Sound waves are longitudinal mechanical vibrations that require a material medium to propagate. Analyzing compressions, rarefactions, speed variations across states of matter, and ultrasound applications is fundamental in Cambridge IGCSE Physics.

A sound wave is produced by a vibrating source (such as a loudspeaker cone or tuning fork).

Compressions and Rarefactions

1. Compressions: Regions where vibrating medium particles are pushed close together, creating local high-pressure regions.
2. Rarefactions: Regions where vibrating medium particles spread apart, creating local low-pressure regions.

Sound cannot travel through a vacuum because there are no particles to transmit mechanical vibrations.

The speed of sound depends directly on the elastic modulus and density of the transmitting medium:

• Solids (~5000 m/s in steel): Tightly packed particles with strong interatomic bonds transmit vibrational energy rapidly to neighboring atoms.
• Liquids (~1500 m/s in water): Moderately packed particles transmit sound faster than gases.
• Gases (~330--350 m/s in air): Particles are far apart; vibrations are transferred only during particle collisions, yielding the slowest wave speed.

Ultrasound is defined as sound waves with frequencies above $20,000\text{ Hz}$ ($20\text{ kHz}$), which is beyond the upper limit of human hearing ($20\text{ Hz to } 20\text{ kHz}$).

1. Echo Sounding and SONAR

Ships use echo sounders to determine ocean depth:

1. An ultrasound pulse is transmitted downward from the ship's hull.
2. The pulse reflects off the seabed and returns to an onboard receiver after time $t$.
3. Total distance traveled by the sound is $2d = v \times t$, so depth $d$ is:

2. Medical Diagnostic Imaging

Ultrasound transducers emit high-frequency sound pulses into the body. Partial reflections occur at boundaries between different tissue types (e.g. muscle and bone). Measuring echo delay times and amplitudes constructs non-invasive 2D/3D images of internal organs and developing fetuses without ionizing radiation hazards.

Advanced Physical Analysis

Sound waves are longitudinal pressure waves consisting of alternating compressions and rarefactions. Speed of sound is highest in solids and lowest in gases due to particle density and elasticity.

Sound is a longitudinal wave traveling fastest in solids (~5000 m/s) and slowest in gases (~330 m/s). Ultrasound (>20 kHz) uses boundary echoes for SONAR ($d = vt/2$) and non-invasive medical scanning.