Frequencies beyond hearing
Definition
Ultrasound
Sound with a frequency greater than 20 000 hertz, above the upper limit of normal human hearing.
Definition
Infrasound
Sound with a frequency less than 20 hertz, below the lower limit of normal human hearing.
- Sound from about 20 Hz20\ \text{Hz}20 Hz to 20 000 Hz20\,000\ \text{Hz}20000 Hz lies within the typical hearing range of a young person.
- The limits describe frequency, not loudness, so an intense ultrasound wave remains inaudible to humans.
- Ultrasound and infrasound are mechanical waves and therefore require a medium.
Wave properties
- For every sound wave, speed, frequency and wavelength are linked by v=fλv=f\lambdav=fλ.
- At a fixed sound speed, ultrasound has a shorter wavelength than audible sound, while infrasound has a longer wavelength.
- Short wavelengths can detect small details, which is useful for imaging and flaw detection.
- Long wavelengths can travel around large obstacles and may propagate over very long distances.
Producing and detecting ultrasound
- A rapidly alternating potential difference can make a piezoelectric crystal change shape and vibrate.
- The vibrating crystal produces ultrasound at the driving frequency.
- Returning ultrasound makes the crystal vibrate and produces an alternating potential difference, so the same type of transducer can act as a detector.
Sources of infrasound
- Large, slow vibrations can produce infrasound, including earthquakes, volcanic activity, ocean waves and large machinery.
- Some animals can produce or detect frequencies outside the human hearing range.
- Detecting a wave does not mean a person hears it; electronic sensors can convert it into a measurable electrical signal.
Example
Comparing wavelengths
- Sound travels at 340 m s−1340\ \text{m s}^{-1}340 m s−1 in air.
- For ultrasound at 40 000 Hz40\,000\ \text{Hz}40000 Hz, λ=34040 000=8.5×10−3 m\lambda=\dfrac{340}{40\,000}=8.5\times10^{-3}\ \text{m}λ=40000340=8.5×10−3 m.
- For infrasound at 10 Hz10\ \text{Hz}10 Hz, λ=34010=34 m\lambda=\dfrac{340}{10}=34\ \text{m}λ=10340=34 m.
- The infrasound wavelength is much larger than the ultrasound wavelength.
Exam technique
- Classify a sound by comparing its frequency with the two numerical boundaries and include the unit hertz.
- When explaining a use, link frequency to wavelength and then to the required effect, such as resolving small structures.
Common Mistake
- Do not write that ultrasound is sound above the loudness threshold; ultrasound is defined only by frequency.
- Use 20 000 Hz20\,000\ \text{Hz}20000 Hz, not 20 000 kHz20\,000\ \text{kHz}20000 kHz.
Self review
- What frequency range defines ultrasound?
- What frequency range defines infrasound?
- Why does ultrasound have a short wavelength in a given medium?
- How can a piezoelectric crystal produce and detect ultrasound?
- Name two natural sources of infrasound.
