Measuring and Calculating Sound Levels

The dB(A) Scale

Local planning officers all around the world use the same scale to determine the sound generated by any potential development, although they may use different criteria. This scale is the decibel (dB) scale to quantify sound measurement. To give you an idea of the scale, look at the table below.

 Sound Level Threshold of Hearing Whisper Talking City Traffic Rock Concert Jet Engine 10 m Away dB(A) 0 30 60 90 120 150

 The dB(A) scale is used in a wide range of fields where sound has to be measured. What the scale actually measures is the intensity of sound over the entire range of frequencies audible to humans (different pitches), and then adjusts this to compensate for the fact that different frequencies are heard with different sensitivity by the human ear. Generally speaking, the frequencies which we hear best are in the medium range, which is where speech falls, as opposed to low or high frequencies. The dB(A) scale displays the result of a calculation based on adding the sound intensity at the most audible frequencies multiplied by higher values, to the intensity of less audible frequencies multiplied by lower values. The result of this calculation is the index number. The dB(A) scale denotes a weighing scheme and is used exclusively for weaker sounds, such as wind turbines. There are two other weighing schemes which are used for louder sounds, (B) and (C). But these scales do not apply to the wind turbines. The dB-scale is not a simple scale, such as weight. 100 kg is 3 kg more than 97kg. The decibel scale is logarithmic. Meaning that an increase in the index of (approximately) 3 means a doubling of sound intensity. So, for example, 100 dB(A) refers to a noise which contains twice the sound energy of on which is 97 dB(A). The reason that sound is measured in this way is because our ears (and brains) perceive sound judged on the logarithm of the sound pressure and not the sound pressure itself. This means that the decibel scale follows human appreciation of loudness. In case you are interested in the exact definitions, take a look at the Reference Manual on Acoustics of this web site - put in link

 Sound propagation over distance Like radiated heat, the energy contained in sound waves and therefore the sound intensity drops relative to the square of the distance to the sound source. So being 200 m from a wind turbine, you will experience a sound level one quarter of what you experience when only 100 m away. Combined with the logarithmic nature of the decibel scale and doubling your distance will reduce the dB(A) level by 6. At distance from the base of a turbine equal to one rotor length (140ft or 43m) where the turbine generates 100 dB(A), you will experience a sound level of approxiamately 55 to 60 dB(A), which is about what a a European clothes dryer emits, or a loud conversation. Four rotor diameters (550 ft or 170m) from the same turbine would be 44 dB(A), which is the equivalent of a quiet living room in a house. At six six rotor diameters (850ft or 260 m) away you would experience 40 dB(A), which would be typically less than the ambient noise at night time.

In the real world you also have to take into account sound reflection and absoption (from hard and soft surfaces). These can play a role depending on the site, to the point of increasing or decreasing the sound level at different distances.

Sounds from several sources

Although the decibel count follows the addition of multiple sound sources, so two wind turbines located at the same distance us would double the sound energy reaching us. So a pair of turbines will mean that the measured sound level will increase by 3 dB(A). Four turbines at that distance means a sound level increased by 6 dB(A). But the vagaries of human hearing mean that you will actually have to install ten turbines, all at the same distance from the target area to perceive a doubling of the subjective loudness (i.e. the dB level has increased by 10).

Pure tone penalties

The human ear (and brain) are able to discern pure tones far more easily than white, or random, noise. This means that many authorities take this into account when calculating sound estimates. This penalises any planned structure which produces noise at pure tones.

Wind Turbine Noise Information in Practice

In compliance with the relevant international standards, turbine manufacturers will specify a theoretical dB(A) level for their products when in use. This will assume the sound produced when the turbine is operating all originates from a single point. However, in fact the noise originates from across the entire surface of the machine and rotor.

This single figure is useful, however, as it allows us to carry out the required calculations. The sound level produced by a modern turbine is typically in the range of 96-101 dB(A). This figure is itself rather pointless for all other purposes, as there will never be a point at which you could experience that level of sound. It is purely used to calculate the sound pressure produced during use to calculate the dB(A) at distance.

Pure tones, with their penalties have by and large been eradicated from all modern wind turbine designs from major manufacturers.

Legal noise limits

This various from jurisdiction to jurisdiction, but generally speaking it is 40 to 45 dB(A) at a distance of 300m. This generally sets a distance of 300 for any wind farm of more than ten turbines, but as with many aspects of wind energy, it really does vary from place to place.

Calculating rather than measuring

Calculating what the potential sound emission will be from wind turbines is an important factor in obtaining planning permission from the local authorities. Especially when installing wind turbines near any populated areas. Even in the most complex terrian, it is usually much simpler to calculate what the potential sound emissions will be than to go in and measure them once the structure is built.

This is simply due to the fact that the background sound will usually be over 30dB(A). The difference in sound levels between the background and 'target' noise has to be over 10dB(A) in order to be accurately measured. This makes it almost impossible to measure the noise of the turbine with any sort of accuracy. Which is what makes the calculation far more important.

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