What is a decibel, exactly?

[The Chase]

Does it represent anything, or was some guy just like one day, 'oh, THIS loud is THIS many decibles...'

???

[Hink]

In electronics and communications, the decibel (abbreviated as dB, and also as db and DB) is a logarithmic expression of the ratio between two signal power, voltage, or current levels. In acoustics, the decibel is used as an absolute indicator of sound power per unit area. A decibel is one-tenth of a Bel, a seldom-used unit named for Alexander Graham Bell, inventor of the telephone.
Suppose a signal has a power of P1 watts, and a second signal has a power of P2 watts. Then the power amplitude difference in decibels, symbolized SdBP, is:

SdBP = 10 log10 (P2 / P1)

Decibels can be calculated in terms of the effective voltage if the load impedance remains constant. Suppose a signal has an rms (root-mean-square) voltage of V1 across a load, and a second signal has an rms voltage of V2 across another load having the same impedance. Then the voltage amplitude difference in decibels, symbolized SdBV, is:

SdBV = 20 log10 (V2 / V1)

Decibels can also be calculated in terms of the effective current (amperage) if the impedance remains constant. Suppose a signal delivers an rms (root-mean-square) amperage of A1 through a load, and a second signal delivers an rms amperage of A2 through another load having the same impedance. Then the current amplitude difference in decibels, symbolized SdBA, is:

SdBA = 20 log10 (A2 / A1)

When a decibel figure is positive, then the second signal is stronger than the first signal. When a decibel figure is negative, then the second signal is weaker than the first signal. In amplifiers, the gain, also called the amplification factor, is often expressed in decibels. A circuit amplifies only if the decibel figure for the output-to-input power ratio (SdBP) is positive.

In sound, decibels are defined in terms of power per unit surface area on a scale from the threshold of human hearing, 0 dB, upward towards the threshold of pain, about 120-140 dB. As examples: the sound level in the average residential home is about 40 dB, average conversation is about 60 dB, typical home music listening levels are about 85 dB, a loud rock band about 110 dB, and a jet engine close up is 150dB.

Decibel units are commonly used in audio equalizers, both the hardware kind and the software kind, as a convenient reference point while editing. Boosting an equalizer band whose center point is 1000 by 3 dB means that you have raised the volume level of that frequency band by 3 dB as it relates to the other frequencies in the sound. A typical equalizer has a range for boosting or diminishing a sound level of +/-18 dB.

see....

[james0tucson]

Does it represent anything, or was some guy just like one day, 'oh, THIS loud is THIS many decibles...'

???

The unit of measure "decibel" actually has different meanings depending on what you're measuring and how you're measuring it.

In recording, you're often measuring attenuation from the maximum signal in negative decibels, with zero being the full signal, and positive being gain, which basically extends the scale. This measurement is called "dBFS" because it represents attenuation from a maximum amplitude (and anything above zero is "clipping"). [Saturation isn't really "gain", I know, I'm trying to keep this simple.]

When measuring loudness, we refer to dB of Sound Pressure Level, which is a relative scale, where 0dB
is the threshold of measurement, usually calibrated to the threshold of human hearing, but sometimes calibrated to the limits of instrumentation. It's a logarithmic scale, which allows a manageable range of values to represent a huge range of measurement.

A reasonable office ambience is about 50dB. Traffic noise, measured at the street, is about 70dB. A big diesel engine up close is about 90dB. A jackhammer, or a loud club is on the order of 100dB. A gunshot at arms length is about 140dB. A jet engine at 30 meters is 150dB.

Decibel measurements are meaningless without also giving an idea of distance. An explosion in Idaho doesn't cause hearing damage to people in Texas...


Measuring sound level becomes really complex when you're dealing with the human hearing domain, since human perception is not continuous, and is more sensitive to certain ranges than others. If you're measuring electronics you'll use "dBu" or "dBv" to represent a ratio of power to load, essentially.


Anyway your question, yes, "somebody" decided it would be easier to introduce a unit of measure, one that could be represented easily on a slide rule no doubt, because it was getting too complicated to write out the equations for adding amplifier circuits together every time it was needed. If you can express amplifier gain in a standard unit, you don't need the other details just to add them together, and this was a big problem for the first people who tried to push a modulated electrical signal through a few miles of wire.

Alexander Graham Bell gets all the mindshare for being the inventor of the telephone, but a whole lot of people worked on making the idea practical.

There is some controversy as to whether the term "decibel" may actually predate the term "bel". At any rate, "decibel" dates to 1929, from a paper by W.H. Martin, a Bell researcher. He coined the term to describe an arbitrary "Transmission Unit" that had been in use before then.

Because the unit of measure describes a *ratio* and not an absolute metric, it can be used for all different situations, such as saying how "loud" a concert is at the 20th row, or how "loud" the same concert is averaged throughout the stadium, or how "loud" the same concert is compared to its "quietest" moment, or how "quiet" the crowd is before the concert starts. It's all relative.

[The Chase]

Thanks, mate! I've got it now.