Sound Insulation at Low Frequencies
Doctoral thesis, 1997
The airborne sound transmission through building partitions at low frequencies was studied. The definition of the sound reduction index as a measure of sound insulation has been examined by means of extensive numerical simulations in 3-D domains.
The simulations were based on the wave theoretical acoustics. The governing equation was the Helmholtz equation with acoustic pressure as the primary variable and with added damping term due to flow resistivity. The formulation of the Finite Element and the Modal Approach models is reviewed.
In order to draw reliable conclusions on the basis of simulation results, it is essential to demonstrate that the simulations give accurate predictions of the real situations. The simulation of sound reduction index evaluation experiments carried out in two different laboratories is, therefore, reported. The results of simulations are compared with the experimental results. The agreement is satisfactory. In one case, the discrepancy between the two evaluations exceeds 1 dB for only a few bands in the frequency range covering 3rd octave bands between 50 Hz and 500 Hz. In the other case the discrepancy is close to 3 dB. The reasons for higher discrepancy are explained in the thesis.
It has been clearly, quantitatively demonstrated that the sound reduction index at low frequencies is strongly affected by the room dimensions, the source position, and the reverberation time - the characteristics of the rooms. Hence, the sound reduction index indicates the property of the coupled room-partition-room system rather than the property of the partition itself. This is, in fact, true at any frequency, but it is most apparent at low frequencies, when the modal density of the rooms is low.
Finite Element Method
sound reduction index