Basics of underwater acoustic transducer

71% of the earth's surface area is the ocean. The ocean contains abundant biological and mineral resources, which is the second space for human survival and development in the future. Sonar is used for an underwater detection device, is an important helper for human development of the ocean, and is an indispensable part of the naval and civil navigation industry. The function of the sonar device is to listen to the underwater useful signal and convert it into an electrical signal for viewing; or to generate an electrical signal and then convert it into an acoustic signal to propagate in the water medium, and then reflect it back and receive it after encountering the target. It is converted into an electrical signal for listening or observation, thereby determining the orientation and distance of the measuring object. In the conversion process of this underwater electroacoustic signal, the key equipment is the underwater acoustic transducer or the transducer array.

Application of underwater acoustic transducer

At present, underwater acoustic transducers have been widely used in many fields such as industry, agriculture, national defense, transportation and medical. Here are just a few of the applications for underwater detection:

(1) Application in sounding: In order to ensure navigation safety, sounding sonar should be installed for both warships and civilian ships; special channel inspection vessels are equipped with high precision and full-featured depth sounders. Depending on the depth of sounding, the frequency and power of the sounding transducer are also very different. The frequency ranges from 10 kHz to 200 kHz, and the power ranges from several watts to tens of kilowatts. Among them, high frequency and low power are used for inland rivers or shallow seas, and low frequency and high power are used for oceanic and deep depths. The requirements for such transducers are beam stabilization and sharp main beam.

(2) Application of underwater piezoelectric transducers in positioning and ranging: Measuring the speed of the ship to the ground, mostly using doppler sonar, four transducers with the same performance to arrange the direction of the left and right sides perpendicular to the keel. The general operating frequency is between 100kHz and 500kHz.

(3) Applications in marine surveys and submarine stratigraphic exploration: Submarine geological surveys mainly use low-frequency large-aperture sonar. Towed sonar is the largest array of acoustic arrays on the active carrier today with the longest distance. In the underwater imaging, high-frequency side-view sonar is usually used. Two linear arrays are arranged symmetrically along the keel in the left and right sides of the ship. Each of them emits a fan-shaped directional beam to the sea floor, and then receives reflected waves from the seabed. The intensity of the uneven reflection wave is different, and images with different brightness appear on the displayed image. Because the operating frequency is higher acoustic signal is attenuated faster, and the range of the action is not far. The frequency range of the test is now several tens of kilohertz to 500 thousand. It is classification of underwater acoustic transducers.

Underwater ultrasonic transducer can be divided into electric, electromagnetic, magnetostrictive, electrostatic, piezoelectric and electrostrictive according to different electromechanical energy conversion principles. For example, piezoelectric ceramics developed in the middle of the century are piezoelectric after high-voltage DC polarization treatment. Therefore, it is called electrostrictive material and is the mainstream of today's piezoelectric transducers, especially in ultrasonic transducers. The field has an extremely wide range of uses. The underwater acoustic transducer can be divided into the following categories according to different vibration modes:

(1) Longitudinal vibration transducer: its vibration direction is parallel to the longitudinal direction. The stress wave propagates in the length of the transducer, and its resonant fundamental frequency depends on the length and is the most widely used type in sonar systems.

(2) Cylindrical transducer: A piezoelectric ceramic tube (or ring) is used to mount the desired length through a suitable mechanical structure. It can be made into a horizontal transducer with horizontal non-directionality and vertical directivity control. It is a type of sonar system that is second only to the longitudinal transducer. It is also a standard hydrophone commonly used in hydroacoustic metrology. And one of the selection of standard transmitters.

(3) Bending vibration transducer: The bending vibration transducer has the advantages of small size and light weight at low frequencies (compared with transducers of the same active material at the same frequency), and the vibration form has curved beams, curved discs, curved plates, etc.

(4) Bending extension transducers: Bending extension transducers are generally composite transducers that combine two modes of vibration. For example, a longitudinally stretchable vibrating bar and a different type of curved casing are combined into a plurality of types of curved extension transducers, and a circular planar radial vibration active component can be combined with a bowl-shaped curved casing to form a type II bending extension. 

(5) Spherical transducer: The spherical transducer made by the respiratory vibration of the hollow piezoelectric ceramic spherical shell has the advantage of good spatial symmetry. It is commonly used as a point source hydrophone.

