Glossary

Ratio of the absorbed (and possibly transmitted) portion of sound to the incident sound.
Absorption coefficient α = 1 - reflection coefficient ρ

(Notional) area with an absorption coefficient of α = 1 that has the same absorption as the entire surface of a room together with the objects it contains.
A = α * S

The base level is the A-weighted sound pressure level of the level frequency distribution of any given noise that is exceeded during 95% of the measurement time.

The A-weighted energy-equivalent continuous sound level of any given noise, referred to a defined period of time. The rating level is a measure for characterising the sound immission acting on a location. It serves primarily to assess the effect of noise on people at the workplace, in the case of new construction, or where transport routes are substantially altered. Its purpose is to evaluate the burden on those affected by noise.
Since the rating level usually has to be determined in advance, it is calculated on the basis of extensive studies from the standardised average level established for a source and from corrections that depend on the immission location, such as the distance from the source. If, in a relevant situation (for example the construction of a new road), rating levels determined at neighbouring properties exceed the statutory limits, measures to reduce the rating levels must be initiated (e.g. noise barriers in the case of traffic noise).

The insertion loss (IL) describes the attenuation of a signal caused by a component that is inserted into a signal path. This may be, for example, a passive filter or a silencer.

Single-number quantity used to describe sound events with an arbitrary time history of the sound pressure level. The energy-equivalent continuous sound level  is calculated as the sound pressure level that, acting continuously, is energy-equivalent to any given noise.

For the resilient mounting of machines and foundations, particularly where a low natural frequency (f0 < 10 Hz) is required. They usually consist of cylindrical helical compression springs housed in a metal casing, whose spring stiffness is dimensioned according to the required tuning. Because of their low inherent damping, the risk of vibrations building up while passing slowly through resonance is greater than with rubber-metal elements. In such cases, and when isolating sudden impact forces (forging operations), additional damping by friction or viscous dampers is required.

Sound pressure level originating from sources other than the one being measured. It is determined by switching off the source under test and recording the sound pressure level. If the total sound pressure level is less than 3 dB above the extraneous noise level, the measurement cannot be evaluated.

Acoustic laboratory room with boundary surfaces that reflect sound well, so that a long reverberation time and a very uniform sound field are established. It is mostly used to measure the absorption coefficient or the sound power level and should have a volume of between 100 m³ and 400 m³.

Prevention of the propagation of structure-borne sound by reflection at a resilient layer. Because different wave types are coupled within solid bodies and because the resilient layers have limited thickness, the principles of structure-borne sound insulation cannot be presented as clearly as those of airborne sound insulation. In general, the softer the resilient intermediate layer and the more heavily it is loaded, the greater the structure-borne sound insulation.

Used to screen noise emitted by a linear or planar sound source (e.g. roads, railway lines, factory installations), so that at an immission location to be protected (e.g. residential buildings, hospitals) the noise is attenuated far enough for the statutory limits to be met. They can be supplemented by passive noise control measures (e.g. acoustic windows).

Describes the time interval in seconds within which the sound pressure in a room falls to one thousandth of its initial value after a sound source suddenly falls silent. This corresponds to a decrease in sound pressure level of 60 dB. According to Sabine’s reverberation formula:
T60 = 0.161 * V / A

Acoustic measurement room with boundary surfaces that absorb sound almost completely (usually wedge-shaped or pyramidal porous absorbers), so that sound propagation approximates free-field conditions. An anechoic room has a sound-absorbing lining with an absorption coefficient of α > 99% above a lower limiting frequency fu.

Logarithmic quantity in dB for the airborne sound insulation of a building element (wall, ceiling). It is defined as the ratio of the sound intensity striking a wall to the total sound intensity transmitted through the wall.
R = 10 * log (I1 / I2)

A distinction is drawn mainly between absorptive, restrictive and reactive silencers (interference silencers). The effectiveness of silencers is characterised by the transmission loss or the insertion loss. Common fields of application are ventilation ducts, motor vehicles and industrial plants.

Logarithmic measure in dB describing sound pressure. It is a sound field quantity that normalises the RMS value of the sound pressure to a reference value of p0 = 20 μPa:
Lp = 20 * log (p / p0)

Denotes the emission of sound from a sound source. The measured quantity associated with sound emission is the source strength, given as the sound power level Lw in dB.
Sound immission: describes the effect of sound at a location. The local physical effect of the noise is the sound pressure present at that location. Sound immission depends on the sound emission and on the sound propagation. The sound immission level is used for the quantitative description of sound immission in the context of noise control.

Logarithmic measure in dB describing the total sound power emitted by a sound source. It is a sound energy quantity that normalises the sound power to a reference value of P0 = 10-12 W:
Lw = 10 * log (P / P0)

Sound levels corrected by a frequency or time weighting, or level values related to a reference curve. The most common weightings follow the A or C weighting curves.

A constructional measure for reducing the noise of machines or for protecting people. A high sound reduction index and an absorbent lining are important factors for effective noise control.

Logarithmic measure in dB for the disturbing noise in adjacent receiving rooms when floors are excited by structure-borne sound (using a standard tapping machine). Note that high values indicate poor impact sound insulation.