Electroacoustic Evaluation of Smartphone-Based Hearing Aid Applications

The National Institute of Deafness and Communication Disorders of the National Institutes of Health under Award Number 5R21DC016241-02.

Stick It in Your Ear: A Systematic Approach to Earmold Selection

How to Improve Audiology Services: The Patient Perspective

Ins, Outs of Frequency-lowering Processing

Results reported from two clinical trials of a real-ear system within a hearing aid

Hearing aid outcomes with open- and closed-canal fittings

When fitting hearing aids, there are three guidelines that should always be followed by the dispenser: restore audibility; so that the amplified sound is above the user's threshold, limit the output; so that the amplified signal does not exceed the user's discomfort level, and do no harm; so that the amplified signal is not unintentionally or undesirably altered by the hearing aid. The first two of these three guidelines are obvious. The first is to present sounds above the user's threshold of hearing in order to create an acoustic environment with the maximum amount of audible speech cues. The second ensures that discomfort level is not exceeded, so that the user does not wince or remove the hearing aids when loud sounds are present. The third guideline—do no harm—is less obvious. This indicates that it is important that the hearing aids produce no alteration of the signal other than that which the fitter intends. This is a subtle way of saying that, among other things, well-fitted hearing aids should not distort the desired sound. The topic of distortion is the subject of this issue of Trends in Amplification. The intention of this issue is to explain why distortion occurs in some hearing aids; to explain how distortion is measured; to summarize what can be done to prevent distortion; and to summarize the perception of distortion by the hearing aid wearer. Distortion in hearing aids can be broadly defined as the generation of undesired audible components that are present in the output, but which are not present in the input. When distortion occurs, hearing aids produce undesired elements at the output through the interaction of the processed signal with some internal non-linear mechanism. These undesired components may interfere to some degree or other with the reception of sound by the listener. If these added elements are small compared to the overall signal level, they may effectively cause no interference at all. If they are large, they can be so disruptive to the listener that the desired sound becomes irritating or even incomprehensible. All audio systems inevitably contain some amount of distortion. The practical problem for the listener is the type of distortion that is present and the level that is acceptable or tolerable in hearing aids before the distortion becomes disruptive to speech intelligibility and sound quality. It is known that highly distorted speech in quiet can remain intelligible (Licklider, 1946); however, as dispensers often experience, intelligibility is not the only measure of whether or not a user will accept and wear hearing aids. Gabrielsson and Sjogren (1979b) have shown that overall perceived sound quality is important to users when selecting a hearing aid. Punch (1978) has shown that listeners with mild to moderate sensorineural hearing losses retain the same ability to make distinctions in sound quality judgments as listeners with normal hearing. This implies that good sound quality is as equally important and desired by listeners with a hearing impairment as it is for listeners with normal hearing. Three characteristics of distortion in hearing aids modify the sound delivered to the user: the type of distortion, the relative amount of distortion at different frequencies, and the variation of distortion with different input levels. Since the most important goal for fitting hearing aids is to restore or facilitate communication ability, undistorted sound is important for optimum speech intelligibility and sound quality. However, it is important to also recognize that there are other reasons to provide undistorted amplification of sound. For example, undistorted and pleasant reproduction of music may be an important sensory experience for some listeners. This issue concentrates on different types of undesired modification to the waveshape of the sound, primarily by total harmonic distortion (THD) and intermodulation distortion (IMD). However, in a more general sense, any undesired component added to the sound by hearing aids is a form of distortion. Thus, this issue also discusses sound generated by hearing aids with no signal present at the microphone. This artifact is more commonly called internal noise. Internal noise fits within the definition of distortion given earlier, since internal noise is an undesired product generated in the output of hearing aids that is not present at the input. In some cases, distortion and internal noise are so closely linked that they cannot always be separated into discrete entities. For example, instability in the frequency and shape of the clock pulses in a Class D output stage (often loosely called clock jitter) results in distortion of the signal. However, this artifact may be audibly revealed in the output sound primarily as noise, rather than as distortion of the waveform. According to this broad definition of distortion, undesired noises such as “motorboating” and other acoustic feedback oscillations can also be considered to be forms of distortion. However, acoustic feedback and other similar audible artifacts occurring within hearing aids will not be discussed here. These types of distortion have been discussed in detail by Agnew (1996b) in a previous issue of Trends in Amplification. In this issue distortion is discussed in two general categories: Undesired modification of the waveform of the incoming sound by some mechanism occurring within the hearing aids. The severity of this effect may or may not vary with the acoustic level of the incoming sound. This type of audible artifact is generally what most dispensers refer to as “distortion”. This will be the subject of Part I of this issue, and will follow the definition used for distortion in this text. The production of undesired audible components in the output sound, regardless of whether or not there is any acoustic input to the hearing aid. This is generally called “noise”. This will be the subject of Part II of this issue. The first part of this issue discusses the causes and effect of distortion. First, different types of distortion are described, followed by an explanation of how they are measured. This is followed by a discussion of the sources of distortion that may be present in hearing aids and the various mechanisms by which distortion is created. The final section of Part I discusses the effects of distortion on the hearing aid wearer. For the reader having difficulty understanding the technical concepts in the first part of this tutorial, a good overview from a different perspective has been presented by Kuk (1996). The second part of this issue discusses the causes and effects of internal noise in hearing aids. This part starts with a discussion of sources of internal noise and continues with a description of methods for the measurement of internal noise. This part concludes with a discussion of the perception of internal noise and its effects on the user of hearing aids. At the end of the text is an intentionally long list of references, which is intended to serve as a reasonably comprehensive resource for the reader seeking further information on distortion and noise in hearing aids.

