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Underwater Acoustics

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Underwater Acoustic Signal Processing

Part of the book series: Modern Acoustics and Signal Processing ((MASP))

Abstract

The topic of this chapter is underwater acoustics, with an emphasis on the aspects important to signal processing. The inherent assumption of distance in remote sensing applications implies the most important aspect of underwater acoustics is how propagation affects the signal of interest. Several topics related to acoustic propagation in the ocean are covered, including time- and frequency-domain characterizations of the wave equation, the propagation loss term in the sonar equation, and the effects of source motion, refraction and boundary reflection on an acoustic wave. The properties of ambient noise relevant to sonar-equation analysis are described, including which sources dominate different frequency regimes. The target strength term in the sonar equation and target impulse response are defined in terms of the scattered response of a signal from an object of interest, including an explanation of how the scattering depends on the acoustic wavenumber of the sensing system and the size of the object (i.e., ka). Finally, the reverberation level term in the sonar equation and a statistical and spectral characterization of reverberation are presented.

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Notes

  1. 1.

    In this text the only portion of the pressure measurement considered is that arising from acoustic signals; static pressure is assumed to be constant over time and removed during sensing.

  2. 2.

    The acoustic pressure is assumed to be on-average zero in order to simplify various results in later sections; it may not be strictly accurate in all cases, especially if T is not overly large.

  3. 3.

    Particle oscillation in the same direction as a wave is traveling defines a longitudinal wave. When an acoustic wave travels through the ocean bottom, it may also include a transverse wave with particles oscillating in the perpendicular direction.

  4. 4.

    These units are codified by international standard in [17, Sect. 3.3.1.4].

  5. 5.

    Note that the propagation loss factor is one over the propagation factor (i.e., L p = 1∕F MP) in the notation of [9, Sect. 3.2.2.1].

  6. 6.

    Nepers are the units of the natural logarithm of a ratio of a measured (i.e., field) quantity such as pressure, so 1 Np = 20log10(e) dB.

  7. 7.

    Sound speed and density values for the various examples in this section are from [9, Sect. 4.4]; for example, medium clay has c b = 0.9846c w and ρ b = 1.331ρ w. The acoustic impedances are computed assuming c w = 1500 m/s and ρ w = 1027 kg/m3.

  8. 8.

    Note that the square root of (3.140) is also commonly defined as the Rayleigh parameter.

  9. 9.

    Note that (3.158) differs from that presented in [5, eq. 3.83] by the − 4π scale noted in Sect. 3.2.3.1.

  10. 10.

    Because for real x > 0, where J ν(x) and Y ν(x) are Bessel functions of the first and second kind, respectively.

  11. 11.

    A property of the Sturm-Liouville system eigenvalues.

  12. 12.

    Note that some radar texts (e.g., [53, Sect. 5.2]) differentiate RCS from scattering cross section by restricting RCS to the polarization the antenna can sense.

  13. 13.

    The rectangular pulse function is one when the magnitude of the argument is less than 0.5 and zero otherwise; see Sect. 4.2.1.

  14. 14.

    This description describes the general trend of G t; most objects will exhibit nulls and peaks with frequency.

  15. 15.

    This essentially requires a flat target spectral response over the frequency band of the source waveform.

  16. 16.

    As described in [52, Sec. 2.3.2], scattering strength does not have units of dB relative to a unit volume or area.

  17. 17.

    This distinction was not made for volume reverberation, although it is relevant.

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  1. Ellicott City, MD, USA

    Douglas A. Abraham

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Abraham, D.A. (2019). Underwater Acoustics. In: Underwater Acoustic Signal Processing. Modern Acoustics and Signal Processing. Springer, Cham. https://doi.org/10.1007/978-3-319-92983-5_3

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  • DOI: https://doi.org/10.1007/978-3-319-92983-5_3

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