As frequency increases for a given diameter crystal, the angle of beam divergence:

As frequency increases for a given diameter crystal, the angle of beam divergence:

• A. Decreases
• B. Remains unchanged
• C. Increases
• D. Varies uniformly through each wavelength

Answer: (A)

When a finite transmitting aperture emits ultrasound, diffraction from the aperture determines how wide the main beam is. The angular width of that beam is roughly proportional to the wavelength divided by the aperture diameter. For a given crystal diameter, increasing frequency shortens the wavelength (since wavelength is inversely related to frequency in the same medium). A shorter wavelength diffracts less, so the main beam becomes narrower. Hence the angle of beam divergence decreases as frequency increases.

When a finite transmitting aperture emits ultrasound, diffraction from the aperture determines how wide the main beam is. The angular width of that beam is roughly proportional to the wavelength divided by the aperture diameter. For a given crystal diameter, increasing frequency shortens the wavelength (since wavelength is inversely related to frequency in the same medium). A shorter wavelength diffracts less, so the main beam becomes narrower. Hence the angle of beam divergence decreases as frequency increases.