# Acoustics - facts and information

Frequency (Hz)
Frequency is defined as the number of vibrations of sound waves per second. The unit of frequency measurement is hertz (Hz). 1 Hz corresponds to one cycle (the horizontal length of one cycle of the sound wave) per second. Sound waves are vibrations which can move through air, water or solid objects. Human beings have an acoustic range of between 20 and 20,000 Hz.

Sound Level i.e. Sound Pressure Level [dB (A)]
The sound level measures the strength of a noise i.e. sound. It is measured in decibel (dB) and in the human ear it is defined as dB (A). Whispering corresponds to approximately 30 dB (A) and normal to loud speech corresponds to approximately 50 - 60 dB (A). The pain threshold for the human ear is roughly 130 dB (A).

Spectrum
A spectrum defines the noises or sounds whose sound waves contain many tones in various frequencies or it refers to pure tones which have only one frequency.

Audibility
The term refers to the suitability of a room (as an office, concert hall, lobby, classroom etc.) for the sound which is expected e.g. verbal communication, musical performance etc.

Reverberation Time (T60)
In room acoustics the reverberation characteristics of a room are determined by the reverberation time which has been established. The term describes the sound absorption of a room in terms of time until the sound level is no longer audible. The reverberation time is given in T60 / T and reflects the period of time in which the sound pressure level in the room dies away to one thousandth of its original level after the source of the sound stops; this corresponds to a decrease of 60 dB in the sound level. Therefore, the longer the reverberation time, the longer the sound pressure level (reverberation) is audible in the room.

Sound Insulation
If a room is used for various purposes i.e. by several people at the same time, then a partition to act as a sound barrier (for mutual noise protection) might be necessary; this is a movable sound -proof wall which can be placed anywhere in the room and takes the sound- absorbing requirements of the room into consideration. In addition, these partitions can also be used to provide visual cover.

Sound Absorption
With every vibration a part of the kinetic energy is converted into heat / heat loss. Depending on the source of the sound, we distinguish between airborne sound absorption (when the sounds to be absorbed e.g. from speech or the playing of a musical instrument spread through the air) and structure-borne sound absorption (when the sounds to be absorbed stem from solid objects e.g. someone walking across a floor).

Sound Absorption Coefficient (α)
The sound absorption coefficient reflects the relationship between the absorbed proportion of the sound and the power of the sound. If there is total reflection of the sound, α = 0; if there is total absorption of the sound, α = 1.

Equivalent Sound Absorption Area A
If the sound absorption coefficient of a structural element is multiplied by its area / m², you get the sound absorbing value / m² i.e. the equivalent sound absorption area A.