Unilateral Amplification Research Articles

Hearing in Noise With Unilateral Versus Bilateral Bone Conduction Hearing Aids in Adults With Pseudo-conductive Hearing Loss.

The role of bilateral bone conduction amplification in patients with bilateral conductive hearing loss is unclear because cranial attenuation is usually considered negligible, and both cochleae can be stimulated with similar efficacy by each device. The aim of the study was to determine if bilateral bone-conduction hearing aids can improve hearing in noise in a homogeneous group of normal-hearing subjects with bilateral pseudo-conductive hearing loss. Prospective, comparative. Department of Communication Sciences and Disorders in University of Haifa, Israel. Department of Communication Sciences and Disorders in University of Haifa, Israel. Induction of bilateral pseudo-conductive hearing loss of more than 35 dB using earplugs and earmuffs. Hearing quality on blinded comparison of unilateral versus bilateral amplification with bone-conduction hearing aids under different locations of noise. Unilateral and bilateral amplification had similar efficacy when both signal and noise were presented from the front. However, bilateral amplification was significantly better when signals were presented from the front and noise was presented from both sides (SNR -10: 92% vs. 84%, p = 0.001; SNR -15: 84% vs. 78%, p = 0.005). Analysis of subject responses on blinded questionnaires revealed that 81% found hearing easier with bilateral amplification. Noise was reported to be more disturbing with unilateral amplification by 55% of the subjects and with bilateral amplification, by 9%. Bilateral amplification with bone conduction devices can improve understanding in noise in the binaural squelch setting. Subjective improvements with bilateral bone-conduction aids included better sound quality and reduced noise disturbance. These findings are consistent with the binaural mechanism of spectral release from masking.

Hearing-aid use and its determinants in the UK National Health Service: A cross-sectional study at the Royal Surrey County Hospital

Objectives: The aim of this study was to investigate the rate of and factors contributing to non-adherence to hearing-aid use in the UK National Health Service. Design: A cross-sectional postal questionnaire survey. Study sample: A questionnaire, including the International Outcome Inventory for Hearing Aids, was sent to all patients fitted with hearing aids at the Royal Surrey County Hospital between 2011 and 2012 (N = 1874). A total of 1023 questionnaires were completed and returned (response rate of 55%). Results: A total of 29% of responders did not use their hearing aids on a regular basis (i.e. used them less than four hours per day). Non-regular use was more prevalent in new (40%) than in existing patients (11%). Factors that reduced the risk of non-regular use included bilateral versus unilateral amplification, and moderate or severe hearing loss in the better ear. 16% of responders fitted with bilateral amplification used only one of their hearing aids. Conclusions: The level of non-regular use of hearing aids in NHS found in this study was comparable to those for other countries. Additional support might be needed for patients at a higher risk of non-regular use.

Brainstem processing following unilateral and bilateral hearing-aid amplification

Following previous research suggesting hearing-aid experience may induce functional plasticity at the peripheral level of the auditory system, click-evoked auditory brainstem response was recorded at first fitting and 12 weeks after hearing-aid use by unilateral and bilateral hearing-aid users. A control group of experienced hearing-aid users was tested over a similar time scale. No significant alterations in auditory brainstem response latency or amplitude were identified in any group. This does not support the hypothesis of plastic changes in the peripheral auditory system induced by hearing-aid use for 12 weeks.

Hearing aid fitting and unilateral auditory deprivation: behavioral and electrophysiologic assessment

The phenomenon of Late-Onset Unilateral Auditory Deprivation was first reported in 1984. However, a high number of unilateral hearing aid fittings are still carried out in cases of bilateral hearing loss, justified by non-auditory factors such as cost, vanity, misinformation and public health policies. ObjectiveTo carry out behavioral and electrophysiological assessment of the auditory performance of adults using unilateral amplification compared with individuals exposed to bilateral symmetric auditory stimulation. MethodThirty five adults, all with symmetric bilateral sensorineural hearing loss, regular users of unilateral hearing aid, bilateral hearing aids and not users of hearing aids, were assessed on behavioral and electrophysiological tests. ResultsVariance analysis revealed that in the unilaterally fitted group, P300 latency was significantly greater in ears with auditory deprivation compared with those fitted with the hearing aid (p < 0.05). This same group also had poorer performance on the Sentence Recognition Test in Noise held in free field. ConclusionThese results corroborate findings in the literature showing that unilateral auditory deprivation can lead to physiological and perceptual changes.

