Underwater Acoustics: Hydrophones


Hydrophones: microphones for underwater sound

The lightly edited text below is taken from a TNO presentation on hydrophones and underwater sound from 1974.


The history of underwater sound

Sound as perceived by humans reaches our ears as vibrations that propagate through the air. The fact that there are also sounds underwater and that these sounds propagate faster and over larger distances than in the air is unknown to many because these sounds are rarely heard. To do this, you have to swim with your head underwater, something you do not do every day. Underwater sound is virtually impossible to hear above water. This is related to the large difference between air and water in terms of specific gravity and compressibility. As a result, underwater sound is almost completely reflected against the water surface and only a fraction of it penetrates to the air above.
According to historical writings, the existence of sound underwater was already known to Leonardo da Vinci. In 1490 he described how he could hear the rowing strokes of distant ships by listening to a tube that he stuck one end into the water. He noticed then that sound underwater remained audible over much greater distances than above water. The phenomenon of sound underwater was considered only a curiosity at that time. In the 17th century, Isaac Newton calculated that sound must travel at a much higher speed underwater than in air due to the lower compressibility and higher specific gravity of water. In 1820, François Beudant rang an underwater bell while simultaneously giving a flag signal near Marseilles. A swimmer at a distance signalled when the bell was heard. Beudant derived a sound speed of 1,500 m/s. In 1827, the speed of sound underwater was measured in Lake Geneva by Calladon and Sturm. They came to a speed of 1,435 m/s.

This print shows the primitive measuring method for determining the speed of sound under water. At the moment the hammer hits the bell, magnesium powder is ignited by the burning fuse. An observer at a great distance takes the flashes of light and starts the chronometer, which is stopped again when the bell sounds. Due to the measuring distance of 5-10 km, these tests had to be carried out at night with clear visibility.
This print shows the primitive measuring method for determining the speed of sound underwater. The moment the hammer hits the bell, a burning fuse ignites magnesium powder. An observer at a great distance notices the flash of light and starts the chronometer, which is stopped again when the bell sounds. Due to the measuring distance of 13.5 km, these tests had to be carried out at night with clear visibility.

In those days, people made noise underwater by ringing a bell underwater or by hitting the ship’s hull with a hammer. Microphones did not yet exist and signals were received by listening with the ear on the inside of the ship’s hull. Nevertheless, it was discovered that the sounds were reflected off the bottom and the banks of the lake.

Only after the Titanic disaster in 1912 did people think of the possibility of discovering icebergs and cliffs in time by listening underwater to their echoes when they had sent out sound signals themselves. The sounds could be captured underwater using (telephone) microphones that were placed on the inside of the ship’s hull. Many ships were then equipped with underwater bells and microphones to prevent collisions during fog (Fessenden oscillator).
With similar facilities, it was possible to determine the water depth under the keel (‘echo sounder‘).

In 1914 the underwater loudspeaker was invented, making use of the piezoelectric properties of quartz crystals. It was now possible to transmit sounds underwater electrically. Better constructions for underwater microphones were also devised. 

The threat of submarines during the Second World War led to the development of devices that could detect this invisible danger using sound signals. These devices were codenamed ”ASDIC” (Anti Submarine Detection Investigation Committee) and later ”SONAR” (Sound Navigation And Ranging). 

After that, more and more applications were found for underwater sound. For example, it turned out that schools of fish can produce strong echoes at certain frequencies. Fishing ships were soon equipped with special fishing sonars. As biologists became more interested in the behaviour of fish, they started to make more use of the possibilities offered by underwater acoustics. Fish were provided with miniature sound transmitters (pingers) that continued to emit sound bursts for months and sometimes even years. With the help of suitable listening devices, the movements of the fish could then be followed. It was also discovered that many other sea animals produce sounds, such as dolphins. They are equipped with a highly developed sonar device that they use when searching for food, for navigation and also for mutual communication.

Recent developments in the field of exploration and extraction of minerals under the seabed increasingly require devices that work with underwater sound. There are now echo sounders whose sound signals can penetrate deep into the seabed so that echoes can be received from deeper layers of the earth. The location of a well for oil or gas extraction can be marked on the seabed with sound sources (pingers). Pingers can also be used as an aid for accurate positioning at sea by placing these pingers on the seabed at known positions and making use of the time differences with which simultaneously transmitted signals are received.

The use of underwater sound is also becoming increasingly important for military purposes, not only for detecting submarines and guiding torpedoes but also for telephone contact with submarines. For all these applications, sensitive and reliable microphones for underwater sound, called ”hydrophones” are required.

