Sound: Class 10 Physics

Key points

Sound is produced by a vibrating body, such as a guitar string, a tuning fork or the skin of a drum. The vibrations push on the surrounding particles and the disturbance travels outward as a wave. Sound needs a material medium (solid, liquid or gas). It cannot travel through a vacuum. A bell ringing inside a jar becomes silent when the air is pumped out.

Sound travels as a longitudinal wave. The particles of the medium vibrate back and forth along the same line in which the wave moves. This creates regions where the particles are crowded together, called compressions (high pressure), and regions where they are spread apart, called rarefactions (low pressure). One compression and one neighbouring rarefaction make one complete wave.

Terms used to describe a wave:

• Wavelength (λ): the distance between two successive compressions or two successive rarefactions. Unit: metre.
• Frequency (f): the number of complete waves passing a point in one second. Unit: hertz (Hz).
• Time period (T): the time for one complete wave, T = 1/f. Unit: second.
• Amplitude: the maximum displacement of a particle from its rest position.
• Wave speed (v): v = f × λ. This holds for every kind of wave.

The speed of sound depends on the medium. It is smallest in gases, larger in liquids and largest in solids, because the particles are closer together and pass the disturbance faster. In air at 0 °C it is about 332 m/s. In water it is roughly 1500 m/s and in steel more than 5000 m/s. In air the speed rises with temperature by about 0.6 m/s for each degree Celsius, so v ≈ 332 + 0.6 t, where t is the temperature in °C. Changes in pressure alone do not change the speed of sound in air.

Three characteristics of a musical sound:

• Loudness depends on the amplitude of the wave. A bigger amplitude carries more energy and sounds louder. Loudness is measured by intensity level in decibel (dB).
• Pitch depends on frequency. A high frequency gives a high pitch (a whistle) and a low frequency gives a low pitch (a drum). Humans hear from about 20 Hz to 20 000 Hz.
• Quality (timbre) lets us tell two instruments apart even when they play the same note at the same loudness. It depends on the shape of the wave, which comes from the extra frequencies (overtones) that are mixed with the main note.

Intensity is the energy passing through a unit area in one second. Its unit is watt per square metre (W/m²). The faintest sound the ear can detect has an intensity of about 10⁻¹² W/m². Intensity falls as we move away from the source because the energy spreads over a larger area. The decibel scale is logarithmic, so a rise of 10 dB means the intensity is ten times larger.

Sound reflects from hard surfaces in the same way as light. The angle of incidence equals the angle of reflection, and the incident ray, the reflected ray and the normal lie in one plane. A reflected sound that is heard separately from the original is called an echo. The ear keeps a sound for about 0.1 s, so the reflected sound must arrive at least 0.1 s later to be heard as an echo. With a sound speed of about 340 m/s, the reflecting wall must be at least 17 m away.

Echoes have practical uses. Ships use echo sounding (sonar) to measure the depth of the sea, d = v × t / 2, where t is the time for the sound to go down and come back. Bats find insects by echoes. Long echoes spoil the sound in large halls, which brings us to acoustics.

Acoustics is the study of how sound behaves in a room. In a hall, repeated reflection causes reverberation, which blurs speech. Designers cover walls and ceilings with soft, rough materials such as curtains, carpets and perforated boards, which absorb sound. Curved ceilings and sounding boards are used to direct sound toward the audience.

Ultrasound is sound with a frequency above 20 000 Hz, which is beyond the hearing range of humans. It is used in medical scanning (for example to view an unborn baby), in cleaning delicate parts, in finding cracks inside metal, and in sonar. Infrasound has a frequency below 20 Hz. Elephants and earthquakes produce it.

Noise is an unpleasant and unwanted sound with an irregular wave pattern. Long exposure to loud sound (above about 85 dB) can damage hearing, raise blood pressure and disturb sleep. Noise can be reduced by planting trees, using silencers on engines, wearing ear protection and keeping horns and loudspeakers at low volume.

Study tip: Learn the three characteristics of sound with the quantity each one depends on (loudness: amplitude, pitch: frequency, quality: wave shape). In numericals write v = f λ or d = v t / 2 first, substitute with units, and state the final answer with its unit. For echo questions always check whether the given time is the round trip.

Definitions and flashcards

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Short questions with model answers

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Common mistakes

• Writing that sound travels fastest in gases. It is fastest in solids and slowest in gases.
• Mixing loudness with pitch. Loudness depends on amplitude, pitch on frequency.
• Forgetting to halve the time or the distance in echo problems. The sound goes to the wall and returns.
• Saying that sound is a transverse wave or that it can pass through a vacuum.
• Leaving out units, or writing frequency in m/s and speed in Hz.

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