Environmental correction factor K 2 26102012 Environmental Correction

Environmental correction factor K 2 26/10/2012 Environmental Correction Factor 1

Environmental correction factor K 2 SPL vs distance from sound source K 2 , also named DLf, Is the difference between the total SPL value and the freefield SPL value 26/10/2012 Environmental Correction Factor 2

Environmental correction factor K 2 We can compute K 2 making the differnce between the formulas of the semireverberant sound field and of the free field: Enveloping surface’s area = 2 pd 2 Q=2 (source in a reflecting plane) This formula provides the “theoretical K 2” value 26/10/2012 Environmental Correction Factor 3

Experimental results 26/10/2012 Environmental Correction Factor 4

Typical packaging workshops These buildings are wide, but not tall 26/10/2012 Environmental Correction Factor 5

Typical packaging workshops These buildings are wide, but not tall 26/10/2012 Environmental Correction Factor 6

Measurement of sound level vs distance An omnidirectional sound source (dodechaedron, Q=1) is employed Dodechaedron 26/10/2012 S. L. M. Environmental Correction Factor 7

Measurement of sound level vs distance The experimental chart obtained looks as this one: 26/10/2012 Environmental Correction Factor 8

Measurement of sound level vs distance Comparison with theoretical curve Here the experimental SPL is significantly larger than theoretical one 26/10/2012 Here instead it is smaller Environmental Correction Factor 9

Measurement of sound level vs distance The same happens in other similar buildings… 26/10/2012 Environmental Correction Factor 10

K 2 value vs distance 26/10/2012 Environmental Correction Factor 11

K 2 at teh workplace (surface S’) 26/10/2012 Environmental Correction Factor 12

Results of the experiments • In many industrial buildings “large and short” the envioronmental effect measured at the workplace is much larger than what theory predicts • Often the owner of the factory acts against the supplier of machinery, in the wrong assumption that they are too noisy, whilst the cause of the high SPL value is mostly due to the building, and not to the machines • This can be ascertainled only performing a direct measurmenet of the enviornmental correction factor K 2 at the workplace • Whenever K 2 is very large, it can be expected that the SPL will reduce significantly thanks to an enviromental treatment based on sound absorption 26/10/2012 Environmental Correction Factor 13

K 2 estimation based on a new empirical formula 26/10/2012 Environmental Correction Factor 14

The Farina/Fornari Formula • The formula was obtained by fitting the experimetal results measured in dozens of industrial workshops: In which T is the reverberation time, H is the room height and the terms within brackets at denominator represents an “apparent volume” of the large, short building 26/10/2012 Environmental Correction Factor 15

Verification of the new formula 26/10/2012 Environmental Correction Factor 16

Evaluation of effectiveness of room treatment • The Farina/formari formula allows for easy assessment of the effectiveness of a room treatment • For example, let’s consider a building with these data: • We now compute the value of K 2 twice, before and after the room treatment, employing the Farina/Fornari formula. 26/10/2012 Environmental Correction Factor 17

Evaluation of effectiveness of room treatment • The Farina/Fornari formula shows a relevant sound reduction due to room absorption treatment: at 10 m we get a reduction of 5. 5 d. B(A) 26/10/2012 Environmental Correction Factor 18 04/11/2010 Il Fattore di Correzione Ambientale instead of 1. 7 d. B(A) as forecasted by the Sabine’s formula

Conclusions • Employing the “traditional” formula for the environmental correction factor causes significant errors in “large and short” buildings, as most factories are. • Nowadays the new EN 415 -9: 2009 standard allows for correct experimental extimation of the “true” value of the environmental correction factor • The direct measurement of K 2 is easy and straightforward • Alternatively, K 2. can be estimated quite accurately thanks to the Farina/Fornari formula • In these buildings, often an absorption treatment produces much better results than what predicted by the traditional formulation 26/10/2012 Environmental Correction Factor 19