3.13 - Sound Waves & Hearing

Sound is a longitudinal wave that needs a medium

• A sound wave is produced by a vibrating object, which passes vibrations into the particles of the surrounding material.
• Sound is a longitudinal wave: the particles vibrate back and forth along the same direction the wave travels.
• As they vibrate, particles bunch into regions of high pressure called compressions, and spread apart into regions of low pressure called rarefactions.
• The wave is this travelling pattern of compressions and rarefactions. The particles vibrate about fixed positions - it's the wave, not the material, that travels.

Sound needs a medium

• Because it is carried by vibrating particles, sound needs a medium - a solid, a liquid, or a gas.
• Sound cannot travel through a vacuum (such as space) because there are no particles to pass the vibrations on.

The speed of sound in different materials and measuring it

• Sound travels at different speeds in different materials: in general, it travels fastest in solids, slower in liquids, and slowest in gases.
• The denser the medium, the more closely packed its particles are, so vibrations pass from particle to particle more quickly.
• In air, sound travels at roughly 330 to 350 m/s, depending mainly on air temperature.

Measuring the speed of sound in air

Two common lab methods are used.

Method 1 - Two microphones and a known wavelength

A signal generator and speaker produce sound of a known frequency. Two microphones connected to an oscilloscope each display a trace. Starting with both by the speaker, one microphone is moved away until the traces align again at exactly one wavelength apart. The speed is calculated using $v=f\lambda$.

Method 2 - Distance and time directly

Two microphones are placed a measured distance apart in a line with a sound source. A data logger records the time the sound reaches each microphone to find the travel time. Speed is calculated using $\text{Speed}=\frac{\text{distance}}{\text{time}}$.

A data logger is used because sound travels too fast over short distances to be timed accurately with a stopwatch.

Reflection of sound and echoes

• Sound waves reflect off hard, flat surfaces. A reflected sound wave is called an echo.

• Sound obeys the law of reflection: the angle of incidence equals the angle of reflection, measured from the normal.

• Soft surfaces (such as carpets and curtains) absorb sound rather than reflect it.

• An echo is heard after the original sound because the reflected wave has to travel further to reach the ear.

Refraction of sound between materials

• When a sound wave passes from one material into another, its speed changes.
• Because $v=f\lambda$ and the frequency stays the same across the boundary, the wavelength changes: it gets longer when the wave speeds up and shorter when it slows down.
• This change in speed causes the sound wave to change direction (refract).
• Sound speeds up when it enters a denser material.