Ultrasound waves have a frequency higher than the upper limit of hearing…

The idea

Ultrasound is defined by frequency and nothing else: it is a longitudinal sound wave with a frequency above 20 000 Hz, so a human cannot hear it. Medical scanners typically use between 1 MHz and 15 MHz. The high frequency matters because, from the wave equation v = f λ, at a fixed speed a higher frequency means a shorter wavelength, and a shorter wavelength resolves smaller detail and reflects cleanly from small boundaries. In soft tissue the speed is about 1500 m/s, so a 3 MHz wave has a wavelength of about 0.5 mm. Ultrasound obeys all the usual wave rules: it needs a medium, it is reflected and refracted at boundaries, and it is absorbed as it travels, which is why deep structures give weaker echoes.

What earns the marks

• Defining ultrasound as sound of frequency greater than 20 000 Hz.
• Stating that it lies above the upper limit of human hearing so cannot be heard.
• Using v = f λ correctly to find a wavelength from a given ultrasound frequency.
• Linking a shorter wavelength to finer detail in the image.

Common mistakes and how to put them right

• Calling ultrasound an electromagnetic wave, or saying it travels at the speed of light.

• Saying ultrasound is louder or more powerful rather than higher in frequency.

• Substituting the speed of sound in air when the question gives a speed in tissue.

• Leaving the frequency in MHz when the speed is in m/s, giving a wavelength out by a factor of a million.

Exam tips

• Convert MHz to Hz before any wave equation calculation and show the conversion.
• Define ultrasound by comparing it with 20 kHz; a definition without that comparison rarely scores.
• Use the speed the question gives you, not a remembered value, when tissue or water is involved.