![]() ![]() If the intensity of the sound source is much greater than the background, then the sound near the source may approximate inverse square drop off- this region is often called the "direct sound field". In any real environment - at least in any enclosed room - reverberant sound and background sound are always present and the inverse square law does not apply. ![]() (Note that decibels can be used to express any sound ratio in addition to the use for comparing to hearing threshold.) If the sound intensity in an auditorium followed the inverse square law, there would be a 20 decibel drop from the nearest listener to the most distant one if the nearest were at 10 ft and the most distant at 100 ft: If the distance from the sound source is doubled, the intensity is down to one fourth, soĪ doubling of distance from the source should give you a 6 dB drop if the inverse square law holds. In an open area with no reverberation, the sound intensity from a point source will drop off according to the inverse square law: Subjective experiments have indicated that for a given sound the intensity must be increased by about 10 decibels to be perceived as twice as loud - a tenfold power increase! This might be borne in mind if you consider paying a lot more money for a 70 watt stereo rather than a 50 watt - the perceived loudness difference is slight. This is not as dramatic a change in loudness as you might expect from power doubling - the ear's loudness response is approximately logarithmic. Doubling the power of a source increases the sound level by 10 log 10(2) = 3 decibels. The just noticeable change in sound intensity is about 1 decibel. Normal music is 40dB-100dB and conversational speech about 75dB. ![]() The threshold of pain is about 10 13 I 0 or 130 decibels. I 0 = the intensity for the standard threshold of hearing at 1000 Hz = 10 -16 watts/cm 2.īy examining the definition when I 0 is inserted for I, giving a 1 in the logarithm, you can see that the level I 0 is zero decibels. The reference level for such measurements is Where I = sound intensity = sound power per unit area (watts/cm 2 is a typical unit). Physics 4060, Acoustics Laboratory Physics 4060: Acoustics Laboratoryīecause of the large range of power associated with audible sound (10 14 or larger) it is customary to use the logarithmic decibel scale for sound intensity measurement. ![]()
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