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The measurement rate of acoustic reduction (R) in an insulating material is determined in special designed enclosures according to UNE EN ISO 140-1 standard. The construction of a testing room that complies with the standards requires a significant financial investment, that too often can not be provided by general machinery manufacturers. The outdoor machines have to be designed considering the noise emission limits established in the European Directive 2000/14, making the installment of insulating materials in enclosures of machines necessary. Suppliers of insulating materials for industrial use does not normally provide the insulating properties of their products, which are often obtained from rock wool, polyethylene, polyurethane, synthetic rubber, etc. It is difficult to test these materials that are characterized by low density surface in a room standardized test procedure that is based on the measurement of sound pressure. The main problem is getting the validation of the measurement when the rate of acoustic reduction is relatively low, as in the case of materials mentioned above. An alternative procedure is proposed in this Doctoral Thesis based on the measurement of sound intensity. The vector nature of this parameter allows just the flow of energy through a given surface to be measured. The validation of the measurement is achieved with the highest degree of accuracy by applying the criteria included in the standard UNE EN ISO 9614. A steel plate 6 mm thick incorporated with a sound source for amplification of a pink noise signal is designed for carrying out the tests. Faced with the source is placed a piece of material to test. The source is connected and the transmitted sound intensity is measured with a probe consisting of two microphones located opposite one another. Previously it has been determined the flow of acoustic energy that impinges on the piece. The difference between the two measures provides directly the rate of acoustic reduction. The main advantage of the described test procedure is that it can be done in a room with normal dimensions and without specific conditioning. The vibroacoustic behaviour of the testing box and its influence on the measures is another important aspect to be considered. A test of modal analysis has been carried out with a steel plate similar to the enclosures of the box. A mathematical model based on the Statistical Energy Analysis SEA has been developed to estimate the paths of energy transmission from the sound source to the point of measurement with the sound intensity probe. The commercial software AUTOSEA2 LT is used. General conclusions regarding the alternative method of measurement and specific ones related to the capacity of insulation of the tested materials have been obtained. As for the general conclusions, the limitations of measuring with the intensity probe at low and high frequencies have been proved, validating measurements with an accuracy of 0.5 dB according to the criteria set forth in the standard UNE EN ISO 9614 Part 3 have been obtained, and a new test procedure for easy determination of the rate of acoustic reduction of an insulating material with low density surface has been established. Finally, the devised procedure allows future developments in the field of vibroacoustic, such as the application of the principle of reciprocity and the determination of acoustic impedance of materials by application of techniques for measuring of acoustics intensity.

Técnicas avanzadas de medida en intensimetría acústica para la caracterización de materiales aislantes