Gas Turbine Handbook : Principles and Practices

150 Gas Turbine Handbook: Principles and Practices
more to it to determine the total sound power a device emits.
So why is sound power so important? It is primarily used for
the calculation of sound level and is used in modeling turbine noise
emissions as well as all other types of equipment. The sound level
at a particular distance is calculated by 5 :
Lp = Lw + D C – A dB (10-4)
where:
D C = the directivity correction based on distance and angle between
the source of noise (equipment) and the receiver
(where the sound level is desired to be known).
A = the attenuation of the sound between the source and the
receiver.
As an analogy, think of a light bulb, which has a power level
of watts (L W ) but the brightness of the light (Lp) is determined by
how far away you are (D C ) and whether an object blocks it or if the
light is absorbed (A) by a dark surface. Sound behaves in a similar
manner.
Decibels
Decibels (dB) are a convenient way of handling very large or
very small scalar values and as described above, are a logarithmic
function that requires a special method for combining decibels. In
certain applications the sound levels, in decibels, L1, L2, etc., of two
or more sources are summed. However, pressure and energy can be
summed, not decibels, so the decibel levels are converted to energy
levels, summed and then put back into decibel form:
Summation = 10 Log [10 (L1/10) +
10 (L2/10) + 10 (L3/10) + …10 (Ln/10) ] dB
(10-5)
Similarly, an average of several measurements is calculated by:
Average = 10 Log {1/n[10 (L1/10) +
10 (L2/10) + 10 (L3/10) + …10 (Ln/10) ]} dB
(10-6)