Programming hearing instruments to make live music more enjoyable

THE ARGUMENT FOR FITTING BIMODALLY If you see a child tomorrow with a hearing loss in both ears, will you recommend one hearing aid or two? The obvious answer is two. You would have a hard time finding a dispensing professional today who does not agree that the benefits of bilateral hearing aid fitting make it the standard of care for those with binaural hearing loss. While the benefits of binaural hearing and the advantage of bilateral fitting are beyond the scope of this article (e.g., see Litovsky et al.,1 Kochkin2), these facts are undisputed in hearing healthcare circles. The industry's confidence in bilateral hearing aids is supported by current trends in fitting. In 1980 only 27% of hearing aid fittings were bilateral.3 Today, it is an amazing 86% for those with binaural hearing loss.4 So, what is bimodal fitting and why should dispensing professionals care? Bimodal fitting means different stimuli are presented to each ear. For the purposes of this paper, it means a cochlear implant in one ear and a hearing aid in the other. But, you may ask, don't cochlear implant audiologists take care of that? The answer is no, at least not usually. Personal experience (first author), communication with cochlear implant audiologists, and the literature5 suggest that most hearing aids in bimodal devices are fitted outside the cochlear implant center. Thus, if you have a patient who receives a cochlear implant in one ear, you will most likely be the one responsible for the continuing care of the hearing aid in the contralateral ear. It is in the best interests of both your patient and you to know how to optimize the hearing aid fitting for the best bimodal performance. If you fit hearing aids on children, the question is not if you will be responsible for managing a child with bimodal devices, but rather when. The number of unilateral cochlear implant recipients who continue to use contralateral hearing aids is clearly increasing (Figure 1). The conventional wisdom that cochlear implants and hearing aids should not be used simultaneously is archaic,6,7as we will show in this paper.Figure 1: Percentage of unilateral cochlear implant users choosing to wear a hearing aid in the contralateral ear. Sources: Tyler et al.,8 Cowan and Chin-Lenn9.BIMODAL DEVICE USE IN CI WEARERS Significant advances over the years in cochlear implant technology, speech-coding strategies, and surgical techniques have resulted in substantial improvements in the auditory-only speech-understanding abilities of cochlear implant recipients.10 As a result, the candidacy criteria approved for cochlear implantation in the United States has progressively expanded. When Cochlear Corporation, Ltd., introduced the original Nucleus® cochlear implant in 1985, the only candidates approved by the Food and Drug Administration were adults with profound bilateral sensorineural hearing loss of post-linguistic origin who had 0% open-set speech recognition using hearing aids. Now, under the FDA criteria approved in 2005, candidates can be adults or children aged 12 months and older, and can have either pre- or post-lingual onset of hearing loss. Although mid- and high-frequency hearing must still be profound (hearing thresholds >90 dB HL), low-frequency hearing loss can be moderate for adults (hearing thresholds >40 dB HL) and severe for children over age 2 (hearing thresholds >70 dB HL). Further, best-aided pre-operative speech-recognition criteria have been raised from 0% to <60%. Figure 2 shows the current criteria for each age group.Figure 2: Current FDA-approved audiometric and speech-recognition criteria for cochlear implantation with the Nucleus device, by age group. (For children, the open-set word-recognition test recommended is the Lexical Neighborhood Test [LNT] or Multisyllabic Lexical Neighborhood Test [MLNT], which are available from www.auditec.com.)For persons with bilaterally profound sensorineural deafness (the purple-shaded area in Figure 2), cochlear implants are clearly the intervention of choice because many obtain little or no benefit from hearing aids. However, for children aged 2 years and up and for adults, there is a range of low-frequency thresholds (the green and yellow areas, respectively) that fall within the approved audiometric range for cochlear implants. Hearing aids often fail to provide adequate performance for these patients,11but a unilateral cochlear implant alone does not provide all