period one
Fundamentals of Sound
notes
Lp = 20 log10
sound pressure, µPa
20 Pa
or 10 log10
sound pressure, µPa
20 Pa
2
Again, these reference values can be considered the threshold of hearing. The multiplier 20 is used in the
Logarithmic Addition of Decibels
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higher |
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to the |
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add | 0 | 0 | 5 | 10 | 15 |
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50 dB + 44 dB = 51 dB
Figure 17
Measuring sound using a logarithmic scale means that decibel values cannot be added arithmetically. Instead, logarithmic addition must be used to add two or more sound levels. This involves converting the decibel values into ratios of sound intensity, adding these ratios, and then converting the sum back into decibels. The mathematics become rather
To demonstrate the use of this figure, consider the example of adding a 50 dB sound to a 44 dB sound. The difference between these two sounds is 6 dB. Therefore, 1 dB is added to the higher of the two sounds (50 plus 1) to arrive at the logarithmic sum of 51 dB.
Also, notice that the logarithmic sum of two sounds of equal magnitude (0 dB difference) results in a 3 dB increase. Therefore, adding two 50 dB sounds would result in a combined sound level of 53 dB.
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