- 1). Select a reference sound power. The reference sound power in air is typically 10^(-12) watt, or 1 trillionth of a watt. This is generally the lowest sound power that the human ear can detect under ideal circumstances.
- 2). Choose a test sound, such as normal speech. This type of sound has an absolute power of about 10^(-5) watts, or 0.00001 watts.
- 3). Derive the relative power of a test sound with the formula L = 10 log (P1/P2). The "L" in this equation is the relative power of the test sound in dB, "P1" is the absolute power of the test sound, and "P2" is the absolute power of the reference sound.
- 4). Calculate the relative power of the test sound. The reference sound power is 10^(-12) and the test sound power is 10^(-5), so the relative power of the reference sound is 10 log (10^(-5)/ 10^(-12)) = 70 dB.
- 5). Compute the relative power of other common sounds. Loud speech has an absolute power of 0.001 watts, or a relative power of 90 dB. A helicopter has an absolute power of 0.01 watts, or a relative power of 100 dB. A chain saw has an absolute power of 0.1 watts, or a relative power of 110 db. A jackhammer produces a sound power of about 1 watt, or 120 dB. This noise level can cause hearing loss within a short period.
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