Pathways

Figure: iStockphoto.comFigure: Frank Musiek, PhD, Pathways EditorHearing healthcare professionals often emphasize periphery function when prescribing amplification, paying little consideration to central auditory function and dysfunction. Immersed in treating hearing sensitivity, they may fail to consider central auditory processing even though it is well known that the auditory system, from the outer ear through the auditory cortex, is a major contributor to effective auditory processing. This focus, however, can lead to missing a phenomenon known as binaural interference, which may be why bilateral amplification is ineffective in some patients.Figure: Jennifer B. Shinn, PhDIt has been standard practice for nearly 50 years to fit hearing-impaired patients with bilateral amplification whenever possible. (J Am Acad Audiol 2005;16[8]:574.) Hearing healthcare providers also routinely counsel patients on the benefits of binaural amplification, emphasizing improved localization and the ability to hear with background noise. We tell patients that two hearing aids are better than one, but is this always the case? Most clinicians would say yes, and perhaps it is better peripherally, but that is not always true with central auditory function. BILATERAL VS. UNILATERAL AMPLIFICATION Patients experience binaural interference in their better ear when bilateral performance is poorer than unilateral performance. One study showed binaural interference in eight percent to 10 percent of the elderly population. (J Am Acad Audiol 1993;4[2]:122.) Other investigators have also confirmed those results, and agreed that binaural amplification in cases of binaural interference is contraindicated. (J Am Acad Audiol 2001;12[5]:261 and 2005;16[8]:574; J Basic Clin Physiol Pharmacol 2007;18[3]:201; Int J Audiol 2010;49[9]:613.) Investigators evaluating patients using the QuickSIN test, which measures the ability to hear in noise, and the dichotic digits test, which is a central auditory test that assesses binaural integration skills, found that older patients performed better when listening to speech recognition in noise with unilateral rather than bilateral amplification. (See FastLinks.) (J Am Acad Audiol 2005;16[8]:574.) This underscores the need for using a variety of listening environments when evaluating the benefits of binaural amplification. Another study found that binaural amplification may not always be beneficial for speech recognition in noise in elderly patients. (J Basic Clin Physiol Pharmacol 2007;18[3]:201.) The authors systematically studied the benefits of bilateral versus unilateral amplification to compare unilateral versus bilateral speech recognition in noise and to investigate the relationship between monaural and binaural hearing aid performance and central auditory function. They concluded that bilateral amplification may not provide the best speech recognition in noise for elderly patients. This finding was made by observing behavior and through objective electrophysiological measures. (J Am Acad Audiol 1993;4[2]:122.) Binaural interference should also be considered in pediatric fittings. One study on a child who demonstrated asymmetric word recognition found that he began to demonstrate adverse behaviors when bilaterally amplified. (J Am Acad Audiol 2007;18[6]:515.) The researcher documented binaural interference when patients were aided unilaterally (90% vs. 36%) and bilaterally (56%). A more recent investigation demonstrated that early intervention using amplification may improve localization in children with unilateral hearing impairment, but older children experienced interference and may not benefit. (J Am Acad Audiol 2010;21[8]:522.) ASSESSING AUDITORY PROCESSING Binaural amplification may help patients in quiet situations, but a degree of binaural interference may occur with significant background noise. Clinicians should be aware that this may be attributed to a central, not peripheral, phenomenon. These sometimes subtle deficits in auditory function may cause ineffective amplification in binaural integration tasks, such as dichotic digit tests and listening in noise. Given these findings, we must go beyond the pure-tone audiogram during fittings, and look at other measures such as auditory system stress. Clearly we need more education and training on the importance of evaluating the entire auditory system, not just the periphery. A recent survey found that while didactic courses in auditory processing have improved, only a limited number of clinicians perform these assessments. (J Am Acad Audiol 2007;18[5]:428.) Even more alarming is that the majority of respondents reported spending only an average of one hour a week assessing central auditory function. Training and education are needed to change this trend. It is critical that clinicians thoroughly understand the auditory system and the role it plays in everyday practice. Clinicians who do not specialize in auditory processing may want to consider routinely screening for central auditory processing deficits in patients with symmetrical hearing. This will generate appropriate recommendations for the best course of treatment. FastLinks Check out HJ's article on dichotic digits tests at http://bit.ly/HJdichotic. Read past Pathways columns in a special collection at http://bit.ly/PathwaysCollection. Check out HJ's R&D Blog at http://bit.ly/HJblogRD. Click and Connect! Access the links in The Hearing Journal by reading this issue on thehearingjournal.com. Comments about this article? Write to HJ at [email protected]. Follow us on Twitter at twitter.com/hearingjournal and like us on Facebook at www.facebook.com/HearingJournal.

