By Dr. Peter Dahl
Principal Engineer, Applied Physics Laboratory and
Associate Professor, Mechanical Engineering
University of Washington, Seattle WA
A hydrophone, or underwater microphone was installed on Ocean Watch in order to provide the Around the Americas team with a simple, uncomplicated way to record and study the underwater sound during the voyage. Sound provides one of the most important ways to study the underwater environment.
Before we discuss the hydrophone measurements, we will provide some background information about the basic properties of sound and why it is used to study the oceans. Sound is generally used to study the underwater environment because water is far more absorptive of light than air, whereas sound travels well in water. Sound is a mechanical wave that is produced when a sound source sets the surrounding molecules (of gas, solids, or liquids) into motion. In fluids, the speed at which sound travels depends on the compressibility and density of the medium through which it is traveling. In sea water, however, this translates to sound speed being dependent on temperature, salinity and depth. Sound can be used ‘passively’, where someone only listens to the sounds around them, or ‘actively’, where someone generates a sound and then listens to echoes produced when the sound is scattered or reflected by underwater objects or fish. In most settings, the underwater environment will always have some kind of ambient or background noise – whether due to man-made sources such as boat engines, or due to natural sources such as marine mammals or snapping shrimp.
One of the science goals for Around the Americas was to obtain measurements of ambient noise in ports-of-call, and other protected and sheltered waters as the team circumnavigates the continents of North and South America. It is of interest to understand the variation of underwater sound levels with latitude as it is influenced by snapping shrimp populations that are more prevalent within tropical latitudes. Snapping shrimp are crustaceans and one of their most interesting characteristics is the sound a single shrimp can produce when its claw is snapped shut at very high speed. The ambient noise environment can often dominated by the chorus of sounds from many shrimp.
As Ocean Watch entered lower latitudes, beginning with a stop in the port of Charleston S.C., interesting sound measurements, likely associated with snapping shrimp either from the genus Synalpheus or Alpheus, began to emerge. In particular, the trend that emerged was a change in the ambient noise level with latitude, which is illustrated in the following two figures. Figure 1 shows the locations of the various ports-of-call and Figure 2 shows the sound pressure spectrum (showing the different frequency contributions to the overall sound) for three low-latitude ports compared with measurements made in the port of Barrow, Alaska. The horizontal-axis corresponds to sound frequency, plotted logarithmically; the vertical-axis shows the sound level observed for a given frequency, expressed in decibels (dB), which is also a logarithmic scaling. Sound frequencies within the nominal range 2 – 20 kHz (red, dashed line) are characteristic of the sound spectrum produced by snapping shrimp. The spectral estimates were made by Robert Burns, an undergraduate research fellow at the University of Washington Applied Physics Laboratory. Go to the following link (MP3 file) to listen to a sampling of the sounds from snapping shrimp recorded on October 16th 2009 by Dr. Michael Reynolds from the Ocean Watch while it was in Charleston Heritage Marina , South Carolina. This location is close to the 33rd parallel-or the approximate northern boundary of snapping shrimp.
As the Around the Americas cruise team enters more tropical waters we anticipate the ambient sound levels in the 2-20 kHz range to further increase, and then eventually to decrease once again as they enter high latitudes in the southern hemisphere.
For more information about snapping shrimp and underwater acoustics, please go to the ‘Discovery of Sound in the Sea’ project web site (http://www.dosits.org) developed by the University of Rhode Island’s (URI) Office of Marine Programs (OMP) in partnership with Marine Acoustics, Inc. of Newport, RI.
Fig. 1 Ports within which the ATA team has measured underwater ambient noise.
Fig. 2. Pressure spectrum of ambient noise from the last three lower-latitude ports, compared with one from Barrow, Alaska. (The peak in the Barrow spectrum is possibly machinery noise.) Sound frequencies within the nominal range 2 – 20 kHz (red, dashed line) are characteristic of the sound spectrum produced by snapping shrimp.