Sound reduction index

The sound reduction index (in the technical literature very often sound reduction index ) is a logarithmic measure and describes the ability of a component or transition between two sound-conducting components or media to insulate the sound . It is defined as the ratio of the sound intensity hitting a wall to the total sound intensity transmitted through the wall : ${\displaystyle \textstyle R}$ ${\displaystyle \textstyle I_{1}}$ ${\displaystyle \textstyle I_{2}}$

${\displaystyle R=10\,\mathrm {dB} \,\lg {\frac {I_{1}}{I_{2}}}=20\,\mathrm {dB} \,\lg {\frac {p_{1}}{p_{2}}}=10\,\mathrm {dB} \,\lg {\frac {1}{\tau }}=-10\,\mathrm {dB} \,\lg \tau \ \,}$.

The transmittance is , and denote the respective sound pressure . ${\displaystyle \textstyle \tau }$ ${\displaystyle \textstyle p_{1}}$ ${\displaystyle \textstyle p_{2}}$

A high degree of sound insulation means a low degree of transmission and good sound insulation properties. The sound reduction index depends on the frequency and the angle of incidence . A sound reduction index can also be specified for diffuse sound incidence and is then calculated from the transmittance weighted over all directions of incidence. It is also common to describe the frequency dependence by specifying it as a third-octave or octave band spectrum. ${\displaystyle \textstyle \theta }$

Single numbers

The sum of all third octave attenuation (here it is 28 dB), which are lower than that of the reference curve, must not exceed 32 dB. If this is fulfilled, the sound reduction index is read off at 500 Hz.

In building acoustics , so-called single number specifications have become established for the sound reduction index. They enable the characterization of the sound insulation of a component (e.g. a wall) without considering the frequency dependency, which is much more understandable for laypeople and simplifies the setting of requirements. The evaluated sound reduction index is determined by comparing the third octave or octave band spectrum of the sound reduction index with a reference curve (typical course for solid components) specified in the DIN EN ISO 717-1 standard. The assessed building sound reduction index describes a sound reduction index determined in this way, which was measured or calculated for a component in the installed state ("on the building"). The problem with the assessed sound reduction index is that the frequency dependency is lost (for non-solid components it is no longer possible to infer the qualitative course of the sound insulation curve) and only a frequency range from 100 to 3150 Hz is considered. Since the range below 100 Hz in particular is perceived as very annoying by users and residents, the weighted sound reduction index can only be used to a limited extent as a measure of the effect of the sound insulation. The spectrum adjustment values and according to DIN EN ISO 717-1 (notation:) only partially remedy this shortcoming. It is always better to consider the frequency as a function of the frequency, as shown in the graphic opposite. ${\displaystyle \textstyle R_{\mathrm {w} }}$ ${\displaystyle \textstyle R_{\mathrm {w} }'}$ ${\displaystyle \textstyle C}$ ${\displaystyle \textstyle C_{\mathrm {tr} }}$ ${\displaystyle \textstyle R_{\mathrm {w} }(C;C_{\mathrm {tr} })}$

Common values ​​for are: ${\displaystyle \textstyle R_{\mathrm {w} }(C;C_{\mathrm {tr} })}$

Soundproofing of double and triple insulating glass

Structure (mm)					Sound insulation (dB)
Disc 1	SDR 1	Disc 2			specification ${\displaystyle \textstyle R_{\mathrm {w} }(C;C_{\mathrm {tr} })}$	normal	high- frequency	low frequency
6th	14 ares	4th			35 (−1; −5)	35	34	30th
10	20 ares	6th			40 (−1; −5)	40	39	35
VSG 4 + 0.38 + 4	16 ares	10			45 (−2; −6)	45	43	39
VSG 6 + 0.76 + 6	24 ares	VSG 4 + 0.38 + 4			50 (−2; −8)	50	48	42
VSG 8 + 0.76 + 6	24 ares	VSG 4 + 0.76 + 6			52 (−2; −6)	52	50	46
Disc 1	SDR 1	Disc 2	SDR 2	Washer 3	specification ${\displaystyle \textstyle R_{\mathrm {w} }(C;C_{\mathrm {tr} })}$	normal	high- frequency	low frequency
4th	16 ares	4th	16 ares	4th	34 (−2; −6)	34	32	28
4th	12 kr	4th	12 kr	4th	35 (−2; −6)	35	33	29
VSG 4 + 0.76 + 6	8 kr	5	8 kr	6th	43 (−2; −7)	43	41	36

Measurement

There are various options for measuring the sound reduction index. A method that is widely used in building acoustics is particularly easy to carry out. A configuration of two rooms is assumed, between which the component to be characterized (e.g. a wall) with the interface is located. A diffuse sound field is assumed in both rooms and the mean sound pressure level is measured, which occurs when a powerful sound source is set up in one of the rooms (transmitting room). In the other room, the reception room, the equivalent absorption area is also determined by measuring the reverberation time . The sound reduction index for diffuse sound incidence can then be determined from the difference between the mean sound pressure level: ${\displaystyle \textstyle S}$ ${\displaystyle \textstyle A}$ ${\displaystyle \textstyle \Delta L}$

${\displaystyle R=\Delta L\,+\,10\,\mathrm {dB} \,\lg {\frac {S}{A}}\,}$

Another possibility, which is mainly used in the laboratory, is to measure the sound intensity when the sound intensity is known . ${\displaystyle \textstyle I}$ ${\displaystyle \textstyle I_{0}}$

calculation

The calculation of the sound reduction index for single-shell components (e.g. sheet steel, brick wall) can be carried out using Berger's law of mass. This relationship shows that the sound reduction index of such components increases with the area-related mass. However, the law of mass only applies until the onset of the track adjustment effect, also known as coincidence , i.e. H. only below the coincidence cutoff frequency. More precise models are much more complicated. Analytical (purely physical, e.g. from the wave equation ), empirical (from measurement data) or numerical (e.g. statistical energy analysis SEA) methods can be used to determine the sound reduction index very precisely or for complex components .

See also

• Degree of absorption
• Sound reduction index of doors

Web links

• Ackermann, Lutz: Simulation of the sound transmission through walls . Braunschweiger Schriften zur Mechanik, Vol. 43-2002 (2002)

Individual evidence

1. ↑ bksv.com
2. ↑ see for example Building Acoustics - Basics of Airborne Sound Insulation of Components . uni-essen.de