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

92 CHAPTER 5. SOUND INSULATIONMateriaal Densiteit E modulus c L f crit (dikte 10 mm)Aluminium 2.7 70 5367 1192Lood 11.1 16 1265 5057Beton 2.5 48 4618 1385CFRP 1.5 1.5 1054 6071Kurk 0.18 0.032 444 14400Glas 2.5 65 5374 1190Staal 7.8 210 5469 1170PU 1.2 0.025 152 42065Table 5.2: Materiaalparameters en coïncidentiefrequentie van enkele materialenthatinsulationatlowfrequencies isvery bad. Theeffect ofthislowfrequencycoincidence effect can be reduced by applying an additional material witha high critical frequency (thin material with high speed of sound) and areasonable insulation at low frequencies on the wall. An example of such amaterial is lead foil. For a thickness of 1 mm f crit = 50 kHz.The graph that summarizes the insulation behavior of a wall as a functionof the frequency is shown in Figure 5.5.5.2.4 Insulation of double wall constructionsDouble walls are used a lot in constructions. Think of double glazing, cavitywalls, ... . The main reason for their use is the high thermal insulation, butthe acoustic properties are also improved. The following practical guidelinescan be used for double walls :1. By doubling the mass the sound insulation raises with 6 dB accordingto the mass-frequency law (in reality it is only 5 dB). Appart from themass itself the air cavity also influences the sound insulation :For a distance between the walls of 2 to 4 cm the sound insulationincreases with 4 dB with respect to the mass-law.Foradistance between thewallsof5to10cmthesoundinsulationraises with 9 dB with respect to the mass-law.2. The experimental frequency law has now a slopeof 6 to 8 dB per octave(in comparison to 5 dB for the experimental law of single walls).3. In the cavity in between the double walls an acoustic resonance canoccur. This can lead to an important reduction in the sound insulationat the resonance frequencies.