Proc. of the International Conference on Computer & Communication Engineering 2014 (ICCCE 2014) 23-25 September 2014, Kuala Lumpur, Malaysia Underwater Acoustic Noise Characteristics Of Shallow Water In Tropical Seas Ahmad Zuri bin Sha'ameri 1 ,Yasin Yousif Mohammed 1,2 , Nor Hisham bin Khamis 1 1 Faculty of Electrical Enginnering, Universiti Teknologi Malaysia, 81310, Skudai, Johor, Malaysia. 2 College of Engineering, University of Mustansiriyah, Baghdad, Iraq. Abstract— In the underwater communication and target locating, the biggest challenge is to reduce the effect of underwater acoustic noise (UWAN). An experimental model is presented in this paper for the noise characteristics of the acoustic underwater channel in shallow water. Data is measured from different depths located in the Tanjung Balau, Johor, Malaysia. Most applications assume that the noise is additive and Gaussian. However, the UWAN is not just thermal noise but a combination of turbulence, shipping and wind noises. Thus, it is appropriate to assume UWAN as colored rather than white. Site-specific noise, especially in shallow water often contains significant non-Gaussian components. The real-time noise data are analyzed for different depths to determine the statistical properties such as power spectral density (PSD), autocorrelation function and probability density function (pdf). The results show the UWAN has a non-Gaussian pdf, and is colored with a power spectral density that decays at a rate of approximately 20 dB/decade. Also, the power decreases with increasing depth as the distance from the surface at approximately 10 dB. Keywords—Underwater communications, Underwater acoustic noise, color noise, non-Gaussian statistics, non-white statistics, noise distribution. I. Introduction The capability to communicate and perform target locating efficiently underwater has important applications, including oceanographic studies, offshore oil prospecting, and defense operation[1]. In general, electromagnetic signals which are used for terrestrial wireless communication (e.g., cellular phones) are not suitable because they are highly attenuated in underwater. On the other hand, sound waves propagate very well underwater and potentially provide the best solution for underwater communication. The main challenge is to find the usable frequency band for sound (around 10kHz) and the mitigate various disturbances in the form of noise generated by both natural (seismic, wind, etc.) and manmade (shipping, other machinery noises)[2]. The underwater environment consists of ambient noise and site-specific noise[3]. Ambient noise is always present in the background of the deep sea. Site-specific noise for example exists for ice cracking in polar region and acoustic noise due snapping shrimp in warmer waters. Other sources are from turbulence, breaking waves, rain, and distant shipping. While ambient noise is often approximated as Gaussian, in practice it is colored exhibiting a decaying power spectral density (PSD). The rate of decay is at approximately 18 dB/decade[3]. Noise observed on site has significant non-Gaussian components[3]. As the attenuation of sound in the ocean is a frequency-dependent process, the ocean acts as a low-pass filter for ambient noise. The ambient noise PSD is thus usually (1/f n ) where noise has more power at lower frequencies and less power at higher frequencies[4]. Several papers show that the noise in underwater acoustic communications does not follow the normal distribution. The actually probability density function (PDF) with extended tails shaped characterizes for this type of noise, emphasizing impulsive behavior due to the high incidence of large amplitude noise events [5-7]. The main goal of this paper is to determine the statistical properties of underwater acoustic noise. The paper is organized as follows: in Section II, a brief introduction of characteristics of ambient noise, the statistical properties are described in Section III, results explain in section IV and Conclusions are summarized in section V. II. Underwater Acoustic Noise Characteristics of underwater acoustic noise (UWAN) in the ocean have been well defined [5]. There are four components (outlined below), where each has a dominating influence at the different portions of the frequency spectrum. The contributions of the major noise sources can be expressed through empirical formulae, which provide power spectral densities of each source relative to frequency f [kHz] in [dB re μ Pa per Hz][6, 7].The power spectrum due to turbulence, shipping, wind and thermal noise are expressed as: N t (f) = 17 − 30 log f . (1) N s (f) = 40 + 20(s − 5) + 26 log f − 60 log(f + 0.03) . (2) N w (f) = 50 + 7.5w 1/2 + 20 log f − 40 log(f + 0.4). (3) N th (f) = −15 + 20 log f . (4) where f is the frequency in kHz. The total noise power spectral density for a given frequency f [kHz] is then: S xx (f) = N t (f) + Ns(f) + N w (f) + N th (f) . (5) Fig.1 shows the empirical noise power spectrum densities in deep water for different conditions of shipping activities