Sound waves moving through water may encounter differences in densities and sounds speeds because of numerous objects, inhomogeneities, and rough boundaries (seafloor and sea surface). These scatter the sound energy in many directions.
Reverberation is the total of all scattering received at a particular location. The reverberation is typically heard as a long, slowly decaying sound with several sharper peaks caused by backscattering from stronger scatterers. Many acoustic instruments seek to make use of the direct return from an object, the echo or backscatter. In active sonar, reverberation can be a limiting factor in the detection of the echo signal as the reverberation can be more intense than the returning echo and the ambient noise (see sonar equation page). Reverberation alters characteristics of the original signal and it tends to persist over time intervals longer than the duration of the original signal. The following sections describe the three types of reverberation.
Suspended particles and inhomogeneities of the ocean water cause scattering. Scatterers can be small, like suspended sediment, bubbles, or plankton, or larger, like fishes or whales. In most ocean environments, marine life is the primary source of volume reverberation. The degree of scattering depends on the wavelength of the sound relative to the size of the particle, the particle density, and particle shape. Inhomogeneities are fluctuations in salinity, density, or temperature that cause scattering. These can be ocean fronts, eddies, or stratified layers of the ocean with a density gradient at the boundary on large or small scales.
Volume reverberation usually decreases with increasing ocean depth, because at greater depth fewer particles exist and density gradients and other inhomogeneities are reduced. One exception is the Deep Scattering Layer (DSL), a concentrated layer of marine organisms that creates strong scattering that can sometimes resemble surface scatter. For more information, please visit the page on sound scattering layers.
Sea Surface Reverberation
A rough sea surface can be a highly effective but also highly variable scatterer. This roughness can have scales ranging from millimeter ripples to large storm waves. Scattering at the sea surface is complex due to the variability of wind and waves and the presence of bubbles. This scattering depends on the angle at which the sound approaches the sea surface, the wind speed, the wavelength, and the presence of bubbles. Greater scattering generally occurs at smaller wavelengths and higher wind speeds.
In polar regions, sea ice may cause reverberation levels as much as 40 dB greater than in an ice-free environment. The roughness of the underside of the ice is a significant factor affecting the reverberation. For more information, please visit the page on how sea ice affects sound travel.
Bottom reverberation is another type of surface reverberation but is more complicated to predict than sea surface reverberation. Bottom reverberation is dependent on the angle of incidence, the sound wavelength, the surface roughness, seafloor type (rock or sediment, and composition), sediment layering, grain size, and grain size distribution. The relationship between grain size and reverberation is unclear in many cases; surface roughness may be a better indicator of scattering strength. When the grain size of the bottom is representative of sand (0.0625 to 2 millimeters), individual grains may act as scatterers along with the surface as a whole at some frequencies. Understanding bottom reverberation may also be complicated by additional factors, such as porosity and sound transmission into the seafloor and scattering from subsurface sediment layers.
Reverberation in Shallow Water
As a sound propagates in shallow water, it interacts multiple times with the ocean surface and bottom, creating reverberation throughout the water column. Marine organisms and suspended sediments are generally more abundant in shallow water. For these reasons, reverberation levels tend to be higher in shallow water than in the open ocean. Received levels of reverberation in shallow water can be greater than natural ambient noise levels. The high levels of reverberation in shallow water can limit the performance of active sonars. Reverberation is also a consideration in understanding how impulsive sounds in shallow water affect marine life. Reverberation can also limit the ability of passive acoustics to detect marine life when monitoring anthropogenic activities like pile driving or seismic airgun surveys.
Additional links on DOSITS
- Science of Sound > How do you characterize sound?
- Science of Sound > Sound Movement > How fast does sound travel?
- Science of Sound > Sound Movement > Why does sound get weaker as it travels?
- Science of Sound > Sound Movement > How does sound move?
- Science of Sound > Advanced > Cylindrical vs. Spherical Spreading
- Science of Sound > Advanced > How does sound travel in shallow water?
- Science of Sound > Advanced > SONAR equation
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