๐ Production and Propagation of Sound
- Sound is produced by the vibration of objects (e.g. a guitar string, vocal cords, a tuning fork).
- Sound travels in the form of longitudinal waves โ the particles of the medium vibrate back and forth in the same direction as the wave travels.
- Sound requires a material medium to travel. It cannot travel through vacuum.
- A longitudinal wave consists of alternating compressions (regions of high pressure) and rarefactions (regions of low pressure).
Bell jar experiment: When air is pumped out of a glass jar containing a ringing electric bell, the sound becomes fainter and eventually inaudible โ proving sound needs a medium to travel.
โก Speed of Sound in Different Media
| Medium | Approximate Speed |
|---|---|
| Air (at 0 ยฐC) | 332 m/s |
| Air (at 25 ยฐC) | ~346 m/s |
| Water | ~1500 m/s |
| Steel | ~5000 m/s |
- Sound travels faster in solids than in liquids, and faster in liquids than in gases.
- Speed of sound in a medium increases with temperature.
- Speed of sound in air is much less than the speed of light โ that is why we see lightning before hearing thunder.
๐ Characteristics of a Sound Wave
| Term | Definition | Unit |
|---|---|---|
| Frequency (f) | Number of complete vibrations (cycles) per second | Hertz (Hz) |
| Time period (T) | Time taken to complete one vibration; T = 1/f | Second (s) |
| Amplitude (A) | Maximum displacement of a vibrating particle from its rest position | Metre (m) |
| Wavelength (ฮป) | Distance between two consecutive compressions or rarefactions | Metre (m) |
Speed of Sound (v) = Frequency (f) ร Wavelength (ฮป)
๐ต Characteristics of Musical Sound
Pitch
- Pitch is the sensation of how high or low a sound is. It depends on frequency.
- Higher frequency โ higher pitch (shriller sound). Lower frequency โ lower pitch (graver sound).
- A woman's voice has higher pitch than a man's voice.
Loudness
- Loudness is the measure of the strength or intensity of sound. It depends on amplitude.
- Greater amplitude โ louder sound. Measured in decibels (dB).
- Normal conversation โ 60 dB. Pain threshold โ 120 dB.
Quality (Timbre)
- Quality (or timbre) helps distinguish between two sounds of the same pitch and loudness produced by different sources. It depends on the waveform of the sound.
- E.g. a guitar and a flute playing the same note sound different due to their different quality.
๐ก Audible Range and Beyond
| Category | Frequency Range | Examples |
|---|---|---|
| Infrasound | Below 20 Hz | Earthquakes, communication by elephants and whales |
| Audible sound | 20 Hz to 20,000 Hz (20 kHz) | Human speech, music, everyday sounds |
| Ultrasound | Above 20,000 Hz | Bats and dolphins (echolocation), medical sonography, sonar, industrial cleaning |
๐ Echo
- An echo is the repetition of sound caused by the reflection of sound waves from a hard surface.
- For an echo to be heard distinctly, the reflecting surface must be at least 17 metres away (since the human ear can distinguish two sounds only if there is a gap of at least 0.1 s between them; speed ร time = 340 ร 0.1 = 34 m โ distance = 17 m).
- Reverberation: Multiple reflections of sound that produce a prolonged sound โ heard in big halls and auditoriums.
- Uses of echo: Sonar (detecting submarines, depth of sea), medical ultrasonography (imaging), echolocation by bats.
๐จ Noise Pollution
- Noise pollution refers to unwanted, unpleasant, or harmful sound in the environment.
- Sources: traffic, loudspeakers, industrial machinery, crackers, aircraft.
- Effects: hearing loss, stress, sleep disturbance, high blood pressure.
- Control measures: sound-absorbing materials, green belts (trees), noise barriers, regulating loudspeakers, proper vehicle maintenance.
๐ท๏ธ Key Terms
Vibration
Longitudinal wave
Compression
Rarefaction
Frequency
Amplitude
Wavelength
Time period
Hertz
Pitch
Loudness
Decibel
Quality / Timbre
Infrasound
Ultrasound
Echo
Reverberation
Sonar
Noise pollution