(6) Shear vibration transducer: The shearing vibration in which the direction of vibration and the direction of polarization are parallel and the direction of the driving electric field is perpendicular to the direction of vibration can meet certain special use requirements. This is the form of a 1MH underwater transducer such as a dental calculus.

 3. Main parameters of underwater acoustic transducer

The main performance indicators of underwater acoustic transducer are underwater working frequency, operating frequency range, frequency bandwidth, emission sound source level (acoustic power) and emission response, directivity, receiving sensitivity and receiving sensitivity response, emission efficiency, quality factor, Impedance, maximum working depth, size and weight.

1) Working frequency

The operating frequency or operating frequency range of a hydroacoustic transducer is typically determined by the operating frequency of the sonar device. The impedance, directivity, sensitivity, transmit power, size, etc. the transducer are all functions of frequency. In general, the transmit transducer is calculated for its performance index in the limited frequency band around the resonant frequency or near the resonant frequency, with maximum emission efficiency at and near this frequency. For a wideband receive transducer the resonant frequency of piezoelectric transducer should be much higher than the upper limit of the receive band to ensure a flat receive response within the wideband and to calculate its receive response at the resonant frequency and below. Frequency sonar transducers range in frequency from tens of Hz to several kilohertz, while small target detection sonar transducers range from tens of kilohertz to hundreds of kilohertz.

(2)Directivity

Whether it is a transducer or a transducer array, their transmit response or receive response will change with respect to their direction. This is where the transducer is directional, and the sound waves emitted by the transmitting transducer are the same as those emitted by the searchlight. Since the transducer has directivity, it can concentrate the sound energy to a certain position to make the energy more concentrated. A large number of transducers are used to form a larger array. The directivity is sharper at the same frequency, the energy is more concentrated, and the transmission distance is farther. The signal-to-noise ratio is larger and the distance is longer in the receiving state. It is impedance (or admittance) characteristics.

The transducer can be viewed as a simple series-parallel equivalent circuit near the resonant frequency. Each resistor, capacitor, or inductor in the circuit represents the inherent characteristics of the transducer, which is the transducer impedance (or admittance) characteristic. The impedance characteristics of the transducer are mastered to match the input circuit of the transmitter's final loop or receiver. The impedance (or admittance) of a transducer is a complex number that is a function of frequency and can generally be expressed as: Z(w) = R(w) + jX(w) (in ohms).In the mechanical resonance, the dynamic varistor tends to zero, and the static capacitive reactance can be tuned with a matching inductor. This can be regarded as a pure resistance. Electrical impedance of piezoelectric transducer is typically in the range of tens of ohms to thousands of ohms.

(4) Transmit power

The function of the Submarine range finder is to convert the electrical power of the electronic transmitter into mechanical power of mechanical vibration, and then convert the mechanical power into acoustic power for transmission. The transmitted sound power refers to the physical quantity of the transducer that radiates energy into the medium per unit time. The unit of power is expressed in watts. The transmit power of the transducer is limited by factors such as rated voltage (or current), dynamic mechanical strength, temperature, and dielectric characteristics.

(5) Launch response

The ability to fully reflect the performance of the transmitting transducer is the emission response, mainly the emission voltage response and the emission current response. The definition of the emission voltage response SV is the ratio of the free field apparent sound pressure Pf generated by the transmitting transducer at a distance of d0 m from its effective acoustic center in the specified direction and the voltage U applied to the input of the transducer: SV=Pfd0 /U. The emission voltage response is usually expressed in decibels.

The emission current response is the ratio of the free field apparent sound pressure Pf generated by the transmitting transducer at a distance of d0 m from its effective acoustic center in the specified direction and the current I applied to the input of the transducer: SI = Pf d0 / I . The emission voltage response is usually expressed in decibels.

(6) Receive sensitivity

The field voltage sensitivity of the transducer refers to the point at which the receiving transducer's open center voltage U(w) is at the output and the center of the sound in the free field (assuming the receiving transducer is not present). The ratio of sound pressure Pf(w) is M(w). For receiving transducers, it is desirable to receive incident acoustic signals over a wide range of frequencies, while piezoelectric transducers typically operate over a wide frequency range below the resonant frequency.

 (7) Fluctuation of receiving sensitivity

wideband receiving transducers require a relatively flat receive response over the frequency range used. It is usually specified that the receiving voltage sensitivity fluctuation is ±1.5dB in the operating frequency band.