the known benefits that arise from listening with two ears rather than one. Binaural benefits from perception of interaural differences in time and intensity are well known to improve speech-recognition performance, particularly in background noise, due to a combination of head shadow, binaural redundancy, and binaural squelch effects (e.g., see Byrne, 198112 for a review). Further, bilateral inputs provide the potential for good localization ability. Finally, a strong argument can be made for bilateral stimulation, especially in children, in light of the impact of auditory deprivation on perception. When a hearing-impaired ear remains unaided, speech-recognition ability in that ear significantly deteriorates over time,13,14 and there appears to be a limited window of opportunity for auditory system stimulation if the patient is to achieve maximal binaural functioning.15 Bilateral implantation is not for everyone. For example, there might be significant usable hearing in one ear. There may be insurance reimbursement or financial barriers. Parents may worry about surgery or preserving one ear for possible future technology or treatments. These concerns may or may not be well-founded. Insurance reimbursement is not the obstacle it once was. Cochlear brand implants are designed to be “backward compatible” so future advances can be applied to implants done today. Cotanche reported that treatment, e.g., hair cell regeneration, may be 20 years or more away.16 However, unilateral versus bilateral implantation in children is ultimately the parents' choice and their wishes must be respected. The less expensive, non-invasive fitting of a hearing aid on the ear contralateral to a cochlear implant allows preservation of hearing in that ear and may provide the benefits of binaural stimulation. SUMMARY OF THE LITERATURE The bimodal fitting approach was first reported in the literature in the early 1990s (e.g., Shallop et al., 199217). Concerns were initially expressed that patients might be unable to combine the two very different sound sources for central processing. Fortunately, this has not proven to be the case. In fact, some researchers have argued that bimodal stimulation may provide “complementary” cues for processing of signals that may be advantageous to speech perception.18 Specifically, the lower frequencies provided by the hearing aid can provide information about the fundamental frequencies of a talker's voice and vowel information, while the mid- and high-frequency information from the cochlear implant can provide information needed on manner and place of articulation of consonants. It has also been suggested that localization ability, sound quality, and music perception may be enhanced by bimodal devices compared with bilateral cochlear implants.19,20 Studies have reported significant speech-recognition improvements for bimodal listening compared to either the patients' pre-operative bilateral hearing aid use or their post-operative use of the hearing aid or cochlear implant alone. This has been shown in adults17,21–23 and in children.24–26 For example, in a study by Luntz et al.,26 12 subjects (3 post-lingually impaired adults and 9 pre-lingually impaired adults and children aged 7 and older) were tested on sentences in noise after 7 to 12 months of using bimodal devices. Both speech (at 55 dB HL) and noise (at 45 dB HL) were presented from a frontal loudspeaker. Average speech-recognition scores were only 12.9% for the hearing aid alone and 60.7% for the cochlear implant alone, but bimodal listening produced an average score of 75.6% correct. Localization abilities have been shown to improve with bimodal devices relative to use of either device alone for some, although not all, adult2728 and pediatric1,24 patients. Many users of bimodal devices have also reported higher levels of satisfaction and perceived benefit than with hearing aids worn pre-implantation, although cosmetic and handling concerns of using the two devices have sometimes been expressed,29 emphasizing the need for sufficient counseling and training. It is also important to consider that children may need more time to learn to use bimodal cues.25 There is debate over the relative effectiveness of bilateral cochlear implants versus bimodal devices. Overall, however, the published literature on bimodal devices has been quite positive (e.g. see Ching et al. for a review18). A judicious approach would be to fit a hearing aid contralaterally to the implant on patients