TABLE OF CONTENTS Sound waves are simply vibrations of the particles making up a solid, liquid, or gas. As an energy form they are therefore an example of mechanical energy, and it follows that, since there must be something to vibrate, sound waves cannot exist in a vacuum.
The only human sense that can detect sound waves is hearing, and that sense is restricted to a relatively narrow range of vibration frequencies called the audible range . It follows that there will be vibration frequencies that are so low or so high that they cannot be detected by the human ear.
The unit of frequency is the hertz, abbreviated as Hz, defined as one cycle of vibration per second. Sounds below approximately 16 Hz are below the limit of human hearing and are called subsonic vibrations, and sounds above approximately 20,000 Hz are too high to be heard and are called ultrasonic vibrations. Between those two values, in the audible range, it is more common to use the term pitch to refer to frequency; a high-pitched sound means high audible frequency, and low-pitched means low audible frequency. A piano key pitched at middle C is at a frequency of 260 Hz.
Abbreviations are used for high frequencies; 1000 Hz is shortened to 1 KHz (one kilohertz), 1,000,000 Hz becomes 1 MHz (one megahertz), and a billion cycles per second becomes 1 GHz (one gigahertz). In ultrasonic flaw detection, most testing is carried out in the...
