first to consider the permissible noise levels as outlined in the previous section. The present section will concentrate on reduced external noise. Noise ingress to an enclosure may basically be by two means: structure-borne sound or airborne sound. Structure-borne sound is Mass (kgm/m? ) that which travels as vibration in the fabric of a building. Concrete, metal water pipes etc. are good conductors of sound and vibrations Figure 8.12 The 'Mass Law' created in one part of a building can often be a nuisance throughout Psychoacoustics 8/13 per unit area will in general increase the sound reduction by only about 5 dB. Typical mass/unit area figures for a few well-known materials are given in Table 8.10. Table 8.10 Typical mass per unit area Mass in kgm/m2 With Rockwool Material surface area 6mm glass 100mm breeze block 12mm plasterboard with studs 115mm brickwork 150mm concrete Without Rockwool There are light-weight partitions which can be effective. Figure 0 125 250 500 1K 2K 4K 8.13 illustrates in simplified form such a partition and Figure 8.14 shows the sound reduction against frequency graph for such a partition. Frequency (Hz) It is a common misconception that sound absorbing materials are also sound insulators. Polystyrene ceiling tiles seem often to have Figure 8.14 Sound reduction characteristic for the been used for this purpose and it may be useful to point out that such partition shown in Figure 8.13. (Courtesy Rockwool Ltd.) tiles, while effective for thermal insulation, are too light to have any value for sound isolation. Further, their sound absorption qualities are also negligible, so they are of almost no use in trying to reduce 8.6 Psychoacoustics reverberation time. Effective reduction of airborne noise involves the careful sealing By this term we mean the somewhat obscure region between the of all crevices, however small. These can sometimes be difficult to physical characteristics of sound and the psychological interpreta- locate. Typical sources of trouble are places where services such as ti