Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 156

Figure 2: Narrowband (top), 1/3 octave band (middle) and 1/1 octave band
(bottom) spectra of the same acoustic pressure signal.

the amount of acoustic energy generated by the source. In contrary, the sound pres-
sure level, L p depends on the observer position and quantifi es the amount of sound
energy reaching the observer.
Pure tones are rarely found in practice, most devices emit sound composed by a
set of different frequencies:

⎡ ⎤

⎛ xt ⎞

∑

ptx '( , ) = A cos 2 p ±

⎢ ⎥

⎜ ⎟ (8 )

⎝ l T ⎠

⎣ ⎦

i i i

Thus, the frequency spectrum of the acoustic pressure has to be determined as
well to characterize the sound. There are three types of commonly used spectra:
narrowband, 1/3 octave band and 1/1 octave band. In this context band refers to a
given frequency interval over which the amplitudes are averaged. For an octave band
the upper limiting frequency is exactly the double of the lower limiting frequency.
For a 1/3 octave band the ratio of the upper and lower limiting frequencies is 2 1/3 .
For a narrow-band frequency the width of the bands is constant and 'small' enough
to capture pure tones and thus gives the most details about the spectrum. Figure 2