# Ultrasonics - Key terms

### FREQUENCY:

The number of waves passing through a given point during the interval of one second. The higher the frequency, the shorter the wavelength.

### HERTZ:

A unit for measuring frequency, equal to one cycle per second. If a sound wave has a frequency of 20,000 Hz, this means that 20,000 waves are passing through a given point during the interval of one second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second) or megahertz (MHz; 106 or 1 million cycles per second.) Hence 20,000 Hz—the threshold of ultrasonic sound—would be rendered as 20 kHz.

### INFRASOUND:

Sound of a frequency between 20 Hz, which places it outside the range of audibility for human beings. Its opposite is ultrasound.

### LONGITUDINAL WAVE:

A wave in which the individual segments vibrate in the same direction as the wave itself. This is in contrast to a transverse wave, or one in which the vibration or harmonic motion occurs perpendicular to the direction in which the wave is moving. Waves on the ocean are an example of transverse waves;by contrast, the shock waves of an explosion, the concentric waves of a radio transmission, and sound waves are all examples of longitudinal waves.

### TRANSDUCER:

A device that converts energy into ultrasonic sound waves.

### ULTRASOUND:

Sound waves with a frequency above 20,000 Hz, which makes them in audible to the human ear. Its opposite is infrasound.

### WAVELENGTH:

The distance, measured on a plane parallel to that of the wave itself, between a crest and the adjacentcrest, or the trough and an adjacent trough. On a longitudinal wave, this is simply the distance between waves, which constitute a series of concentric circles radiating from the source.

### WAVE MOTION:

Activity that carries energy from one place to another without actually moving any matter.