In this lesson we will learn how sound waves transfer energy and how sound waves can be characterised.
By the end of this lesson you will be able to:
- Define what sound waves are and explain how they are generated.
- Describe how sound waves consist of compressions and rarefactions, and represent these graphically.
- State the frequency range of human hearing and define ‘infrasound’ and ‘ultrasound’.
- Compare sounds with different intensities and decibel levels.<
- Perform calculations involving sound waves.
- Describe the behaviour of sound waves when they encounter different types of boundaries.
(Image: Skitterphoto, Pixabay)
- Sound is a form of energy that is transferred as mechanical, longitudinal waves.
- Sound waves can travel through solids, liquids and gases, but not through a vacuum.
- Sound waves involve alternating regions of high pressure (compressions) and low pressure (rarefactions).
- On a graph, compressions are represented by peaks and rarefactions are represented by troughs.
- Like all waves, the frequency, wavelength and velocity of a sound wave are mathematically related, as shown by the following formula:
- The frequency of a sound wave is related to its pitch – the higher the frequency, the higher the pitch.
- The amplitude of a sound wave is related to its loudness – the greater the amplitude, the louder the sound.
- The loudness (intensity) of sound is measured using the decibel scale.
- This uses the units decibels, which have the symbol dB.
- The decibel scale is a logarithmic scale, which means that for every 10 dB increase in sound level, there is a 10-times increase in sound intensity.
- Humans can detect sounds ranging from 20 Hz to 20 kHz, but this varies with age, sex, lifestyle and environmental factors.
- Some animal can detect sound waves with frequencies outside this range.
- Infrasound is sound with frequencies below 20 Hz.
- Ultrasound is sound with frequencies above 20 kHz.
- Three outcomes are possible when a sound wave encounters an object:
- Transmission – the sound wave travels through the object.
- Reflection – the sound wave bounces back off the object.
- Absorption – the sound wave is ‘trapped’ by the object and converted to heat energy.
- Hard surfaces reflect sound waves more than soft surfaces.
- Thicker or more dense objects absorb more sound; thinner or less dense objects transmit more sound.
- Low frequency sounds are more likely to be transmitted; high frequency sounds are more likely to be reflected or absorbed.
(Image: © Dan Russell)
(Header image: Thaut Images, Adobe Stock)