Microphones are not yet hydrophones

In principle, it is possible to make microphones, which are intended to pick up airborne sound, waterproof so that they can be used as hydrophones. Microphones are now manufactured in large numbers and the simple types are very cheap. However, it turns out that microphones that are sensitive to airborne noise have low sensitivity underwater. The cause is the difference in specific gravity and compressibility of air and water.
Sound is a vibrating movement of matter, accompanied by a periodic variation in pressure that propagates at a certain speed (the speed of sound).

Air has a low specific gravity and a high compressibility, which means that the sound vibrations are accompanied by a relatively low amplitude of pressure, while the air molecules vibrate at a relatively high speed. An air microphone is therefore always equipped with a very light and thin membrane that is easily carried along by the weak pressure vibrations. An electrical signal is then derived in some way from the vibration of that membrane.
Water, on the other hand, has a high specific gravity while its compressibility is low. Sound under water is therefore accompanied by a relatively high pressure amplitude, while the water molecules hardly move. A membrane set in motion by the water can therefore never vibrate with a greater amplitude than the water itself. An air microphone underwater can therefore only emit a weak signal, even if the pressure amplitude is high.
The mobility of the membrane of a hydrophone will therefore have to be adapted to the inertia of the water, which leads to a high stiffness of the membrane and a generally very robust construction of the hydrophone housing.

Operating principles

There are a large number of physical principles on which the operation of a microphone and a hydrophone can be based. The five most interesting will be mentioned here:

  • The carbon microphone.
    This is used in almost all [classic] telephones all over the world. The operation is based on a change in the resistance of coal grains that are pressed against each other by the pressure of the membrane that absorbs the sound. When a direct current is passed through the package of carbon granules, a voltage is created across the microphone terminals that fluctuates in accordance with the sound vibrations against the membrane. Due to the large deflections that the membrane must undergo to cause a reasonable change in resistance, this microphone is not suitable as a hydrophone.
  • The condenser microphone.
    This consists of a stretched metal foil that is located a short distance from a metal plate and forms a capacitor with it. Air vibrations set the metal foil in motion, which changes the capacity. An electrical charge supplied through a high resistance changes the voltage across the condenser microphone in accordance with the sound vibrations. This microphone is also not suitable as a hydrophone due to the required large deflections of the membrane.
  • The electromagnetic microphone.
    There are many variants of this. The principle is based on a coil that in some cases is driven by a membrane and then moves in a magnetic field. In other cases the coil is stationary and a piece of iron in the coil moves in a magnetic field or the membrane itself is made of iron. In all cases, the magnetic flux in the coil changes in accordance with the sound vibrations. The coil then emits an electrical voltage through induction. Due to the large deflections required for a reasonable induction voltage, this microphone is also less suitable as a hydrophone.
  • The magnetostrictive or piezomagnetic microphone.
    According to the principle of magnetostriction (or piezomagnetism), the magnetizability of a ferromagnetic material changes under the influence of an elastic deformation. Nickel and Vanadium permendur (49% Nickel, 49% Cobalt and 2% Vanadium) exhibit this effect to a strong extent. Because the deformation of the metal requires large forces while the displacements are small, this principle is extremely suitable for hydrophones. The construction consists of a closed magnetic circuit of magnetostrictive material in which a magnetic field is maintained, usually with the aid of a permanent magnet. The sound field deforms the magnetic circuit, resulting in a change in the magnetic flux that is converted into an alternating voltage using a wire winding.
  • The piezoelectric microphone.
    This makes use of the property of some crystals and some polycrystalline substances so that an electric field strength is created in the material under the influence of a mechanical deformation. With the help of electrodes, this field strength can be taken as an electric voltage which is proportional to the deformation of the crystal.
    Because crystals deform only slightly under high pressure, they are ideally suited for use in hydrophones. Due to the great sensitivity of this type of material and the simplicity of construction, it is also widely used in air microphones. Through an ingenious trick, the small force that the membrane produces during a large deflection is transferred to the crystal as a large force with a small deformation of the crystal.

Of the five types mentioned, piezoelectric hydrophones appear to have such favourable properties that only this type will be discussed in more detail below.

Piezoelectric hydrophones

The oldest known piezoelectric crystal is quartz. The first good hydrophones were based on quartz crystals. This material has extremely stable properties, even under extreme conditions of temperature and pressure and it is insoluble in water. However, the crystals are expensive and processing is difficult. Furthermore, quartz crystals are relatively insensitive. The generated piezoelectric voltage is relatively low compared to later discovered piezoelectric crystals. Not only were they much more sensitive than quartz, they were also much cheaper and easier to process. The best known of these is Seignette salt, which was recently used in virtually all crystal microphones and gramophone elements.
In the 1940s and 1950s, people preferred to use the less sensitive ADP for hydrophones (Ammonium Dihydrogen Phosphate) because it is stronger and more stable than Seignette salt. The disadvantage of all these crystals is that they dissolve easily in water. This is an unfavourable property that makes these crystals less suitable for use in hydrophones because even a small leakage can lead to the dissolution of the crystal.

