The basics of room acoustics: Theory & terms

We at Acoustic Bulletin writes for both acousticians and sound enthusiasts of all types and levels of experience. This post is for anyone who is just entering the field of room acoustics. It contains only very simple and basic descriptions of the the main terms, but should provide enough detail to give you a firm grasp in most situations concerning room acoustic design. 

Sound: Amplitude & pitch

Sound arises when particles in the air are set into motion. This happens, for example, when we speak. The movements of the particles create pressure waves that are transported through air at a speed of approx. 343 meters per second. Sound waves can basically be described in two ways: The amplitude, which is an expression of how much energy a sound contains, and the wave length that determines “the tone” or more precisely the pitch of the sound.

Sound level

The amount of energy contained in a sound wave determines the distance from wave crest to trough and is called the sound level and is measured in decibels (dB). The decibel scale is not linear, but logarithmic. This means that people experience 10 dB as a doubling of the sound level. Comfortable sound levels range from 0 to 120 dB, which is also called “the pain threshold”.

Frequency bands

A sound consists of many waves with different wave lengths, which together form the timbre of a given sound. This ratio of waves cycles pr. second is also called frequency. It is measured in Hertz (Hz) and is determined by the length of a given wave. The audible spectrum is roughly between 20 and 16,000 Hz, going from a dark to a bright pitch. For the sake of simplicity, this spectrum is often divided into frequency bands where each band represents an average of a frequency range. Within room acoustics, you typically work with the frequency bands 125, 250, 500, 1,000, 2,000 and 4,000 Hz.

Reverberation time

The main descriptor in room acoustics is reverberation time. The reverberation time (T) specifies the time period in which sound energy dissipates in a space. It roughly represents how noisy a space is. Reverberation time is measured in seconds within specific frequency bands. For example, a regulatory requirement set for stairwells at schools could be T ≤ 1.3 within the frequency range 500-2,000 Hz, which means that the reverberation time must be 1.3 seconds or less within the frequency bands 500, 1,000 and 2,000 Hz.

In short, reverberation time depends on the ratio between the volume of a room and the absorption area. The larger the room is, the longer the reverberation time will be. This is particularly important if you for example are dealing with rooms of double height and/or open environments. On the other hand, the reverberation will decrease the larger the equivalent absorption area of a room is (more below). A room’s reverberation time will often be represented as a graph where the y-axis represents reverberation time and the x-axis is the frequency band as shown below:

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Absorption area

A room’s absorption area (A) is an expression of the total amount of sound absorption that the room contains. The larger the absorption area of a room is, the faster the sound energy will die off. Materials absorb sound different depending on the frequency, which is why the regulatory requirements set absorption area requirements over a frequency range (125-4,000 Hz).

For large rooms such as open learning spaces and office environments, norms in some countries set demands on absorption area per volume rather than reverberation time alone. Typical building materials and interior decor contribute astonishingly little to the absorption area, while acoustic materials contribute significantly.

When it comes to acoustic materials, it is important to keep in mind that their actual area is not necessarily equal to their absorption area. It depends on the quality of the materials. You should therefore always ask or look for the absorption classes of the acoustic products, which are listed from class A and lower. The better the quality of acoustic material used is, the more it will contribute to the absorption area of the room, and the fewer square meters of acoustic material are needed.


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