MYP Physics 4.3 Sound Waves Notes

Sound Waves Transfer Energy Without Transferring Matter

1. Sound is produced when an object vibrates, disturbing the particles around it.
2. These vibrations cause nearby particles to vibrate, which then pass the disturbance to neighbouring particles.
3. Sound is classified as a mechanical wave, meaning it requires particles to transfer energy.
4. Sound cannot travel through a vacuum because there are no particles available to vibrate and pass on the energy.
5. This explains why sound cannot be heard in outer space.

Sound Is A Longitudinal Compression Wave

1. Sound in air is a longitudinal wave.
2. That means the particles of the medium vibrate parallel to the direction the wave travels.
3. The sound wave moves through the medium, but the particles do not move along with the wave.
4. Particles simply oscillate around their fixed positions.
5. Energy is transferred as vibrations pass from particle to particle.

Compression and Rarefaction

1. Compressions are regions where particles are close together.
2. These regions have higher pressure and higher density.
3. Rarefactions are regions where particles are farther apart.
4. These regions have lower pressure and lower density.
5. As the wave travels, compressions and rarefactions move through the medium.
6. The spacing between compressions represents the wavelength of the sound wave.

Media Through Which Sound Can Travel

1. Sound can travel through solids, liquids, and gases.
2. It cannot travel through a vacuum, for example, the space.
3. The speed of sound varies depending on the medium.
4. In general:
   1. Sound travels slowest in gases
   2. Faster in liquids
   3. Fastest in solids
5. This is because particles are closer together and interact more strongly in solids.

Wave Speed Depends On The Medium And Conditions

1. The speed of sound is a measurable wave quantity.
2. Speed of sound is the distance travelled by a sound wave per second.
3. In air at room temperature, the speed of sound is about 330–340 m s⁻¹.
4. Sound travels faster in warmer air.
5. Temperature affects particle kinetic energy and vibration transfer.
6. IT is calculated by: $$\text{speed} = \frac{\text{distance}}{\text{time}}$$
7. 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

1. Work in a large open space with two students separated by a measured distance $d$.
2. 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).
3. Student B starts a stopwatch when they see the clapper strike and stops it when they hear the sound.
4. Calculate the speed using $v=\frac{d}{t}$.

Echo Method

1. 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.
2. The sound travels to the wall and back, so the total distance is $2d$:

$$v=\frac{2d}{t}$$

Frequency Determines Pitch, And Humans Hear A Limited Range

1. Pitch is how high or low a sound seems, and it mainly depends on frequency.
2. Frequencies above human hearing are called ultrasound.
3. Many animals can detect ultrasound, for example:
   1. dogs (up to roughly 40 to 50 kHz),
   2. dolphins and bats (often above 100 kHz), which use it for echolocation (finding objects by interpreting echoes).
4. Some very low-frequency sounds below 20 Hz are called infrasound.
5. Even when not consciously heard, infrasound can sometimes be associated with sensations such as unease.
6. It is calculated by $$f = \frac{1}{T}$$

Amplitude Relates To Loudness, And Decibels Are Logarithmic

1. Loudness is a human perception related to sound wave amplitude (pressure variation) and also to the frequency sensitivity of the ear.
2. In practice, sound level is often measured in decibels (dB).

Human Hearing Range

1. Human hearing is limited to a specific frequency range.
2. Humans can hear frequencies from approximately 20 Hz to 20,000 Hz.
3. Sounds outside this range still exist as sound waves.
4. Hearing range decreases with age.

Loudspeakers Convert Electrical Signals Into Sound

1. A loudspeaker produces sound by vibrating a cone (also called a diaphragm) to push and pull on the surrounding air, creating compressions and rarefactions.
2. In a common moving-coil loudspeaker:
   1. A coil of wire is attached to the cone.
   2. An alternating current (AC) flows through the coil.
   3. The coil sits in a magnetic field from a permanent magnet.
   4. The magnetic field exerts a force on the current-carrying coil.
   5. When the current reverses direction, the force reverses, so the cone oscillates.
3. The cone vibrates at the same frequency as the AC signal, so the electrical waveform is converted into a sound wave.

Microphones Convert Sound Into Electrical Signals

1. A microphone performs the reverse process.
2. In a moving-coil (dynamic) microphone:
   1. Incoming sound waves make a diaphragm vibrate.
   2. The diaphragm is attached to a coil that moves in a magnetic field.
   3. Because the coil moves, it experiences a changing magnetic environment.
   4. An alternating voltage is induced in the coil.
3. So, sound energy is transformed into an electrical signal that can be amplified, recorded, or transmitted (for example, in telephones).