This simulation demonstrates how sound waves travel through matter as longitudinal pressure disturbances. A source emits wavefronts that expand outward (or advance as plane waves), reaching a medium interface. At the boundary, part of the wave reflects back and part transmits into the new medium. The simulation renders the incident wavefronts in blue, reflected wavefronts as dashed orange arcs, and transmitted wavefronts in a colour specific to the second medium. The pressure field overlay uses pixel-level shading to show compressions (blue) and rarefactions (dark red) in real time.
Sound speed varies dramatically between media: roughly 343 m/s in air, 1480 m/s in water, and 5960 m/s in steel. The simulation captures this by stretching transmitted wavefronts proportionally to each medium's speed multiplier. Enabling absorption causes the transmitted amplitude to decay exponentially with distance, modelling how energy is lost to heat in dense media. Switching between point and plane sources shows how wavefront geometry affects reflection and transmission geometry at the interface.
What is sound wave propagation?
Sound propagates as a mechanical wave: a series of compressions and rarefactions moving through a material medium. Molecules are pushed together and pulled apart in the direction of travel, transferring energy without net displacement of matter. Unlike light, sound cannot travel through a vacuum because it needs particles to vibrate.
Why does sound travel faster in water than in air?
Speed of sound equals the square root of the bulk modulus divided by density. Water is far less compressible than air, so its bulk modulus is much higher. Even though water is denser, the stiffness wins: sound reaches about 1480 m/s in water versus 343 m/s in air at room temperature.
What happens when sound meets a boundary between two media?
Part of the wave reflects back into the original medium and part transmits into the new one. The fractions depend on the acoustic impedance mismatch (impedance = density times wave speed). A large mismatch, such as air-to-steel, reflects most energy; a closer match, such as water-to-tissue, transmits more, which is why ultrasound gel is used in medical imaging.
A point source radiates energy equally in all directions, producing spherical (or circular in 2D) wavefronts whose amplitude drops as 1/r with distance. A plane source generates flat wavefronts that maintain constant amplitude over distance in an ideal medium, like a large flat loudspeaker. Real sources are somewhere in between depending on wavelength and aperture size.
As a sound wave travels through a medium it loses energy to internal friction, heat conduction between compression and rarefaction zones, and molecular relaxation. This absorption increases with frequency and varies strongly between materials. The simulation models it as exponential decay of amplitude beyond the interface, controlled by a characteristic absorption length.
Higher frequency means shorter wavelength (wavelength = speed/frequency). Short-wavelength waves are absorbed more quickly in most media and diffract less around obstacles. Lower frequency waves travel farther, which is why foghorns use low frequencies and why bass notes pass through walls more easily than treble ones.
Acoustic impedance Z = rho times c, where rho is the medium density and c is the sound speed. It determines how much sound is reflected at an interface. When two media share the same impedance, no reflection occurs at all, a principle exploited in impedance-matching layers used in ultrasound transducers and anti-reflection coatings.
No. Sound is a mechanical wave that requires a medium to propagate. In a vacuum there are no molecules to compress and rarify, so no pressure disturbances can form or travel. The simulation shows this with the Vacuum option, where wavefronts cease to form regardless of the source.
Huygens principle states that every point on a wavefront acts as a secondary point source of wavelets, and the new wavefront is the envelope of all those wavelets. This explains how wavefronts bend around obstacles, how they reconstruct after passing through an aperture, and how reflection and refraction angles arise from simple geometric construction. The expanding ring model in this simulation directly embodies Huygens wavelets.
In the pressure-field overlay, blue shading indicates compressions where molecules are packed closer together and pressure is above ambient. Dark shading marks rarefactions where molecules spread apart and pressure falls below ambient. These alternate bands move outward from the source at the wave speed, and their spacing equals the wavelength.