How do we emit ultrasound?

Emitting ultrasound 

Ultrasound is generated by a vibration

A sound is generally generated by the vibration of a thing.

For example. d voice is made by shaking vocal cords to vibrate the air. Similarly a vibration generates an ultrasound.Various kinds of methods are known to generate vibration. Among them,for ultrasound,the method using a piezoelectric ceramic,converting electrical energy to vibrating energy,is widely utilized.

It is the nature of ultrasound that the cycle of vibration generated is shorter or its frequency is higher.

What are piezoelectric ceramics?

The special ceramics that can stretch and shrink by applying voltage

Piezoelectric ceramics are a polycrystalline ceramics that are made by sintering highly purified powder like titanium oxide or barium oxide.

When piezoelectric ceramics are exerted forcefully (mechanical energy is applied),they generate electric voltage (normal piezoelectric effect),

Conversely,when piezoelectric ceramics are applied with voltage,they stretche or shrink depending on the polarity of voltage (converse piezoelectric effect).In this manner， piezoelectric ceramics can convert electric energy to mechanical energy,and vice versa (Piezoelectric effect).

By applying polarization treatment (a direct high voltage is placed on ceramics under a high temperature to even up the orientation of spontaneous polarization),the piezoelectric effect is imparted to ceramics.

Emitting ultrasound using piezoelectric ceramics

Placing alternating voltage on a piezoelectric ceramic..... 

By installing electrodes on a piezoelectric ceramic and placing an alternating voltage, the ceramic vibrates to emit sound due to the piezoelectric effect.The frequency of alternating current determines the frequency of sound,and ultrasound is generated when the frequency isd more than 20kHz.

A vibration using a resonance frequency can generate ultrasound having a larger amplitude.As a resonant frequency depends on the shape of ceramics,a piezoelectric ceramic is made in a shape suitable for obtaining a necessary frequency.