Sound Waves Transfer Energy Without Transferring Matter
Sound
Sound is a type of wave, a traveling disturbance that transfers energy from one place to another.
- Sound is produced when an object vibrates, disturbing the particles around it.
- These vibrations cause nearby particles to vibrate, which then pass the disturbance to neighbouring particles.
- Sound is classified as a mechanical wave, meaning it requires particles to transfer energy.
- Sound cannot travel through a vacuum because there are no particles available to vibrate and pass on the energy.
- This explains why sound cannot be heard in outer space.
- A "Mexican wave" in a stadium moves around the crowd even though each person only moves up and down and returns to their seat position.
- Sound travels through air in a similar way: the disturbance moves on, while the air molecules mainly vibrate in place.
When a guitar string vibrates, it causes the surrounding air particles to vibrate, producing sound.
Sound Is A Longitudinal Compression Wave
Longitudinal wave
A wave in which the oscillations of the medium are parallel to the direction the wave travels.
- Sound in air is a longitudinal wave.
- That means the particles of the medium vibrate parallel to the direction the wave travels.
- The sound wave moves through the medium, but the particles do not move along with the wave.
- Particles simply oscillate around their fixed positions.
- Energy is transferred as vibrations pass from particle to particle.
Compression and Rarefaction
Compression
A compression is a region in a longitudinal wave where the particles of the medium are close together, resulting in a high-pressure area.
Rarefaction
A rarefaction is a region in a longitudinal wave where the medium's particles are spread apart, creating a low-pressure region.
- Compressions are regions where particles are close together.
- These regions have higher pressure and higher density.
- Rarefactions are regions where particles are farther apart.
- These regions have lower pressure and lower density.
- As the wave travels, compressions and rarefactions move through the medium.
- The spacing between compressions represents the wavelength of the sound wave.
- A common misconception is that sound is "made of moving air that travels to your ear."
- In normal sound waves, the air does not flow from the source to the listener.
- The air particles oscillate, while the disturbance (energy) propagates.
Media Through Which Sound Can Travel
- Sound can travel through solids, liquids, and gases.
- It cannot travel through a vacuum, for example, the space.
- The speed of sound varies depending on the medium.
- In general:
- Sound travels slowest in gases
- Faster in liquids
- Fastest in solids
- This is because particles are closer together and interact more strongly in solids.
Closer particle spacing allows vibrations to be passed on more quickly.
- Light and radio waves do not need a medium because they are electromagnetic waves.
- Sound relies on matter.
Wave Speed Depends On The Medium And Conditions
- The speed of sound is a measurable wave quantity.
- Speed of sound is the distance travelled by a sound wave per second.
- In air at room temperature, the speed of sound is about 330–340 m s⁻¹.
- Sound travels faster in warmer air.
- Temperature affects particle kinetic energy and vibration transfer.
- IT is calculated by: $$\text{speed} = \frac{\text{distance}}{\text{time}}$$
- It is measured in metres per second (m s⁻¹)
Measuring The Speed Of Sound Using Time Delay
A simple way to measure sound speed is to measure a time delay between a visible event that produces a sound and the moment the sound is heard.
Method With A Visual "Clap" Signal
- Work in a large open space with two students separated by a measured distance $d$.
- Student A produces a loud sound with a clear visual cue at the same moment (for example, using a clapper made from two hinged pieces of wood).
- Student B starts a stopwatch when they see the clapper strike and stops it when they hear the sound.
- Calculate the speed using $v=\frac{d}{t}$.
Echo Method
- If you stand a measured distance $d$ from a large flat wall and make a sharp sound, you can time the interval $t$ between the sound and the echo.
- The sound travels to the wall and back, so the total distance is $2d$:
$$v=\frac{2d}{t}$$
- Improve accuracy by using a larger distance (so the delay is longer), repeating several times, and averaging your results.
- For echo timing, choose a hard, flat wall to get a clear reflection.
Frequency Determines Pitch, And Humans Hear A Limited Range
- Pitch is how high or low a sound seems, and it mainly depends on frequency.
- Frequencies above human hearing are called ultrasound.
- Many animals can detect ultrasound, for example:
- dogs (up to roughly 40 to 50 kHz),
- dolphins and bats (often above 100 kHz), which use it for echolocation (finding objects by interpreting echoes).
- Some very low-frequency sounds below 20 Hz are called infrasound.
- Even when not consciously heard, infrasound can sometimes be associated with sensations such as unease.
- It is calculated by $$f = \frac{1}{T}$$
Audible Range
The range of frequencies that a typical human can hear, about 20 Hz to 20 kHz.
Ultrasound
Sound with frequency above the upper limit of human hearing (about $20\ \text{kHz}$).
- Higher frequency sounds have a higher pitch.
- Lower frequency sounds have a lower pitch.
Amplitude Relates To Loudness, And Decibels Are Logarithmic
- Loudness is a human perception related to sound wave amplitude (pressure variation) and also to the frequency sensitivity of the ear.
- In practice, sound level is often measured in decibels (dB).
- Because the decibel scale is logarithmic, adding two identical sound sources does not "double the dB."
- Two equal sources increase level by about 3 dB.
Sound Waves
Human Hearing Range
- Human hearing is limited to a specific frequency range.
- Humans can hear frequencies from approximately 20 Hz to 20,000 Hz.
- Sounds outside this range still exist as sound waves.
- Hearing range decreases with age.
Loudspeakers Convert Electrical Signals Into Sound
- A loudspeaker produces sound by vibrating a cone (also called a diaphragm) to push and pull on the surrounding air, creating compressions and rarefactions.
- In a common moving-coil loudspeaker:
- A coil of wire is attached to the cone.
- An alternating current (AC) flows through the coil.
- The coil sits in a magnetic field from a permanent magnet.
- The magnetic field exerts a force on the current-carrying coil.
- When the current reverses direction, the force reverses, so the cone oscillates.
- The cone vibrates at the same frequency as the AC signal, so the electrical waveform is converted into a sound wave.
Diaphragm
A thin, flexible surface that vibrates to create or detect sound waves.
Microphones Convert Sound Into Electrical Signals
- A microphone performs the reverse process.
- In a moving-coil (dynamic) microphone:
- Incoming sound waves make a diaphragm vibrate.
- The diaphragm is attached to a coil that moves in a magnetic field.
- Because the coil moves, it experiences a changing magnetic environment.
- An alternating voltage is induced in the coil.
- So, sound energy is transformed into an electrical signal that can be amplified, recorded, or transmitted (for example, in telephones).
- This relies on electromagnetic induction.
- Changing magnetic conditions produce an induced voltage.
- The faster the change, the larger the induced signal.
- Why does sound require a medium to travel?
- How do particles move in a longitudinal sound wave?
- Write the formula used to calculate the speed of sound.
- State the wave equation and explain each term.
- How does frequency affect pitch?
- Why does sound travel faster in solids than in gases?