Saltier and colder water lie beneath less salty, warmer water. Because of SOFAR, sound emitted at a certain depth bounces between these various layers and can travel for hundreds of miles. This up-and-down bending of low-frequency soundwaves allows soundwaves to travel great distances without the signal losing significant energy. By placing hydrophones at the axis of the sound channel, researchers can record sounds such as whale calls, earthquakes, and manmade noise occurring vast distances from the hydrophones.
In some instances, low-frequency sounds can be heard across entire ocean basins. Only certain sound waves stay in the sound channel without hitting the ocean surface or seafloor. Sound waves that start upward from the source at steeper angles are still refracted, but not sharply enough to avoid hitting the ocean surface.
Similarly, sound waves that start downward from the source at steeper angles will not be refracted sharply enough to avoid hitting the seafloor.
Sound loses energy whenever it hits the ocean surface or seafloor. Whenever sound reflects from the rough ocean surface or seafloor, some sound energy is scattered and lost. A sound wave that hits the ocean surface or seafloor many times will be too weak to be detected. Sound that does not hit the ocean surface or seafloor will still lose energy to absorption. Low- frequency sounds lose very little energy to absorption , however. The result is that low-frequency sounds that do not interact with the ocean surface or seafloor can be detected after traveling long distances through the ocean.
The amount of absorption increases as the frequency of the sound increases, and higher frequency sounds are therefore only detectable at shorter distances. The distances at which sounds can be detected depend on the frequency, how loud the source is, and how loud the background ambient noise is. Sound waves traveling in the sound channel follow many different paths. When the sound source and receiver are located at the depth of the sound speed minimum, called the SOFAR or sound channel axis , sound waves travel nearly straight down the axis and Cycle one complete vibration of a particle through a wave, e.
Sound channel axis. On the left, sound speed profile from mid-latitudes. On the right are shown only the paths sound travels from a source at m depth to a receiver at m depth that is km away from the source.
Contrast this image with the image toward the top of the page where all paths a sound travels from a sound source are shown. Adapted from Figure 1. Although sound travels away from a sound source in all directions, only sound traveling away from a source on paths that leave the source at specific angles will reach a receiver at a specific location.
The sound waves traveling on these different paths have slightly different travel times. A single explosive source will therefore be heard as a number of separate arrivals, leading to the characteristic signature of a SOFAR transmission building up to its climax:. The final pulse of sound is typically the loudest and comes from the sound wave that travels nearly on the sound channel axis.
Although this sound wave travels the shortest distance, it travels in the region near the sound speed minimum where the sound speed is lowest. The paths that sound will take for a source near the ocean surface are quite different.
If the deep sound channel extends up to the surface, rays that depart from the source nearly horizontally will not hit the ocean surface or seafloor. Sounds that travel on these paths can be detected at long ranges, just as is true for sounds traveling away from a deep source that do not interact with the ocean surface or seafloor.
Sound paths from a source near the surface come together, or converge, creating regions of higher sound pressure at about the same depth as the source every km away from it. These regions of higher sound pressure are called convergence zones. In between the convergence zones, there are regions of lower sound pressure called shadow zones.
On the right are the paths followed by sound waves as they travel away from a sound source located at a depth of 50 m. The rays come back together near the surface at a range of about 55 km, forming a convergence zone.
The rays do not reach the region near the surface between the source and the convergence zone, forming a shadow zone. Search for:. Home Science of Sound Sound What is sound? How do you characterize sounds? Amplitude Intensity Frequency Wavelength How are sounds made? What happens when sound pressures are large?
Sound Movement How fast does sound travel? Why does sound get weaker as it travels? Sound Spreading Sound Absorption How does sound move? Reflection Refraction Scattering Reverberation How does sound travel long distances? An underwater earthquake heard miles away in the eastern Pacific. Sign up for our weekly newsletter. Skip to content. Share this Facebook Twitter Email. Brought to you by The Pulse. The Pulse Go on an adventure into unexpected corners of the health and science world each week with award-winning host Maiken Scott.
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