between 5 and 10 dB from 1 kHz to 4 kHz when compared to subjects’ own devices.
|FIGURE 6. Average real-ear probe-microphone output measures of the initial first-fit of the Absolute Power RIC (solid curves) compared to subjects’ own BTE hearing aids (dashed curves) in response to recorded speech material (Verifit “Carrot Passage”) presented at 50 dB, 65 dB, 75 dBSPL, and a puretone sweep presented at 85 dBSPL (all measurements recorded using the Audioscan Verifit).|
Following 6 weeks of wearing the RIC, real-ear measures were repeated to document the preferred user settings (ie, final-fit). In Figure 7, the real-ear aided responses for 50 dB, 65 dB, 75 dB, and 85 dBSPL inputs are displayed for both the RIC (solid curves) and subjects’ own BTE hearing aids (dashed curves). Based on subjective assessment of real-world hearing aid performance, subjects requested an average increase in gain for all input levels by approximately 4 to 8 dBSPL. Compared to subjects’ own BTE hearing aids, the final preferred settings for the Absolute Power RIC showed an average increase in gain/output between 1 and 10 dBSPL across the frequency range (for each input level). This suggested that subjects preferred the added stable gain that the Absolute Power RIC provided.
|FIGURE 7. Average real-ear output measures for the final-fit of the Absolute Power RIC (solid curves) compared to subjects’ own devices (dashed curves) in response to recorded speech material (Verifit “Carrot Passage”) presented at 50 dB, 65 dB, 75 dBSPL, and a puretone sweep presented at 85 dBSPL (all measurements recorded using the Audioscan Verifit).|
Speech understanding in noise. To evaluate subjects’ ability to understand speech in noise, the Hearing in Noise Test (HINT) was conducted using 20-sentence lists presented at 0° azimuth with uncorrelated speech-shaped noise generated at 65 dBSPL from seven speakers surrounding the subjects.13 Subjects were tested in three conditions: 1) Using their own BTE hearing aids in omnidirectional mode; 2) Absolute Power RIC hearing aids in omnidirectional mode; and 3) Absolute Power RIC hearing aids in directional mode. Not all of the subjects’ own BTE hearing aids had directional capability, which prevented the testing of that condition.
The average HINT results are shown in Figure 8. The average signal-to-noise ratio (SNR) scores in both omnidirectional conditions were comparable (p=1.000). However, the average SNR score in the RIC directional condition was -0.53 dBSNR, demonstrating a statistically significant difference between both omnidirectional conditions and the directional condition (p<.0003). This result represented an average improvement in speech understanding in the directional mode by 4.75 dB, which is consistent with the reported directional benefit in individuals with mild-to-moderate hearing loss.14,15
|FIGURE 8. Average HINT in noise results for each test condition (with 1 standard deviation).|
Real-world performance. At the start of the clinical trial, each subject completed a custom questionnaire based on experiences with their own power BTE hearing aids. The same outcome measure was repeated at the conclusion of the study based on their trials with the Absolute Power RIC hearing aid. The 14-item questionnaire asked subjects to rate their perceived level of difficulty (using a 5-point scale with 1 indicating a high level of difficulty and 5 representing little difficulty) in various real-world environments. The questionnaire separated the 14 items into three distinct groups: home, work, and social events. Average ratings for each environment (Figures 9 to 11) indicated that the test devices significantly reduced subjects’ perceived level of difficulty in most environments when compared with their own power BTE hearing aids.
|FIGURE 9. Average difficulty ratings (with 1 standard deviation) between own devices (green bars) and the Absolute Power RIC (blue bars) for “Home” environments (n=5 participants). The mean difference between groups is statistically significant (p<.005) for each category (except the “TV-Drama” condition).|
|FIGURE 10. Average difficulty ratings (with 1 standard deviation) between own devices (green bars) and the Absolute Power RIC (blue bars) “Work” environments (n=5 participants). The mean difference between groups is statistically significant (p<.005) for each category (except the “Work-Telephone” condition).|
|FIGURE 11. Average difficulty ratings (with 1 standard deviation) between own devices (green bars) and the Absolute Power RIC (blue bars) for “Social” environments (n=5 participants). The mean difference between groups is statistically significant (p<.005) for each category.|
Results of this clinical trial indicate that the Absolute Power RIC met or exceeded the preferred gain requirements of experienced users of traditional power BTEs as measured by a real-ear probe-microphone system. This additional gain, especially in the high frequencies, could be the result of feedback cancellation, which allowed more stable gain resulting in a wider bandwidth.
