Sound Waves

Sound is a wave made by vibrating objects (like guitar strings or vocal cords). These vibrations pass through a medium (air, water, or solid) and transfer energy without moving the medium along with them.

What kind of wave is sound?

Sound is a longitudinal wave: the vibrations of particles are parallel to the direction the wave travels. It creates compressions (squashed regions) and rarefactions (spread-out regions) that move through the medium.

Key wave ideas

• Amplitude: how large the vibration is. Bigger amplitude → louder sound.
• Frequency (measured in hertz, Hz): how many vibrations per second. Higher frequency → higher pitch.
• Wavelength (λ): distance between adjacent compressions (or rarefactions).
• Wave speed (v): how fast the wave travels through the medium.

These are linked by the wave equation:

$$v = f\,\lambda$$

Hearing range and ultrasound

• Humans hear roughly 20 Hz to 20 000 Hz (20 kHz).
• Ultrasound is sound with frequency greater than 20 kHz. It is used for medical imaging of soft tissues, checking for cracks in materials (non-destructive testing), and sonar for measuring distance or depth.

Speed of sound

• In air: approximately 330–350 m/s (about 340 m/s).
• Sound generally travels faster in solids than in liquids, and faster in liquids than in gases.
• Sound needs a medium. It cannot travel in a vacuum.

Reflection and echoes; diffraction

• Echo: a reflected sound.
• Sound diffracts (spreads) around corners and through doorways. Longer wavelengths (lower frequencies) diffract more, which is why you can hear bass notes around a corner.

Measuring speed of sound (distance–time)

Stand a known distance from a visible event that makes sound (e.g., someone claps wooden blocks). Time the interval between seeing the action and hearing it. Use v = distance ÷ time. Repeat and average.