Presets
Playback
Source and air
Wavelength equals the speed of sound divided by the frequency; cold air is slower, so waves get shorter.
Room, barrier and second source
Rigid walls reflect and build standing waves. A gap narrower than a wavelength spreads sound in all directions.
Model checks
The science
Sound as a pressure wave
For small disturbances the pressure obeys the wave equation ∂2p/∂t2 = c2 ∇2p. The speed of sound in air is c ≈ 331.3 √(1 + T/273.15) m/s and the wavelength is λ = c / f. The room is solved with a second-order finite-difference scheme on a 200 x 125 grid with a time step set by the Courant condition c Δt/Δx ≤ 1/√2.
What to look for
- Spreading. In two dimensions the amplitude falls as
1/√r, so the level drops by 3 dB for each doubling of distance (a 3D point source would lose 6 dB). - Reflection and standing waves. With rigid walls the waves bounce and form fixed nodes and antinodes; the room modes of a rectangle are at
f = (c/2)√((nx/Lx)2 + (ny/Ly)2). - Diffraction. Through a gap of width
athe waves spread strongly whenais not much larger than λ, and move as a beam when it is. - Interference. Two sources in phase reinforce along some directions and cancel along others.
Honest simplifications
- Two dimensions only (a line source), so levels are relative to the level at 1 m, not absolute sound pressure levels.
- Air damping is a simple linear term and absorbing walls use a sponge layer, not a real impedance model. Wind, humidity, and the human hearing weighting are not modelled.
- The simulation runs in slow motion (about 70 times slower than real sound).
What to try
- Load Rigid room, standing waves and wait: the pattern becomes stationary with fixed quiet lines.
- Load Diffraction through a gap and narrow the gap below the wavelength shown in the last graph.
- Switch on the second source and slide its phase to 180 degrees to turn constructive interference into cancellation.
- Click the room to move the microphone and watch its level change.