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<blockquote data-quote="audioholic" data-source="post: 2431132" data-attributes="member: 549629"><p>Close, but not quite. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif</p><p></p><p>"2.9 How does sound decay with distance?</p><p></p><p>At distances large compared to the size of the source, sound intensity diminishes according to the inverse square law.</p><p></p><p>I = Io/D^2</p><p></p><p>This is relatively simple to reliably calculate, provided the source is small and outdoors where no echoes occur. (But indoor calculations in a reverberant field are rather more complex. )</p><p></p><p>If the noise source is outdoors and its dimensions are small compared with the distance to the monitoring position (ideally a point source), then as the sound energy is radiated it will spread over an area which is proportional to the square of the distance. This is an 'inverse square law' where the sound level will decline by 6dB for each doubling of distance.</p><p></p><p>Line noise sources such as a long line of moving traffic will radiate noise in cylindrical pattern, so that the area covered by the sound energy spread is directly proportional to the distance and the sound will decline by 3dB per doubling of distance.</p><p></p><p>Close to a source (the near field) the change in SPL will not follow the above laws because the spread of energy is less, and smaller changes of sound level with distance should be expected.</p><p></p><p>If the observation position very close to the source, at a distance that is small compared to the size of the source, the sound level changes very little with location in that source area. One may be able to determine the "virtual center" of the whole sound field, whence inverse square law calculations can proceed in reference to that distance, for locations outside the source area.</p><p></p><p>The surrounding environment, especially close to the ground, and in the presence of wind &amp; vertical temperature gradients, has a great effect on the sound received at a distant location. Ground reflection affects sound levels more than a few feet away (distances greater than the height of the sound source or the receiver above the ground). Wind and air temperature gradients affect all sound propagation beyond 100 meters over the surface of the earth. Sound propages well downwind (traveling with the wind), and very lirrle upwind. When the ground surface is cooler than the air just above it ("inversion"), typically late at night and just before dawn, sound will travel great ditances across the landscape even without any wind.</p><p></p><p>In addition it is always necessary to take into account attenuation due to the absorption of sound by the air, which may be substantial at higher frequencies. For ultrasound, air absorption may well be the dominant factor in the reduction.</p><p></p><p>*** 2.10 What is the sound power level?</p><p></p><p>(See ACCULAB Reference Sound Source on this site:</p><p></p><p><a href="http://www.campanellaacoustics.com/rssman.htm" target="_blank">http://www.campanellaacoustics.com/rssman.htm</a> )</p><p></p><p>Sound power level, Lw, is often quoted on machinery to indicate the total sound energy radiated per second. It is quoted in decibels with respect to the reference power level. The reference level is 1pico-watt (pW) [1x10^(-12) watts]. One watt of radiated sound power is represented as "Lw=120 dB re one picowatt". If the reported sound power is in terms of A-Weighted spectral weighting, a suffix, A, is applied to form dB(A).</p><p></p><p>The sound pressure level (SPL) resulting from sound power (Lw) being radiated into free space, e.g. over a paved surface, is computed from</p><p></p><p>SPL = Lw - 20*log® - 11 dB re 20 uPa (R in meters)SPL = Lw - 20*log® - 0.7 dB re 20 uPa (r in feet)</p><p></p><p>If instead the sound is emitted over a reflecting plane such as a hard surface, three (3) decibels are added to the SPL.</p><p></p><p>For example, a lawn mower with sound power level 100 dB(A) will produce at a sound pressure level (SPL) of about 89dB(A) at the operator (you) position over grass and 92 dB(A) when the mower is operated over a hard surface such as your driveway. At your neighbor's yard 50 feet (15m) away, the SPL will be is 65 dBA." - <a href="http://www.campanellaacoustics.com/faq.htm#basic_decay" target="_blank">http://www.campanellaacoustics.com/faq.htm#basic_decay</a></p></blockquote><p></p>
[QUOTE="audioholic, post: 2431132, member: 549629"] Close, but not quite. [IMG]//content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif[/IMG] "2.9 How does sound decay with distance? At distances large compared to the size of the source, sound intensity diminishes according to the inverse square law. I = Io/D^2 This is relatively simple to reliably calculate, provided the source is small and outdoors where no echoes occur. (But indoor calculations in a reverberant field are rather more complex. ) If the noise source is outdoors and its dimensions are small compared with the distance to the monitoring position (ideally a point source), then as the sound energy is radiated it will spread over an area which is proportional to the square of the distance. This is an 'inverse square law' where the sound level will decline by 6dB for each doubling of distance. Line noise sources such as a long line of moving traffic will radiate noise in cylindrical pattern, so that the area covered by the sound energy spread is directly proportional to the distance and the sound will decline by 3dB per doubling of distance. Close to a source (the near field) the change in SPL will not follow the above laws because the spread of energy is less, and smaller changes of sound level with distance should be expected. If the observation position very close to the source, at a distance that is small compared to the size of the source, the sound level changes very little with location in that source area. One may be able to determine the "virtual center" of the whole sound field, whence inverse square law calculations can proceed in reference to that distance, for locations outside the source area. The surrounding environment, especially close to the ground, and in the presence of wind & vertical temperature gradients, has a great effect on the sound received at a distant location. Ground reflection affects sound levels more than a few feet away (distances greater than the height of the sound source or the receiver above the ground). Wind and air temperature gradients affect all sound propagation beyond 100 meters over the surface of the earth. Sound propages well downwind (traveling with the wind), and very lirrle upwind. When the ground surface is cooler than the air just above it ("inversion"), typically late at night and just before dawn, sound will travel great ditances across the landscape even without any wind. In addition it is always necessary to take into account attenuation due to the absorption of sound by the air, which may be substantial at higher frequencies. For ultrasound, air absorption may well be the dominant factor in the reduction. *** 2.10 What is the sound power level? (See ACCULAB Reference Sound Source on this site: [URL="http://www.campanellaacoustics.com/rssman.htm"]http://www.campanellaacoustics.com/rssman.htm[/URL] ) Sound power level, Lw, is often quoted on machinery to indicate the total sound energy radiated per second. It is quoted in decibels with respect to the reference power level. The reference level is 1pico-watt (pW) [1x10^(-12) watts]. One watt of radiated sound power is represented as "Lw=120 dB re one picowatt". If the reported sound power is in terms of A-Weighted spectral weighting, a suffix, A, is applied to form dB(A). The sound pressure level (SPL) resulting from sound power (Lw) being radiated into free space, e.g. over a paved surface, is computed from SPL = Lw - 20*log® - 11 dB re 20 uPa (R in meters)SPL = Lw - 20*log® - 0.7 dB re 20 uPa (r in feet) If instead the sound is emitted over a reflecting plane such as a hard surface, three (3) decibels are added to the SPL. For example, a lawn mower with sound power level 100 dB(A) will produce at a sound pressure level (SPL) of about 89dB(A) at the operator (you) position over grass and 92 dB(A) when the mower is operated over a hard surface such as your driveway. At your neighbor's yard 50 feet (15m) away, the SPL will be is 65 dBA." - [URL="http://www.campanellaacoustics.com/faq.htm#basic_decay"]http://www.campanellaacoustics.com/faq.htm#basic_decay[/URL] [/QUOTE]
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