Glossary

dB (decibel) - The scale on which sound pressure level is expressed. It is defined as 20 times the logarithm of the root-mean-square pressure of the sound field and reference pressure (2 x 10^-5 Pa).

Direct Sound - Sound that arrives at the listener’s position directly from the sound source, i.e. without being reflected from any objects or surface.

First Reflections - Normally defined as the sound reflections that reach the listening position up to approximately 20 ms after the direct sound.

Flutter Echoes - Repeated sound reflections caused by sound waves travelling between parallel reflective surfaces such as walls.

Reverberation - An acoustical phenomenon that occurs in enclosed spaces, when sound persists in that space as a result of repeated reflection or scattering from surfaces enclosing the space or objects within it.

Reverberation Time (s) - A measure of the degree of reverberation in a space. It is equal to the time required for the level of a steady sound to decay by 60 dB after it has been turned off.

Room Modes - At specific frequencies, called room resonance frequencies, standing waves are created within rooms. These frequencies depend on the dimensions and shape of the room. This group of resonance frequencies are normally referred to as room modes. When a sound source generates sound with frequencies equal or close to the room resonance frequencies, the room response will be enhanced and patterns of maximum pressure levels and minimum pressure levels will be produced. The shape of these patterns differs with the room resonance frequency.

Sound Absorption - The portion of the sound energy that is absorbed and not returned when a sound wave hits a surface.

Sound Diffusion - Sound diffusion occurs when a sound wave hits a complex surface such as a diffuser and its energy is distributed in many directions.

Sound Reflection - The portion of the sound energy that is returned when a sound wave hits a surface.

Standing Waves - A standing wave is originated from the interaction of two sound waves with equal frequency and amplitude but travelling in opposite directions. Unlike the travelling waves, the standing waves do not cause a net transport of energy, since the two waves that form it are carrying equal energy in opposite directions. The resulting standing wave alternates between maximum and zero amplitude.