Spec links: 4.6.1.1 Transverse and longitudinal waves · 4.6.1.2 Properties of waves · 4.6.1.4 Sound waves (Physics only) (HT) · 4.6.1.5 Waves for detection and exploration (Physics only) (HT) · Reading time: about 4 minutes
The first photo
For many families, the first picture of a new baby is taken months before the birth. It is a grainy black-and-white image on a screen. You can see a tiny head, a curled-up body and sometimes a beating heart. No camera is used and no light gets inside the body. The picture is made with sound – sound so high that no human can hear it.
Doctors in Glasgow began using this idea to look at unborn babies in the 1950s. Today it is a routine part of pregnancy care.
What is a sound wave?
Waves transfer energy from one place to another without transferring matter. There are two types.
In a transverse wave, the oscillations are perpendicular (at right angles) to the direction of energy transfer. Ripples on water are an example.
In a longitudinal wave, the oscillations are parallel to the direction of energy transfer. The particles squash together in compressions and spread apart in rarefactions. Sound waves are longitudinal.
Humans can hear sounds with a frequency from about 20 Hz to 20 000 Hz (20 kHz). Sound above 20 kHz is called ultrasound. Medical scanners use frequencies of a few million hertz (megahertz, MHz).
Speed, frequency and wavelength
All waves obey the wave equation:
wave speed = frequency × wavelength, or v = f λ
Speed is in metres per second (m/s), frequency in hertz (Hz) and wavelength in metres (m).
Ultrasound travels through soft body tissue at about 1500 m/s. A scanner uses a frequency of 3 MHz, which is 3 000 000 Hz. What is the wavelength?
λ = v ÷ f = 1500 ÷ 3 000 000 = 0.0005 m, which is 0.5 mm.
Such a short wavelength helps the scanner show small details.
Listening for echoes
The probe sends short pulses of ultrasound into the body. When the waves meet a boundary between two different tissues, such as fluid and bone, some of the wave is reflected. The rest carries on to the next boundary.
The reflected waves are echoes. The probe detects them and a computer times each one. The pulse travels to the boundary and back again, so:
distance to boundary = speed × time ÷ 2
Suppose an echo returns after 0.0001 s (one ten-thousandth of a second):
distance = 1500 × 0.0001 ÷ 2 = 0.075 m, which is 7.5 cm.
The computer does thousands of these sums every second. It uses the times and strengths of the echoes to build up the moving picture on the screen.
Why use sound?
X-rays can also see inside the body, but they are ionising radiation. They can damage cells and DNA, which is a special worry for a growing baby. Ultrasound is not ionising. At the levels used in scans it is thought to be very safe, so it can be used again and again during a pregnancy.
The same idea is used in other places too. Ships use pulses of sound to measure the depth of the sea. This is called echo sounding.
Key words
- Transverse wave: a wave in which the oscillations are perpendicular to the direction of energy transfer.
- Longitudinal wave: a wave in which the oscillations are parallel to the direction of energy transfer, showing compressions and rarefactions.
- Frequency: the number of waves passing a point each second; unit hertz (Hz).
- Wavelength: the distance from a point on one wave to the equivalent point on the adjacent wave.
- Ultrasound: sound waves with a frequency higher than the upper limit of human hearing (above 20 kHz).
- Echo: a reflected sound wave; ultrasound is partially reflected at a boundary between two different media.
Questions
- What is the range of normal human hearing? [1 mark]
- Is sound a transverse or a longitudinal wave? [1 mark]
- What happens to ultrasound when it reaches a boundary between two different tissues? [2 marks]
- Explain the difference between a transverse wave and a longitudinal wave. Give one example of each. [3 marks]
- Explain why ultrasound, rather than X-rays, is used to scan unborn babies. [2 marks]
- A scanner produces ultrasound with a frequency of 5 MHz. The speed of ultrasound in soft tissue is 1500 m/s. Calculate the wavelength. Give your answer in metres. [3 marks]
- A ship sends a pulse of sound to the sea bed. The echo returns after 0.80 s. The speed of sound in sea water is 1500 m/s. Calculate the depth of the sea. Give the unit. [3 marks]
- Describe how an ultrasound scanner can be used to find the distance from the skin to a baby inside the body. [6 marks]
Show answers
- 20 Hz to 20 kHz / 20 000 Hz (1)
- Longitudinal (1)
- Some of it is reflected (as an echo) (1); the rest is transmitted / carries on (1)
- Transverse: oscillations perpendicular to the direction of energy transfer (1); longitudinal: oscillations parallel to the direction of energy transfer (1); one correct example of each, e.g. ripples on water / electromagnetic waves and sound waves (1)
- X-rays are ionising and can damage cells / DNA of the baby (1); ultrasound is not ionising / is (thought to be) safe, so can be used repeatedly (1)
- 5 MHz = 5 000 000 Hz and λ = v ÷ f = 1500 ÷ 5 000 000 (1); λ = 0.0003 (1); m (allow 3 × 10-4 m) (1)
- distance = speed × time ÷ 2 = 1500 × 0.80 ÷ 2 (1); = 600 (1); m (1) (an answer of 1200 m scores 1 mark)
- Indicative content:
• probe placed on the skin (with gel) sends a pulse of ultrasound into the body
• ultrasound travels through tissue at a known speed (about 1500 m/s)
• at the boundary between different tissues, e.g. fluid and the baby, some ultrasound is reflected
• the reflected pulse / echo is detected by the probe
• the time between sending the pulse and receiving the echo is measured
• distance = speed × time
• divide by 2 because the pulse travels there and back
• many echoes from different boundaries are used to build an image
Level 3 (5–6 marks): a clear, logically ordered description including reflection at a boundary, timing of the echo and correct use of distance = speed × time ÷ 2 with the reason for halving; Level 2 (3–4 marks): describes sending and detecting a reflected pulse and links time to distance, but may omit halving or the boundary idea; Level 1 (1–2 marks): simple statements, e.g. ultrasound reflects off the baby, or the time is measured
More reading: See all 30 GCSE Physics comprehension articles