Successful and unsuccessful users of bilateral amplification: Differences and similarities in binaural performance

Bilateral amplification seems to be the best solution for bilaterally hearing-impaired persons. Nevertheless, some individuals are unsuccessful with this strategy. The goals of the present study were to develop tests to improve the diagnostic test battery before rehabilitation of hearing-impaired persons with bilateral or unilateral amplification, and to evaluate the tests with normally-hearing subjects and with two groups of hearing-impaired persons. The latter two groups contained 11 successful and 11 unsuccessful users of bilateral amplification respectively. Hearing thresholds, speech recognition in noise, signal analysis ability, binaural abilities, and dichotic tests were used in the investigation. The subjects answered a questionnaire and hearing aid gain curves were measured. The results for the two groups were similar for peripheral hearing functions and binaural performance. The unilateral amplification group showed significantly worse results in speech-in-noise and dichotic tests. Spatial aspects within the questionnaire were correlated to amplification preference. We therefore suggest the inclusion of speech-in-noise, dichotic tests, and questions on spatial orientation into the diagnostic scheme before rehabilitation with hearing aids.SumarioLa amplificación bilateral parece ser la mejor solución para las personas con daño auditivo bilateral. Sin embargo, algunos individuos no tienen éxito con esta estrategia. Las metas del presente estudio fueron desarrollar pruebas para mejorar la batería diagnósticas de exámenes antes de la rehabilitación de personas hipoacúsicas con amplificación unilateral o bilateral, y evaluar las pruebas en personas con audición normal, y con dos grupos de personas con daño auditivo. Los últimos dos grupos incluían 11 usuarios exitosos y 11 usuarios no exitosos de amplificación bilateral, respectivamente. Se utilizaron en esta investigación umbrales auditivos, reconocimiento de lenguaje en ruido, capacidad de análisis de la señal, capacidades binaurales y pruebas dicóticas. Los sujetos contestaron un cuestionario y se midieron las curvas de ganancia para auxiliares auditivos. Los resultados para los dos grupos fueron similares para las funciones auditivas periféricas y para el desempeño binaural. El grupo de amplificación unilateral mostró resultados significativamente peores en las pruebas dicóticas y de lenguaje en ruido. Los aspectos espaciales dentro del cuestionario se correlacionaron con las preferencias en la amplificación. Sugerimos, por tanto, la inclusión de pruebas dicóticas, de lenguaje en ruido y preguntas sobre orientación espacial en el esquema diagnóstico antes de la rehabilitación con auxiliares auditivos.

The Benefits of Bilateral Versus Unilateral Amplification for the Elderly: Are Two Always Better than One?

