Acoustic Glossary


The properties of a room or building, which determine how sound is transmitted within it.

Decibel (db)

A logarithmic unit used to measure sound and electronics, indicating a ratio of power or intensity relative to a particular reference level. Its use is central to acoustics, particularly as people interpret loudness much closer to a logarithmic scale than a linear one. 0 db is about the lowest level most people can hear.


The rate at which a source produces complete cycles of sound waves. Frequency is measured in hertz: 1Hz = 1 cycle per second. Low-pitched sounds have low frequencies and high-pitched sounds have high frequencies.


People’s perception of the intensity of sound. Intriguingly, this does not reliably correspond to the level of the sound – our ears sense noise in different ways. This explains why certain repetitive noises are perceived to have lower ‘loudness’ than their volume would suggest.


When something oscillates in a stationary position it causes vibrations, which often create unwanted noise; for example, a washing machine on spin cycle. Vibration often poses a challenge when trying to reduce noise levels.


Refers to sound which bounces off a surface, or several surfaces. Overall, it is dependent on the texture and structure of the relevant surface(s). Reflection can significantly diminish quality of acoustic conditions – for example, concave surfaces are a particular challenge as they focus reflections on one area.


The prolongation of sound in a space caused by its reflection from multiple surfaces, from its emission until it becomes inaudible.

Reverberation time

The time it takes for reflections of a direct sound to decay by 60dB below the level of the direct sound. Key factors that affect it are: size and shape of the space and the materials used to construct it. Rooms used for speech typically require a shorter reverberation time as this enhances intelligibility. However, there is no ideal reverberation time, as different rooms have different functions.

Sound absorption coefficient

The sound absorption coefficient is a scalar representation of the amount of sound absorbed when striking a particular surface.

Speech intelligibility

The ability for speech to be understood. Indoors, reverberations play a key role in defining the level of intelligibility. Strong intelligibility is the ideal platform for a relaxing, productive atmosphere. For satisfactory communication, the average speech level should exceed that of any interfering noise by 6dB.