About 1950 it was discovered that some ceramic materials made from barium titanate, lead zirconate and related substances exhibit a strong piezoelectric effect. The technology of these piezoelectric ceramics has advanced considerably since then. Ceramic elements are also used in lighters and ignition mechanisms for gas heaters and combustion engines, which was previously not possible with crystals. Piezoelectric ceramics, like quartz, are insoluble in water. Unlike quartz, they have a high sensitivity. Due to the possibility of mass production from cheap raw materials, the price of this material can be kept low (some types of whitewash consist of barium oxide, titanium oxide, lead oxide or mixtures thereof). These substances serve as raw materials from which barium titanate and lead zirconate are manufactured.

Another advantage of piezoelectric ceramics is that they can be manufactured in various shapes and sizes. In the past, crystals had to be sawn out of the mother crystal at certain angles to the crystallographic axes. As a result, the shape was limited to rectangular blocks, the largest possible size of which was determined by the size of the mother crystal. The shape and dimensions of ceramic elements are determined by the possibilities offered by ceramic technology. One now has much greater freedom to adapt the shape of the piezoelectric element to the desired hydrophone construction.

Limitations on hydrophones

The usability of a hydrophone is characterized by three factors:

  • The sensitivity
    The sensitivity is equal to the ratio between the electrical voltage that the hydrophone emits and the sound pressure that the hydrophone absorbs. For weak sounds, one needs sensitive hydrophones, for strong sounds less sensitive ones. The sensitivity is determined by the properties of the piezoelectric material, the distance between the electrodes and the construction of the hydrophone.
  • The electrical capacity
    The electrical capacitance between the electrodes is an important factor that plays a role when the hydrophone is connected to a long cable. When the electrical capacitance of the cable is greater than the capacitance of the hydrophone, the cable causes a voltage drop that is greater as the capacitance of the cable increases. Like the sensitivity, the capacity of the hydrophone is also determined by the properties of the piezoelectric material, the distance between the electrodes and the construction of the hydrophone.
  • The resonance frequency
    Each hydrophone behaves as a system in which masses and spring elements are coupled to each other so that the phenomenon of resonance occurs at several frequencies. Resonance makes the hydrophone insensitive to frequencies higher than the resonant frequency. The construction of the hydrophone must therefore be designed in such a way that no resonances occur in the frequency range that one wants to listen to. This condition is usually met by a compact construction using light materials, such as magnesium, aluminium, titanium, glass and plastics. Of the metals mentioned, titanium is preferred because it is extremely resistant to the corrosive effects of seawater and has good mechanical and elastic properties, with certain favourable properties being further enhanced in some alloys. Finally, the thermal expansion coefficient is relatively low, which is favourable when adhesive bonds are used with glass and ceramic materials, whose expansion coefficient is also low.


The TNO Physics Laboratory has been working for years on designing and manufacturing hydrophones for all kinds of applications. Some of the most common types will be covered next.

A hydrophone in its simplest form
A hydrophone in its simplest form

A hydrophone in its simplest form contains two blocks of piezoelectric ceramic material sandwiched between a metal membrane and the bottom of a sturdy metal box. The blocks can be either rectangular or cylindrical. They are arranged relative to each other in such a way that they both supply the same electrical voltage to the common electrode when the membrane is pressed. The outgoing cable is only shown schematically: the penetration through the wall of the box must of course be properly watertight. The thread of the cover must also be sealed with gasket material. The dimensions of the hydrophone should correspond to the frequency range to be listened to: a hydrophone with outer dimensions of the order of 10 millimetres can be used for frequencies up to 100 kHz. For a hydrophone ten times larger, with outer dimensions close to 100 millimetres, the frequency range only extends to close to 10 kHz. However, this large hydrophone is ten times more sensitive than the small one.

Better adjustment

The above hydrophone is far from optimally dimensioned. The piezoelectric element is much stiffer than the surrounding water so the sound pressure cannot deform the element sufficiently.

Increased sensitivity
Increased sensitivity

The figure above shows a possible improvement. As with a hydraulic press, the pressure on the membrane is concentrated on the much smaller cross-section of the piezoelectric element, which therefore undergoes a greater deformation and therefore delivers a greater electrical voltage. The diameter and length of the piezoelectric element are related to the resonance frequency and sensitivity, allowing better adaptation of the hydrophone to the surrounding medium.
The ceramic element can be manufactured in one piece and the electrodes can later be applied with conductive paint. The sensitivity and electrical capacitance between the electrodes can be changed within certain limits.

