Section 2: Physics of Ultrasound

2.11: Signal-to-Noise Ratio
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.