Sound reduction index

The sound reduction index R is a logarithmic measure and describes the ability of a component or a transition between two sound-conducting components or media, to insulate the sound.

Definition

The sound reduction index R is the ratio of the incident on a wall of sound intensity to the whole through the wall of the transmitted sound intensity:

The transmittance is.

A high sound reduction means a low transmittance and good Schalldämmvermögen. The sound reduction index depends on frequency and angle of incidence. A sound reduction can also be specified for diffuse sound incidence and is then calculated from the weighted over all directions of incidence averaged transmittance. Also common is the description of the frequency dependence by specifying as a third or octave band spectrum.

Single number

In building acoustics, so-called single number for the sound reduction index have been established. They allow the characterization of the acoustic insulation of a component (e.g., a wall) without consideration of the frequency dependence of which is essential to understand for the layman and simplifies the set of requirements. The weighted sound reduction index is determined ( typical curve for solid components) by comparing the third or octave band spectrum of the sound reduction index with a 717-1 specified in the standard DIN EN ISO reference curve. The weighted apparent sound reduction index denotes a so- Discovered sound reduction index, which has been measured or calculated for a component in the mounted state ( " the building "). The problem with the weighted sound reduction index is that the frequency dependence is lost (for non- solid components can no longer be closed on the qualitative course of Schalldämmkurve ) and only one frequency range 100-3150 Hz is considered. Since especially the range below 100 Hz by users and residents is perceived as very disturbing, the weighted sound reduction index can only be partially used as a measure of the effect of sound insulation. The correction terms according to DIN EN ISO 717-1 resolve this shortcoming only partially. Better is always a frequency-dependent analysis.

Measurement

To measure the sound transmission loss, there are several possibilities. Particularly easy to perform a procedure that is widely used in building acoustics. This is based on a configuration of two spaces between which the component to be characterized (e.g., a wall ) located at the separation surface. In both rooms, a diffuse sound field is assumed and measured the average sound pressure level which is established when, in one of the rooms ( the source room ) is situated a powerful sound source. In another room, the reception room, in addition the equivalent absorption area is determined by measuring the reverberation time. The sound reduction index for diffuse sound incidence can then be determined from the difference of the mean sound pressure level:

A further possibility, which is used mainly in the laboratory, the measurement of the sound intensity with a known sound intensity.

Calculation

The calculation of sound insulation can for single components (eg steel, brick wall ) done with the help of Berger 's mass law. This relationship shows that the sound reduction of such components increases with the mass per unit area. However, the mass law applies only to the onset of the track adaptation effect, also called coincidence, ie just below the coincidence frequency limit. More accurate models are much more complicated. There are many models to determine the sound reduction index very closely or for complicated components. This can analytically ( purely physical, such as from the wave equation), empirically ( from measured data ) or numeric ( eg Statistical Energy Analysis SEA).