Ceramic tube as a hydrophone element

A drawback of this construction is that the rod-shaped ceramic element tends to buckle sideways when a strong pressure impulse falls on the membrane at an angle. This can be prevented by replacing the solid piezoelectric element with a thin-walled hollow cylinder.

Hollow cylinder hydrophone
Hollow cylinder hydrophone

The ceramic element can be manufactured in one piece. The electrodes can later be applied with conductive paint. The sensitivity and electrical capacitance between the electrodes can be changed within certain limits.

Hydrophone with an O-ring seal
Hydrophone with an O-ring seal

By replacing the flexible edge of the membrane with a seal with an O-ring, an even more compact construction is achieved. A central bolt straight through the hydrophone clamps the now loose front part firmly against the ceramic cylinder and puts it under axial pressure. This is necessary because the adhesive connections and the ceramic itself have only a low tensile strength, but can withstand high pressure. Due to permanent pretension, the hydrophone is more resistant to shocks in the axial direction. If the cross-section of this bolt is small compared to the cross-section of the ceramic material, it does not hinder an unhindered transfer of the sound pressure to the piezoelectric element.

Bending vibrators

A completely different construction of hydrophones is possible by using the effect that a deflecting membrane is stretched on the convex side and compressed on the concave side. A thin disk of piezoelectric material, glued to one side of the membrane, will then emit an electrical voltage proportional to this deflection.

A bending vibrator
A bending vibrator

The box is filled with air so as not to hinder the free movement of the membrane. An annular groove along the outer edge of the membrane ensures that the deformation of the ceramic disk is approximately equal in all places. By choosing the correct thickness of the membrane, its stiffness can be adjusted to the compressibility of the water.

Unlike the previously mentioned hydrophones, this type is not suitable for use deep underwater. The high hydrostatic pressure would compress the membrane too much, which could cause the ceramic disc to rupture. Alternative designs lead to other problems and make the hydrophone expensive and bulky. The difference in thermal expansion between the metal membrane and the ceramic disc can also cause the disc to break. The metal membrane must also be very resistant to the corrosive effect of seawater. Zirconium, although expensive, is one such metal. 
All hydrophones described so far consist of sturdy metal boxes, one wall of which is more or less flexible and is therefore sensitive to sound. These hydrophones are intended to be mounted on the side of a ship (below the waterline), a buoy or other submerged object, with the sensitive surface facing the water.

Spherical hydrophone

A hydrophone, which can be suspended freely in the water and which is then sensitive in all directions, can be constructed in the simplest way with two half spheres of piezoelectric ceramic material coated internally and externally with a conductive layer (electrode) and glued together into a closed ball. The electrical connection to the inner electrode is led out through a hole. The outer electrode is grounded. The sphere and the connection to the cable are surrounded by a protective layer of rubber or synthetic resin. Such a ball with a diameter of 29 millimetres resonates at approximately 100 kHz and can therefore be used as a hydrophone up to that frequency. Due to the even distribution of the hydrostatic pressure, such balls can be used deep underwater without the risk of breaking the ceramic.

A spherical hydrophone
A spherical hydrophone

Tube-shaped hydrophones

In a tube-shaped hydrophone, the ceramic element has the shape of a hollow cylinder that is made waterproof using two covers. These can be made of both metal and plastic. The tubular shape of the ceramic element has the advantage that this shape lends itself easily to mass production by extrusion. The price can therefore be lower than that of spherical elements, while there is also more freedom in arranging the electrodes, for example by metallizing the two cylinder surfaces. A higher sensitivity is achieved at the expense of the electrical capacitance between the electrodes by designing these electrodes as several narrow strips parallel to the axis. 

A tube-shaped hydrophone
A tube-shaped hydrophone


The photo below shows a cutaway model of a hollow cylinder hydrophone. The hydrophone is mounted between O-rings in a watertight mounting with a watertight electrical contact plug. The electrodes on the ceramic tube completely cover both cylinder surfaces. The electrical connections are led out through the bottom of the hydrophone capsule. The space behind the capsule is filled with silicone oil. The white washer under the hexagonal nut is a lock washer. This hydrophone, which has a gas wire on the outside, can be installed in all kinds of underwater constructions.

Cutaway model of a hydrophone. (1) hydrophone capsule (2) ceramic tube (3) front piece (4) O-rings (5) bushings (6) retaining ring (7) contact plug
Cutaway model of a hollow cylinder hydrophone.
(1) hydrophone capsule
(2) ceramic tube
(3) front piece
(4) O-rings
(5) bushings
(6) retaining ring
(7) contact plug

By assembling a large number of hydrophones, one large cylindrical 360-degree listening post can be created for underwater use, such as the system at test from 1965.

System of 216 hydrophones (from 1965)
System of 216 hydrophones (from 1965)