What are room modes in acoustics?

Room modes (also called standing waves or eigenmodes) are resonant frequencies at which a room strongly amplifies sound. They occur when the room dimensions are integer multiples of half-wavelengths: f_n = n�c/(2L) for axial modes in a rectangular room of length L. In a rectangular room there are axial (one axis), tangential (two axes), and oblique (all three axes) modes. At low frequencies, rooms typically have sparsely spaced modes leading to severe coloration of sound � bass notes at certain pitches are dramatically boosted while adjacent pitches are attenuated.

How does the Doppler effect change sound frequency?

The Doppler effect occurs when there is relative motion between a sound source and observer. If the source moves toward the observer at speed v_s in a medium where sound travels at c, the observed frequency is f_obs = f_source � c/(c - v_s). As the source moves away: f_obs = f_source � c/(c + v_s). In this simulation, the moving source travels at ~0.4c (Mach 0.4), so approaching frequency is boosted by factor 1/(1-0.4) = 1.67� and receding frequency is reduced to 1/(1+0.4) = 0.71� � a clearly audible pitch shift straddling the original frequency.

What is the FDTD method used in this simulation?

FDTD (Finite-Difference Time-Domain) solves the acoustic wave equation ?�p/?t� = c�?�p by replacing derivatives with finite differences on a regular grid. Space is discretised into cells of width ?x and time into steps ?t. The stability condition (Courant-Friedrichs-Lewy criterion) requires the Courant number r = c�?t/?x = 1/v2 � 0.707 for 2D grids. This simulation uses r = 0.4 giving R� = 0.16, well within the stability limit. Damping is added via the term a�?p/?t which models absorption by wall materials. Boundary conditions use a sponge (exponential decay) layer at open edges to absorb outgoing waves.

How does noise cancellation work physically?

Noise cancellation (active noise control) works by superimposing an anti-phase pressure wave on the original noise. If two identical sinusoidal sources are separated by half a wavelength and driven 180� out of phase, their pressure fields add destructively along the axis between them (destructive interference). The Noise Cancel preset places two sources separated by ~0.22 � domain width with inverted polarity. Where the waves meet with equal amplitude and opposite phase, pressure cancels to near-zero � visible as dark bands in the pressure field.

Home▸Room Acoustics & Architectural Sound▸Acoustic Wave Simulator – Room Modes, Doppler Effect & Reverberation

🧪 Acoustic Wave Simulator – Room Modes, Doppler Effect & Reverberation

Interactive 2D FDTD acoustic wave simulator. Visualise pressure waves in open and closed pipes, room resonant modes, Doppler shift from a moving source, long reverberation in a reflective chamber, and destructive interference for noise cancellation. Free.

Room Acoustics & Architectural Sound2DModerate60 FPS

⚙ Under the hood

This simulation demonstrates the principles of acoustics and wave propagation. Users can observe how sound waves interact with surfaces to create room modes, Doppler effects, and reverberations.

Acoustic WavesReverberation

2D · HTML5 Canvas 2D · 60 FPS target · runs fully client-side, no install