## Terminology and basic concepts

Two distinctly different measures are used to describe wind turbine noise. These are the sound power level LW of the source (i.e., the wind turbine) and the sound pressure level LP at a location. Because of the response of the human ear, a logarithmic scale is used based on reference levels that correspond to the limit of hearing. The units of both LP and LW are the decibel (dB).

A noise source is described in terms of its sound power level, LW:

where W is the total sound power level emitted from the source (in Watts) and W0 is a reference value of 10~12 W.

The sound pressure level LP is defined as

where P is the RMS value of the sound pressure and Po is a reference value of 2 X 10~5 Pa.

By simple algebraic manipulation it may be seen that the addition of n sound pressure levels (expressed in dB) is carried out as shown:

Thus, adding two sound pressure levels of the same magnitude results in an increase of 3 dB. Table 9.2 gives an indication of the typical range of sound pressure levels.

The human ear is capable of detecting sounds between 20 Hz and 20 kHz and spectral analysis is typically undertaken over this range. A narrow-band spectrum, with a defined bandwidth of measurement, gives the fullest information of the signal and may be used to detect particular tones. However, it is conventional to use octave and 1/3-octave bands for broadband analysis. The upper frequency of an octave band is twice that of the lower frequency while for the 1/3-octave band the upper frequency is 3 2 times the lower frequency.

It is common to weight the measurements to reflect the response of the human ear with frequency. This is done by applying the so-called A-weighted filter. Measurements made with this filter are referred to as dBA or dB(A). Table 9.3 shows the centre frequencies of the octave bands together with the A-weighting in dB. It may be seen that frequencies below 250 Hz and above 16 kHz are heavily attenuated.

An equivalent sound pressure level Leq,T is the value of a continuous steady sound that, within the specified time interval (T) has the same mean square sound pressure level as the sound under consideration which varies with time (International Energy Agency, 1994)

A similar calculation may be undertaken using A-weighted values to give LAeq,T

Table 9.2 Examples of Sound Pressure Levels

Example Sound pressure level

Table 9.2 Examples of Sound Pressure Levels

Example Sound pressure level

 Threshold of hearing

## Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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