1. A sound pressure expressed in dB above the standard sound pressure of 20 µPa. The following reference pressures are in common use: (a) 2 X 10E-4 microbar, (b) 1 microbar. Reference pressure (a) is in general use for measurements concerned with hearing and with sound in air and liquids, whereas (b) has gained widespread acceptance for calibration of transducers and various kinds of sound measurements in liquids. Unless otherwise explicitly stated, it is to be understood that the sound pressure is the effective (root-mean-square) sound pressure. It is to be noted that in many sound fields the sound pressure ratios are not the square roots of the corresponding power ratios.

2. The power level of sound being the sound pressure squared, referenced to 20 mPa2 measured in dB. Commonly, how loud the sound is measured in decibels. Speech is around 70-80 dB at three feet. Normal background noise on average is 50-60dB. 120dB is the threshold of pain. 200dB is 50lbs of TNT detonated 10 feet away. Source: Church Audio & Acoustics Glossary

3. Ten times the logarithm of the ratio of the time-mean-square pressure of a sound, in a stated frequency band or with a stated frequency weighting, to the square of the reference sound pressure of 20 micropascal. Unit, decibel (dB); symbol, Lp. Source: Glossary of Terms in Noise Control Engineering

4. (abbreviated SPL): Ten times the base-10 logarithm of the time-mean-square sound pressure, in a stated frequency band (often frequency-weighted), divided by the squared reference sound pressure of 20 • Pa, the threshold of human hearing.

SPL = 10 × log10[p2÷pref2]

Where

p2 = time-mean-square sound pressure; and

pref2 = squared reference sound pressure of 20 • Pa.

5. Given in decibels (dB) is an expression of loudness or volume. A 10 dB increase in SPL represents a doubling in volume. Live orchestral music reaches brief peaks in the 105 dB range and live rock easily goes over 120 dB. Source: http://www.owenscorning.com/around/sound/glossary.asp

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