TYPE OF PRODUCTS : DEFINITIONS

- Acoustic marker
- Actuator
- FREE-FLOODING Transducer
- Hydrophone
- Hydrophone with remote supplied preamplifier
- Hydrophone with voltage preamplifier
- Impedance
- Transducer
- Transponder

MAIN CHARACTERISTICS OF TRANSDUCERS AND HYDROPHONES

- Directivity
- Noise level
- Reception sensitivity (Sh :dB ref :Volt/цPa)
- Sound level (Sv :dB ref :цPa/Volt)
- Transmission sensitivity (Sv :dB ref :цPa/Volt)

TYPE OF PRODUCTS

Acoustic marker
This is an acoustic transmitter that produces impulses continuously and at a given frequency. The recurrence and the length of the impulse are constant.

Actuator
An actuator, also called an activator, is a transducer that guarantees the opposite transformation of the sensor. Often the phenomena concerned resemble both conversions, one close to the source and the other close to the destination, we say that these phenomena are reversible.
Another definition : an actuator, also called an activator, is an electro-magnetic system that allows the transformation of electrical energy into mechanical energy, into a form that is most often the movement of a mechanical part. It is generally guided by a sensor.

FREE-FLOODING Transducer
Refer to definition of Transducer

The FREE-FLOODING transducer is a transducer in which the piezo-electric parts remain under equal pressure. Their acoustic characteristics remain almost completely constant, whatever the depth at which they are operating. They are mainly used for underwater applications in deep water.
Note/ The maximum submersion acceptable for a FREE-FLOODING transducer is almost limitless.

Hydrophone
The hydrophone is a sensor capable of detecting acoustic signals in a liquid environment (comparable to the functioning of a microphone in an aerial environment). It is generally fitted, as close to the piezo-electric material as is possible, with a preamplifier to adapt, in level and in bandwidth, the electrical signal emitted from the piezo-electric material. The hydrophones thus only use the direct effect of piezo-electricity.
The two main characteristics are :
-    Their noise level (in dB reference (volts/√Hz)-limiting their capacity to detect weak signals).
-    Their reception sensitivity ( in dB reference volt/μPa).

Note/A hydrophone is more often configured to function far from its frequency of resonance, thus one obtains a flat response rather than high performance.
A hydrophone can thus be only used in reception, it is the equivalent of the microphone which is used in air.

Hydrophone with remote supplied preamplifier
Refer to definition of Hydrophone

The hydrophones with remote supplied preamplifier consist of a hydrophone and an integrated preamplifier, placed as close as possible to the piezo-electric material supplied by a constant current.
This type of power supply is most often used when the hydrophone is connected across a long cable thus providing the hydrophone with a greater immunity to electromagnetic noise.

Hydrophone with voltage preamplifier
The hydrophones with voltage preamplifier consist of a hydrophone and a preamplifier, placed as close as possible to the piezo-electric material, of a high input impedance.
They are used in most applications, they enable the sensitivity of the hydrophone to be increased, and allow it to be connected under a low output impedance, and thus without loss of sensitivity.

Impedance
The electrical impedance of a transducer is the relationship between the voltage on its contacts to the current which crosses it, with a phase difference of an angle Φ . This impedance, close to the frequency of resonance of the transducer, is comparable to the resistance Rp and a capacitor Cp connected in parallel.

Transducer
A transducer is a vibratory electro-mechanical device capable of transforming electrical energy into mechanical or acoustic energy or the opposite. The transducer is able to use both effects of piezo-electricity (direct effect or the inverse effect, comparable both to a microphone and a loud-speaker in aerial acoustics). The two main characteristics are :
- Its transmission sensitivity (in dB reference μPa/volt).
- Its reception sensitivity (in dB reference volt/μPa).
One can optimise all their characteristics by producing groups of sensors. Thus it is possible to produce arrays of forms and dimensions designed specifically for their particular applications.

Note/A transducer, to optimise its performance, is most often of a dimension that allows it to function close to its frequency of resonance, rather than in respect to bandwidth.
A transducer can thus be used for transmission as well as for reception.

Transponder
It is an acoustic receiver/transmitter which, on reception of an acoustic interrogation signal transmitted wireless, responds on another frequency.
The acoustic transponder is an apparatus built to be installed aboard a mobile underwater vehicle of a ROV or AUV type or on a fixed underwater installation.

MAIN CHARACTERISTICS

Directivity
The directivity of a transducer (transmitter or receiver) characterises the sensitivity variation, to transmission or to reception, of this transducer in relation to a particular observation direction called the axis of the transducer. In practice, and due to the symmetry introduced through the design of the transducers, only two directivity graphs in two orthogonal planes between them have been determined, called horizontal plane and vertical plane.

Noise level
The background noise of a hydrophone is the characteristic that will limit its capacity to detect weak acoustic signals.
The background noise of a hydrophone is made up of thermodynamic murmur of the sensitive element, of murmur induced by the resistance to entry of the preamplifier and the murmur of the preamplifier itself.

Reception sensitivity (Sh :dB ref :Volt/цPa)
The voltage response in free field of a transducer used as a receiver, is the relationship between the voltage at the output contact of the transducer, measured on open circuit, to the sound pressure in free field existing in an undisturbed acoustic field, for each undisturbed acoustic field a progressive wave plane is heard.

Sound level (Sv :dB ref :цPa/Volt)
The sound level of an electro-acoustic transducer used to transmit sound waves corresponds to the apparent sound pressure at a distance of 1 metre from the acoustic centre of the transducer for a given electrical voltage.

Transmission sensitivity (Sv :dB ref :цPa/Volt)
The voltage response to the transmission of an electro-acoustic transducer used to transmit sound waves is the relationship between apparent sound pressure at a distance of 1 metre from the acoustic centre of the transducer in a given direction and the electrical voltage applied to its contacts.