Sound waves

How it works

Sound is a longitudinal wave: particles of the medium oscillate along the direction of travel and form compressions and rarefactions, regions of higher and lower pressure. The distance between neighbouring compressions is the wavelength λ. The medium sets the speed of sound – 343 m/s in air at 20 °C, about 1480 m/s in water and 5960 m/s in steel – and the wavelength is λ = v / f. Two tones with close frequencies alternately add up and cancel, so the loudness pulses at a beat frequency equal to the difference of the frequencies.

λ = v / fT = 1 / fbeat frequency = |f₂ − f₁|

Try it yourself

1. In “Sound wave” mode keep 440 Hz in air: the wavelength is 0.78 m and the period 2.27 ms.
2. Click “One octave higher” and raise the frequency from 220 to 440 Hz. Double the frequency means half the wavelength: 1.56 m shrinks to 0.78 m while the speed of sound stays the same.
3. Choose “Sound underwater”. With the same 440 Hz tone the wavelength grows to about 3.36 m, because sound travels about 4.3 times faster in water.
4. Try “Can you hear the pulse?” (440 + 444 Hz) and enable sound. The loudness pulses four times a second because the frequencies differ by 4 Hz.

Model limitations

The animation shows a simplified wave without attenuation and with a greatly exaggerated particle displacement; real molecular displacements are tiny. The speed of sound is fixed for each medium. Playback through speakers is not a calibrated loudness measurement.

Related experiments

• Doppler effect
• Waves and interference
• Fourier series