The sound level produced by a modern turbine is typically in the range of 96-101 dB(A). This single figure is useful, however, as it allows us to carry out the required calculations. However, in fact the noise originates from across the entire surface of the machine and rotor. This will assume the sound produced when the turbine is operating all originates from a single point. In compliance with the relevant international standards, turbine manufacturers will specify a theoretical dB(A) level for their products when in use. Wind Turbine Noise Information in Practice ![]() This penalises any planned structure which produces noise at pure tones. This means that many authorities take this into account when calculating sound estimates. The human ear (and brain) are able to discern pure tones far more easily than white, or random, noise. ![]() 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. Four turbines at that distance means a sound level increased by 6 dB(A). So a pair of turbines will mean that the measured sound level will increase by 3 dB(A). In case you are interested in the exact definitions, take a look at the Reference Manual on Acoustics of this web site - put in linkĪlthough 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. This means that the decibel scale follows human appreciation of loudness. 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. ![]() So, for example, 100 dB(A) refers to a noise which contains twice the sound energy of on which is 97 dB(A). Meaning that an increase in the index of (approximately) 3 means a doubling of sound intensity. The dB-scale is not a simple scale, such as weight. But these scales do not apply to the wind turbines. There are two other weighing schemes which are used for louder sounds, (B) and (C). The dB(A) scale denotes a weighing scheme and is used exclusively for weaker sounds, such as wind turbines. The result of this calculation is the index number. 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. 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. 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. ![]() However, since the air violently flooding a jet engine produces strikingly ordered acoustic fields, studying these engines does present an opportunity to study turbulence itself.The dB(A) scale is used in a wide range of fields where sound has to be measured. The chaotic nature of turbulence has perplexed physicists since antiquity. As for what explains turbulence itself, we are still mystified. Physicists explain the loud noise as the “amplification of turbulence fluctuations”. It is the turbulence caused by the high-velocity air exiting the nozzle and ramming into the low-velocity air outside that is responsible for the production of such powerful sound waves. The conclusion is that the slower the jet, the quieter it is. This might sound like a paltry change, but since the decibel scale is logarithmic, the reduction is actually immense. Again, consequently, the jet’s velocity is decreased, but the reduction in intensity is around 15 decibels. Mixer in Jet engine and Hush kit (Photo Credit : Alf van Beem / Wikimedia Commons)Īnother way to decrease the whining is to use longer nozzles, such that the turbulence of the expanded gas is more controlled.
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