The use of bilateral hearing aids is based on the assumption that the human auditory system functions best when both ears receive incoming acoustic information. There is evidence, however, that some elderly individuals perform better while using unilateral as opposed to bilateral amplification. The main objective of the present study was to compare speech recognition in noise in elderly hearing-impaired patients initially fitted with bilateral hearing aids while they used unilateral versus bilateral amplification. A secondary goal was to investigate the association between performance with one versus two hearing aids and central auditory function as measured by a dichotic test, and to evaluate the effect of increasing age on these two measures. Twenty-eight patients (mean age 72.8 years, range: 62-86) with bilateral symmetrical mild-to-severe sensorineural hearing loss fitted with digital hearing aids, participated in the study. Speech recognition in noise was assessed in three conditions: (1) aided right ear, (2) aided left ear, (3) aided bilaterally, using the AB open-set monosyllabic word test at a signal-to-noise ratio of +10 dB. Speech stimuli were presented at 70 dB SPL via a loudspeaker located at 0 degree azimuth and the noise was presented via a second loudspeaker located at 180 degrees azimuth. In addition, dichotic listening abilities were evaluated using the threshold-of-interference test. The results of the present study indicated comparable mean group performance while using unilateral versus bilateral amplification. For the most of the patients (71%), however, speech recognition in noise was better while using unilateral amplification to the 'better' ear compared to bilateral amplification. While the performance in the dichotic test was not correlated with speech recognition in noise with bilateral versus unilateral amplification, the performance in these two tests deteriorated significantly with increasing age. Our results suggest that for elderly patients, bilateral amplification may not always be advantageous for speech recognition in noise. As most patients continue to use bilateral amplification, it is clear that there are listening situations in which bilateral amplification provides benefit. Sensitive performance measures that will tap critical aspects of daily binaural listening situations are therefore required. Finally, the deterioration in performance of the aging auditory system underscores the need to 'tailor' auditory habilitation to individual needs.

Binaural Interference in a Child: A Case Study

A case study of a child (KB) who demonstrated binaural interference is reported. KB wore unilateral amplification from 1.6 to 4.6 years of age, at which time word-recognition scores under phones were markedly asymmetric, reflecting significantly better performance for the aided ear than the unaided ear, despite similar unaided pure-tone sensitivity. Suspecting the asymmetry in word-recognition performance might be the result of auditory deprivation, bilateral amplification was prescribed at 4.6 years of age. Three months later, adverse changes in the child's behavior were reported. At 5.3 years, significant interaural asymmetry was noted in word-recognition scores under phones, in unilateral-aided word-recognition scores (90% vs. 36%), and in the bilateral-aided score (56%), supporting the presence of binaural interference.

Bilateral or unilateral amplification: Is there a difference? A brief tutorial

“… the basic question for the clinician faced with providing benefit to the aurally impaired individual is not whether two ears are better than one, but rather, ‘Are two hearing impaired ears coupl...

Effects of bilateral versus unilateral hearing aid fitting on abilities measured by the Speech, Spatial, and Qualities of Hearing scale (SSQ)

