A new face drilling rig for narrow tunnels and ... - Advanced Mining

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Fig. 5: Forms of enveloping surfaces Enveloping surface, enveloping surface dimension The link between the energy size (sound power level) and the field size (sound pressure level) is established by the enveloping surface. According to DIN 45635 the sound power level is determined by the enveloping surface method. To this end the average sound pressure level is taken, at appropriate distance, on a measuring surface enveloping the machine (enveloping surface). As enveloping surfaces are possible simple geometric forms like balls, cylinders, cuboids. The mostly common enveloping surfaces in the plant engineering are cuboids, cylinders and balls or parts of them. The indication of the position and the distance from the test object is an essential part when measuring the sound pressure The measuring surface S, or better the enveloping surface, is the surface in [m²] enveloping the test object and resulting from the measuring distance. The enveloping surface dimension is defined as the tenfold decade logarithm of the ratio that the enveloping surface bears to the reference surface of 1m². Here, it may be noted that only in case of a hemispherical radiation, where the noise source is in the middle of the ball, no reflections occur on surfaces forming the boundary. The reflections intensify the sound pressure level on the enveloping surface. The sound power of the machine fixed on that basis would be incorrect and needs to be corrected. That is why a measurement to be taken in a dead room or in a free field would be ideal. ⎛ S LS = 10× lg ⎜ ⎝ S0 S = 1m² 0 Issue 04 | 2010 ⎞ ⎟ ⎠ L S [ dB] S[ m²] The sound power level results, as a sum, from the energetically taken mean of the sound pressure level obtained over all measuring points and the enveloping surface dimension TRANSFER OF TECHNOLOGY Vollkugel Halbkugel Viertelkugel Achtelkugel L = L + L L W p n W = { 10 × lg( ∑ n i= 1 S 1 10 Lpi 10 )} + L From this equation can be seen at once that the sound power level for the plant about that is talked here is always higher than the sound pressure level. The sound power level only becomes lower than the sound pressure level, if the quotient resulting from the enveloping surface divided by the reference surface of 1m² will get smaller than 1, because the logarithm will be negatively then. But this goes only for enveloping surfaces that are smaller than 1m², a rare fact in case of equipment used to handle bulk material. With the help of a small example shall be made clear the yet important link between the three sizes sound pressure level, sound power level and enveloping surface. On the slewing crane, about that is talked in more detail later on, there has been taken, at a distance of 18.5m, a sound pressure level of Lp = 67,6 dB. Basing on a hemispherical enveloping surface and on a sound pressure level that is equal over the whole enveloping surface, the sound power of the slewing crane is calculated, as follows: From it results, according to definition, an enveloping surface dimension of S www.advanced-mining.com 74
Now, it is enabled the sound power level of the slewing crane to be calculated. A-weighting All values have been represented in the form of linear values till now. Because of the dependence on the frequency of the human ears, as shown in figure 4, the linear values are adapted to the frequency selectivity of the ears by means of filters. As a function of the noise source, there is the A, B, C and D-weighting. In general and when rating A-Bewertung Table 4: A-weighting 10 0 -10 -20 -30 -40 -50 -60 -70 -80 Issue 04 | 2010 Filter TRANSFER OF TECHNOLOGY 1 10 100 1000 10000 100000 Frequenz (Hz) the noise of cranes too, use is made of the A-weighting. The A-weighted sound levels are characterized by the index A, and on top of that, they get emphasized by an A in brackets, added to the decibel dB(A). Filters (third-octave filters, octave filters) In the sphere of acoustic metrology, third-octave filters and octave filters proved to be a good instrument for rating the noise spectrum. This type of filters are capable of letting uninfluenced the power part within the bandwidth, but they are able to suppress the remaining spectrum above and below the limit frequency. Both filter types are characterized by the indication of the centre frequency, the bandwidth and the position of the limit frequencies, defined by the standards DIN 45652 and EN ISO 266. A+Bewertung Abb. 6: Filterkurve A-Bewertung www.advanced-mining.com 75
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