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Addition of sound level contributions can be performed as a logarithmic addition. Let’s add two contributions, Lp1 and Lp2; the resulting sound pressure level Lptot will be

Lptot = 10 log ((10A(Lp1/10)) + (10A(Lp2/10)))

Should one find logarithms unmanageable, all is not lost: it is possible to use a simple table of additions (Table 2.1). Starting with the highest of the two sound level values to be added (which is noted X in Table 2.1), one looks up the difference to the value to be added. A couple of examples are given in Section 2.3.6.2.

By the way, one can derive an important consequence for noise control purposes: Adding a noise source whose contribution is no greater than the original sound level minus 15 dB will not affect the overall sound level value.

# Eq uivalent Sound Levels and Statistical Sound Levels

How does one describe a temporally fluctuating noise? A simple way is to make reference to its energetic value and use the equivalent sound level, given the symbol Leq, which represents a nonfluctuating signal containing as much acoustic energy as the signal under study over the period of time considered. Incidentally, the A-weighted value, which is widely used in surveys, is given the symbol LAeq.

Table 2.1 Result of adding to a sound level Lp, of X dB a second level Lp2

 Adding X dB and: Result X dB X + 3 dB X + 1 dB X + 3.5 dB X + 2 dB X + 4.1 dB X + 3 dB X + 4.8 dB X + 4 dB X + 5.5 dB X + 5 dB X + 6.2 dB X + 6 dB X + 7.0 dB X + 7 dB X + 7.8 dB X + 8 dB X + 8.6 dB X + 9 dB X + 9.5 dB X + 10 dB X + 10.4 dB X+ 11 dB X + 11.3 dB X + 12 dB X + 12.3 dB X + 13 dB X + 13.2 dB X + 14 dB X + 14.2 dB X + 15 dB X + 15.1 dB X + 16 dB X + 16.1 dB

Are we done then? Of course not. For the same Leq value the temporal fluctuations of a noise can be very different, and people can be sensitive to such variations. For example, a 65 dB(A) LAeq value can be reached close to a major highway where it is a continuous rumble, but it can also be reached close to a country road if a single motorcycle passes by ! Clearly enough, something else is needed. One then uses the notion of statistical sound levels, given the symbol Lx, which states the sound level reached or exceeded for x% of the analysis time. Several standards and regulations currently use L50 (which will be less than the Leq value for a fluctuating noise), as well as L10 (which gives a fair idea of the highest noise levels) and L90 (which gives a fair idea of the lowest noise levels), with the difference giving the dynamic of the noise.

What about the fluctuations of noise over a whole day? Legal writers and standard writers have eventually come up with the notion of day-evening-night, where the energetic sum is weighted according to the period of the day (i.e., there is a penalty of 5 dB for evening noise and 10 dB for night noise): where Ld, Le, and Ln are the day (7:00 a.m to 7:00 p.m.), evening (7:00 p.m. to 11:00 p.m.), and night (11:00 p.m. to 7:00 a.m.) values, respectively, of the noise levels. Please note that while the durations of each period are clearly defined in the European Union, each member state can adjust the corresponding time limits. Found a mistake? Please highlight the word and press Shift + Enter Related topics