When an object vibrates in a medium (usually air or water), it affects the medium surrounding the object and causes particles to vibrate. This creates a continuous vibration in the air or water particles, which spreads to the surroundings and forms a wave. When these waves reach our ears, we perceive them as sound (within the appropriate size and frequency). If the medium is air, the frequency of the energy wave can be heard by humans within a certain range (20 to 20,000 Hz), and this is usually called “sound” or “sound wave.” The frequency range that humans cannot hear (above 20 kHz) is called ultrasonic waves and is no different from the sounds and frequencies we can hear.
Sound is produced when certain types of solid objects, such as wood, metal, or a taut string, are struck in the air (or water). This generates a sound with a specific frequency, that is, the pitch of the sound, depending on the material and shape of the object. Therefore, it is possible to design and manufacture a vibrator with the desired frequency and acoustic characteristics, and this is called an acoustic transducer. The wave generated here causes a chain movement of air particles and spreads to the surroundings. Because of the spreading form of this wave, it is called a wave of condensation. In other words, as the air is compressed, a dense part with a higher density than the surrounding atmospheric pressure and a small part with a lower density appear periodically, and this is shown in the picture below. At this time, the air particles move in the same direction as the wave, so it is also called a longitudinal wave.
When these waves progress and reach our ears, they are transmitted to the brain via the outer ear and inner ear and nerves, and are recognized as language or sound through various analyses. The human ear can only perceive a certain frequency range (audible frequencies). Frequencies other than this are usually called ultrasonic waves, and only the frequency is different from the sounds we speak with our mouths and hear with our ears, but other physical characteristics are the same. As these sound waves pass through a medium, they undergo complex phenomena such as transmission, reflection, refraction, scattering, absorption, and diffraction. Many types of sensors are manufactured using these phenomena.
When ultrasonic waves are transmitted within the human body, human tissues undergo subtle vibrations, which are accompanied by various medical phenomena. Cells can be activated or destroyed using the heat and cavitation phenomenon caused by the vibration of human tissue. Imaging diagnosis is also possible using reflection characteristics at the interface between tissues. Moving type diagnosis (heart, fetal, etc.) is possible using the Doppler effect.
Generation of Ultrasound
When you hit the xylophone with a stick, different sounds are produced depending on the length of the keyboard. Long ones produce low sounds, and short ones produce high sounds. The same phenomenon can be seen in pianos and guitars. A xylophone keyboard is a long, square bar that, if made of the same material, produces a sound with a frequency inversely proportional to its length. Once the shape and material characteristics are defined like this, the natural resonance frequency in a specific direction is determined.
Piezoelectric ceramics can be molded and fired in powder form, so they can be made into various desired shapes. Some representative shapes are shown below.
Above, the arrow represents the direction of vibration, the letters represent the length, and in the formula below, Fs represents the resonance frequency and N represents the frequency constant for each type.
Example of ultrasound generation
Piezoelectric ceramics have weak tensile strength, so to generate more powerful ultrasonic waves, a structure assembled with metal must be created as shown in the following figure, and this is used to generate powerful ultrasonic waves. This structure is a structure in which metal is inserted into both sides of four piezoelectric ceramics and fastened with bolts. The frequency is determined by the diameter and the total assembled length, and the piezoelectric ceramics can be compressed by the fastening force of the bolts to drive within the breaking tensile strength. This is a structure developed by Langevin in France and is called Bolt Clamped Langevin Transducer, Langevin Transducer, and BLT Transducer.
The figure below shows the impedance characteristics of the Lanjuvan oscillator as a graph. The resonance frequency is 27,800 Hz, the resonance impedance is 43 Ohm, the anti-resonance frequency is 28,900 Hz, and the anti-resonance impedance is 7,500 Ohm. Between resonance/anti-resonance frequencies, the phase becomes +90 degrees, so it changes to almost an inductor component, and in other frequency ranges, it becomes -90 degrees, becoming a capacitor component.
This can be expressed as an electrical equivalent circuit as shown in the figure below.
