Class 9 Science Chapter 10: Sound Waves: Characteristics and Applications — Notes, Examples & Practice

Sound is produced by vibrating objects: a guitar string, the skin of a tabla, your vocal cords. Touch a ringing bell gently and you can feel it vibrate.

A vibrating object pushes the air particles next to it together, making a region of high pressure — a compression — then pulls back, making a region of low pressure — a rarefaction. These pass from particle to particle. The particles only oscillate back and forth about their mean positions; it is the disturbance (energy) that travels to your ear, not the air itself.

Because the particles vibrate parallel to the direction the wave travels, sound is a longitudinal wave (unlike water ripples or light, which are transverse). Sound is a mechanical wave: it needs a medium (solid, liquid or gas) and cannot travel through a vacuum — a ringing bell in a jar becomes silent as the air is pumped out.

The distance between two consecutive compressions (or rarefactions) is the wavelength λ (metre). The time for one complete oscillation is the time period T (second). The number of oscillations per second is the frequency f, measured in hertz (Hz): $f = \dfrac{1}{T}$.

The amplitude is the maximum disturbance of the medium from its normal (mean) value — for sound, the maximum change in pressure or particle displacement.

In one time period the wave moves forward by one wavelength, so its speed is $v = \dfrac{\lambda}{T} = f\lambda$. Speed depends on the medium and its temperature (sound is faster in warmer air), not on the frequency.

Pitch is how high or low a sound seems, and it depends mainly on frequency: a whistle or a bird's song has high frequency; a lion's roar or a drum has low frequency. A woman's or child's voice usually has a higher pitch than a man's.

Loudness depends on the amplitude (and the energy carried): hitting a drum harder makes larger vibrations and a louder sound. Loudness is measured in decibels (dB); long exposure to sounds above about 80–85 dB, such as loud earphones, can damage hearing permanently.

Quality (timbre) lets you tell a flute from a violin playing the same note at the same loudness: the waveform has a different shape because each instrument produces a mixture of frequencies. A sound of a single frequency is a tone; a pleasant mixture is a note; an unpleasant one is noise.

Sound reflects from hard surfaces like walls, cliffs and buildings, obeying the same laws as light: the angle of incidence equals the angle of reflection.

An echo is a reflected sound heard separately from the original. Our brain keeps a sound for about 0.1 s, so the echo must return at least 0.1 s later. In air at 344 m/s, the sound travels $344 \times 0.1 = 34.4$ m in that time, there and back — so the reflecting surface must be at least about 17.2 m away.

In a big hall, many reflections overlap and the sound persists — reverberation. Too much makes speech blurred, so auditoriums use sound-absorbing materials (curtains, panels, carpets, soft seats). Curved reflectors are used on purpose in megaphones, stethoscopes and concert-hall ceilings to direct sound to the audience.

Humans hear frequencies from about 20 Hz to 20,000 Hz (20 kHz); the upper limit falls with age. Below 20 Hz is infrasound — produced by earthquakes, whales and elephants. Above 20 kHz is ultrasound — used by bats, dolphins and some insects; dogs can hear some ultrasound.

Ultrasound can travel in a narrow beam and reflects strongly from boundaries, which makes it very useful: ultrasound scanning (sonography) images internal organs and checks the growth of a baby in the womb; ultrasonic cleaning shakes dirt off hard-to-reach parts; it detects cracks in metal blocks; it can break kidney stones into fine grains.

SONAR (Sound Navigation And Ranging) on ships sends ultrasound pulses down and measures the time for the echo from the sea bed or a submarine: depth $= \dfrac{v \times t}{2}$. Bats use the same idea, echolocation, to fly and hunt in the dark.

The outer ear (pinna) collects sound and funnels it down the ear canal to the eardrum, a thin membrane. Compressions push the eardrum in; rarefactions let it move out — so it vibrates at the frequency of the sound.

In the middle ear, three tiny bones — the hammer, anvil and stirrup — amplify these vibrations and pass them to the inner ear.

In the inner ear, the fluid-filled, snail-shaped cochlea converts the vibrations into electrical signals that travel along the auditory nerve to the brain, which interprets them as sound. Loud noise can damage the delicate hair cells of the cochlea, and they do not grow back.