In a previous page, the effect that frequency and wavelength have
on flaw detectability was discussed. However, the detection of
a defect involves many factors other than the relationship of
wavelength and flaw size. For example, the amount of sound that
reflects from a defect is also dependent on the acoustic impedance mismatch
between the flaw and the surrounding material. A void is generally
a better reflector than a metallic inclusion because the impedance
mismatch is greater between air and metal than between two metals.
Often, the surrounding material has competing reflections. Microstructure
grains in metals and the aggregate of concrete are a couple of
examples. A good measure of detectability of a flaw is its signal-to-noise
ratio (S/N). The signal-to-noise ratio is a measure of how the
signal from the defect compares to other background reflections
(categorized as "noise"). A signal-to-noise ratio of
3 to 1 is often required as a minimum. The absolute noise level
and the absolute strength of an echo from a "small"
defect depends on a number of factors, which include:
- The probe size and focal properties.
- The probe frequency, bandwidth and efficiency.
- The inspection path and distance (water and/or solid).
- The interface (surface curvature and roughness).
- The flaw location with respect to the incident beam.
- The inherent noisiness of the metal microstructure.
- The inherent reflectivity of the flaw, which is dependent on
its acoustic impedance, size, shape, and orientation.
- Cracks and volumetric defects can reflect ultrasonic waves
quite differently. Many cracks are "invisible" from
one direction and strong reflectors from another.
- Multifaceted flaws will tend to scatter sound away from the
The following formula relates some of the variables affecting
the signal-to-noise ratio (S/N) of a defect:
Rather than go into the details of this formulation,
a few fundamental relationships can be pointed out. The signal-to-noise
ratio (S/N), and therefore, the detectability of a defect:
- Increases with increasing flaw size (scattering amplitude).
The detectability of a defect is directly proportional to its
- Increases with a more focused beam. In other words, flaw detectability
is inversely proportional to the transducer beam width.
- Increases with decreasing pulse width (delta-t). In other
words, flaw detectability is inversely proportional to the duration
of the pulse produced by an ultrasonic transducer. The shorter
the pulse (often higher frequency), the better the detection
of the defect. Shorter pulses correspond to broader bandwidth
frequency response. See the figure below showing the waveform
of a transducer and its corresponding frequency spectrum.
- Decreases in materials with high density and/or a high ultrasonic
velocity. The signal-to-noise ratio (S/N) is inversely proportional
to material density and acoustic velocity.
- Generally increases with frequency. However, in some materials,
such as titanium alloys, both the "Aflaw"
and the "Figure of Merit (FOM)" terms in the equation
change at about the same rate with changing frequency. So, in some
cases, the signal-to-noise ratio (S/N) can be somewhat independent