Ultrasonic represents a special branch of general acoustics, the science of mechanical oscillations of solids, liquids and gaseous media. With reference to the properties of human ear, high frequency inaudible oscillations are Ultrasonic or supersonic. In other words, while the normal range of human hearing is in between 16Hz and 16 kHz, Ultrasonic frequencies lie between 20 kHz and 500 MHz. Expressed in physical terms, it is the sound produced by mechanical oscillation of elastic media.
The occurrence of sound presupposes the existence of material it can present itself in solid, liquid or gaseous media. Wet processing of textiles uses large quantities of water, and electrical and thermal energy. Most of these processes involve the use of chemicals for assisting, accelerating or retarding their rates and carried out at elevated temperatures to transfer mass from processing liquid medium across the surface of the textile material in a reasonable time.
Scaling up from lab-scale trials to pilot plant trials is difficult. In order for ultrasound to provide its beneficial results during dyeing, high intensities are required. Producing high intensity, uniform ultrasound in a large vessel is difficult.
Ultrasound reduces processing time and energy consumption, maintains or improves product quality, and reduces the use of auxiliary chemicals. In essence, the use of ultrasound for dyeing will use electricity to replace expensive thermal energy and chemicals, which have to be treated in wastewater.
Ultrasound energy is sound waves with frequencies above 20,000 oscillations per second, which is above the upper limit of human hearing. In liquid, these high-frequency waves cause the formation of microscopic bubbles, or cavitations. They also cause insignificant heating of the liquid. Ultrasound causes cavitational bubbles to form in liquid. When the bubbles burst, they generate tiny but powerful shock waves.
In a solid, both longitudinal and transverse, waves can be transmitted whereas in gas and liquids only longitudinal waves can be transmitted. In liquids, longitudinal vibrations of molecules generate compression and refractions, i.e., areas of high pressure and low local pressure. The latter gives rise to cavities or bubbles, which expand and finally, during the compression phase, collapse violently generating shock waves. The phenomena of bubble formation and collapse (known as cavitations) are generally responsible for most of Ultrasonic effects observed in solid/ liquid or liquid/liquid systems.
The figure below shows the waves produced by ultrasound