Ultrasound

Definition and Frequency Range

Ultrasound refers to sound waves with frequencies above 20 kHz (20,000 Hz), which is beyond the upper limit of human hearing. Humans typically hear sounds between 20 Hz and 20 kHz, so ultrasound waves are inaudible to us.

Ultrasound is widely used in both medical and industrial fields due to its ability to travel through different materials and reflect off boundaries. It is especially useful for imaging and detecting objects or flaws inside materials without damaging them.

Example: If an ultrasound echo returns after 0.01 seconds and the speed of sound in the medium is 1500 m/s, the distance to the reflecting object is calculated as $\frac{1500 \times 0.01}{2} = 7.5$ metres.

Production and Detection

Ultrasound waves are produced and detected using devices called ultrasound transducers. These rely on the piezoelectric effect, where certain crystals generate an electric signal when compressed and conversely change shape when an electric signal is applied.

When an alternating electrical signal is applied to the crystal in the transducer, it vibrates at ultrasonic frequencies, producing ultrasound waves. These waves travel through a medium and reflect off boundaries between materials with different densities.

The reflected ultrasound waves (echoes) return to the transducer, causing the crystal to vibrate and generate an electrical signal. This signal is then processed to create an image or detect the presence of objects.

Applications of Ultrasound

Medical imaging: Ultrasound is commonly used in hospitals to create images of internal body structures, such as unborn babies during pregnancy (scanning). It is safe because it does not use ionising radiation.

Industrial flaw detection: Ultrasound is used to find cracks or faults inside metal parts or welds without damaging them. The ultrasound waves reflect differently from flaws, allowing technicians to locate defects.

Cleaning and physiotherapy: Ultrasound waves can clean delicate objects by causing tiny bubbles to form and collapse in liquids (ultrasonic cleaning) through a process called cavitation. In physiotherapy, ultrasound helps to promote healing by increasing blood flow and warming tissues.

Properties and Behaviour in Media

Ultrasound waves can travel through solids and liquids but not through gases as effectively. This is because ultrasound waves are longitudinal and require particles to be close together to transmit vibrations efficiently; gases have particles that are much further apart, so ultrasound travels poorly through them.

They travel faster in solids than in liquids because particles are closer together, allowing vibrations to pass more quickly.

When ultrasound waves meet a boundary between two materials with different densities, some of the waves are reflected back, and some are refracted (change direction). This reflection and refraction are key to ultrasound imaging and distance measurement.

Ultrasound is used to measure distances by timing how long it takes for an echo to return. This principle is used in echo sounding, such as measuring the depth of the sea or locating objects underwater.

For example, if an ultrasound pulse is sent from a ship to the seabed and the echo returns after 0.5 seconds, the depth can be calculated using the speed of sound in water (about 1500 m/s):

$$\text{Distance} = \frac{\text{Speed} \times \text{Time}}{2}$$

The time is divided by 2 because the ultrasound pulse travels to the seabed and back.

So, the depth is:

$$\frac{1500 \times 0.5}{2} = 375 \text{ metres}$$