How does backing (damping) material influence pulse duration and bandwidth?

• A. It lengthens the pulse and narrows the bandwidth
• B. It broadens the pulse and narrows the bandwidth
• C. It shortens the pulse and broadens the bandwidth
• D. It has no effect on the pulse duration

Answer: (C)

Damping with backing absorbs energy and suppresses ringing of the transducer after the drive pulse. This makes the acoustic pulse die away much faster, so the time-domain pulse is shorter. A shorter pulse in time corresponds to a broader range of frequencies in the spectrum, so the bandwidth increases. If damping were weak, the pulse would linger longer and the spectrum would be narrower. So the option describing a shortened pulse with a broadened bandwidth captures the true effect of backing material. The other statements would imply longer pulses, narrower bandwidth, or no change, which don’t reflect how damping shapes the pulse.