Åke Olofsson

Dose and time-dependent protection of the antioxidant N-l-acetylcysteine against impulse noise trauma

Noise-induced hearing loss is one of the most common causes of hearing disability, and at present there is no effective biological protection or cure. Firearms and some industrial equipment can generate very high levels of impulse noise, which is known to cause sensorineural hearing loss. It has been shown that antioxidants such as N-L-acetylcysteine (NAC) can protect the inner ear from oxidative damage. The present study investigates whether NAC (i.p.) can protect the cochlea from impulse noise trauma. Rats were exposed to 50 noise pulses at 160 dB SPL peak value. Electrophysiological hearing thresholds were assessed with auditory brainstem response (ABR) up to 4 weeks after noise exposure. Animals exposed to impulse noise, without treatment of NAC, had larger threshold shifts in the frequency range 4-40 kHz than animals injected with NAC. Hair cell loss was significantly reduced using a schedule of three NAC injections in the rats. These results suggest that NAC can partially protect the cochlea against impulse noise trauma.

The efficacy of N-acetylcysteine to protect the human cochlea from subclinical hearing loss caused by impulse noise: A controlled trial

In military outdoor shooting training, with safety measures enforced, the risk of a permanent, noise-induced hearing loss is very small. But urban warfare training performed indoors, with reflections from walls, might increase the risk. A question is whether antioxidants can reduce the negative effects of noise on human hearing as it does on research animals. Hearing tests were performed on a control group of 23 military officers before and after a shooting session in a bunker-like room. The experiments were repeated on another group of 11 officers with peroral adminstration of N-acetyl-cysteine (NAC), directly after the shooting. The measurements performed were tone thresholds; transient-evoked otoacoustic emissions, with and without contralateral noise; and psycho-acoustical modulation transfer function (PMTF), thresholds for brief tones in modulated noise. Effects from shooting on hearing thresholds were small, but threshold behavior supports use of NAC treatment. On the PMTF, shooting without NAC gave strong effects. Those effects were like those from continuous noise, which means that strict safety measures should be enforced. The most striking finding was that the non-linearity of the cochlea, that was strongly reduced in the group without NAC, as manifested by the PMTF-results, was practically unchanged in the NAC-group throughout the study. NAC treatment directly after shooting in a bunkerlike room seems to give some protection of the cochlea.

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

Abstract 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. Sumario La 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 effect of presentation level and compression characteristics on sentence recognition in modulated noise

The effect of fast-acting compression on speech recognitionin fully modulated (FUM) noise in listeners with normaland impaired hearing was investigated in two experiments. We wanted to determine the relationships between the benefitfrom compression and some audiological factors.Furthermore, the sensitivity to changes in compressionparameters was also evaluated. The results showed thattwo-thirds of the listeners performed worse with fast-actingcompression than with linear processing. Normal-hearinglisteners showed the most benefit from compression. A significantrelationship was found between benefit from compressionand speech-to-noise ratio at threshold (SNRT) inslightly modulated (SM) noise. Pure-tone threshold wasfound to be a weak predictor of benefit from compression.No relationship was found between benefit from compressionand the release of masking for the FUM noise. Thevariability in the results across different compressionparameters was related to SNRT in SM noise. The resultssuggest an inverse relationship between benefit from compressionand the severity of the suprathreshold hearing loss. Sumario Se investigó en dos experimentos el efecto de una compresiónde acción rápida sobre el reconocimiento dellenguaje con ruido totalmente modulado (FUM), ensujetos con una audición normal o alterada. Quisimosdeterminar la relación entre el beneficio de la compresióny algunos otros factores audiológicos. Más aún, se evaluóla sensibilidad a los cambios en los parámetros de compresión.Los resultados mostraron que dos tercios de lossujetos mostraron un beneficio negativo de la compresión.Los sujetos normo-oyentes mostraron el mayorbeneficio de la compresión. Se encontró una relación significativaentre el beneficio de la comprensión y larelación señal/ruido a nivel del umbral (SNRT), conruido levemente modulados (SM). Se encontró que elumbral tonal puro fue un débil pronosticador del benefi-cio de la compresión. No se halló relación entre el benefi-cio de la compresión y la liberación del enmascaramientopara el ruido FUM. La variabilidad en los resultadossegún los diferentes parámetros de compresión se relacionócon el SNRT en ruido SM. Los resultados sugierenuna relación inversa entre el beneficio de la compresión yla severidad de la hipoacusia supra-liminar.

Evaluation of the usefulness of a silicone tube connected to a microphone in monitoring noise levels induced by drilling during mastoidectomy and cochleostomy.

Abstract Conclusion: The use of a silicone tube and a microphone enables peroperative noise recordings in cadavers and should be safe to apply in vivo. Our preliminary data indicate that noise levels during inner ear surgery reach levels that can cause noise-induced hearing loss. Our method is easily performed and enables future uncomplicated and safe noise recordings and we suggest future application in vivo to expand knowledge regarding peroperative noise levels. Objectives: To evaluate the safety and utility of a silicone tube connected to a microphone probe in noise recordings during middle ear surgery and to achieve preliminary results regarding drill-related noise levels. Methods: Peroperative noise recordings were obtained during mastoidectomy and at the round window during cochleostomy in a cadaver model by means of a silicone tube connected to a microphone and a Matlab® computer program. Results: Our method enabled recordings of radiated noise levels close to the drill ranging from 84 to 125 dB SPL during drilling in cortical bone and from 85 to 117 dB during drilling in the mastoid cavity. During cochleostomy noise levels ranged from 114 to 128 dB SPL when recordings were made close to the round window. Maximal noise levels were underestimated due to microphone overload above 80 Pa.

