Phased Array 

PA – Phased Array is the name given to a special type of ultrasonic probe.

An array is a group of transmitters, receivers or transmitter/receivers, generally called array elements. When used as a transmitter, firing the elements at different times can lead to interference between the sound waves produced by each individual element. This interference can be both constructive (waves add together) and destructive (waves cancel).

It is this interference which gives the array probe its main advantage – the ability to change beam shape and angle depending upon the timing at which elements are fired. When an array is used as a receiver, the difference between the times at which a pulse arrives at each array element contains information about the location of the pulse source.

Your ears are a simple two-element receiving array. By operating together they allow you not only to hear the sound but also identify the direction of a sound source. So it is with an ultrasonic array: by adding the signals received by each receiver or element of the array in a particular manner,
it is possible to 'listen' to sound arriving along a particular angular direction or from a particular point. When operating in the transmit mode it is possible to send the sound beam out along a particular angular direction or focus it at a particular point.

Conventional twin-crystal probes are not classed as an array because one crystal is used to transmit and one is used to receive. The phased refers to the fact that in order to achieve the desired beam angle or focus, the phase of the signals transmitted or received on the elements need to be adjusted relative to each other.

The manipulation of the sound beam requires that the elements in the array are small relative to the wavelength and that the spacing between the elements is also small relative to the wavelength. Consequently, the performance of an array probe depends upon the number of the elements, the size of the elements and the distance between them. Special signal processing equipment is required to operate the array. 

Phased arrays have been used for a long time in radar, sonar and medical applications but
NDT ultrasonics has the disadvantage of having to get the sound into the metal and having to cope with both longitudinal and shear wave modes and the relatively small wavelengths involved.

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