1. Overview
Sound is a form of energy transfer produced by vibrating sources. Understanding sound waves is crucial as they are the primary way we communicate and navigate our environment, ranging from simple speech to advanced medical imaging and underwater exploration.
Key Definitions
- Longitudinal Wave: A wave in which the vibration of the medium is parallel to the direction the wave travels.
- Frequency: The number of vibrations per second, measured in Hertz (Hz).
- Amplitude: The maximum displacement of a point on a wave from its rest position (determines volume).
- Pitch: How "high" or "low" a sound is, determined by its frequency.
- Echo: The reflection of a sound wave from a surface.
- Ultrasound: Sound waves with a frequency higher than the upper limit of human hearing (above 20,000 Hz).
- Medium: The substance (solid, liquid, or gas) through which a sound wave travels.
Core Content
Production and Nature of Sound
- Vibrating Sources: Sound is produced whenever an object vibrates. These vibrations push and pull on surrounding air molecules, creating a wave.
- Longitudinal Nature: Sound travels as a longitudinal wave. The particles of the medium oscillate back and forth in the same direction that the wave is moving.
- Medium Requirement: Sound cannot travel through a vacuum. It requires a medium (particles) to transmit the vibrations.
- A bell jar experiment showing a ringing bell inside a vacuum where no sound is heard once the air is removed.
Human Hearing
- The approximate range of audible frequencies for a healthy human ear is 20 Hz to 20,000 Hz (or 20 kHz).
Speed of Sound
- The speed of sound in air is approximately 330 – 350 m/s.
- Determining Speed (Experimental Method):
- Two people stand a measured distance apart (e.g., 500 meters) in a large open field.
- Person A fires a starting pistol or clashes two cymbals together.
- Person B starts a stopwatch when they see the flash/action and stops it when they hear the sound.
- Speed is calculated using: $\text{Speed} = \frac{\text{Distance}}{\text{Time}}$.
Characteristics of Sound
- Pitch: Controlled by frequency. Higher frequency = higher pitch (a "squeaky" sound).
- Loudness: Controlled by amplitude. Larger amplitude = louder sound.
- Two wave traces; one with short peaks (quiet) and one with tall peaks (loud), both having the same spacing between peaks.
Echoes
- An echo is simply a reflected sound wave. When sound hits a hard, flat surface, it bounces back toward the source.
Extended Content (Extended Curriculum Only)
Compression and Rarefaction
Longitudinal waves consist of:
- Compressions: Regions of high pressure where particles are bunched together.
- Rarefactions: Regions of low pressure where particles are spread apart.
Speed in Different Media
The speed of sound depends on how close the particles are to one another.
- Solids (Fastest): Particles are tightly packed, allowing vibrations to pass quickly.
- Liquids: Slower than solids, but faster than gases.
- Gases (Slowest): Particles are far apart, so it takes longer for vibrations to transfer.
- Order: $v_{\text{solid}} > v_{\text{liquid}} > v_{\text{gas}}$
Uses of Ultrasound
- Non-Destructive Testing (NDT): Ultrasound is beamed into metal pipes or moving parts. If there is a crack or flaw, the sound reflects back earlier than expected.
- Medical Scanning: Ultrasound reflects off different boundaries of soft tissue (e.g., scanning an unborn baby). It is safer than X-rays as it is non-ionizing.
- Sonar (Sound Navigation and Ranging): Used by ships to find the depth of the sea or locate fish.
- Calculation Note: In Sonar and Echo-location, the sound travels to the object and back.
- $\text{Total distance traveled} = \text{Speed} \times \text{Time}$
- $\text{Depth/Distance to object} = \frac{\text{Speed} \times \text{Time}}{2}$
Key Equations
- Wave Equation: $v = f\lambda$
- $v$ = speed (m/s)
- $f$ = frequency (Hz)
- $\lambda$ = wavelength (m)
- Speed Equation: $v = \frac{d}{t}$
- $d$ = distance (m)
- $t$ = time (s)
- Echo Distance: $d = \frac{v \times t}{2}$
Common Mistakes to Avoid
- ❌ Wrong: Thinking sound travels faster in air than in water or solids.
- ✅ Right: Sound requires a medium and travels fastest in solids because the particles are closer together.
- ❌ Wrong: Forgetting to divide by two in echo/sonar calculations.
- ✅ Right: Always check if the question asks for the total distance traveled (there and back) or the distance to the object (divide by 2).
- ❌ Wrong: Confusing pitch and loudness.
- ✅ Right: Remember that Amplitude = Loudness and Frequency = Pitch. Changing one does not affect the other.
- ❌ Wrong: Assuming sound can travel through a vacuum like light does.
- ✅ Right: Sound is a mechanical wave; it cannot travel through a vacuum.
Exam Tips
- Check Units: Ensure frequency is in Hz (not kHz) and time is in seconds before using equations.
- Echo Questions: Read carefully! If the question says "The sound took 2 seconds to return to the ship," you must divide that time by 2 to find the depth, or calculate the total distance and then divide by 2.
- Drawing Waves: If asked to draw a "louder sound of a lower pitch," make the waves taller (amplitude) and the peaks further apart (frequency).