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Gas Turbine Acoustics and Noise Control 151
To extract the ambient or a contaminating noise level (L 1 ) from a
total operating noise level (L 2 ) to determine what the source is (and
L2 should be at least 3 dB higher than L1):
Source = 10 Log [10 (L2/10) - 10 (L1/10) ] dB (10-7)
Reporting or calculating sound levels in excess of one decimal point
is unwarranted.
Frequency and Bandwidth
When we speak of acoustics most think of music, stereo systems
and other elements of the science of producing sound in a pleasurable
manner. Unwanted sound is called noise. It is this unwanted sound
with which we are concerned. Sound or noise emanates at particular
frequencies (discrete) or groups of frequencies (broad-band) that we
can hear or sometimes feel. The unit of frequency is the hertz (Hz)
having units of 1/sec (cycles per second is no longer used). The noise
signature or emissions of equipment can span thousands of discrete
frequencies, and thus, can be very challenging to analyze (in fact, the
cited equations used in this chapter are all a function of frequency
but for simplicity are not annotated as such). Industry Standards
grouped these discrete frequencies into bands of frequencies known
as octaves, which means a doubling and is a geometric-logarithmic
function where 1,000 Hz has been defined as the datum point; that
is, an octave above 1k Hz is 2k Hz, an octave down is 500 Hz, etc.
By grouping these frequencies into just nine octave bands (31.5 – 8k
Hz) makes noise control work much easier and somewhat more manageable.
Other frequency bandwidths are used too, such as one-third
octave band, and what is commonly known as narrowband analysis
where the frequency spectrum can be measured in fractions of a
hertz. This is typically the approach when identifying the source of
a particular sound. Table 10-1 lists one-third and octave bands that
are commonly used for sound measurement and analyses.
The Weighted Sound Level
The human ear receives sound at many frequencies, at many
different amplitudes and all at the same time. It acts as a filter and
it shifts sensitivity based on the amplitude and frequency of the
sound. It performs poorly at low frequencies, very well at middle