BASICS OF ACOUSTICS: Groups Of Controlled Parameters 3

6.5 The fifth group of parameters: the duration of the probe pulse and the PET’s own noise – the dead zone.
The dead zone is the area of the controlled object near the input surface, in which defects are not detected by the echo method.
It refers to the parameters of the method, because when changing material, the size of this zone is determined by height, and depends on several parameters.

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Fig.7.6 – Illustration of the “dead zone”


The main reason for the appearance of the dead zone is the presence of a probing pulse in the A-scan, which can contain from 4 to 12 periods of oscillations. Accordingly, the longer the duration of the probe pulse and the duration of the noise of the PET, the larger is the dead band.
The magnitude of the dead zone for inclined transducers depends on the angle of entry. The larger the angle of entry, the less the dead zone, since the acoustic axis is closer to the surface.
Calculation of the dead zone can be carried out according to the formula:

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Where SI is the duration of the probe pulse, μs; N – duration of the noise of the PET, μs.

6.6 Sixth group of parameters: resolution of equipment – resolution on range and front
The resolution of the equipment is the minimum time between two echoes visible on the A-scan as separate.

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Fig.7.7 – Determination of resolving power: equipment (ΔTmin), range (ΔRmin), front (ΔХmin)

Separate signals recognize two echo signals, if the level of their fusion is below the nearest maximum by more than 6 dB.
Range resolution is the minimum distance between two reflectors located at different distances along the beam signals from which they are seen as separate on the A-scan.
The resolution on the front is the minimum distance between two reflectors located at different distances along the beam signals from which they are seen as separate on the A-scan.