What’s the Hydrophone detection

A hydrophone is a transducer that converts an underwater sound pressure signal into an electrical signal. When the pressure (acoustic disturbance) on the piezoelectric material piezo ceramic tubes changes, the charge distribution inside the piezoelectric material changes proportionally and is reflected in the form of a voltage signal, so it can be extracted through the electrode on the surface of the piezoelectric element. These charges are amplified by a voltage amplifier or a charge amplifier, and the signal processing oscilloscope displays an image that reflects the waveform of the sound wave. Thus, the sound pressure measurement in the ultrasonic sound field is completed in a very straightforward manner. Traditional materials used for ultrasonic sound field testing are piezoelectric ceramics and PVDF (polyvinylidene fluoride). Piezoelectric ceramics have high hardness and sensitivity, and can withstand a certain range of sound pressure in the HIFU field at low power, but the sound intensity is increased. 

When the large piezo cylinder tube transducer is easily broken, the linear dynamic range is small, and the acoustic impedance is high, so that the hydrophone has a certain interference to the measure sound field.The PVDF acoustic impedance is close to the acoustic impedance of water, with good acoustic impedance matching, soft texture, easy processing is stable chemical properties, with wide frequency response and excellent linearity. The dynamic range is larger than that of piezoelectric ceramic hydrophones. Therefore, PVDF is currently generally used for measurement. It can improve the uneven frequency response produced by piezoelectric ceramics and reduce the interference to the measuring sound field as long as the film is thin enough. PVDF is available in both film and needle types. The diameter of the film type is greater than 5 cm, while the diameter of the needle is less than 1 mm, which is easily damaged in the HIFU sound field. The size of the HIFU focal region is about 1.1mm ×2.1mm×3.2mm. PVDF has the disadvantage of low spatial resolution, and it has edge effect. The volume cannot be made very small. It is limited by temperature. When the temperature reaches 60°C, it will be depolarization occurs, the reuse rate is low, and the hydrophone measurement requires a mechanical method for point-by-point scanning. Even if a plane of 10×10 cm 2 is scanned, it takes several hours at the fastest, so a few simple lines are used, piezo tube for hydroacoustic transducers describe the sound field distribution becomes inevitable.

The use of high frequency piezoelectric ceramic hollow spheres as hydrophones has unique advantages in terms of geometry, size and sensitivity. The ball has a diameter of 0.7 to 1 mm, a resonance frequency of 1.8 to 2.7 MHz, and a sensitivity twice that of a pin hydrophone. It has excellent stability and is subjected to four times the pressure of a pin hydrophone. It is an ideal sensor for measuring high intensity sound fields. A new type of hydrophone for HIFU sound field measurement is reported,who is indicating that the sensor can measure sound power during HI FU treatment, thus sensitive piezoelectric transducer is ensuring accurate delivery of energy during treatment, and measurement of radiation force. Compared with hydrophone measurements, its components are durable and have a small temperature impact. In 2006, Zanelli and Howard designed a hydrophone that effectively avoids cavitation damage. the piezoelectric ceramic is placed in a metal shield to provide a smooth outer surface for the cavitation nucleus on the surface. The possibility of occurrence is reduced to a minimum. In the degassed deionized water, the sound field measurement of the transducer with a frequency of 1.50 MHz, a diameter of 100 mm and a focal length of 150 mm has achieved good results. However, the linear dynamic range of piezoelectric ceramics is insufficient, affecting the upper limit it uses in HIFU measurements.

About fiber inspection,Optical fiber sensing has anti-electromagnetic interference, small size, high spatial resolution, wide response bandwidth and extremely fast response speed, and has been widely used in many fields. The optical fiber detection of the ultrasonic sound field refers to a method of obtaining a sound field signal by analyzing optical signals such as light intensity and optical phase modulated by the sound field in the optical fiber. Commonly used for end face method, fiber grating method and acousto-optic diffraction method. It is proposed to measure the sound field by using the change of the reflected light at the end of the fiber, that is the end face method. It is coated with a multi-layer medium at the end of the fiber. When the sound wave is incident on the multilayer medium, it causes elastic deformation of the medium. At each level, there is reflected light, so the total of piezoelectric ceramic tube reflected light is reflected light from all layers. As a result of the interference, the sound pressure at the end face of the fiber can be measured by measuring the change in the reflected light intensity.It is also pointed out that the nonlinear deviation of this type of sensor is less than 5% at -3-30MPa. In 1996, based on this, a multi-layer coated fiber sensor prototype was proposed and designed.

 He believes that the sensor can be used for high-energy shock waves and low-energy diagnostic ultrasound measurements. However, the sensor probe has limited impact resistance in the HIFU sound field. Kouch has improved it with a single-coated fiber and uses michelson interferometry to improve sensitivity. A single-film optical fiber with a titanium plate surface is used as an arm of the interferometer. Under the action of the sound field, the end face of the fiber will move slightly. This small displacement can be detected by an interferometer. The light source used in the experiment was a 2 mW He2He laser source, and the purpose of the photodiode was to reduce noise. Compared with the method of measuring the change of light intensity, the sensitivity is higher, but the optical path system is more complicated, and the vibration isolation requirement is high, which affects its practical application.