Ultrasound

Ultrasound is sound with a frequency higher than what humans can hear. Humans hear about 20 Hz to 20 000 Hz. Ultrasound has frequency $f > 20\,\text{kHz}$ (20 000 Hz).

Key ideas

• Ultrasound is a longitudinal wave: particles in the medium vibrate back and forth along the direction the wave travels.
• It needs a medium (air, water, or solid) and cannot travel in a vacuum.
• Echoes are reflections of sound. Ultrasound reflects strongly from boundaries between different materials.
• Speed depends on the medium (about 330–350 m/s in air; much faster in water and solids). Frequency does not change the speed in one medium.
• Higher frequency means shorter wavelength, which can give finer detail in scans, but it does not mean the sound travels faster.

How ultrasound is used

• Medical scanning (soft tissue): A probe sends pulses of ultrasound into the body. Reflections from tissues return to the probe and a computer builds an image. Gel is used to remove air gaps so sound enters the body.
• Non-destructive testing (NDT): Engineers send ultrasound into metal or plastic parts (like a bridge cable). Cracks reflect the waves, showing hidden damage without breaking the object.
• Sonar: Boats send ultrasound into water. The echo from the seabed or fish returns. Time taken tells the depth or distance.

Echo timing: finding distance

For a pulse that goes there and back, the total distance is twice the one-way distance. If the wave speed is $v$ and the measured time is $t$:

$$\text{depth (one way)} = \frac{v\times t}{2}$$

Common misconceptions

• “Higher frequency travels faster.” In one medium, speed is fixed by the medium; frequency changes wavelength: $v = f\lambda$.
• “Gel in scans is for cooling.” The gel removes air so ultrasound enters the body efficiently.

Summary

• Ultrasound: $f > 20\,\text{kHz}$; longitudinal; needs a medium.
• Used in medical imaging, NDT, and sonar.
• Depth or distance from echoes: $\text{one-way distance} = \dfrac{v t}{2}$.