JOURNAL OF SOUND AND VIBRATION Journal of Sound and Vibration 290 (2006) 519–523 Short Communication Coherence measurement for early contact detection between two components B.C.B.N. Suryam à , K.K. Meher, Jyoti K. Sinha, A. Rama Rao Vibration Laboratory, Reactor Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Received 26 February 2004; received in revised form 6 April 2005; accepted 20 April 2005 Available online 12 July 2005 Abstract In process and power plants, there are several structural components which may come in contact with the neighbouring components over a period of time. For example, the coaxial tubes often used for conveying hot fluids, where the detection of contact between the tubes at an early stage is important for safety and performance considerations. The paper presents the observation on the coherence behaviour for the detection of early contact based on the simple laboratory experiments. r 2005 Elsevier Ltd. All rights reserved. 1. Introduction In vibration analysis, the ordinary coherence between two vibration signals is defined as [1] Cohðg 2 Þ¼ jS xy ðoÞj 2 S yy ðoÞS xx ðoÞ , (1) where S xx ðoÞ and S yy ðoÞ are the power spectral densities of two signals, xðtÞ and yðtÞ, and S xy ðoÞ is their cross-power spectrum at an angular frequency ðoÞ. The coherence between two signals indicates the degree to which two signals are linearly correlated at a given frequency. In experimental modal analysis [2], coherence is a well-known tool for assessing the linear correlation between the structural responses xðtÞ due to a given input excitation yðtÞ to the structure when ARTICLE IN PRESS www.elsevier.com/locate/jsvi 0022-460X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsv.2005.04.013 à Corresponding author. Tel.: +91 22 2559 1501; fax: +91 22 2550 51 51. E-mail address: vilred@magnum.barc.ernet.in (B.C.B.N. Suryam).