Slide 1: Making Sounds & Vibrations
Sounds are produced by vibrating objects[cite: 16]. These vibrations travel through a medium (solid, liquid, or gas) to reach our ears.
- Stringed Instruments: Plucking or bowing a string causes it to vibrate[cite: 10]. The vibrations are transmitted to the instrument body and the surrounding air.
- Wind Instruments: Blowing into or across a tube causes an internal air column to vibrate[cite: 10]. Longer or wider tubes produce deeper (lower pitch) notes.
- Percussion Instruments: Striking a surface (such as a drum skin or xylophone key) causes it to vibrate directly[cite: 10].
When sound reaches our ears, it causes the eardrum to vibrate[cite: 11]. These vibrations pass through tiny bones to the inner ear, where electrical signals are sent to the brain[cite: 11].
Slide 2: The Speed of Sound & Echoes
Sound travels through air at approximately 330 m/s (or 1200 km/h)[cite: 11]. This is much slower than the speed of light (300,000,000 m/s), which explains why you see lightning before hearing thunder, or see a cricketer hit a ball before hearing the sound[cite: 12, 13].
Echoes:
An echo is a reflected sound wave bouncing off a hard surface (like a tall wall or cliff)[cite: 12, 13].
Formula: $\text{Speed} = \frac{\text{Total Distance}}{\text{Total Time}} = \frac{2 \times \text{Distance to Wall}}{\text{Time for Echo}}$
Speed of Sound in Different Materials:
Sound travels fastest in solids and slowest in gases because particles in solids are packed close together, allowing vibrations to pass on rapidly[cite: 14].
| Medium | Material | Approximate Speed (m/s) |
|---|---|---|
| Gas | Air (0°C) | 330[cite: 14] |
| Gas | Carbon Dioxide | 260[cite: 14] |
| Liquid | Water / Sea Water | 1500 - 1530[cite: 14] |
| Solid | Wood (Oak) | 3800[cite: 14] |
| Solid | Iron / Steel | 5100[cite: 14] |
Slide 3: Measuring the Speed of Sound in the Lab
The speed of sound can be measured in a school laboratory using the Time-of-Flight method[cite: 13].
Slide 4: Pitch, Loudness & Oscilloscope Traces
An oscilloscope converts sound vibrations received by a microphone into a visual wave trace on a screen[cite: 15].
- Pitch: How high or low a note sounds[cite: 15]. Higher pitch corresponds to a higher frequency (more complete wave cycles per second, measured in Hertz, Hz)[cite: 15].
- Loudness: How loud a sound is[cite: 15]. Greater loudness corresponds to a larger amplitude (height of the wave from center line to crest)[cite: 15].
- Period ($T$): Time taken for one complete vibration cycle[cite: 15]. $\text{Frequency } f = \frac{1}{\text{Period } T}$[cite: 15].
Range of Hearing:
The human ear can detect frequencies between 20 Hz and 20,000 Hz (20 kHz)[cite: 15]. As people grow older, their ability to hear high-pitched sounds (above 15 kHz) gradually decreases[cite: 15].
Slide 5: How Sounds Travel - Compressions & Rarefactions
Sound is a longitudinal wave. It moves through a medium as a series of pressure variations where air particles vibrate back and forth parallel to the direction of wave travel[cite: 16].
- Compression: A region where air particles are squashed close together (high pressure)[cite: 16, 17].
- Rarefaction: A region where air particles are spread further apart (low pressure)[cite: 17].
Important Note: Sound cannot travel through a vacuum because there are no particles or molecules to vibrate and transmit energy[cite: 16, 17].