The Speech, Spatial and Qualities of Hearing Scale (Gatehouse & Noble, ) was applied to three independent clinical groups: 144 people prior to being fitted with amplification; 118 people with six months experience with unilateral amplification; and 42 people with six months experience with bilateral amplification. For traditional speech hearing contexts (one-on-one, in groups, in quiet, in noise) there was benefit with one aid, and no further benefit with two. By contrast, hearing speech in demanding contexts (divided or rapidly switching attention) showed benefit with one aid and further benefit with two. In the spatial domain, directional hearing showed some benefit with one hearing aid, and particular further benefit in distance and movement discrimination from fitting with two. There was some benefit from unilateral fitting for elements of the qualities domains (clarity, naturalness, recognisability, segregation of sounds), with no consistent sign of further benefit from two. Bilateral fitting added benefit with respect to listening effort. Two hearing aids offer advantage in demanding and dynamic contexts; these contexts are argued as significant in the maintenance of social competence and emotional wellbeing. The present results go toward establishing the real-world advantages of bilateral hearing aid fitting and suggest that previous, inconclusive clinical findings reflect inquiry limited to more traditional areas of hearing function.SumarioSe aplicó la escala de cualidades auditiva, espacial y del lenguaje (Gatehouse & Noble, ) a tres grupos clínicos independientes: 144 personas, antes de adaptarles auxiliares auditivos; 118 personas con 6 meses de experiencia con amplificación unilateral y 42 personas con 6 meses de experiencia con amplificación bilateral. Se encontró beneficio de la amplificación unilateral en el contexto de habla tradicional (uno a uno, en grupo, en silencio y en ruido) y no hubo beneficio posterior en el modo bilateral. En contraste, bajo condiciones de discriminación del lenguaje en contexto demandante (atención dividida o cambio de rápido de atención) hubo beneficio con un solo un auxiliar, y mayor beneficio con dos. En el dominio espacial, hubo cierto beneficio en la audición direccional con un solo auxiliar auditivo y particularmente resultó benéfica la condición bilateral en la discriminación de distancia y movimiento. Hubo cierto beneficio de la condición unilateral para elementos de cualidad (claridad, naturalidad, fácil discriminación, segregación de sonidos) sin signos consistentes de beneficio con amplificación bilateral. La amplificación bilateral benefició el menor esfuerzo para escuchar. Hay ventajas con dos auxiliares auditivos en contexto demandante o dinámico; estos contextos son considerados como significativos en el mantenimiento de la competencia social y el bienestar emocional. Estos resultados se encaminan a establecer las ventajas de la amplificación bilateral en el mundo real y sugieren que los hallazgos clínicos previos e inconclusos, reflejan información limitada a áreas más tradicionales de la función auditiva.

Fitting strategies and candidature criteria for unilateral and bilateral hearing aid fittings

In this paper, hearing aid fitting strategies are reviewed and candidature criteria for unilateral and bilateral fittings are discussed. Fitting strategies are primarily governed by the objectives of hearing aid fitting that can be set in different ways. Based on common goals, fitting strategies can be broken down into three stages: (1) basic fitting, (2) fine-tuning and (3) verification and validation. A fine-tuning concept basing on so-called meta-controllers is described and the issue of bilateral fitting strategies is discussed. In a first order approach, bilateral hearing aids can be fitted in parallel as unilateral aids, followed by bilateral fine-tuning, making sure that loudness is balanced and overall loudness is accepted. Parallel fitting of bilateral aids is more favourable than sequential fitting to avoid rejection because of a negative experience with unilateral amplification due to difficulties in disadvantageous listening situations. Candidature criteria are presented by means of a flow chart structuring the decision making process and arriving at the conclusion that bilateral fitting should be considered the normal case. If there is any doubt about the benefit of bilateral fitting, a trial with bilateral aids should be performed.

Unilateral versus Bilateral Amplification for Adults with Impaired Hearing

This study compared unilateral and bilateral aided speech recognition in background noise in 28 patients being fitted with amplification. Aided QuickSIN (Quick Speech-in-Noise test) scores were obtained for bilateral amplification and for unilateral amplification in each ear. In addition, right-ear directed and left-ear directed recall on the Dichotic Digits Test (DDT) was obtained from each participant. Results revealed that the vast majority of patients obtained better speech recognition in background noise on the QuickSIN from unilateral amplification than from bilateral amplification. There was a greater tendency for bilateral amplification to have a deleterious effect among older patients. Most frequently, better aided QuickSIN performance was obtained in the right ear of participants, despite similar hearing thresholds in both ears. Finally, patients tended to perform better on the DDT in the ear that provided less SNR loss on the QuickSIN. Results suggest that bilateral amplification may not always be beneficial in every daily listening environment when background noise is present, and it may be advisable for patients wearing bilateral amplification to remove one hearing aid when difficulty is encountered understanding speech in background noise.

