When propagating sound encounters a boundary between materials with different acoustic impedances ( Z), some sound waves are reflected back ( R) while others continue through. Sound with a frequency above 20 kHz is known as ultrasound. The audible frequency range for a human ear is around 20 Hz to 20,000 Hz (20 kHz). Higher frequencies result in high-pitch sounds like a whistle, whereas lower frequencies are perceived as low-pitch sounds like rolling thunder. To the human ear, sound frequency manifests as pitch. Thus, if an object emits 20 waves per second, it has a frequency of 20 Hz. The number of waves produced in one second by a vibrating object is known as the sound’s frequency ( f) and is measured in hertz (Hz). In essence, sounds are pressure waves emitted by a vibrating object. If exposed to an electric AC signal, the piezoelectric material will start to vibrate as it alternatingly contracts and expands, thereby emitting sound waves. This is known as the converse piezoelectric effect. Similarly, exposing a piezoelectric material to an electrical field will cause it to deform, either expanding or contracting, depending on the direction of the field. This conversion of mechanical energy to electrical energy is known as the direct piezoelectric effect. When a piezoelectric material is exposed to physical stress, it will polarize and generate a voltage across its surface. Piezoelectricity is an electro-mechanical phenomenon found in certain asymmetric crystal structures such as quartz and various polycrystalline ceramics. How is the piezoelectric effect used to generate ultrasound?īy Louise Møller Bierregaard and Malte Aarenstrup Launbjerg
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