Sound Waves: Pitch, Loudness and Speed
Watch a sound wave pass through the air. A loudspeaker pushes on the air and a row of particles wobbles back and forth about fixed spots, so patches where they are squeezed together (compressions) and spread apart (rarefactions) travel along while each particle stays where it is. Raise the frequency for a higher pitch and a shorter wavelength, raise the amplitude for a louder sound, and change the medium: sound goes 343 m/s in air, 1007 m/s in helium, 1480 m/s in water and 5960 m/s in steel. A graph shows the pressure along the wave and a microphone trace shows it over time, with the wavelength, period, speed and approximate sound level worked out for you. For instance, a 440 Hz note in air has a wavelength of 0.78 m.
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What you can adjust
- Frequency
- 20 – 2000 Hz
- Amplitude (loudness)
- 1 – 10
- Medium
- 0 – 3
How many times each particle moves back and forth every second. A higher frequency is a higher pitch. 440 Hz is the note A above middle C. The waves are drawn slowed down, with the wavelength squeezed so every setting fits on the screen; the numbers shown are the real ones.
How far the particles move and how strongly the air is squeezed. A bigger amplitude is a louder sound. Doubling the amplitude adds about 6 dB. The sound level shown is approximate, with an amplitude of 5 set at about 70 dB.
What the sound travels through. Sound goes about 343 m/s in air at 20 °C, 1007 m/s in helium, 1480 m/s in water and 5960 m/s in steel. The frequency stays the same, so a faster medium means a longer wavelength.
About this simulation
What it shows
A loudspeaker on the left pushes a grid of air particles. Each one moves back and forth about its own spot (three are marked in orange), while bunched-up and spread-out patches travel to the right. Below, a graph of the pressure along the wave and a microphone trace of the pressure at one spot over time.
The key idea
Sound is a vibration passed from particle to particle, and nothing travels along with it. The frequency sets the pitch, the amplitude sets the loudness and the medium sets the speed, so the wavelength is the speed divided by the frequency (λ = v ÷ f).
Try this
Run the defaults: 440 Hz in air has a wavelength of 0.78 m. Set Frequency to 880 Hz and it halves to 0.39 m. Set Medium to Water and it stretches to 3.36 m.
The picture is slowed down by hundreds of times so you can follow it, and the wavelength is squeezed so that every setting fits (a higher pitch still shows more waves than a lower one). The particles are drawn far larger and fewer than real air. The numbers shown are the real ones, except the sound level, which is approximate: a relative scale with an amplitude of 5 set at 70 dB.