To vary the wavelength of sound used to test a part, which of the following should be changed?

The wavelength of sound in ultrasonic testing is directly related to the frequency of the sound wave. When the frequency increases, the wavelength decreases, and conversely, when the frequency decreases, the wavelength increases. This relationship is governed by the fundamental wave equation, which states that the wavelength is equal to the speed of sound divided by the frequency.

In ultrasonic testing, varying the frequency allows for adjustments in the ability to detect flaws of different sizes and types within materials. For instance, higher frequencies may be used for detecting smaller flaws but have a shorter wavelength, therefore, they may not penetrate as deeply into materials. Conversely, lower frequencies can penetrate deeper but are less sensitive to small defects.

Adjusting the diameter of the transducer, the electrical pulse voltage, or the pulse repetition rate does not directly change the wavelength of the sound being used for testing. While these factors may affect the sensitivity, resolution, or energy of the ultrasonic wave, they do not alter the fundamental relationship between sound frequency and wavelength. Thus, changing the sound wave frequency is the correct approach to vary the wavelength in ultrasonic testing.