An echo sounder, often called a depth sounder or fathometer, is a device used in marine navigation and hydrographic surveying. It measures the distance from a vessel to the seafloor or submerged objects. Operating on acoustic principles, the device transmits a sound signal downward and calculates depth based on the time it takes for the signal to return. This technology allows vessels to safely navigate and enables the detailed mapping of underwater topography.
The Mechanism of Sound Generation
The sound produced by an echo sounder originates from a transducer, typically mounted on the hull of a ship. This transducer serves as both the speaker and the microphone, converting energy between electrical and acoustic forms. When activated, a transmitter unit generates a short burst of alternating current electricity.
This electrical pulse is sent to the transducer, which rapidly vibrates a material to project a precisely directed acoustic wave into the water. This outward-traveling sound wave is the “noise” the echo sounder creates. After transmission, the transducer switches roles to act as a receiver, listening for the returning sound.
The transmitted sound travels until it strikes a boundary, such as the seabed or a school of fish, causing energy to reflect back as an echo. The transducer captures this weak acoustic echo and converts the mechanical vibration back into an electrical signal. A timing mechanism then precisely measures the elapsed time between transmission and reception, calculating the distance using the known speed of sound in water.
Acoustic Characteristics of the Pulse
The sound emitted by an echo sounder is not a continuous tone but a brief acoustic event called a pulse. This pulse is characterized by high frequency and short duration, which are necessary for achieving the high resolution needed for accurate depth measurement. The operational frequency dictates the sounder’s application, involving a trade-off between penetration depth and resolution.
For deep-water mapping, echo sounders use lower frequencies, often 20 to 25 kilohertz (kHz), because lower-frequency sound travels farther before being absorbed by the water. Systems designed for shallow-water navigation or fish finding utilize higher frequencies, sometimes reaching 200 kHz or up to 400 kHz. These higher frequencies provide greater detail and a narrower beam but lose energy more quickly, limiting their effective range.
The duration of the acoustic pulse is short, typically lasting less than 10 milliseconds. This short pulse length enables the system to distinguish between objects that are close together, such as fish just above the seabed. The sound is often described as a sharp “ping” or “click” when the frequency is low enough for human hearing, but high-frequency pulses are completely inaudible.
Sound Intensity and Marine Life Implications
While the acoustic pulse is brief, the sound is powerful at its source, measured in terms of acoustic intensity or source level. Source levels vary by device size and purpose, but they can be high, often exceeding 200 decibels (dB). This high intensity ensures enough acoustic energy reaches the bottom and returns as a detectable echo, even in deep water.
The intensity of the sound experienced by marine life, known as the received level, drops rapidly with distance from the vessel. This decrease is due to the spreading of the sound wave and absorption by the seawater. However, the high-frequency output of many echo sounders overlaps with the hearing and echolocation ranges of marine mammals, particularly toothed whales and dolphins.
The noise from echo sounders contributes to the overall background noise in the marine environment, despite the short pulses. For species relying on sound for navigation, communication, and foraging, this added noise can cause acoustic masking. Studies indicate that for some marine mammals, the received level from these devices is sufficient to cause avoidance behavior, especially when vessels are in close proximity.