T.O. 33B-1-1
5-12
5.1.6.1.1
Frequency Bandwidth.
The above discussion on frequency pertains to the peak frequency used in an inspection. In all cases, the ultrasonic
instrument and search unit produces a band of ultrasonic energy covering a range of frequencies. The range is
expressed as bandwidth. Ultrasonic inspection procedures can be sensitive to frequency. Therefore, the inspection
results can be affected by variation in the bandwidth of the inspection system. For example, certain inspections use loss
of back reflection as criteria for rejection. Frequencies that are too high can lead to diminished or complete loss of back
reflection due to the sound being scattered by a rough inspection surface, large grain structure in the test material, or
small irrelevant discontinuities. In other words, improper choice of peak frequency and bandwidth of the inspection
system (instrument and transducer) can produce irrelevant indications that affect inspection results. Both the
instrument and the transducer affect the bandwidth of the inspection. Therefore, it is important to have a reference
standard of the same material manufactured with the same manufacturing process and having the same surface
conditions as the test part, so the inspection results will be the same for different inspection systems. Instruments are
constructed to pulse the search unit, and measure the response in different ways with respect to bandwidth. Some
instruments use a broadband pulser and a broadband amplifier. With these instruments, the bandwidth is controlled by
the search unit. A given search unit has a maximum response at the natural resonant frequency of the transducer
element. However, the element will also respond at other frequencies. The search unit response to these other
frequencies is controlled by its internal construction. Modern instruments are designed to be operated in either narrow
band or broad band modes to accommodate a variety of transducers. A broader bandwidth means better resolution; and
a narrow bandwidth means greater sensitivity. Ultrasonic systems are generally designed with respect to bandwidth to
provide a reasonable compromise between resolution and sensitivity.
5.1.6.2
Sound Beam Characteristics.
The sound beam does not propagate uniformly through the volume defined by the straight-sided projection of the
transducer face. Side lobes exist along the outer edges of the beam near the transducer face, and sound intensity is not
uniform throughout the beam.
5.1.6.2.1
Dead Zone.
In contact scanning, there is an area beneath the search unit in which there can be no inspection. When a transducer is
excited, it vibrates for a finite amount of time during which it cannot act as a receiver for a reflected echo. If a defect
were close to the surface, the reflected sound would arrive back at the transducer while it is still transmitting. A dead
or no-inspection zone is inherent in all ultrasonic equipment. In some types of equipment, the dead zone is not obvious
because the length of the transmitted pulse can be electronically suppressed. The dead zone can be estimated
experimentally. Generally, the shorter the pulse length, the shallower the dead zone will be.
5.1.6.2.2
Near Field.
Extending from the face of the search unit is an area characterized by wide variations in sound beam intensity. This
area is called the near field (Fresnel zone) as shown in Figure 5-12. The length of the near field, can be calculated by
the following equation:
N
D
D f
v
=
=
2
2
4
4
l
Where:
N = near field length (inches)
D = diameter of transducer element in a round search unit or maximum diagonal of transducer element
in a rectangular or square search unit (inches)
l = wavelength of sound in the test material (inches)
f = frequency (Hertz)
v = velocity (inches per second)
