Velocity:
The distance covered by the wave in
unit time.
Frequency:
The number of complete disturbances (cycles) in unit time. Usually
expressed in Hertz.
Wavelength:
The distance between successive points of equal phase
in a wave.
Properties
of Acoustic Plane Wave
Wavelength, Frequency and Velocity
Among the properties of waves propagating in isotropic solid materials are wavelength, frequency,
and velocity. The wavelength is directly proportional
to the velocity of the wave and inversely proportional to the
frequency of the wave. This relationship is shown by the following
equation.
The applet below shows a longitudinal and transverse wave. The
direction of wave propagation is from left to right and the movement
of the lines indicate the direction of particle oscillation. The
equation relating ultrasonic wavelength, frequency, and propagation
velocity is included at the bottom of the applet in a reorganized
form. The values for the wavelength, frequency, and wave velocity
can be adjusted in the dialog boxes to see their effects on the
wave. Note that the frequency value must be kept
between 0.1 to 1 MHz (one million cycles per second) and the wave
velocity must be between 0.1 and 0.7 cm/us.
As can be noted by the equation, a change in frequency will
result in a change in wavelength. Change the frequency in the
applet and view the resultant wavelength. At a frequency of .2
and a material velocity of 0.585 (longitudinal wave in steel)
note the resulting wavelength. Adjust the material velocity to
0.480 (longitudinal wave in cast iron) and note the resulting wavelength.
Increase the frequency to 0.8 and note the shortened wavelength
in each material.
In ultrasonic testing, the shorter wavelength resulting from
an increase in frequency will usually provide for the detection
of smaller discontinuities. This will be discussed more in following
sections.
