Reflection, absorption and diffraction

When a sound wave meets a surface or an obstacle, several things can happen at the same time. How much of each depends on the size of the obstacle compared with the wavelength.

Reflection

A hard, smooth, large surface reflects sound like a mirror reflects light: the angle of incidence equals the angle of reflection. Reflections give rooms their character and also cause problems such as echoes, flutter and comb filtering, see Comb filtering.

A surface behaves as a good mirror only if it is larger than the wavelength. At 1 kHz (34 cm) a panel of 1 m reflects well. At 100 Hz (3.4 m) it does not.

Absorption

Part of the energy is not reflected but turns into heat in the surface or its material. The fraction absorbed is the absorption coefficient α, from 0 (perfect reflector) to 1 (perfect absorber), see RT60 and absorption.

Porous materials such as mineral wool absorb because air moving in their fibres loses energy to friction. They work best where the particle velocity is high, a quarter wavelength away from a hard wall. A resonant absorber works around a chosen frequency, see the absorber sizing tool.

Transmission

Some energy passes through the surface into the space behind it. Heavy, airtight constructions transmit less. Isolation is a different job from absorption, see Absorption, diffusion and isolation.

Diffraction

Waves bend around obstacles and through openings. The effect is strong when the obstacle is similar in size to the wavelength, and weak when it is much larger.

• Low frequencies bend around objects, so you hear bass behind a wall or around a corner.
• High frequencies form a shadow behind the obstacle.
• An opening smaller than the wavelength acts almost like a new point source.

This is why a thin screen does nothing for bass, and why your own head casts a shadow only for high frequencies, see How we hear direction.

Scattering

A rough or structured surface sends reflected sound in many directions. It is most effective for wavelengths comparable with the size of the structure. Diffusers are designed for this, such as the QRD diffuser.

A quick reference

Compute the wavelength with the frequency and wavelength tool.

Draft: written by an AI assistant, awaiting technical review.