What is a sound pressure decibel?

What is an audio decibel, or specifically a sound pressure decibel? This topic can be very confusing since there are so many different variations of what we commonly refer to as the 'decibel.' This article will specifically simplify the answer to some rules of thumb for the novice. It is important to note that there is a difference between the electical/mathematical decibel, and the arbitrary perceived loudness.

An audio decibel is a unit of measure that we use to refer to, among other things, sound pressure level. A decibel is 1 tenth of a Bel (named after Alexander Graham Bell) and is a logarithmic relative measurement. It is important to note that this is different from voltage, amperage, optical, or any other measurement that uses the dB scale. For sound pressure level, 0 dB starts at the threshhold of human hearing, which is about the sound of a mosquito at 10 feet. 120 dB is the sound of a loud rock concert, 134 dB the threshold of pain. Typical human conversations are around 70 dB. This represents a very wide range of sound intensity, so a logarithmic scale is used as measurement. These are always a ratio. Here is the actual notation:

SPL = 20 log_10 (P1/Pref)

where SPL is Sound Pressure Level, P1 is the level being measured, and Pref is the reference level, typically 0 db. Decibel logs are always log base 10.

What are the practical rules of thumb?

A 1 decibel change is Just Barely Noticeable.
A 3 decibel change is roughly twice the power intensity, but
A 10 decibel change is percieved as twice the loudness.

And therefore, long throw faders (100mm or longer) are preferable since they give you finer control on the mixing console!

Hope this helps!

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