Noise reduction of an acoustical enclosure --- Mechanisms and prediction accuracy
Noise reduction (NR) of an acoustical enclosure with flexible boundary walls has been predicted using t he Statistical Energy Analysis (SEA) method by several authors. Although it is useful for a rough NR estimation, a lar ge discrepancy often exists between the predicted and measured NR levels. Moreover, some physical mechanisms which may affect NR prediction were not addressed in the e xisting SEA models. The sources of the discrepancy were identified by investigating the limitation of SEA f or system energy transfer in the entire frequency r ange of noise transmission, and the effect of enclosure wall coup ling and sound-structural coupling on the NR and its prediction accuracy. This paper presents a modified SEA model, which includes the non-resonant response and more accurate transmission coefficient of finite panels, and comp ares the model prediction with experimental results . A reasonable agreement between the prediction and experiment was observed.
Predicting the sound insulation of single leaf walls: extension of Cremer's model.
In his 1942 paper on the sound insulation of single leaf walls, Cremer [(1942). Akust. Z. 7, 81-104] made a number of approximations in order to show the general trend of sound insulation above the critical frequency. Cremer realized that these approximations limited the application of his theory to frequencies greater than twice the critical frequency. This paper removes most of Cremer's approximations so that the revised theory can be used down to the critical frequency. The revised theory is used as a correction to the diffuse field limp panel mass law below the critical frequency by setting the nonexistent coincidence angle to 90 degrees. The diffuse field limp panel mass law for a finite size wall is derived without recourse to a limiting angle by following the average diffuse field single sided radiation efficiency approach. The shear wave correction derived by Heckl and Donner [(1985). Rundfunktech Mitt. 29, 287-291] is applied to the revised theory in order to cover the case of thicker walls. The revised theory predicts the general trend of the experimental data, although the agreement is usually worse at low frequencies and depends on the value of damping loss factor used in the region of and above the critical frequency.
Accurate estimation of the mean sound pressure level in enclosures
The accurate determination of the mean sound pressure level is the basis for the determination of the sound power of a source in enclosures. The relation between the sound power of the source under investigation and the mean sound pressure level to be measured in the room can be derived either by means of wave theory or statistical room acoustics. It is shown that both models yield the same relation in the limiting case of high frequencies. Taking this relation as a starting point, it is first possible to include the modal behavior into the statistical model which is important in order to extend the validity of the statistical model to the low-frequency range. Second, the correction term for the potential energy stored near the room boundaries, which is known as the Waterhouse correction, is improved by taking into account the absorption characteristic of the room walls. The improved relation gives a higher accuracy for the determination of the mean sound pressure level in enclosures especially in the low...