Sound Waves
Section: Waves | Syllabus: Cambridge IGCSE Physics (0625)
Production and Transmission of Sound
- Sound is produced by a vibrating source, such as a tuning fork, a loudspeaker cone, a guitar string, or vocal cords
- The vibrating source makes the particles of the surrounding medium vibrate, and this vibration passes from particle to particle away from the source
- Sound is a longitudinal wave: the particles of the medium vibrate parallel to (along) the direction in which the wave, and its energy, travels
- Sound needs a medium (a solid, liquid or gas) to travel through - it cannot travel through a vacuum, because there are no particles to vibrate
A vibrating tuning fork producing a sound wave in air
Demonstrating that Sound Needs a Medium
- An electric bell is suspended inside a bell jar connected to a vacuum pump, and switched on so that it can be heard ringing
- As air is pumped out of the jar, the sound of the bell becomes quieter, even though the hammer can still be seen striking the bell
- As air is let back into the jar, the sound gets louder again
- This shows that sound needs particles of a medium to travel, since removing the air removes the medium that carries the sound to the listener
Compression and Rarefaction
Extended Only
- As a vibrating source moves outward, it pushes particles of the medium closer together, forming a compression - a region of higher pressure
- As the source moves back, particles spread further apart, forming a rarefaction - a region of lower pressure
- Compressions and rarefactions travel outward from the source, one wavelength apart, carrying the sound energy through the medium
Compressions and rarefactions in a sound wave
Speed of Sound
- The speed of sound in air is approximately 330-350 m/s (a commonly used value is 340 m/s)
Extended Only
- In general, sound travels faster in solids than in liquids, and faster in liquids than in gases
- This is because particles are closer together in solids than in liquids, and closer together in liquids than in gases, so vibrations pass from particle to particle more quickly
Measuring the Speed of Sound in Air
- Stand a measured distance d from a large flat wall, such as the side of a building
- Make a sharp sound, such as a clap, and start a stopwatch at the same instant
- Stop the stopwatch the instant the echo is heard, to find the time t
- The sound travels to the wall and back, a total distance of 2d, so the speed of sound is found using:
- Formula:
- v = (2d/t)
- Where:
- v = speed of sound (m/s)
- d = distance to the wall (m)
- t = time between making the sound and hearing the echo (s)
- Formula:
- Repeating the measurement several times and calculating an average value for t improves the accuracy of the result
Worked Example: Measuring the Speed of Sound
Muchindu stands 170 m from a cliff face, claps once, and hears the echo 1.0 s later. Calculate the speed of sound.
- Given: d = 170 m, t = 1.0 s
- v = (2d/t)
- v = (2 × 170/1.0) = 340 m/s
Echoes
- An echo is a reflection of sound waves, heard as a repetition of the original sound
- Sound reflects well off hard, flat surfaces, such as walls and cliffs, and is absorbed by soft surfaces, such as curtains and carpets
Loudness and Pitch
- Loudness depends on the amplitude of a sound wave: a larger amplitude produces a louder sound, and a smaller amplitude produces a quieter sound
- Pitch depends on the frequency of a sound wave: a higher frequency produces a higher pitch, and a lower frequency produces a lower pitch
How amplitude affects loudness and frequency affects pitch
The Audible Range and Ultrasound
- The human ear can detect sound frequencies from approximately 20 Hz to 20 000 Hz (20 kHz) - this is the audible range
- Ultrasound is sound with a frequency higher than 20 kHz, above the upper limit of human hearing
Uses of Ultrasound
Extended Only
- Ultrasound pulses are partly reflected whenever they reach a boundary between two different media; measuring the time taken for these reflections to return allows a distance or a depth to be calculated
Non-Destructive Testing of Materials
- Ultrasound pulses are directed into a material, such as a metal girder or an aircraft wing
- A crack or flaw inside the material reflects some of the ultrasound early, producing an extra reflected pulse that would not occur in a flawless sample
- This detects internal flaws without damaging or cutting open the material being tested
Medical Scanning of Soft Tissue
- Ultrasound pulses are directed into the body and reflect from boundaries between different soft tissues, such as between amniotic fluid and a foetus
- The reflected pulses are detected and processed to build up an image, for example to monitor an unborn baby during pregnancy
- Unlike X-rays, ultrasound scanning does not use ionising radiation, so it is considered safe for this use
Sonar
- A transmitter below a ship sends a pulse of ultrasound downward through the water
- The pulse reflects off the seabed, or off an object such as a shoal of fish or another vessel, and is detected on its return
- The depth or distance is calculated from the time taken and the speed of sound in water, using:
- Formula:
- d = (vt/2)
- Where:
- d = depth or distance to the reflecting surface (m)
- v = speed of sound in water (m/s)
- t = total time for the pulse to travel there and back (s)
- Formula:
Using a reflected ultrasound pulse to calculate a depth
Worked Example: Sonar Depth Calculation
Muumbe's ship sends an ultrasound pulse toward the seabed. The reflected pulse returns after 0.4 s. The speed of sound in water is 1500 m/s. Calculate the depth of the water.
- Given: v = 1500 m/s, t = 0.4 s
- d = (vt/2)
- d = (1500 × 0.4/2) = 300 m
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