As discussed on the radiated fields page, near the ultrasonic transducer there are significant fluctuations in the sound intensity due to constructive and destructive interference of the multiple waves which originate from the transducer face. Because of acoustic variations within this field, called the near field, it can be extremely difficult to accurately evaluate flaws in materials when they are positioned within this area.

However, at some point the pressure waves combine to form a relatively uniform front. The area where the ultrasonic beam is more uniform and spreads out in a pattern originating from the center of the transducer is called the far field. Knowing where the far field starts is important since optimal detection occur when flaws are located at the start of far field since this is where the sound wave is well behaved and at its maximum strength. The transition point between the near field and the far field (sometimes referred to as the "natural focus" of an unfocused transducer) can be calculated if the frequency and diameter of the transducer and the speed of sound in the material are known.

Example Calculation

Calculate the end of the near field when using a 2.25 MHz, 0.375 inch diameter transducer to inspect a component made of brass. The sound velocity in brass is 0.1685x10⁶ inch/second

Near Field Formula

        

Where:	N = Near Field Length or Transition from Near Field to Far Field
	D = Diameter of the Transducer
	F = Frequency of the Transducer
	l = Wavelength
	V = Velocity of Sound in the Material

Substitute the values into the formula.

        

Complete the square and cancel terms where possible.

Multiply.

Divide.

The near field will extend into the material 0.469 inches from the transducer face. Within this near field area, it is hard to predict the signal amplitude from a reflector.