Sound

Level 1

What is sound?

Sound is a mechanical wave: it is a vibration that travels through a material medium, such as air, water or a solid. The molecules of this medium move closer together and spread apart, transmitting the energy of the vibration like a wave on the surface of water.

  Examples :
- The human voice: vocal cords vibrate in the air.
- A drum: the drumhead vibrates to produce sound.
- Guitar strings: they vibrate to create musical notes.

Propagation of a sound wave in air

● Compression (molecules close together) → direction of propagation ● Rarefaction (molecules far apart)

Propagation of sound

Sound needs a medium to travel. In a vacuum, there are no molecules to transmit the vibration, so nothing can be heard. The speed of sound varies depending on the medium:

  • Air: 340 m/s
  • Water: 1500 m/s
  • Steel: 5000 m/s

The denser and more rigid the medium, the faster sound travels.

  Example :
If you place your ear on a table, you hear a noise coming from the other side faster than through the air.

Sound volume (current)

140 dB

 
 
 

0 dB

— dB Select an example
 
 

The volume of a sound depends on the amplitude of the wave: the larger the amplitude, the louder the sound. Current is measured in decibels (dB).

  Examples :
- Whisper: 30 dB
- Normal conversation: 60 dB
- Rock concert: 120 dB

Sound pitch (frequency)

The frequency of a sound determines whether it is low-pitched or high-pitched and is measured in Hertz (Hz).

  • Low frequencies (20-200 Hz) → low-pitched sounds
  • Medium frequencies (200-2000 Hz) → medium-pitched sounds
  • High frequencies (2000-20000 Hz) → high-pitched sounds

  Examples :
- A bass guitar: 50 Hz (low-pitched)
- A flute: 2000 Hz (high-pitched)

This measurement makes it possible to establish the frequency range in which the human ear perceives sounds: from 20Hz to 20,000Hz

🔊 Before using this device, turn down your device's volume to a low level. High frequencies can be unpleasant at high volume.

440 Hz Audible range
 
Infrasound
Audible range
Ultrasound
 

1 Hz 20 Hz 20 kHz 40 kHz
Synthesized pure tone — no file required

Timbre and musical instruments

Timbre allows us to distinguish two sounds that have the same pitch and the same volume. It depends on the frequency composition of a sound, called harmonics.

  Examples :
- A C note played on a piano and on a guitar does not sound the same: the timbre is different.
- The voices of two people singing the same note can be distinguished thanks to timbre.

Technologies using sound

Sound is used in many scientific and medical applications:

  • Ultrasound: for medical ultrasound scans, cleaning, or industrial testing.
  • Audio systems: microphones, loudspeakers, recordings.
  • Sonar: uses ultrasounds to detect objects underwater.
  • Infrasound: for monitoring earthquakes, volcanic eruptions, tornadoes, or nuclear tests.

  Examples :
- Ultrasound scan to visualize a fetus.
- Sonar to locate a submarine or a school of fish.

Echo and reflection of sound

When sound meets a surface, it can be reflected. If the distance is sufficient, the wave returns to the source in the form of an echo. Echoes are used in many technologies such as sonar and animal echolocation.

  Examples :
- Shouting in a valley or in a large empty room: you hear the same sound come back a few seconds later.
- Bats use echoes to detect obstacles and hunt.

The Doppler effect

The Doppler effect occurs when a sound source moves relative to an observer. The sound seems higher-pitched as the source gets closer and lower-pitched as it moves away.

  Examples :
- Ambulance siren approaching then moving away.
- Passing train: the whistle sound changes pitch.