VRIJE UNIVERSITEIT BRUSSEL Acoustics - the Dept. of ...

82 CHAPTER 5. SOUND INSULATIONthe transmission of the sound (and vibrations) energy from one room to theother. This indirect way of transmission throughall those other boundaries /paths (other than directly through the partition which separates the rooms)is called ’flanking’ transmission (see Figure 5.1). Consequently a distinctionmustbemadebetweenthesoundinsulationRofaseparatingstructurewhichis called the ’sound reduction index’ in the international ISO140 norm, andthesoundinsulation D n between two roomswhich iscalled ’the normalizedlevel difference’ in the ISO140 norm.Figure 5.1: Flanking (2 to 4) and direct (1) transmission.It is obvious that in practice, i.e. in the acoustics of a construction,only the sound insulation D n between two rooms matters because flankingtransmission is always present. Normally we will perform the so-called ’fieldmeasurements’ of the insulation between two given rooms, where we ignorethesoundpathbetweenthesource-andreceivingroom. ThesoundinsulationD n is expressed in dB. To characterize the quality of a partition constructionby measurements all the flanking effects must be eliminated and thus a fieldmeasurement is not an option. A ’laboratory-only measurement’ must beperformed (see Figure 5.2) Such a laboratory consists of two rooms : thesource- and receiving room are independent with regard to vibrations, andthe sound transmission occurs only through the structure under test. Thisenables us to determine the sound insulation R (this is typically larger thanD n ).We will now give the formula that enables us to experimentally determinethe sound insulation as function of the frequency. Let S represent theseparation plane consisting of the wall to be tested (10 m 2 is required for