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How Do Underwater Microphones Work? (Complete Guide to Hydrophones)


Have you ever wondered how underwater microphones work? If so, then you have come to the right place! In this guide, we explain how an underwater microphone, or a hydrophone, works and how it can record sounds and underwater noise.

What Microphone is Used for Recording Underwater Sounds?

How Do Underwater Microphones Work? (Complete Guide to Hydrophones)

A microphone used to detect and record noise and sounds in the underwater world is called a hydrophone or a piezo hydrophone. Hydrophones are often used as scientific instruments to detect underwater sound waves, monitor underwater noise, track aquatic ecosystems, observe underwater marine life and activities, detect submarines and incoming boat vessels, and oversee underwater excavation activities.

The first underwater microphone designed by scientists was used during World War I to detect enemy submarines or keep track of a submarine crew to keep them safe. Using hydrophone arrays or multiple hydrophones was common back then to keep a surveillance system below water. These hydrophone arrays were placed in the Atlantic Ocean back in the 1950s to help monitor any submarine activity during the Cold War.

While a hydrophone array is still used to keep a surveillance system of borders, scientists now also use this method to record seismic activities, such as to track fish movements, locate submarines, and other underwater sound activities that may help progress marine biology learning and development. Environmental groups also use these to protect marine life and study acoustic pollution.

Regardless of the intended application, the main feature and beauty of a hydrophone is its ability to record distant and low-level sounds amidst deep depths and to a certain underwater pressure limit.

What Are the Different Types of Hydrophones?

How Do Underwater Microphones Work? (Complete Guide to Hydrophones)

Hydrophones can be grouped into two categories: ones that can record sounds underwater and ones used for airborne sounds.

Underwater Hydrophones

The three types of underwater hydrophones are seismic, flow-through, and bottom. These hydrophones are placed in deep water and are used for various applications.

Surface Hydrophones

The three types of surface hydrophones are dome, cupped, and bridge.

Other hydrophones can also be further categorized as either passive or active. Passive hydrophones have no power source as they detect underwater sounds emanating from ship propellers, explosions, and bubbles. Passive hydrophones are also used to measure sound noise levels and infrasound sources under the water.

Active hydrophones have two types; piezoelectric and magnetostrictive. Most hydrophones are made from a piezoelectric material that allow them to operate under deep pressure and withstand any electrical signals and small voltage signals. The use of a piezoelectric transducer is now common among modern-day hydrophones. On the other hand, magnetostrictive hydrophones operate on changes in magnetic flux density and electrical voltage.

How Does An Underwater Microphone or Hydrophone Work?

How Do Underwater Microphones Work? (Complete Guide to Hydrophones)

If a regular microphone collects sound in the air, hydrophones measure ocean sounds. A hydrophone can detect sound waves and different sounds below the water because of how it is designed.

A hydrophone often has a spherical body because it is placed inside a cylindrical metal or plastic tube encasing. To further protect a hydrophone from getting damaged, a single device needs to use special ceramic materials. A ceramic hydrophone also helps produce small electrical currents and voltage signals across a wide frequency range in various directions.

A hydrophone works by detecting pressure changes below water and converting sound waves into voltage. The hydrophone can determine the electrical signals and electrical output transmitted because the speed and distance of the sound wave picked up in the water are proportional to the pressure change.

For a hydrophone to detect sound pressure waves produced by various sources, the device must have a piezo sensor. This sensor is embedded in the hydrophone and will help ensure that the microphone has a flat frequency response proportional to the magnitude of the sound pressure wave. It’s also important to note that hydrophones listen to sound rather than transmit sound. With that, a hydrophone simply amplifies and records electrical signals.

A single hydrophone has greater sensitivity than all other types of microphones. One hydrophone can pick up an ultra-low frequency infrasound sound wave in the 10-100 Hz range and detect acoustic signals with ultra-high frequency sounds. A hydrophone has a frequency range from 40 Hz to 20,000 Hz, which overall makes it capable of picking up various sound sources from the ocean floor and animal sounds at a far distance. However, this makes the acoustic impedance of a hydrophone unable to pick up airborne sounds.

The number of hydrophones used can also change how these devices record sounds. A single hydrophone can record sounds from any direction whereas a hydrophone array or several hydrophones simultaneously positioned will result in recordings that can be manipulated to listen to (pick up) sounds with greater sensitivity. On the other hand, using directional hydrophones (omni or hemi) will allow you to listen to a sound source in a particular direction because these are often situated in a fixed position.

To retrieve the sound source or electrical signal picked up by the hydrophone, the device is attached to an amplifier. A stereo image can later be created when the electrical signals or recordings are transferred to a computer.

Final Note

How Do Underwater Microphones Work? (Complete Guide to Hydrophones)

Unlike a regular microphone, a hydrophone works in water. Rather than transmitting sounds, it listens and captures sound waves which will later on be converted.

With these devices, we are also able to further understand and appreciate marine life and the water.

Frequently Asked Questions

Do sound waves travel underwater?

Yes, sound waves travel in water. The process in which sound waves refract up and down is called the “sound channel”.

How fast does a sound wave travel underwater?

Sound travels faster 4.3 times in water than they do through the air because of density. However, the distance that sound travels in water can vary depending on the water temperature and pressure.

Naomi Feller

Originally from the East Coast, Naomi started singing as young as 3 years old. In her early teens Naomi made some embarrassing YouTube videos before settling on a love for Podcast editing. When she's not pouring over endless amounts of audio, she lends her expertise to us here at Shout4Music with her crystal clear and finely tuned microphone reviews.

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