The SWL quoted by the manufacturer is often an average over a particular range. This method of rating speakers using SWL is often deceiving because most speakers produce very different SWLs at different frequencies of sound, often varying as much as ☑0 dB throughout the speaker's usable frequency range (it generally varies less in higher quality speakers). This measurement can also be represented as a strict efficiency ratio of audio output ( sound power) to electrical input (electrical power), but this is far less common. The SWL is then measured and the product labeled, something like "SWL: 93 dB 1 W/1 m". Then a particular sound (usually white noise or pink noise) is played through the source at a particular intensity so that the source is consuming one watt of power. The most common means is measuring the sound power level from the speaker with the measuring device placed directly in front of and one meter away from the source. Most audio manufacturers use SWL to describe the efficiency of their speakers. One of the advantages of expressing the noise level of a source in terms of its power level is that one does not have to note any distance from the source. As noise is unwanted sound the trumpet can be perceived to be acceptable when listened to as music but at the same sound power level may be perceived to be noisy if one is trying to sleep. The Trumpet and excavator both have the same sound power of 0.3 watts, but may be judged psychoacoustically to be different levels. Table: Sound power level and sound power of some sound sources Situation
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