Currents are commonly measured with sound. There are several different ways to measure currents with sound. An instrument called an Acoustic Doppler Current Profiler or ADCP is often used to measure the current in specific places like shipping channels, rivers and streams, and at buoys. They are also called Acoustic Doppler Profilers (ADP). ADCPs can be placed on the bottom of the ocean, attached to a buoy or mounted on the bottom of ships.
RAFOS floats (SOFAR spelled backwards) also use sound to measure currents. RAFOS Floats are typically used in the open ocean to measure a current like the Gulf Stream.
An ADCP sends out a sound pulse. The sound pulse is at a very high frequency, from 40kHz to 3,000 kHz. The human ear can hear frequencies up to 20kHz and even dolphins only hear frequencies up to 120kHz. At such high frequencies the wavelength is very small, about 6 mm to 0.5mm.
The sound pulse from the ADCP will reflect off small particles in the water. These small particles may be fine silt or small living creatures like plankton. Even very clear water has many small particles in it. The ADCP listens with a hydrophone for the sound that is bounced off the small particles.
The measurement of currents with sound depends on the Doppler effect. The Doppler effect is a change in frequency of a sound due to the motion of the source of the sound or the motion of the listener. The most common example of the Doppler effect is the change in frequency of a train whistle. As the train comes toward you, the frequency increases. This Doppler effect is because the motion of the train is squeezing the sound waves. As the train moves away, the frequency decreases because the train's movement is stretching out the sound waves. The Doppler shift also occurs in the water.
In the animation below, the sound source is moving toward Observer B and away from observer A. Observer B will hear a higher frequency sound and Observer A will hear a lower frequency sound.
The ADCP sends out a sound that reflects off small particles and returns to the ADCP. If those particles are in a current, then those particles are moving with the current. There will be a Doppler shift in the frequency of the sound that reflects off the small particles and returns to the ADCP. That Doppler shift can be used to calculate the current speed. Most ADCPs have 3 or 4 sound sources that work together. By using several sources, the ADCP can tell the direction of the current as well as its speed. The ADCP can also tell at what depths in the water column the current is moving by how long it takes the sound to return to the ADCP.
RAFOS floats (SOFAR spelled backward) are floating instruments designed to move with the water and track the water's movements. The float contains a hydrophone and signal processing circuits, a microprocessor and a battery. The RAFOS float has an accurate clock with which to determine the arrival times of acoustic transmissions. The RAFOS Float keeps track of its own position by listening for the signal from sound sources in the water near the study area. The RAFOS Float uses the time of travel and the phase of the sound to determine its position. Because the RAFOS Float moves with the current, the float's position tracks the path of the current.
A key element of the RAFOS float is that the instrument does not have to be retrieved for its data to be analyzed. The float is designed to return to the surface and telemeter its data to a satellite system at the end of each mission. The RAFOS float can be weighted to float at any depth.