Fields of Ultrasonic Transducers
The sound that emanates from a piezoelectric transducer does
not originate from a point, but instead originates from most of
the surface of the piezoelectric element. Round transducers are
often referred to as piston source transducers because the sound
field resembles a cylindrical mass in front of the transducer.
The sound field from a typical piezoelectric transducer is shown
below. The intensity of the sound is indicated by color, with
lighter colors indicating higher intensity.
Since the ultrasound originates from a number of points along
the transducer face, the ultrasound intensity along the beam is
affected by constructive and destructive wave interference as
discussed in a previous page on wave
interference. These are sometimes also referred to as diffraction
effects. This wave interference leads to extensive fluctuations in the sound intensity near
the source and is known as the near field. Because of acoustic variations within a near field,
it can be extremely difficult to accurately evaluate flaws in
materials when they are positioned within this area.
The pressure waves combine to form a relatively uniform front at the end of the near field. The area beyond the near field where the ultrasonic beam is more uniform is called the far field. In the far field, the
beam spreads out in a pattern originating from the center of the
transducer. The transition between the near field and the far field occurs at a distance,
N, and is sometimes referred to as the "natural
focus" of a flat (or unfocused) transducer. The near/far
field distance, N, is significant because amplitude variations
that characterize the near field change to a smoothly declining
amplitude at this point. The area just beyond the near field
is where the sound wave is well behaved and at its maximum strength.
Therefore, optimal detection results will be obtained when flaws
occur in this area.
For a piston source transducer of radius (a), frequency
(f), and velocity (V) in a liquid or solid medium,
the applet below allows the calculation of the near/far field
transition point. In the Java applet below, the radius (a) and the near field/far field distance can be in metric or English units (e.g. mm or inch), the frequency (f) is in MHz and the sound velocity (V) is in metric or English length units per second (e.g. mm/sec or inch/sec). Just make sure the length units used are consistent in the calculation. To see an example calculation click here.
Spherical or cylindrical focusing changes the structure
of a transducer field by "pulling" the N point
nearer the transducer. It
is also important to note that the driving excitation normally
used in NDT applications are either spike or rectangular pulsars,
not a single frequency. This can significantly alter the performance
of a transducer. Nonetheless, the supporting analysis is widely
used because it represents a reasonable approximation and a good