Intensity and Loudness of Sound ( Read ) | Physics | CK Foundation
distance of a sound-source comes from the loudness variable. Physical . in order to observe any changes in the functional relationship of distance. A doubling of the distance from a noise source will reduce the sound pressure level with 6 decibel. Intensity. The intensity of a sound wave is its power/unit area. I = P/A. In one dimension the intensity is constant as the wave travels. In two or three dimensions.
A Distant Sound
Since total energy must be conserved, the energy per unit area must decrease. The scale that is used to express intensity is based on powers of A sound that is 10 times more intense is assigned a value of 10 dB, and a sound times as intense is given a value of 20 dB. This means that if a sound is 10x times more intense than another sound, it has a decibel reading that is 10x higher than the less intense sound.
Sound level meter Materials can be found on the Internet.
Read about sound wave propagation in air. Formulate a hypothesis to predict how sound wave intensity decreases with distance from a sound source. Identify a fixed sound source.
Using a pedometer, measure and record your distance from this source. Using a sound level meter, measure and record the intensity of the sound in decibels. Repeat these measurements at other distances from the sound source. The measurement locations should fall in a straight line.
Express the distances as relative values with reference to the closest distance to the sound source. Then express the intensities as relative values with reference to the intensity closest to the sound source. Inverse Square Law The energy in sound waves and thus the sound intensity will drop with the square of the distance to the sound source.
In other words, if you move m away from a wind turbine, the sound level will generally be one quarter of what it is m away. A doubling of your distance will thus make the dB A level drop by 6. At one rotor diameter distance 43 m from the base of a wind turbine emitting dB A you will generally have a sound level of dB A corresponding to a European clothes dryer.
The precise relationship between sound level and distance from the sound source is given in a table on the Reference Manual on Acoustics of this web site. In practice, sound absorption and reflection from soft or hard surfaces may play a role on a particular site, and may modify the results shown here. Adding Sounds from Several Sources If we have two wind turbines rather than one, located at the same distance from our ears, naturally the sound energy reaching us will double.
As we have just learned, this means that two turbines will increase the sound level by 3 dB A. Four turbines instead of one at the same distance will increase the sound level by 6 dB A.
You will actually need ten turbines placed at the same distance from you, in order to perceive that the subjective loudness has doubled i. If you wish to learn the details about adding sounds together, take a look at the Reference Manual on Acoustics in this web site.
The Pure Tone Penalty The fact that the human ear and mind discerns pure tones more easily than random white noise, means the authorities may wish to take that into account when doing sound estimates.
They consequently often have rules which specify that you add a certain number to the dB A figure in case you have pure tones present in a sound. Wind Turbine Noise Information in Practice In accordance with international standards manufacturers generally specify a theoretical dB A level for sound emissions which assumes that all sound originates from a central point, although in practice, of course, it will originate from the whole surface of the machine and its rotor.
Sound pressure thus calculated is typically around dB A for modern wind turbines. The figure itself is rather uninteresting, since there will not be a single point, where you can experience that sound level! Rather, it is useful for predicting the sound level at different distances from the wind turbine.
Pure tones have generally be eradicated completely for modern wind turbines, at least in the case of the modern turbines listed in the catalogue on the Wind Power Calculator page. Legal Noise Limits At distances above m the maximum theoretical noise level from high quality wind turbines will generally be significantly below 45 dB A outdoors, corresponding to the legislation in Denmark.