Construction

This transmission suite at the University of Salford is a test facility that meets the requirements of the ISO standard specifications for measuring airborne sound insulation, impact sound insulation, and sound absorption of building elements. It comprises of two reverberation chambers – the source room and the receiving room with a separating wall between them. These rooms are isolated from each other such that any sound transfer from the source to the receiving room only occurs through the partition installed in the aperture of the separating wall. The facility is supported on springs and resilient elements to isolate the chambers from ground borne vibration. For these test chambers, the separating wall has a 𝑅𝑤 value of about 70 dB. Figure 4.11 shows the transmission suite.

Figure 4.11: The transmission suite consisting of the source room (right), receiving room (left) and the mic positions in black in both rooms

For the I-ASCA test, the dual leaf partition was first installed in the separating wall aperture between the source and receiving rooms. The aperture size in the separating wall was 1250 x 1500 mm2 _{(Figure 4.12-left graphic) and the partition size was smaller (910 x}

910 mm2_{). Therefore, after placing the partition in the aperture, a filler wall had to be}

constructed in the remaining aperture space. In the first attempt of the I-ASCA here, the filler wall was entirely built with plasterboard stacks. Next, a single layer facing wall was put up in front and behind of the plasterboard stacks. It was intended that the filler wall would provide insulation to any flanking transmission. The complete construction is depicted in Figure 4.12-right graphic.

Figure 4.12: The aperture in the separating wall between source and receiving rooms prior to the I-ASCA test (left) and on right- the partition (in black) installed in the brick walled aperture

with the filler wall structure around it (in blue). The filler wall cavity here is made up of plasterboard stacks

One of the motivations of the diagnostic tests was that the diagnostic results could be potentially used as a complement to the standard sound insulation tests. Therefore, the tests in the transmission suite provided a good opportunity to perform the combined sound insulation and I-ASCA tests on the test structure in a controlled environment. At first the sound insulation of the construction was measured by ISO 10140 standard test. Figure 4.13 shows the measured sound insulation of the test structure.

Figure 4.13: Sound insulation of the dual leaf partition and filler wall structure measured by ISO 10140 method

Figure 4.14: Schematic for FRF measurements (left) and operational measurements (right) for the I-ASCA test in transmission suite (top view). Red line denotes the interface

Next, the I-ASCA tests had to be performed (see measurement schematic in Figure 4.14). The grid discretisation from previous tests was kept the same to yield a total of 64 sound transmission paths. Next, the accelerance measurement was performed in parts to yield an accelerance matrix ‘A’ as per Eq. (3.5). The validity of these measurements was checked by performing a reciprocity check on the cross diagonal elements of the accelerance matrix. A few plots of reciprocity for nine sets of force-response for randomly chosen paths are shown in Figure 4.15.

Figure 4.15: Reciprocity between nine sets of force-response for randomly chosen paths on the dual leaf partition installed in transmission suite

At the same time of performing the FRF measurements, the vibroacoustic FRF’s had to be measured. For this, three microphones were placed in the receiving room and the FRF’s were measured for these positions. In the operational test, a loudspeaker was placed facing the corner of the source room. Placing the loudspeaker in the corner position allows the excitation of all the source room modes under operational conditions. When driven with a pink noise excitation, all the modes are excited and the due to hard walls and diffusing elements a near diffuse field is created in the room above the Schroeder frequency [141]. Thus, the sound transfer of the partition could be studied for a diffuse field excitation. With the loudspeaker operational, the accelerations of the paths and the pressures at receiver room positions were measured. All operational measurements were referenced to the driving voltage of the loudspeaker to maintain the phase between different measurement sessions. Following Eq. (3.5-3.8) the blocked forces were calculated and the sound pressure radiated by the partition was predicted (Eq. 3.10). Figure 4.16 shows the pressure validation results.

Figure 4.16: Pressure validation for the I-ASCA test on the dual leaf partition in the transmission suite. Predicted pressure by I-ASCA is compared with the measured pressure in narrow band

From Figure 4.16, it was observed that the predicted pressure was lower than the measured pressure, about 4-5 dB lower till 800 Hz. Again, the possible reasons of this mismatching considered were the blocked forces or flanking. The accelerance matrix showed good reciprocity and the operational data was referenced to the excitation voltage to prevent any phase mismatching between different measurement sessions. Thus, the first the blocked force calculated should be correct as their accuracy depends on correct FRF and operational measurement on all paths. In addition, the predicted pressure does not show any peculiar inverse error peaks (except above 800 Hz). Regarding flanking, some transmission through the filler wall was expected as it was attached to the frame of the dual leaf partition (structure borne flanking). In addition, some airborne flanking was also expected at low frequencies through the filler wall. To avoid this flanking transmission from the filler wall, the filler wall has to be rated higher for sound insulation than the test partition as well as acoustically isolated from the frame of the partition. Thus, with these observations a second test was again set up to reduce the flanking transmission.