Which of the following is NOT a factor affecting beam divergence?

• A. Frequency
• B. Crystal diameter
• C. Wavelength
• D. Material color

Answer: (D)

Beam divergence in ultrasonic testing comes from diffraction at the transducer’s finite aperture and the wavelength of the ultrasound. A larger crystal diameter acts as a bigger aperture, reducing spreading and producing a narrower beam. The wavelength (and thus frequency, since wavelength is speed divided by frequency) sets how strongly the beam diffracts: higher frequency means shorter wavelength and less divergence, while lower frequency means longer wavelength and more divergence. Material color doesn’t affect the acoustic wave’s speed, wavelength, or how the beam diffracts, so it does not influence beam divergence.