Oto-traumatic Effects of Computer Simulated Magnetic Coil Impulse Noise: Analysis of Mechanisms

The brief impulse noise artifacts of 1.0 ms or less generated by some magnetic coils used in extracranial magnetic stimulation may induce acoustic trauma. We investigated the effects of these magnetic coil acoustic artifacts (MCAA) on the inner ear by exposing rabbits to computer stimulated impulse noise designed to mimic the impulse noise of the coil in spectrum and acoustic energy. The simulated impulse noise stimuli (50 impulses) were varied in maximum peak sound pressure (160, 157, and 155 dB re: 20 muPascal), rise-time (100 microseconds and 1,000 microseconds) and duration. The frequency spectrum of the simulated impulse noises were kept constant at 0.5 kHz to 7 kHz with peak energy in the 2-5 kHz range. The results indicated that the simulated magnetic coil impulse noise caused extensive cochlear damage and permanent threshold shifts largely equal to those induced by the MCAA. The MCAA created slightly greater PTS than the simulated impulse of the same peak sound pressure. Each of the 3 experimental stimuli induced similar PTS in the auditory range of 0.5 to 16 kHz, with the higher peak sound pressure stimuli (157 and 160 dB) causing greatest hearing loss. Increasing the rise-time of the simulated brief impulse noise from 100 microseconds to 1,000 microseconds did not reduce the level of PTS significantly. The results suggest that for brief acoustic signals of around 1 ms or less, the peak pressure and spectral content rather than the rise-time and duration were the important factors in the development of noise-induced hearing loss.(ABSTRACT TRUNCATED AT 250 WORDS)

Hearing Aid Measurements with Speech and Noise Signals

An increasing number of hearing aid types include one or more features which are intentionally non-linear. In such devices measurement of frequency response and distortion using sweep tone measurements are typically of little relevance. Five different non-linear hearing aid types were used to evaluate three different broad-band measuring methods. The results revealed that these methods were meaningful in estimating average frequency response obtained with a specific input signal, but none of the three methods used in the study was able to evaluate separately the effects of the most important signal modifications: memoryless non-linearity like peak clipping, time-varying gain from AGC and additive internal noise.

Tinnitus and Other Auditory Problems – Occupational Noise Exposure below Risk Limits May Cause Inner Ear Dysfunction

The aim of the investigation was to study if dysfunctions associated to the cochlea or its regulatory system can be found, and possibly explain hearing problems in subjects with normal or near-normal audiograms. The design was a prospective study of subjects recruited from the general population. The included subjects were persons with auditory problems who had normal, or near-normal, pure tone hearing thresholds, who could be included in one of three subgroups: teachers, Education; people working with music, Music; and people with moderate or negligible noise exposure, Other. A fourth group included people with poorer pure tone hearing thresholds and a history of severe occupational noise, Industry. Ntotal = 193. The following hearing tests were used: − pure tone audiometry with Békésy technique, − transient evoked otoacoustic emissions and distortion product otoacoustic emissions, without and with contralateral noise; − psychoacoustical modulation transfer function, − forward masking, − speech recognition in noise, − tinnitus matching. A questionnaire about occupations, noise exposure, stress/anxiety, muscular problems, medication, and heredity, was addressed to the participants. Forward masking results were significantly worse for Education and Industry than for the other groups, possibly associated to the inner hair cell area. Forward masking results were significantly correlated to louder matched tinnitus. For many subjects speech recognition in noise, left ear, did not increase in a normal way when the listening level was increased. Subjects hypersensitive to loud sound had significantly better speech recognition in noise at the lower test level than subjects not hypersensitive. Self-reported stress/anxiety was similar for all groups. In conclusion, hearing dysfunctions were found in subjects with tinnitus and other auditory problems, combined with normal or near-normal pure tone thresholds. The teachers, mostly regarded as a group exposed to noise below risk levels, had dysfunctions almost identical to those of the more exposed Industry group.

The effect of audibility, signal-to-noise ratio, and temporal speech cues on the benefit from fast-acting compression in modulated noise

The objective of the experiment was to investigate three aspects that might contribute to the benefit of fast-acting compression seen in normal-hearing listeners. Six normal-hearing listeners were tested with speech recognition in a fully modulated noise (FUM) either through a fast-acting compressor or through linear amplification. In the first experiment, three different presentation levels of the FUM noise (15, 30, and 45 dB SL) were tested. The second experiment manipulated the control signal of the compressor independently of the audio input signal at four signal-to-noise ratios (−15, −10, −5, and 0 dB). A signal correlated noise version of the speech signal was tested in the third experiment at three speech-to-noise ratios (−20, −15 and −10 dB). Results showed that performance was better with compression than with linear amplification through all of the tested conditions at least when the signal-to-noise ratio was negative. The results suggest that other aspects of the hearing impairment than those simulated here are involved in the degraded performance seen for some hearing-impaired listeners with fast-acting compression.