Sound waves (physics only) (HT only) — AQA GCSE Physics

The ear is a converter: it turns pressure variations in air into vibrations of solids and then into nerve impulses. A sound wave travelling down the ear canal makes the air pressure at the ear drum rise and fall, so the drum, a thin stretched membrane, is pushed in and out at the frequency of the wave. The drum is joined to three small bones, the ossicles, which pass the vibration on and amplify it before it reaches the fluid filled cochlea. Hair cells in the cochlea respond to those vibrations and send electrical impulses along the auditory nerve, and the brain interprets them as sound. The sensation of sound is produced in the brain, not in the ear itself. A larger amplitude gives a louder sound; a higher frequency gives a higher pitch.

A solid part such as the ear drum has its own mass and stiffness, so it responds well only to a band of driving frequencies. If the wave oscillates far faster than the drum can follow, the drum barely moves; if it oscillates very slowly, the pressure change is too gradual to build a useful vibration. So converting a sound wave into vibration of a solid, and back again, is efficient only over a limited frequency range. The same limit applies to every device that does this job: a microphone diaphragm, a loudspeaker cone and the ossicles of the ear all have a working range set by their size, mass and stiffness. Outside that band the transfer becomes inefficient, so waves outside it produce little or no vibration and therefore little or no signal.

Human hearing runs from about 20 Hz to 20 kHz. The conversion of sound waves to vibrations of solids works over a limited frequency range. The ceiling exists because the structures within the ear, such as the ear drum and ossicles, cannot vibrate fast enough to detect higher frequencies. Sounds above the upper limit are ultrasound and those below the lower limit are infrasound. The range of human hearing narrows with age, with the top end falling first. Prolonged exposure to loud noise can also damage the ear and raise the minimum sound level a person can detect, particularly at high frequencies. Exam questions may ask students to interpret graphs showing how the minimum audible level changes with age and noise exposure.

This is the skill the section builds to, and it is Higher tier physics only. You need a worked example in both directions. Sound to vibration: pressure variations in air push on the ear drum, the drum vibrates, the ossicles pass the vibration to the cochlea, and nerve impulses travel to the brain. Vibration to sound: in a loudspeaker a varying current in a coil sitting in a magnetic field feels a varying force, so the coil and cone vibrate, and the moving cone pushes on the air to create the compressions and rarefactions of a sound wave. Each converter works only over a limited frequency range because the vibrating solid has mass and stiffness and cannot follow oscillations that are too fast. For the ear this sets the audible range at 20 Hz to 20 kHz.

These two figures must be recalled. 20 Hz means twenty complete oscillations every second and is the lowest frequency a person with normal hearing can detect; 20 kHz is 20 000 Hz and is the highest. Keep the units straight, because kHz means a thousand hertz, so the audible range spans a factor of one thousand. The numbers matter because they define ultrasound, which is any sound above 20 kHz, and because the wave equation turns them into wavelengths: with the speed of sound in air about 330 m/s, 20 Hz gives a wavelength of about 16.5 m while 20 kHz gives about 1.65 cm. The range is for normal hearing; individuals vary, and the upper limit falls with age and with exposure to loud noise.