Acoustic Theory

Sound is defined as mechanical kinetic energy which is transmitted through elastic media by pressure fluctuations and molecular motion. The sound level is expressed in dB and determines the ‘strength’ of the sound source. The sound of a whistling bird (50 dB) is for example stronger than the sound of a falling leaf (10 dB); the key of a piano can be struck hard or soft. Sound can be one single frequency (e.g. musical notes), but is usually made up of a number of frequencies (e.g. traffic noise). A frequency is expressed in Hz and determines the ‘pitch‘ of the sound source.

The airborne sound insulation of a building element is determined by the difference between the sound level of the room in which a sound source is present (source room) and the room that is screened by the building element from the sound source (receiving room). Sound insulation depends on the frequency of the sound source.

Frequencies can be distinguished into three categories: low tones, mid tones and high tones. The frequency range of urban road traffic is concentrated around the low tones whereas a singing teakettle rather consists of high tones.

Instead of listing the airborne sound insulation by frequency, technical documentation usually indicates a concise global performance. The European standard EN ISO 717-1 describes a method to express the airborne sound insulation by the single-number quantity in which Rw is the weighted sound reduction index in dB.

The sound insulation for air borne sound can be optimized for a given maximal weight of the construction by using double layer constructions instead of a single homogenous layer. To minimize transmission of sound between compartments acoustic insulation such as Knauf Insulation's acoustic range should be used in the cavity between the masses. The idea is to create a mass-spring-mass system by using an attenuation layer between the mass layers. This acts as a spring and decouples the mass layers much like the suspension system in your car decouples the body of the car from vibrations created by imperfections in the road.

In addition, the fiberous nature of Knauf Insulation's acoustic range helps to defuse sound by reflecting sound waves in a variety of directions and consequently minimizing the sound either directly transmitted or reflected. The minimization of the reflected sound energy back into the room reduces both echo and reverberation.In effect, this is the principal applied by having egg boxes on the walls of amateur recording studios.