Sound refraction is the bending of sound waves as they travel through a medium where the speed of sound changes gradually from one region to another. Just as light bends when passing from air into water, sound bends when it crosses a boundary between regions of different temperature, wind speed, or gas composition — because the speed of sound depends on the properties of the medium it travels through.
Refraction is most noticeable outdoors over long distances, where temperature and wind gradients bend sound toward or away from the ground, but it also plays a subtle role inside large enclosed spaces with significant temperature stratification.
Real-World Analogy
Picture a marching band walking in formation across a boundary between pavement and sand. The marchers on pavement move at full speed while the marchers hitting the sand slow down. Because one side of the line slows before the other, the entire formation pivots — the direction of march changes even though nobody intentionally turned.
Sound waves do the same thing. When one part of a wavefront enters a region where sound travels slower (cooler air, headwind), that part falls behind while the rest continues at the original speed. The wavefront pivots, and the sound bends toward the slower region.
Technical Definition
The speed of sound in air depends primarily on temperature:
c = 331.3 + 0.606 x T
Where c is the speed of sound in metres per second and T is temperature in degrees Celsius. At 20 degrees C, sound travels at approximately 343 m/s. At 0 degrees C, it drops to 331 m/s.
Refraction follows Snell's Law, adapted for acoustics:
sin(theta_1) / c_1 = sin(theta_2) / c_2
Where theta_1 and theta_2 are the angles of the sound ray relative to the normal of the boundary, and c_1 and c_2 are the sound speeds in each region.
Temperature Refraction
On a sunny day, the ground heats the air near the surface. The warm air near the ground has a higher sound speed than the cooler air above. Sound waves travelling horizontally bend upward — away from the ground — creating a "shadow zone" where sound is difficult to hear at a distance. This is why outdoor concerts can sound quieter than expected on hot afternoons.
At night, the ground cools rapidly while air above remains warm (a temperature inversion). Now the sound speed is higher above than below, and sound waves bend downward — toward the ground. Sounds carry much farther at night, which is why you can hear distant highway traffic or a train horn that is inaudible during the day.
Wind Refraction
Wind adds a velocity component to the speed of sound. Downwind, the effective sound speed increases with altitude (because wind speed increases with height above ground). Sound bends downward, enhancing propagation. Upwind, the effective sound speed decreases with altitude, bending sound upward and creating shadow zones.
The standard reference for outdoor sound propagation is ISO 9613-2:1996, which includes methods for accounting for meteorological conditions including temperature and wind gradients in the calculation of sound attenuation over distance.
Why It Matters for Design
Refraction has its greatest impact in these scenarios:
Outdoor noise assessment. Environmental noise studies for highways, airports, and industrial facilities must account for refraction. A noise barrier designed using only geometric calculations (straight-line propagation) may underperform during temperature inversions when sound bends over the top of the barrier.
Large indoor spaces. In spaces like aircraft hangars, atriums, and indoor swimming pools, temperature stratification between floor level and ceiling can cause noticeable refraction effects. Heated air rising from a swimming pool, for example, creates a gradient that bends sound downward from the ceiling, potentially concentrating reflected energy at unexpected locations.
Stadia and amphitheatres. Open-air performance venues are subject to refraction from both temperature and wind gradients. Acoustic designers model these effects to ensure that sound reinforcement systems deliver consistent coverage under varying meteorological conditions.
Long corridors and tunnels. Temperature differences between ventilated and unventilated sections of tunnels can bend sound in ways that affect emergency communication systems and alarm audibility.
For typical enclosed rooms (offices, classrooms, meeting rooms), refraction is negligible because the temperature is essentially uniform throughout the space. The room acoustic calculations that AcousPlan performs assume a uniform medium, which is valid for these environments.
How AcousPlan Uses This
AcousPlan's room acoustic calculations assume a uniform speed of sound within the modelled space, which is accurate for the enclosed rooms the platform is designed to analyse. The speed of sound is set at 343 m/s (corresponding to approximately 20 degrees C), consistent with the reference conditions in ISO 3382-2:2008.
For users working on large-volume spaces where temperature stratification may be relevant — such as atriums, natatoriums, or industrial facilities — understanding refraction helps contextualise why measured RT60 values might differ from calculated predictions. AcousPlan's comparison tools allow you to overlay measured data from field measurements against calculated values, making it possible to identify when environmental factors like refraction are influencing results.
Related Concepts
- What is Sound Diffraction? — Bending around obstacles, not through gradients
- What is Sound Reflection? — Bouncing off surfaces rather than bending through media
- What is Sound Wavelength? — The physical scale that determines diffraction and refraction behaviour
- What is Frequency in Acoustics? — The property that determines wavelength and refraction angles
Calculate Now
While refraction is primarily an outdoor phenomenon, understanding it makes you a better acoustic designer. Start modelling your indoor spaces with the AcousPlan Room Calculator and see how material choices — not weather — control the sound field in your rooms.