Objective Assessment of Performance

The sound Insulation of a Besblock Pumice walling construction has been assessed under laboratory conditions in accordance with the requirements of British Standard BS 2750 : Part 3:1980. The method of test involved the construction of a sample of wall of Besblock Pumice blocks between two reverberent test chambers which are isolated from each other to prevent any flanking transmission of sound. A source of steady continuous spectrum sound was generated via loudspeakers in one chamber and the resulting sound levels measured simultaneously in both chambers at each of the 16 one-third octave bands having centre frequencies from 100 HZ to 3150 HZ.

The difference between the two sets of results, with appropriate allowances for the area of the test specimen and the acoustic characteristics of the receive chamber, provides an objective measure of the sound insulation of the specimen known as the Sound reduction Index. It will be appreciated that the 16 different results for each frequency are not a convenient way of assessing the performance of the wall. Therefore all 16 results are rated to give a single figure of sound insulation performance, known as the Weighted Sound Reduction Index (Rw). The methodology for this is defined in BS 5821: Part 1 : 1984.

The Mass Law

The insulation of solid walls against airborne sound is related to the superficial mass of the wall, that is its mass per unit area usually given in Kg/m2. An empirical 'mass law' has been derived from a large number of sound insulation measurements carried out by the National Physics laboratory (NPL). These measurements were performed on a variety of single leaf constructions ranging from lightweight materials, through medium weight materials such as glass and plasterboard to heavy masonry. The NPL study was conducted prior to the introduction of the Rw method and did not include pumice blockwork. By re-working NPL's original data, but restricting the analysis to masonry products a mass law more applicable for predicting the performance of Besblock Pumice blockwork generally in accordance with current rating methods has been derived as follows: Rw = 0.4 + 19.2 log m (db) Equation (1) where m is the superficial mass of the wall (kg/m²)

Estimated Sound Insulation of Solid Besblock Pumice Blockwork Walls

Interpolated values of Weighted Sound reduction Index (Rw) for various thicknesses of solid Besblock Pumice blockwork (1050 kg/m³) are presented in table 1 for both plastered and dry lined finishes. These assessments utilise the 'slope' of the mass law regression formula in Equation (1), modified to take account of the performance of the tested Pumice blockwork wall which proved superior to the modified mass law. The table indicates that the performance of Solid Besblock Pumice blockwork is 4 to 6 dB better than the mass law prediction.		Also presented in Table 1 is the mass of the laid wall for the various thicknesses of blockwork. The quoted mass includes the finish to each side assumed as 10kg/m²/side. For comparison purposes Table 1 also shows the predicted performance of the walls based on the laid weight using the mass law formula in Equation (1)
Block Thickness (mm) Mass of Wall with finish (kg/m²) Predicted Rw from Eqn. (1) (db) Interpolated Rw Plastered Wall (db) Dry Lined (db) 75 103 39 44 43 90 119 40 45 44 100 130 41 46 45 115 147 42 47 46 140 174 43 49 48 150 185 44 49 48 190 229 46 51 50 200 240 46 52 51 215 257 47 52 51 Table 1 Estimated Sound Reduction Index Values for Besblock Pumice Walls.

Weight Ratio from Besblock Pumice Blockwork