This work presents a numerical study on atmospheric sound propagation over rough water surfaces with the aim of improving predictions of sound propagation over long distances. A method for generating pseudorandom sea profiles consistent with sea wave spectra is presented. The proposed method is suited for capturing the logarithmic nature of the energy distribution of the waves. Sea profiles representing fully developed seas for sea states 2, 3, 4, and 5 are generated from the Elfouhaily et al. (ECKV) sea wave spectra. Excess attenuation caused by refraction and surface roughness is predicted with a parabolic equation (PE) solver. A novel method for estimating equivalent effective impedance based on PE predictions at different sea states is presented. Parametric expressions using acoustic frequency and significant wave height are developed for effective surface impedances. In this work, sea surface roughness is on a scale comparable with the acoustic wavelength. Under this condition, the acoustic scattering is primarily incoherent. This work shows the limitations of using an equivalent surface impedance in such incoherent scattering cases.

This study investigates the characteristics of the sound field during the 2013 and 2014 Southern California Behavioral Response Study. Acoustic data were collected before, during, and after simulated mid-frequency active sonar (MFAS) playbacks. A numerical approach, incremental computation method, is developed to quantify the inter-ping sound field during the simulated MFAS playback. Comparisons are made between inter-ping sound field and natural background in three distinctive frequency bands: low-frequency (4.5 kHz) bands. The results show significantly elevated sound pressure levels (SPLs) in the inter-ping interval of the MFAS-frequency band when compared to natural background levels before and after playbacks. No difference is observed between inter-ping SPLs and natural background levels in the low- and high-frequency bands. In addition, the duration of elevated inter-ping sound field depends on the MFAS source distance. The prolonged elevation of sound field beyond the brief sonar ping is likely due to volume reverberation, although multipath propagation can also be a contributor. This phenomenon underscores the need for further studies on potential acoustic masking to marine mammals by MFAS at distances.

This work presents a case study of atmospheric acoustic transmission loss measurements over a mixed domain of water and sand. The aim of this paper is a detailed description of the measurement system and data handling approach. This effort is part of an ongoing project modeling atmospheric acoustic transmission loss in littoral or riverine environments. The goal of the presented experiment is the evaluation of the excess attenuation introduced by a sandy shore. The site is a small pond with a man-made sandy beach area. An acoustic source is placed on the opposite edge of the pond and three 7-microphone arrays are placed in line at ranges of 126 m (water’s edge), 146 m and 166 m. Data is presented in order to highlight the design of the measurement system and post-processing approach. Sound pressure levels predicted by a numerical model, which is presented in detail in a companion work, are included as an illustrative case.

Dirac points in the band structure of acoustic systems are essential features affording classical analogs of quantum condensed matter states. We show that measured dispersion curves near and at the Dirac point of an acoustic graphene analog can be suppressed by strong variations in the impedance boundary between free field and surface wave regimes under certain conditions. Increased Rayleigh scattering and diffractive excitation are shown to increase the dispersed surface wave pressure amplitude, circumventing the impedance-based wave suppression. The improved excitation and scattering conditions for observing acoustic Dirac points for two samples with two distinct operational frequency ranges are reported.