MECANISMOS DE TRANSPORTE DE FOLATOS Y ANTIFOLATOS

For the pressure validation test to be applicable, it is important that the sound transfer occurs through the partition only. However, for in-situ installations other flanking transmission to the receiver room may exist, such as a window or a door separate from the test partition. Such elements will also transmit pressure to the receiving volume and the measured pressure then is a resultant of pressure transmitted by the partition as well as these secondary sources. Then, a pressure validation test may not be applied successfully. Rather in such cases, the on-board validation test can be applied to predict the vibratory response at the interface. This will also confirm if the directly measured blocked pressures are able to predict the vibratory response of the partition. The acceleration on the partition at a point ‘k’ can be predicted as,

Figure 5.22: On board validation results for velocity prediction using directly measured blocked pressure in narrow bands (top plots) and third octave bands (bottom plot)

Figure 5.22 shows the on-board validation results for two reference points on the partition. The predicted and measured accelerations at the reference points are within 2-3 dB in the region from 50-900 Hz. Again, an interesting implication of the direct blocked pressure measurement is that vibratory response of the panel can be predicted relatively faster than inverse methods like I-ASCA.

5.6

Conclusions

The main motivation behind the work presented in this chapter was to develop a faster approach than blocked force based I-ASCA for diagnosing airborne sound transfer through dual leaf partitions separated by a cavity. To achieve this, the direct measurement of blocked forces was considered. Three different approaches were discussed for the direct measurement of blocked pressures (blocked force/path area) which led towards a conclusion that the contact pressure is equal to the blocked pressure and a radiated pressure term. By neglecting the radiated pressure, the blocked pressure can be closely approximated by a direct contact pressure measurement.

The direct blocked pressure measured as contact pressures were studied for cases of unbaffled and baffled partitions. The difference between the contact pressure and blocked pressure is the radiated pressure, which was later estimated to be negligible. The validity of these blocked pressures was determined by using a pressure validation and on board validation test. An important consideration for measuring contact forces is that complete source-receiver-interface of the partition should be accessible as seen for the case of unbaffled partition. In case of airborne flanking, on-board validation may be a better test to validate the blocked pressures.

With this approach, accelerances are not measured and only the vibroacoustic FRF’s were measured. As a result, the measurement time of D-ASCA is significantly reduced compared to I-ASCA. D-ASCA does not involve an inversion process, and therefore the inverse errors are avoided. In addition, any local blocked pressures can be measured independently without a global testing of the whole structure. This means that a local source contribution can be measured without measurements on all paths. As accelerances are not measured, accelerometers are not used, and there is no effect of mass loading on the structure otherwise present in conventional I-ASCA.

It was also observed it was difficult to obtain a good coherence in the vibroacoustic FRF measurements as the source and receiving room were highly reverberant. This problem is especially worse at low frequencies where the signal to noise ratio is very low as the hammer does not input much low frequency energy into the structure. These add to measurement errors and the pressure prediction may then be affected. If this measurement can be automated, then measurement errors may be further minimised and the measurement time can be reduced further. Quite favourably, this is possible by measuring the FRF reciprocally using the well-established principle of vibroacoustic reciprocity. Using the principle of vibroacoustic reciprocity, the vibroacoustic FRF can be measured reciprocally as velocity on the partition due to volume velocity excitation in receiver volume (see Table 3.2).

𝑝𝑘

𝑓_𝑗 = 𝑣_𝑗

𝑄_𝑘 (5.27)

If a scanning laser vibrometer is used to measure the velocities on the panels instead of using accelerometers, then the FRF measurement can be fully automated and may be quicker than direct measurements for large partitions. This would make the measurement approach totally non-invasive to the partition under test. It can also be seen that the measurement can be done with a minimum of three microphones (one for measuring contact pressures, one for pressure validation and one as a receiver for diagnostic test) thereby providing some cost advantages.

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6

COMBINED SOUND

INSULATION AND

DIAGNOSTIC TESTS

6.1

Airborne sound insulation tests with diagnostic tests

The sound insulation of a partition is measured by standard methods under laboratory conditions to give the SRI. The SRI provides the frequency dependence of the sound insulation of the partition. However, as mentioned previously, these SRI values (or single number rating 𝑅𝑊), do not provide any information on the sound transfer

contribution of different paths/elements in the partition which may be important for R&D purposes. With diagnostic tests outlined in Chapter 3 (I-ASCA, I-PCA) and Chapter 5 (D-ASCA), the source and path contributions can be measured. Using this diagnostic data, the weak paths of sound insulation can be identified. If such information is compared with the SRI results, then the weak paths may be identified in specific regions where the partition exhibits low SRI. Thus, the diagnostic tests can run in complement to the standard airborne sound insulation tests under laboratory conditions, which may help in improving the sound insulation of the partition.