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.
A technique called reciprocal transmission can also be used to measure currents with sound.
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 relative to 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 a current and track the current's movements.
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. The RAFOS float can be designed to float at different depths, allowing the full structure of the current to be studied.
Just as a boat going downstream with the current in a river travels faster than a boat going upstream against the current, a sound pulse moving in the same direction as a current travels faster than one moving against the current. Sound pulses transmitted in opposite directions at the same time (called "reciprocal transmissions") will therefore have different travel times. The pulse traveling with the current will have a shorter travel time than the pulse traveling against the current. The difference between the two travel times can be used to compute the current.
Two sources and two hydrophones are necessary to measure current velocity by transmitting sound in opposite directions through the current. High precision measurements are required because the difference in the travel times of oppositely traveling pulses is tiny. Sound travels at about 1500 meters per second in the ocean, while ocean currents typically have speeds of only 0.1 meters per second. Sound traveling with such a current will travel at about 1500.1 meters per second, while sound traveling against the current will travel at about 1499.9 meters per second. (Even strong currents such as the Gulf Stream have speeds of only about 1 meter per second.)
Acoustic Current Meters (ACM) apply this basic principle to measure ocean currents without using propellors or any other moving parts. See for example: http://www.falmouth.com/DataSheets/3DWAVE.pdf The same basic principle has been applied to measure the average current in the Strait of Gibraltar using acoustic sources and receivers (transceivers) separated by about 20 kilometers. It has also been used to make precise measurements of ocean tidal currents using acoustic sources and receivers separated by up to about 1000 kilometers.