The speed of sound in most piezoelectric material ranges from what value?

• A. 4 to 6 mm/us
• B. 2 to 3 mm/us
• C. 6 to 8 mm/us
• D. 1 to 2 mm/us

Answer: (A)

Think about how sound travels in solids: the speed depends on how stiff the material is and how dense it is. Piezoelectric ceramics used in ultrasonic devices are relatively stiff solids, so longitudinal waves move at a few thousand meters per second. In common piezoelectric materials this shows up as roughly 4 to 6 millimeters per microsecond. That range, about 4000–6000 m/s, is the typical speed you’ll see cited for sound in these materials. The other options don’t fit: 2–3 mm/µs is slower (more like shear waves or softer media), 6–8 mm/µs is higher than the usual for these ceramics, and 1–2 mm/µs is more typical of liquids or very soft solids. So 4 to 6 mm/µs is the standard speed.