Interaural asymmetry of hearing loss, Speech, Spatial and Qualities of Hearing Scale (SSQ) disabilities, and handicap

A series of comparative analyses is presented between a group with relatively similar degrees of hearing loss in each ear (n = 103: symmetry group) and one with dissimilar losses (n = 50: asymmetry group). Asymmetry was defined as an interaural difference of more than 10dB in hearing levels averaged over 0.5, 1, 2 and 4kHz. Comparison was focused on self-rated disabilities as reflected in responses on the Speech, Spatial and Qualities of Hearing Scale (SSQ). The connections between SSQ ratings and a global self-rating of handicap were also observed. The interrelationships among SSQ items for the two groups were analysed to determine how the SSQ behaves when applied to groups in whom binaural hearing is more (asymmetry) versus less compromised. As expected, spatial hearing is severely disabled in the group with asymmetry; this group is generally more disabled than the symmetry group across all SSQ domains. In the linkages with handicap, spatial hearing, especially in dynamic settings, was strongly represented in the asymmetry group, while all aspects of hearing were moderately to strongly represented in the symmetry group. Item intercorrelations showed that speech hearing is a relatively autonomous function for the symmetry group, whereas it is enmeshed with segregation, clarity and naturalness factors for the asymmetry group. Spatial functions were more independent of others in the asymmetry group. The SSQ shows promise in the assessment of outcomes in the case of bilateral versus unilateral amplification and/or implantation.

Patient satisfaction with bilateral bone-anchored hearing aids: the Birmingham experience

The Birmingham bone-anchored hearing aid (BAHA) programme has fitted more than 300 patients with unilateral bone-anchored hearing aids since 1988. Some of the patients who benefited well with unilateral aids and who had used bilateral conventional aids previously applied for bilateral amplification. To date, 15 patients have been fitted with bilateral BAHAs. The benefits of bilateral amplification have been compared to unilateral amplification in 11 of these patients. Subjective analysis in the form of validated comprehensive questionnaires was undertaken. The Glasgow benefit inventory (GBI), which is a subjective patient orientated post-interventional questionnaire developed to evaluate any otorhinolaryngological surgery and therapy was administered. The results revealed that the use of bilateral bone-anchored hearing aids significantly enhanced general well being (patient benefit) and improved the patient's state of health (quality of life). The Chung and Stephens questionnaire which addresses specific issues related to binaural hearing was used. Our preliminary results are encouraging and are comparable to the experience of the Nijmegen BAHA group.

Speech intelligibility with bilateral bone-anchored hearing aids: the Birmingham experience

The Birmingham bone-anchored hearing aid (BAHA) programme, since its inception in 1988, has fitted more than 300 patients with unilateral bone-anchored hearing aids. Recently, some of the patients who benefited extremely well with unilateral aids applied for bilateral amplification. To date, 15 patients have been fitted with bilateral BAHAs. The benefits of bilateral amplification have been compared to unilateral amplification in 11 of these patients who have used their second BAHA for 12 months or longer. Following a subjective analysis in the form of comprehensive questionnaires, objective testing was undertaken to assess specific issues such as 'speech recognition in quiet', 'speech recognition in noise' and a modified 'speech-in-simulated-party-noise' (Plomp) test. 'Speech in quiet' testing revealed a 100 per cent score with both unilateral and bilateral BAHAs. With 'speech in noise' all 11 patients scored marginally better with bilateral aids compared to best unilateral responses. The modified Plomp test demonstrated that bilateral BAHAs provided maximum flexibility when the origin of noise cannot be controlled as in day-to-day situations. In this small case series the results are positive and are comparable to the experience of the Nijmegen BAHA group.

Horizontal localization and speech intelligibility with bilateral and unilateral hearing aid amplification: Localización horizontal y discriminación del lenguaje con adaptación unilateral y bilateral de auxiliares auditivos

