Section 1 of 5Sound as a Pressure Wave
Sound is a longitudinal wave that needs a medium, so it cannot travel through a vacuum.
Compressions
- Regions where the particles of the medium are squashed closer together, giving higher pressure and higher density.
Rarefactions
- Regions where the particles are spread further apart, giving lower pressure and lower density.
Travel
- The energy passes along by particles vibrating back and forth, while the particles themselves do not travel with the wave.
Speed
- Sound normally travels fastest in solids, slower in liquids and slowest in gases, because particles are closer in solids.
Section 2 of 5How the Ear Works
The ear converts sound waves in air into vibrations of solid parts, then into electrical signals.
1
The outer ear collects sound waves and directs them along the ear canal towards the eardrum.
2
Pressure variations in the sound wave make the eardrum vibrate with the same frequency as the wave.
3
Three tiny bones, the ossicles, pass on the vibration and increase its force as they carry it to the inner ear.
4
In the cochlea the vibrations move tiny hair cells, which produce electrical impulses in the auditory nerve.
5
The brain interprets the nerve impulses as sound, with the pitch and loudness of the original wave.
Section 3 of 5Range of Hearing
Range of Hearing
Human range
- A healthy young person can hear frequencies from about 20 Hz to about 20 000 Hz.
Ultrasound
- Sound with frequency above 20 000 Hz is too high for humans to hear.
Ageing
- The range usually shrinks with age, and high frequencies are the first to be lost.
Limit
- Hearing is limited because the structures of the ear respond only over a restricted range of frequencies.
| Frequency | Wavelength in air at 330 m/s |
|---|---|
| 20 Hz | 16.5 m |
| 20 000 Hz | 0.0165 m (1.65 cm) |
Section 4 of 5Worked Example
Question
- Sound travels at 330 m/s in air.
- Calculate the wavelength of the lowest frequency a human can hear, 20 Hz.
- Write the wave equationv = f × λ.What the symbols mean: v = wave speed (m s−1) · f = frequency (Hz) · λ = wavelength (m)
- Rearrange for wavelengthλ = v ÷ f.
- Substituteλ = 330 ÷ 20.
- Calculateλ = 16.5 m.
Answer 16.5 m
Section 5 of 5Exam Checklist & Tips
5.1.9 – Key Points for the Exam
- Longitudinal wave A wave in which the vibrations are parallel to the direction of energy transfer.
- Compression A region of a sound wave where the particles are closer together.
- Rarefaction A region of a sound wave where the particles are further apart.
- Frequency The number of waves passing a point each second, measured in hertz.
- Ossicles The three small bones in the middle ear.
Common Mistakes
- Saying sound can travel through a vacuum; it needs particles to vibrate.
- Believing that air particles travel from the source to the ear; they only vibrate about fixed positions.
- Quoting the hearing range as 20 000 Hz to 20 Hz or forgetting the units.
- Describing the ear as sending sound itself to the brain, when it sends electrical impulses.