Because façades mainly are liable to airborne sound, only airborne sound insulation will be further discussed.
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 in figure 2) and the room that is screened by the building element from the so und source (receiving room in figure 2). Sound insulation depends on the frequency of the sound source.
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. Frequencies can be distinguished into three categories: low tones, mid tones and high tones (figure 3). The frequency range of urban road traffic is concentrated around the low tones whereas a singing teakettle rather consists of high tones. Figure 3 arranges examples of sound sources in order of their strength and frequency range.
First this paper examines how airborne sound insulation can be characterized. Subsequently the importance of airborne sound insulation is disc ussed. Finally the factors that influence airborne sound insulation of windows are explored.