In the far field of a uniform ultrasonic beam, sound intensity is maximum at which location?

• A. Minimum at the beam center line
• B. Maximum at the beam center line
• C. Maximum throughout twice the angle
• D. Not related to orientation of the beam

Answer: (B)

In the far field of a uniformly illuminated beam, the intensity is greatest along the beam’s center line because all parts of the aperture reach that direction with essentially the same phase, so their waves add up constructively. Off-axis directions introduce path length differences across the aperture, causing partial cancellation and a lower intensity. This central, on-axis maximum is the main lobe of the pattern for a uniformly illuminated aperture, with intensity falling off as you move away from the centerline. Therefore, the location of maximum intensity is on the beam center line.