Sound Waves
Section: Waves | Syllabus: Cambridge AS Level Physics 9702
What is Sound? Sound is a form of energy that travels as a longitudinal wave through a medium. Sound waves are produced by vibrating objects and require a material medium to propagate. Sound Wave A longitudinal wave caused by vibrations that travels through a medium by compressions and rarefactions of particles.
Type of wave: Longitudinal (mechanical wave) Requires a medium: Cannot travel through a vacuum Caused by: Vibrating objects Speed: Varies depending on the medium Key Point Sound waves need particles to travel.
This is why sound cannot travel through space (a vacuum), but light can. Sound as a Longitudinal Wave In sound waves, particles vibrate parallel to the direction of energy transfer, creating regions of compression and rarefaction.
Compression A region where particles are pushed close together, creating higher pressure. Rarefaction A region where particles are spread apart, creating lower pressure. Particles oscillate back and forth parallel to the wave direction Compressions correspond to high pressure regions Rarefactions correspond to low pressure regions One wavelength = distance from one compression to the next compression Diagram Placeholder [Interactive diagram showing compressions and rarefactions in a sound wave - to be added] Production of Sound Sound is produced when objects vibrate.
The vibrating object causes the surrounding particles to vibrate, creating a sound wave. Examples of Sound Production Tuning fork: When struck, the prongs vibrate back and forth Loudspeaker: The cone vibrates to produce compressions and rarefactions Musical instruments: Strings, air columns, or membranes vibrate Human voice: Vocal cords vibrate as air passes through Diagram Placeholder [Diagram showing a vibrating tuning fork creating compressions and rarefactions - to be added] Transmission of Sound Through Media Sound can travel through solids, liquids, and gases, but not through a vacuum.
The speed of sound varies depending on the medium. Medium Approximate Speed Explanation Solids ~5000 m/s (steel) Particles are close together and strongly bonded, allowing faster transmission Liquids ~1500 m/s (water) Particles are close but less tightly bonded than solids Gases ~340 m/s (air at 20°C) Particles are far apart, so vibrations take longer to transfer Vacuum 0 m/s No particles present, so sound cannot travel Important Sound travels fastest in solids and slowest in gases.
This is because particles in solids are closer together and can transmit vibrations more efficiently. Bell Jar Experiment To demonstrate that sound cannot travel through a vacuum: Place a ringing bell inside a bell jar connected to a vacuum pump As air is removed, the sound becomes quieter In a complete vacuum, no sound is heard (though the bell is still ringing) When air is let back in, the sound returns Diagram Placeholder [Diagram of bell jar experiment showing setup and results - to be added] Speed of Sound in Air The speed of sound in air at room temperature (20°C) is approximately 340 m/s.
This value can be measured experimentally. Measuring Speed of Sound Method using echoes: Stand a known distance from a large flat wall Make a sharp sound (e.g., clap) and start a timer When you hear the echo, stop the timer Calculate: Speed = 2 × distance ÷ time (multiply by 2 because sound travels to wall and back) Method using oscilloscope: Connect two microphones to an oscilloscope at a known distance apart Create a sound between them (e.g., clap or beep) Measure the time delay between the two signals on the oscilloscope Calculate: Speed = distance ÷ time Exam Tip Remember the formula: Speed = Distance ÷ Time.
For echo experiments, don't forget to double the distance! Reflection of Sound Sound waves can be reflected when they hit a hard, flat surface, producing an echo. Echo A reflected sound wave that can be heard as a distinct repetition of the original sound.
Law of reflection: Angle of incidence = Angle of reflection Hard surfaces: Reflect sound well (e.g., walls, cliffs) Soft surfaces: Absorb sound, reducing echoes (e.g., curtains, carpet) Echo condition: You hear an echo if the reflected sound arrives at least 0.1 s after the original Applications of Sound Reflection Sonar: Used by ships and submarines to detect objects underwater Echolocation: Used by bats and dolphins to navigate and hunt Ultrasound scanning: Medical imaging of internal organs and unborn babies Depth measurement: Determining ocean depth Diagram Placeholder [Diagram showing sonar system and how echoes are used to detect objects - to be added] Properties of Sound Waves Pitch and Frequency Pitch How high or low a sound appears to the listener.
High pitch: High frequency (e.g., whistle, birdsong) Low pitch: Low frequency (e.g., bass drum, thunder) Pitch is determined by the frequency of vibration Faster vibrations = higher frequency = higher pitch Loudness and Amplitude Loudness How loud or quiet a sound appears to the listener.
Loud sounds: Large amplitude (big vibratio…
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