Adaptive Filter Based Two-Probe Noise Suppression System for Transient Evoked Otoacoustic Emission Detection MIS ˇ KO SUBOTIC ´ , 1 ZORAN S ˇ ARIC ´ , 1 and SLOBODAN T. JOVIC ˇ IC ´ 2 1 Life Activities Advancement Center, Gospodar Jovanova 35, 11000 Belgrade, Serbia; and 2 School of Electrical Engineering, University of Belgrade, Bulevar Kralja Aleksandra 73, 11000 Belgrade, Serbia (Received 24 May 2011; accepted 3 October 2011; published online 19 October 2011) Associate Editor Berj L. Bardakjian oversaw the review of this article. Abstract—Transient otoacoustic emission (TEOAE) is a method widely used in clinical practice for assessment of hearing quality. The main problem in TEOAE detection is its much lower level than the level of environmental and biological noise. While the environmental noise level can be controlled, the biological noise can be only reduced by appropriate signal processing. This paper presents a new two-probe preprocessing TEOAE system for suppression of the biological noise by adaptive filtering. The system records biological noises in both ears and applies a specific adaptive filtering approach for suppression of biological noise in the ear canal with TEOAE. The adaptive filtering approach includes robust sign error LMS algorithm, stimuli response summation according to the derived non-linear response (DNLR) technique, subtraction of the estimated TEOAE signal and residual noise suppression. The proposed TEOAE detection system is tested by three quality measures: signal- to-noise ratio (S/N), reproducibility of TEOAE, and mea- surement time. The maximal TEOAE detection improvement is dependent on the coherence function between biological noise in left and right ears. The experimental results show maximal improvement of 7 dB in S/N, improvement in reproducibility near 40% and reduction in duration of TEOAE measurement of over 30%. Keywords—Otoacoustic emissions, TEOAE, Biological noise, Adaptive noise cancelation, Hearing testing, Signal processing. INTRODUCTION Analysis of otoacoustic emission (OAE) signal gives reliable information on the quality of the cochlear function. For this reason, methods of measurement and analysis of OAE signal have significant application in clinical practice. 19 It was demonstrated that evoked otoacoustic emissions (EOAE) are most effective, particularly transient evoked otoacoustic emission (TEOAE) and distortion product otoacoustic emission (DPOAE). TEOAE is recognized as adequate method for hearing screening. For elicitation of otoacoustic emis- sion it uses a click stimulus of 80–88 dB peak equivalent sound pressure level (SPL). 9 However, the presence of the stimulus artifacts and noise in ear canal, results in difficulty in identifying the true TEOAE signal. To reduce the stimulus artifacts, Kemp 12 proposed a derived non-linear response (DNLR) technique. This technique was considered in this paper. A serious problem in OAE signal measurement is its extremely low level, between 10 and 20 dB SPL. 19 This is significantly under the level of noise which appears in the auditory canal. This noise may be external (ambi- ent noise) and internal (biological noise). Biological noise in the auditory canal is the consequence of the functioning of human organism (heartbeats, breathing, functioning of internal organs, body movements, etc.) which significantly masks OAE signal, particularly at frequencies below 1000 Hz. 14 In one experiment the level of biological noise measured in unclosed auditory canal was 20 dB SPL, for a healthy young individual. 13 Placing the probe which hermetically closes auditory canal increases SPL of the biological noise to 30 dB SPL. 10,25 In order to provide favorable signal-to-noise ratio (S/N) and to improve reliability of OAE detection, different procedures are applied based on improvement of recording conditions, optimization of stimulus sig- nal characteristics, as well as improvement of the algorithms for processing of OAE signal. 2,15,16,18,21 Taking into account that SPL of ambient noise in quiet room is about 40 dB SPL, 11 and that the acoustic Address correspondence to Misˇ ko Subotic´ , Life Activities Advancement Center, Gospodar Jovanova 35, 11000 Belgrade, Serbia. Electronic mail: ifp2@ikomline.net, sariczoran@yahoo.com, jovicic@etf.rs Annals of Biomedical Engineering, Vol. 40, No. 3, March 2012 (Ó 2011) pp. 637–647 DOI: 10.1007/s10439-011-0430-2 0090-6964/12/0300-0637/0 Ó 2011 Biomedical Engineering Society 637