who show sufficient benefit from the hearing aid and are able to use the binaural cues provided. FACTORS IN FITTING THE HEARING AID Certain aspects of the fitting need to be considered and possibly modified for optimal use of bimodal devices. Dispensing professionals who follow proven, evidence-based protocols for hearing aid fitting, however, will require minimal adaptation of their normal procedure. The American Academy of Audiology has published a Pediatric Amplification Protocol and all professionals dispensing hearing aids to children should be familiar with it.30 Optimization of the hearing aid in bimodal fittings essentially requires three steps. First, the cochlear implant map must be stable. You will need to communicate with the cochlear implant audiologist to know when this has been accomplished. Second, a frequency response should be selected for the hearing aid that will provide the best speech intelligibility. This is established by starting with a hearing aid that has been fitted and verified using a prescriptive formula. While the first author has had success using NAL-NL1,31 and Ching recommended it as an optimal starting point,32 those who are proficient with DSL[i/o]33 or another validated prescriptive approach should not be discouraged from using it as the starting point. From the initial prescription, two alternate frequency responses should be programmed into the hearing aid and adjusted for equal loudness. This is easy in multiple-memory digital hearing aids. As the limits of the hearing aid permit, program one should be the selected prescriptive formula frequency response. Program two should have 6-dB per octave less amplification in the low frequencies (-6 dB at 1000 Hz, -12 dB at 500 Hz, and -18 dB at 250 Hz). Program three should have 6-dB per octave more amplification in the low frequencies (+6 dB at 1000 Hz, +12 dB at 500 Hz, and +18 dB at 250 Hz). Once the programs are established, the child should listen to connected discourse while the audiologist switches between programs to determine which one provides the clearest speech. This can be done by playing a recorded story or watching a child-friendly video. The cochlear implant should be turned off during this frequency response selection process. Ching reported that this procedure is appropriate for children as young as 6 years.32 For younger children, the professional may choose to default to the prescriptive response. Finally, the third step in the fitting protocol is to match overall loudness between the hearing aid and cochlear implant. Both the implant and the aid are turned on and the child is asked to report if the hearing aid is louder or softer than the cochlear implant. The aid is then adjusted accordingly. This can also be done while the child listens to a recorded story or watches a video. A chart like that in Figure 3 can be helpful for this task. Some children might experience loudness discomfort from amplification. If so, Ullauri et al. suggest starting with a lower volume setting on the hearing aid and raising it over time as acclimatization occurs until the level of balanced loudness is achieved.34Figure 3: Loudness balancing scale. Source: Cochlear in-house material.A flow chart for fitting the hearing aid in bimodal devices is shown in Figure 4. This recommended protocol has been validated in children and found to provide good binaural benefits.24 For the reader wishing more in-depth training, a tutorial is available at www.cochlearcollege.com. Ching et al. have also published excellent articles on fitting and adjusting the hearing aid for children wearing bimodal devices.2,35Figure 4: Optimizing the hearing aid in bimodal fitting. Source: Cochlear in-house material.CONCLUSIONS The use of bimodal devices is the recommended treatment option for children who meet cochlear implant candidacy but who either have some usable hearing in one ear or for other reasons get only one implant. Bimodal devices can be a successful alternative to bilateral hearing aids or to one cochlear implant alone. It is important to remember these three vital rules: (1) Work with the implant center to make sure the implant map is stable. (2) Fit the hearing aid frequency response for maximal speech intelligibility. (3) Balance the loudness with the cochlear implant and hearing aid. Bimodal fitting can provide optimal use of the different, but potentially complementary, bilateral cues provided by the acoustic amplifier and the electric stimulation from the implant.

