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Power of sound reference power12/17/2023 To facilitate a "buy quiet" policy, it is important to compare noise production of different machines. If the A-weighted sound pressure level for the operator exceeds 80 dB then the sound power level also must be specified. The noise emission of machines has to be mentioned in the user manual, and in all commercial publications if these contain technical specifications, a requirement of the Machinery Directive. You could use it for such purpose indoors as well, but then it immediately becomes much more complex because reflections and screening have to be taken into account. For example for the assessment of industrial noise and its propagation to the environment. The most common application is to predict sound pressure level, at a certain distance, due to a source or a collection of sources. Again that explains why a number with decibel alone doesn't give information, a full description is required. Sound power level is also expressed in decibels (dB), but with the reference value 1 picoWatt. When converting 'pressure multiplied with surface area' to logarithmic measures, we get the sum of the logarithms, as shown at the bottom of the picture. As the distance increases, the sound pressure decreases, inversely proportional to the increase in surface area. If these conditions are met, then the product of the sound pressure (in Pascal) with the enveloping surface (in square meter) is always the same: after all, there is a constant flow of acoustic energy through the surface, from inside to outside. for this set up with a hemispherical surface there is a minimum measuring distance, related to the size of the source, roughly: radius ≥ 2 * length of the source (this set up is appropriate for outdoor use).there are no sound-reflecting walls or obstacles in the vicinity (reflection from the floor is required!).there are no other relevant sounds in the vicinity than those of the source to be measured.In the situation shown in the picture some restrictions apply: The image above, with a hemispherical measurement surface, helps to explain this. Sound power level is indicated with LW (W in subscript), in practice with "A" because mostly A-weighting is applied. And that sound power level no longer depends on distance and/or position, it only depends on the source and its operating condition. ![]() If we place an (imaginary) enveloping surface around a sound source and measure the average sound pressure level on that surface, we can calculate the sound power level of the source. There is no direct measurement method for this, a sound level meter essentially registers the pressure variation, and thus gives a value in sound pressure level Lp (dB). It is also possible to determine the total sound energy emitted by a source. Therefore, the sound of a source cannot be described clearly with only its sound pressure level the distance at which that sound level is measured is information that is absolutely necessary and sometimes also the orientation relative to the source. The sound pressure level of a certain source depends on the distance to that source, and sometimes also on the direction relative to that source. In practice often including an "A" if A-weighting is applied. And it is required to give a full description of what is expressed in decibel! Sound pressure level is indicated with Lp (p in subscript). A decibel therefore always includes a reference value, which for sound pressure level is 20 microPascal (µPa). Pseudo-unit, because decibel is a relative number, you can express everything in decibels. ![]() That's why we use sound pressure level, which is indicated with the pseudo-unit decibel (dB). A logarithmic scale gives more manageable numbers and such a scale corresponds better with our subjective perception of loudness differences. Air pressure is expressed in Pascal (Newton / square meter), but this is inconvenient to describe the pressure variation we perceive as sound in Pascals for several reasons. Sound is a perceptible variation of the air pressure. This can lead to confusion and sometimes tyring discussions. Unfortunately, it is increasingly used as equivalent to 'sound'. This is a copy of an article originally published in Dutch DecibelĪlmost everyone is familiar with the decibel.
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