MECANISMOS CLÁSICOS DE RESISTENCIA A ANTIFOLATOS

After the sound insulation testing, the diagnostic tests had to be performed. As mentioned earlier, the glazing and frame elements present a path for sound transfer from source to receiving room assuming the transmission through seals can be neglected. Thus, the next step was to measure the sound transfer through these elements using diagnostic tests. Then, following the I-ASCA methodology, the first step of the measurement was to discretise the partition in smaller areas. The discretised partition is shown in Figure 6.3. Due to the different size and dimensions of the glazing and frame as well as test time restrictions, a fine discretisation could not be maintained throughout the window surface. Ultimately, a coarse grid was used on the glazing (15 x 15 cm2 _{path area) while the}

discretisation on the frame was much finer (7.5 x 9 cm2 _{path area). Because the size of the}

glazing was bigger than the frame, more low order modes would be excited in the glazing. This would mean efficient coupling with lower order room modes and more transmission in the low frequency region through the glazing. At low frequencies the grid size can be coarse as grid size increases with frequency. Thus, using a coarse grid on the glazing and a finer grid on the frame made sense. After the discretisation, the following measurements were conducted according to D-ASCA.

Figure 6.3: Discretisation on the glazing and frame elements for diagnostic measurements

1) Contact pressure measurements –A loudspeaker driven by pink noise excitation was set up in the source room facing the corner. The sound field driven by the loudspeaker simulates the airborne excitation on the window. Under operational conditions, the contact pressures against each path were measured using microphones. The measurements were referenced to the loudspeaker voltage. Each measurement was averaged over 60s containing multiple windows. These contact pressures then approximate the blocked pressures on the window.

2) Vibroacoustic FRF’s –Using a force hammer to impact at each path position, the pressures were measured at receiver positions in receiver room. The receiver positions were the same

as the ISO 10140 positions so that the diagnostic results can be averaged over the room volume following standard guidelines.

3) Operational accelerations –The accelerations were also measured on the path positions using accelerometers under operational conditions. The accelerations too were referenced to the driving voltage of the loudspeaker. This was measured to be used in the I-PCA methodology.

Once all the measurements were completed, using the D-ASCA methodology (Eq. (5.22)) the sound transfer through the window in the receiving room was predicted and compared to the measured pressure as a part of the pressure validation. The results of the pressure validation are shown in the Figure 6.4.

Figure 6.4: Pressure validation results for D-ASCA test on the double casement window at 6 receiver positions –comparing the predicted pressure (in blue) to measured pressure (in red) in

narrow band from 30-1000 Hz

The pressure validation results in Figure 6.4 show that the measured and predicted pressure spectrums match well for frequencies above the 60 Hz. The prediction accuracy was quantified in on-third octave bands for the spatially averaged receiving room pressure as shown in Figure 6.5.

Figure 6.5: Averaged receiving room SPL predicted by D-ASCA (in blue) compared to measured pressure (in red) in one-third octave bands from 60-1000 Hz

An excellent agreement (within 1 dB above 63.5 Hz band) between the predicted and measured SPL for the receiver room in one-third octave bands can be seen. The deviations (2-3 dB) in low frequency band of 63.5 Hz are firstly due to the error in measuring the vibroacoustic FRF’s. The hard tip of hammer is not very suitable to impact low frequency force into the structure which results in poor signal to noise ratio at very low frequencies. To overcome this, a softer hammer tip is suitable. This would mean testing the whole structure again with soft hammer tip; however, due to time constraints this test was not done. Also for building acoustics applications the frequency range above 100 Hz is mostly of interest. Secondly, the RT in that band was greater than the length of the time window used for FRF measurement. To obtain an accurate FRF at such low frequency end, it is advisable to use the individual window length greater than the RT [137]. The window length of 5.12 s was therefore not ideal for the FRF measurement below 63.5 Hz band. For building acoustics applications, the frequency range of interest is typically above 100 Hz (but not always), and the prediction above 100 Hz is within 1 dB which shows the potential of the method in predicting the receiver response. Using the methodology described in Section

5.5.3, the radiated pressure was also calculated and the results for radiated pressure compared to the contact pressure on a single patch are shown in Figure 6.6.

Figure 6.6: Comparison between total contact pressure, radiated pressure and blocked pressure at a single patch in narrow band (top) and one-third octave bands (bottom plot) in 0-800 Hz

Figure 6.6 again shows negligible value of the radiated pressure which means that the contact pressure are equal to the blocked pressure in this measurement case. Therefore, the disagreements in the low frequency (<63 Hz) from Figure 6.5 arising due to contact pressure approximation can be ruled out.