Speech intelligibility and horizontal localization of 19 subjects with mild-to-moderate hearing loss were studied in order to evaluate the advantages and disadvantages of bilateral and unilateral hearing aid (HA) fittings. Eight loudspeakers were arranged in a circular array covering the horizontal plane around the subjects. Speech signals of a sentence test were delivered by one, randomly chosen, loudspeaker. At the same time, the other seven loudspeakers emitted noise with the same long-term average spectrum as the speech signals. The subjects were asked to repeat the speech signal and to point out the corresponding loudspeaker. Speech intelligibility was significantly improved by HAs, bilateral amplification being superior to unilateral. Horizontal localization could not be improved by HA amplification. However, bilateral HAs preserved the subjects' horizontal localization, whereas unilateral amplification decreased their horizontal localization abilities. Front-back confusions were common in the horizontal localization test. The results indicate that bilateral HA amplification has advantages compared with unilateral amplification.SumarioSe estudió la inteligibilidad y la localización sonora horizontal en 19 sujetos con hipoacusia de nivel superficial a medio, para evaluar las ventajas y desventajas de la adaptación bilateral y unilateral de auxiliares auditivos (HA). Se colocaron en circulo ocho bocinas que cubrian el piano horizontal de los sujetos. A través de una bocina seleccionada al azar, se trasmitieron señales lingüisticas contenidas en una prueba de frases. Simultáneamente las otras siete bocinas emitian ruido del mismo espectro de duración que la señal lingüistica. Se les pidió a los sujetos que repitieran la señal lingüistica escuchada y que señalaran la bocina que la emitía. La inteligibilidad del lenguaje fue mucho mejor con la adaptación binaural de auxiliares auditivos que con la unilateral. No hubo mejoria en la localización horizontal con la adaptación de auxiliares auditivos. Sin embargo, la adaptación binaural preservó la localización horizontal, mientras que la adaptación unilateral disminuyó esa habilidad. La confusión entre frente y detrás es común en la prueba de localización horizontal. El resultado indica que la adaptación bilateral de auxiliares tiene ventajas sobre la adaptación unilateral.

The time course of effects on intensity discrimination following monaural fitting of hearing aids

Systematic effects on intensity discrimination occur in listeners who have experienced long-term unilateral amplification [Robinson and Gatehouse, J. Acoust. Soc. Am. 97, 1183–1190 (1995)]. For a bandlimited tone complex centered at 3 kHz, the normally aided ear showed better discrimination performance than the unaided ear at sound-pressure levels greater than 85 dB, yet was worse than the unaided ear at lower levels. No effects were found at 0.25 kHz. In the present study, intensity discrimination was investigated prospectively for five listeners with bilateral, sensorineural, hearing impairment following provision of unilateral amplification. The stimuli were the same as those used previously. Weber functions for both ears were similar at both frequencies prior to fitting the aid, and 4 to 12 weeks postfitting. From 15 weeks, the normally aided ear showed better discrimination performance than the unaided ear at 3 kHz for a sound-pressure level of 95 dB. No effects were observed at 0.25 kHz, or for the unaided ear. Hence, changes in intensity discrimination occur in the normally aided ear following long-term exposure to the frequency-gain characteristics of unilateral amplification. Further research should examine the generality of these results to other listeners, and show whether it extends beyond intensity discrimination to other psychoacoustic capabilities.

A nondegenerate electron beam parametric amplifier

In a non-degenerate electron beam parametric amplifier (EBPA), signal and idler frequencies are far apart. This contrasts with the more familiar degenerate type which uses a pump frequency approximately equal to twice the signal frequency, thus placing the idler near the signal. The non-degenerate EBPA shares with its better-known degenerate counterpart some desirable properties, such as unilateral amplification and the ability to take high overload; in addition, it is a single-channel device. On the other hand, it is more difficult to construct.

The Three-Level Solid State Traveling-Wave Maser*

Broadband very-low-noise microwave amplification can be obtained from solid state maser action in a propagating microwave structure. Such a traveling-wave maser produces unilateral amplification with a high degree of gain stability. The theory of the traveling-wave maser is developed and used to compare the gain, bandwidth and gain stability of the traveling-wave maser with that of the cavity maser. The general requirements for traveling-wave maser slow-wave structures are discussed. Theoretical analysis and experimental results are presented for the comb-in-wareguide slow-wave structure. A traveling-wave maser consisting of a ruby-loaded comb structure was tested. A gain of 23 db at 6 kmc with a bandwidth of 25 mc was obtained. Further performance characteristics of this amplifier and one using gadolinium ethyl sulfate are given. Experimental verification of the low noise temperature of solid state masers was obtained.