Additional high frequency gain may also have been achieved through the use of the custom receiver as opposed to standard earmold with #13 tubing used on power BTE instruments, which can attenuate high frequencies due to tubing length and diameter.16 This added stable gain, combined with the directional system of the Absolute Power RIC, increased speech understanding in noise by an average of 4.75 dB when compared to performance in omnidirectional mode.
These results confirm that patients with severe-to-profound hearing impairment do benefit from directionality under controlled conditions, which translates to a real-world advantage. Finally, subjects rated their ability to communicate at home, at work, and in social situations with the Absolute Power RIC consistently better than their experiences using their own power BTE hearing aids.
The Absolute Power RIC was evaluated with the needs of those with severe-to profound hearing loss in mind. Results demonstrated:
- The RIC instrument provides a feedback-free fitting that can meet or exceed gain/output requirements of traditional power BTE users.
- Individuals with severe-to-profound hearing loss can benefit from directionality with advanced DSP features.
- Subjects consistently rated real-world performance of the RIC device higher than their own power BTE devices.
Given these findings, the clinical trial determined that the Absolute Power RIC is a viable hearing aid option for those with severe-to-profound hearing loss and offers a lightweight, cosmetically appealing alternative to the traditional power BTE.
Thanks to Laurel Gregory and Aaron Schroeder for offering valuable insights into the study design and to all who reviewed this article.
- Kirkwood DH. Bucking bad economic news, hearing aid sales rise by 5.4% on way to record year. Hear Jour. 2007;60(12):11-16.
- Hearing Industries Association. Statistical Reporting Program, First Quarter, 2009. Alexandria, Va: Bostrom Inc; April 13, 2009.
- Kuk F, Keenan D, Sonne M, Ludvigsen C. Efficacy of an open-fitting hearing aid. Hearing Review. 2005;12(2):26-30,32.
- Kochkin S. MarkeTrak VII: Hearing loss population tops 31 million people. Hearing Review. 2005;12(7):16-29.
- Rosen S, Faulkner A, Smith DA. The psychoacoustics of profound hearing impairment. Acta Otolaryngologica (Stockholm). 1990;469[supplement]:16-22.
- Hodgson F, Dillon H. Modified hearing aid gain and frequency response requirements for severe/profound losses. Sydney, Australia: National Acoustic Laboratories; 1989.
- Madell JR, Gendel JM. Earmolds for patients with severe and profound hearing loss. Ear Hear. 1984;5(6):349-351.
- Keidser G, Dillon H, Dyrlund O, et al. Preferred low- and high-frequency compression ratios among hearing aid users with moderately severe to profound hearing loss. J Am Acad Audiol. 2007;18(1):17-33.
- Kühnel V, Margolf-Hackl S, Kiessling J. Multi-microphone technology for severe-to-profound hearing loss. Scand Audiol. 2001;30[Suppl 52]:65-68.
- Kochkin S. Open letter to the hearing health care community. Hearing Review. 2004;11(3):14.
- McClymont LG, Browning GG. Characterization of severely and profoundly hearing impaired adults attending an audiology clinic. J Laryngol Otol. 1991;105:534-538.
- American National Standards Institute (ANSI). American National Standard Specification of Hearing Aid Characteristics (ANSI S3.22-2003). New York: Acoustical Society of America; 2003.
- Nilsson M, Soli SD, Sullivan JA. Development of the Hearing in Noise Test for the measurement of speech reception thresholds in quiet and in noise. J Acoust Soc Am. 1994;95(2):1085-1099.
- Hornsby BWY. Factors affecting the benefit of asymmetric directional fittings. Paper presented at: International Hearing Aid Research Conference; 2006; Lake Tahoe, Calif.
- Ricketts T, Mueller HG. Predicting directional hearing aid benefit for individual listeners. J Am Acad Audiol. 2000;11:561-569.
- Ricketts TA, Bentler RA. Impact of “standard” earmold on RECD. Am J Audiol. 1995;4(1):43-45.
Citation for this article:
Pisa JFD. Power to the people: A RIC for severe-to-profound hearing loss. Hearing Review. 2009;16(10):28-34.