On-site non-destructive measurement of termite activity using the spectral kurtosis and the discrete wavelet transform q Juan José González de la Rosa a,b,⇑,1 , Antonio Moreno-Muñoz a,c , Antolino Gallego d , Rosa Piotrkowski d,e , Enrique Castro d a Research Group PAIDI-TIC-168: Computational Instrumentation and Industrial Electronics (ICEI), Spain b University of Cádiz, Area of Electronics, EPSA, Av. Ramón Puyol S/N, E-11202 Algeciras, Cádiz, Spain c University of Córdoba, Area of Electronics, Campus de Rabanales, Leonardo da Vinci building, E-14071 Córdoba, Spain d University of Granada, Dept. of Applied Physics, Research Group PAIDI-TEP-232: Numerical Methods and Signal Processing in Dynamical Systems, Spain e University of San Martin, School of Science and Technology, Argentina article info Article history: Received 26 February 2009 Received in revised form 15 April 2010 Accepted 25 August 2010 Available online 17 September 2010 Keywords: Acoustic emission Discrete wavelet transform Higher-order statistics Insect detection Spectral kurtosis Time–frequency analysis Transient detection abstract In this paper we present the operation results of a portable computer-based measurement equipment, conceived to perform non-destructive testing of suspected termite infestations. Its signal processing nucleus is the spectral kurtosis (SK), whose pattern allows the target- ing of alarms and activity signals. Data have been also de-noised using the discrete wavelet transform (DWT) in order to study its potential complementarity to SK. The DWT keeps the successive approximations of the termite emissions, supposed more non-Gaussian (less noisy) and with less entropy than the detail coefficients. For a given mother wavelet, the maximum acceptable level in the wavelet decomposition tree, which preserves the insects’ emissions features, depends on the comparative evolution of the approximations vs. details’ entropies, and the value of the global SK associated to the approximation of the separated signals. The paper explains the detection criterion by showing different types of real-life recordings (alarms, activity, and background). Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Termites’ activity transients are of high complexity, which in fact is increased by the anisotropic media through which they propagate. Consequently, getting a complete characterization of the vibratory-acoustic emission, may imply the selection of the appropriate model, which best explains the processes of generation, propagation and cap- ture of the emitted signals. Due to the difficulties of modeling these emissions, non-parametric methods for pest targeting have been developed in order to settle the operating fundamentals of field instruments. These methods are mainly based in the power spectrum analysis [1,2]. This paper deals with the performance of a final-version equipment (a signal processing unit based in a laptop com- puter), whose previous prototype’s performance, based in the time–frequency domain analysis of the kurtosis, was described in [1,3]. In this final version, the measurement method is mainly based in the interpretation of the spec- tral kurtosis (SK) graph; the complementary wavelet anal- ysis, is designed to help separate termite emissions from the noisy background, supposed with more entropy, sim- plifying even more the detection criterion. At the same time, we use a simple data acquisition unit, the sound card of a laptop computer (maximum sampling speed at 44,100 Hz), which simplifies the hardware unit. 0263-2241/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2010.08.009 q This work was funded and supported by the Andalusian Government towards the Research Unit PAIDI-TIC-168. ⇑ Corresponding author at: University of Cádiz, Area of Electronics, EPSA, Av. Ramón Puyol S/N, E-11202 Algeciras, Cádiz, Spain. Tel.: +34 956028020; fax: +34 956028001. E-mail address: juanjose.delarosa@uca.es (J.J.G. de la Rosa). URL: http://www.uca.es/grupos-inv/TIC168/ (J.J.G. de la Rosa). 1 Main Researcher of the Research Unit PAIDI-TIC-168. Measurement 43 (2010) 1472–1488 Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement