A novel real-time noise reduction system for the assessment of evoked otoacoustic emissions

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Abstract

A novel noise reduction method and apparatus to be used in connection with the measurement of evoked otoacoustic emissions (EOAE) are presented. The noise reduction method is based on an adaptive noise canceller and requires a noise-only reference microphone placed in the vicinity of the OAE-probe. The method was implemented in real time on a custom built digital signal processing system using an Analog Devices ADSP-2181 digital signal processor. The system interfaces seamlessly with a commercial EOAE acquisition system. Results of a series of experiments show than noise reductions of 7–8 dB can be reached.

Introduction

Evoked otoacoustic emissions (EOAE) are low-level acoustic signals produced by the normally hearing human inner ear [1]. They were first described in 1978 by Kemp [2]. Today, they are used for a number of clinical applications (see [3] for a recent review). Transient evoked otoacoustic emissions (TEOAE) can be measured in the majority (>99%) of normally hearing ears or in ears with a relatively unimportant hearing loss up to approximately 20 dB hearing level (dB HL), but not in ears with a more pronounced hearing loss in excess of approximately 20 dB [4], [5], [6]. Therefore, the main clinical application of EOAE is to differentiate between no or very mild and more pronounced peripheral hearing losses, especially in patients with limited cooperation, such as children or neonates. The attractiveness of EOAE-measurements for clinical purposes stems from its non-invasiveness, its short measuring time (often less than 5 min, including set-up) and the objective nature of its results.

Although there are several methods to measure EOAE, only two of them are widely used in clinical practice [7]: TEOAE and DPOAE (distortion product otoacoustic emissions). In both techniques, a measurement probe containing a sensitive microphone and one or two miniature loudspeakers is inserted into the outer ear canal (cf. Fig. 1). Stimuli — clicks in TEOAE and two sine waves of different frequencies in DPOAE — are emitted through the output transducer(s) and the resulting otoacoustic emissions (OAE) are recorded with the probe microphone (see e.g. [3], [7] for a detailed description). As the OAE are rather low in level (typically up to 20 dB sound pressure level (dB SPL) [7]), the recorded OAE are typically averaged over 260 measurements (TEOAE) or several seconds (DPOAE) in order to improve signal-to-noise ratio (SNR).

Because of the inherently low levels of the recorded OAE, external noise can considerably reduce the usefulness of the method [8]. A number of noise sources have been identified, including equipment noise and noise from unrelated sources [9]. One way to reduce the effect of external noise is to ensure proper sealing of the ear probe [9]. Other methods to eliminate or at least to attenuate external noise, e.g. from unrelated equipment or other persons present in the room, may not be easy to implement, especially in neonatal screening programs, where special rooms for the assessment of OAE may not be practical or easily available. One method to reduce the effect of external noise is to reject time-segments with low SNR, detected by their high total signal level. Such an algorithm is implemented in the ILO88 (Otodynamics, Hatfield, UK) OAE-system [10]. However, as more noisy segments are excluded at higher noise levels, measurement of EOAE may take very long or may even be completely blocked.

In this report, a novel noise reduction scheme for the measurement of EOAE is described. It may be useful to conduct OAE measurements in noisy surroundings as often encountered in clinical settings. As a second application, the noise reduction system may be helpful as a tool for research into the effect of noise on OAE. It is known that noise, which is applied to one ear, has the effect of decreasing the amplitude of EOAE measured at the contralateral ear [11], [12]. However, mainly because of the lack of an appropriate noise reduction system, hitherto little is known about the effect of noise on the level of OAE in the ipsilateral ear.

Section snippets

Description of the noise reduction system

Fig. 1 shows a schematic diagram of an OAE-system using the proposed noise reduction system. A computer based OAE-acquisition system emits either clicks (TEOAE) or pairs of pure tones (DPOAE) via a signal generator/amplifier and a loudspeaker in the ear probe. The ear probe, only slightly larger than a commercially available earphone, contains the loudspeaker(s) and a sensitive microphone and is fitted tightly into the outer ear canal of the ear to be assessed. The microphone in the ear probe

Real-time realization of the noise reduction system

A prototype of the noise reduction system was realized in real time using a custom built digital signal processing (DSP) system built around an Analog Devices ADSP-2181 processor capable of performing up to 33 mega instructions per second (MIPS). The system has been designed to directly interface with the Otodynamics OAE-system consisting of the ILO92 hardware and ILO88 software (both from Otodynamics, Hatfield, UK; see [10] for a detailed description of the system).

Fig. 3 shows a photograph of

Performance of the noise reduction system

From a different area of application of adaptive noise reduction systems, the noise reduction for hearing aids [15], [16], it is known that the performance of adaptive noise reduction systems is usually a complex function of a number of parameters, including reverberation, the spectral qualities of the noise, the number and placement of noise sources, as well as the delay in the target signal path and the length of the adaptive filter. Therefore, the dependence on the performance of the

Discussion

The results of the experiments indicate that a noise reduction of 7–8 dB can be realized using the proposed noise reduction system, as indicated by the plots of test time versus noise level in Fig. 5, Fig. 6. The estimate of the response level — if present — can be performed up to higher noise levels and is not changed significantly by the proposed noise reduction system. In the normally hearing ear, wave reproducibility is generally higher in noise as must be expected from a noise reduction of

Summary

A novel adaptive noise reduction for the assessment of EOAE in noisy environments has been presented. The system requires an additional microphone to serve as a noise-only reference and a prototype real-time system has been realized. A series of 135 measurements using a normally hearing and a sensorineurally hearing impaired ear suggest that noise reductions of 7–8 dB can be achieved in an experimental but realistic setting.

Acknowledgements

We wish to thank Prof. Dr R. Häusler and Ms E. Clamann for their helpful comments and their assistance in preparing this paper.

Martin Kompis received his Diploma and doctoral degree in Electrical Engineering from the Swiss Federal Institute of Technology (ETH), Zurich in 1989 and 1993, and his diploma in Medicine and MD degree from the University of Zurich in 1994 and 1995, respectively. He is currently the Head of Audiology Department at the University-ENT-clinic of Berne, Switzerland. His research interests include biomedical acoustics and signal processing.

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Martin Kompis received his Diploma and doctoral degree in Electrical Engineering from the Swiss Federal Institute of Technology (ETH), Zurich in 1989 and 1993, and his diploma in Medicine and MD degree from the University of Zurich in 1994 and 1995, respectively. He is currently the Head of Audiology Department at the University-ENT-clinic of Berne, Switzerland. His research interests include biomedical acoustics and signal processing.

Markus Oberli received his Diploma in Electrical Engineering from the Berner Fachhochschule, University of Applied Sciences, Burgdorf, Switzerland in 1999. During his studies, he worked on the development of noise reduction systems for the measurement of otoacoustic emissions. He is now with Disetronic Medical Systems AG, Burgdorf, Switzerland.

Urs Brugger received his M.S. and Ph.D. degrees in electrical engineering from the Swiss Federal Institute of Technology, Zürich, Switzerland, in 1975 and 1982, respectively. Since 1982, he is the Professor of Electrical Engineering at the Berner Fachhochschule, University of Applied Sciences, Burgdorf, Switzerland. His present interests are in the field of active and digital filters, signal processing, and the application of signal processing techniques to biological systems.

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