Multichannel Compression Hearing Aids: Perceptual Considerations

Frequency-Lowering Hearing Aids: Verification Tools and Research Needs

Fitting hearing aids involves innovation and compromise. This is especially true in deciding the tightness of the fit, whether of an earmold or a custom product. Tight-fitting instruments are good for obtaining high levels of gain and output, especially in the lower frequencies, and for minimizing feedback problems. However, they often are uncomfortable, sometimes create an unacceptable occlusion effect, and can give the patient a “plugged up” sensation. If the patient has normal or near-normal hearing in the lower frequencies, it is common practice to move toward a more open fitting to alleviate occlusion and let low frequencies into the ear naturally. In this case, the compromise is reduced gain and output, and a greater chance of unwanted acoustic feedback. There is little documentation of the first open-canal (OC) fittings, but we know they received considerable attention immediately after the CROS-type hearing aid was introduced.1 It was quickly discovered that the open earpieces (or tubing only) used for the CROS hearing aid also could be used for ipsilateral high-frequency amplification (hence, the term IROS [ipsilateral routing of signals], referring to a large vent). In the late 1960s, researchers showed that OC fittings could provide useful high-frequency amplification.2-4 In the ensuing decades, there wasn’t much excitement about open fittings, though there was some renewed interest during the Libby horn’s peak of popularity5 and on other occasions when a product designed specifically for high-frequency hearing loss was introduced.6 The ho-hum attitude toward OC fittings that persisted for more than 30 years was stirred up considerably a few years ago when five key aspects were combined in a single OC product: a small “cute” BTE casing, multichannel compression and gain adjustments, a thin tube channeling the sound from the hearing aid to the ear, a comfortable nonoccluding eartip, and, most importantly, effective feedbackreduction algorithms. As reviewed by Mueller,7 these five factors combine to offer a wide range of potential patient benefits, and OC products have rapidly taken over a sizable share of the total hearing aid market. While the peer-reviewed journals have published little about these modern OC fittings, several articles about this style have appeared in the trade journals, and the OC product is a hot topic at hearing aid workshops and seminars. However, there are still some key issues related to these products that need further clarification, and maybe even a couple areas where misconceptions exist. In this paper, we’ll provide some tips on OC fittings, which we hope will clarify more than they confuse.

The use of thin-tube open-ear fitting has become increasingly popular in the last few years. This type of fitting offers improvements in the wearers’ perception of their own voice, especially for people with a mild hearing loss or a highfrequency hearing loss. It also provides an instant fit, is easy to demonstrate, and improves the appearance of behind-the-ear (BTE) hearing aid to wearers. Both the acceptance rate and the return rate of these models are more favorable than the average for hearing aids overall. Although the openness of the fitting may reduce the potential improvement in signal-to-noise ratio (SNR) offered by a directional microphone, Kuk et al. have shown that an open-fit directional microphone provides an SNR improvement of 2-3 dB over the same hearing aid in its omnidirectional microphone mode. 1 On the other hand, thin-tube open-ear fittings may compromise the audibility of the high frequencies for some wearers. 2 In this paper, we will review some of these potential compromises and discuss why frequency transposition may be a solution for the limitations. We will also show that frequency transposition improves the perception of everyday sounds and may enhance consonant recognition for some people fitted with thin-tube openear devices. ISSUES WITH THIN-TUBE, OPEN-EAR FITTINGS

Inability to understand speech in noise has been cited repeatedly as the principal complaint of hearing aid users. While data exist documenting the benefit provided by hearing aids with directional microphones when listening to speech in noise, little work has been done to develop a standard clinical protocol for fitting these hearing aids. Our goal was to evaluate a clinical measure of the acoustic directivity of a directional hearing aid, including its association with a test of speech perception in noise. The performance of two commercially available directional behind-the-ear (BTE) hearing aids was evaluated using the Hearing in Noise Test (HINT) and the Real Ear Aided Response (REAR) on 24 adult participants with symmetric, mild to moderately severe, sensorineural hearing loss. The HINT was conducted with the speech signal presented from 0 degrees and the noise from 180 degrees and either 135 degrees or 225 degrees, depending on the ear tested. REAR was measured at the above three angles using swept pure tones, and these measures were used to compute in situ directivity for each subject and hearing aid. Directional benefit for the HINT was greatest when noise was presented from the azimuth of the published polar diagram null of a given hearing aid in its directional mode (180 or 135/225 degrees). The only significant correlation between HINT and REAR results, however, was found when the noise source was at 180 degrees. These results confirm the validity of using real ear measures as a way to assess directionality in situ, but also indicate the complexity of predicting perceptual benefit from them. These data suggest that factors beyond acoustic directionality may contribute to improvement in speech perception in noise when such improvements are found.

Beyond BTEs: Earmold Opportunities in Clinical Practice