Sound Waves — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions

Section 1 of 8Sound as a wave

Sound as a wave

Production

• Production sound waves are produced by vibrating sources such as strings, loudspeaker cones and vocal cords

Type

• Type sound waves are longitudinal

Medium

• Medium a medium is needed, so sound cannot travel through a vacuum

Particles

• Particles air particles vibrate to and fro about fixed positions while the pattern moves outwards

Effect on solids

• Effect on solids vibrations are transferred to a solid, for example a glass can vibrate and shatter

Section 2 of 8Compressions and rarefactions

Compression

• Compression a region of higher density where the particles are close together

Rarefaction

• Rarefaction a region of lower density where the particles are further apart

Pressure wave

• Pressure wave the pressure of the air varies with the wave, so sound is a pressure wave

Wording

• Wording state that the particles of the medium, not the wave, are closer together or further apart

Drawing

• Drawing lines closer together show compressions and lines further apart show rarefactions

Section 3 of 8Speed of sound

Air

• 330 to 350 m/s, increasing with temperature

Liquids

• Liquids about 1500 m/s

Solids

• Solids about 5000 m/s

Reason

• Reason particles in a solid are closer and respond more quickly to each other's movement

Comparison with light

• Comparison with light sound is much slower than light, so lightning is seen before thunder is heard

Section 4 of 8Frequency and pitch

Frequency

• Frequency the number of vibrations per second, in hertz

Pitch

• Pitch high frequency gives a high pitch and low frequency (long wavelength) gives a low pitch
  • (short wavelength)

Hearing range

• Hearing range about 20 Hz to 20 000 Hz, which reduces with age

Wave equation

• Wave equation v = fλ applies to sound

Section 5 of 8Amplitude and loudness

Amplitude

• Amplitude the maximum displacement of particles, which corresponds to the maximum pressure change

Loudness

• Loudness a larger amplitude gives a louder sound and a smaller amplitude a quieter one

Independence

• Independence changing loudness does not change pitch or speed

Oscilloscope trace

• Oscilloscope trace a taller wave means a louder sound and more waves in the same time means a higher pitch

Section 6 of 8More from the specification

1Speed of Sound Echo Experiment

• The speed of sound in air can be measured by timing echoes from a flat wall: speed v = 2d / t,
• where d is the measured distance to the wall and t is the time taken for the reflected pulse to return.

2Ultrasound Pulse-Echo Depth Measurement

• Ultrasound pulses reflect at boundary interfaces between differing tissue or geological layers,
• allowing depth to be determined from the measured time interval using depth = (speed × time) / 2.

3Techniques for Measuring Sound Speed

• In outdoor timing experiments, measuring distance with a trundle wheel and timing flashes/bangs with electronic gates minimises human reaction time errors in the recorded experimental results.

4Acoustic Frequency Range and Ultrasound

• Sound waves are longitudinal mechanical vibrations;
  • human hearing detects frequencies from 20 Hz to 20,000 Hz,
• while frequencies exceeding 20 kHz are termed ultrasound.

5Speed of Sound Echo Experiment

• The speed of sound in air can be measured by timing echoes from a flat wall: speed v = 2d / t,
• where d is the measured distance to the wall and t is the time taken for the reflected pulse to return.

6Ultrasound Pulse-Echo Depth Measurement

• Ultrasound pulses reflect at boundary interfaces between differing tissue or geological layers,
• allowing depth to be determined from the measured time interval using depth = (speed × time) / 2.

7Techniques for Measuring Sound Speed

• In outdoor timing experiments, measuring distance with a trundle wheel and timing flashes/bangs with electronic gates minimises human reaction time errors in the recorded experimental results.

8Acoustic Frequency Range and Ultrasound

• Sound waves are longitudinal mechanical vibrations;
  • human hearing detects frequencies from 20 Hz to 20,000 Hz,
• while frequencies exceeding 20 kHz are termed ultrasound.

9Distance

• Total length of path travelled, a scalar.

10Ultrasound

• Sound with a frequency above 20 kHz

11Microphone

• Device that converts sound into an electrical signal

12Boundary

• Surface between two media at which waves are partially reflected

13Independent variable

• The variable deliberately changed, here time

14Dependent variable

• The variable measured, here temperature

15Control variable

• A quantity kept the same to make the test fair

16Average speed

• Total distance divided by total time.

Section 7 of 8Key terms

Term	Meaning
Compression	Region of a longitudinal wave where the particles are close together
Rarefaction	Region of a longitudinal wave where the particles are spread apart
Pitch	How high or low a note sounds, set by frequency
Loudness	How loud a sound is, set by amplitude
Pressure wave	Wave in which the pressure of the medium varies

Section 8 of 8Exam technique

1

Saying sound travels in a vacuum — there are no particles to vibrate.

2

Saying the wave is closer together in a compression — the particles of the medium are closer together.

3

Saying a louder sound is faster — speed depends only on the medium.

4

Mixing up pitch and loudness — pitch is frequency and loudness is amplitude.