In practical applications pure harmonic tones are rare.
The character of noise can be dominated by one or more harmonic tones, e.g. for compressor applications.
In cases like that it Fourier analysis is a good way to look at the separate tones.
Noise can also be more random through the frequency domain, such as white noise. In the latter
case noise is present at each frequency. Naturally also combinations are possible.
A number of examples, with the frequency along the xaxis and the sound pressure level
along the yaxis:
The analysis of a sound spectrum is important to find the tones or frequency areas
that need to be silenced. As other graphs noise spectra can be represented by
histrograms, tabulations or by connecting the points.
As described above the data is often provided in tabulations.
The frequency bands are also defined on a logaritmical scale. The bands are from the
lower to the upper limit of the band. The octave band central frequency is:
These have been defined by I.S.O. (values for 31.25 and 62.5 were rounded off):
31.5  63  125  250  500  1000  2000  4000  8000  16000  Hz 
The 16000 Hz octave band is hardly ever used in industrial noise control.
The boundaries between the octave bands can simply be calculated:
As the frequencies of the octave bands are different it can be proven that the
individual effective sound pressures squared can be added up to provide the overal level of
sound pressure squared. This leads to:
For more precise analyses terts  octave bands are used. These will provide a closer approximation
of the continuous spectrum in reality. Distrurbing tonal noises can be located with more
precision.
Analog to the octave bands the terts octave bands are:
octave band central frequency 
terts  octave band central frequency in Hertz 


31.5  25  31.5  40  
 
63  50  63  80  
 
125  100  125  160  
 
250  200  250  315  
 
500  400  500  630  
 
1000  800  1000  1250  
 
2000  1600  2000  2500  
 
4000  3150  4000  5000  
 
8000  6300  8000  10000  
 
16000  12500  16000  20000  
 
It is important to have a parameter that provides a value for the negative
effect on hearing and disturbance. The most widely used parameter is the
A  weighting, thus sound pressure levels in dB(A)
The sensitivity of the ear is not the same for all frequencies. The A  weighting
is based on curves that show the subjective auditive perception
or sensitivity of the human ear at about 40 to 60 dB.
As there was a very clear correlation between the A  weighted sound pressure
levels and hearing loss of industrial workers this weighing has become very popular.
The practical meaning of the Aweighing is good for applications up to about
90 dB(A). At higher sound pressure levels the ear looses its lower sensitivity
for low frequency noise and becomes more vulnerable.
The B , C , and D weightings also exist. The D  weighting was developed
specifically for the aircraft industry and gives an extra penalty for
high frequency noise in line with the nuisance that people experience
by jet engines.