Definition
A unit used to express the ratio between two power levels, typically sound or signal intensity. One bel equals a tenfold increase in power. Because the bel is a large unit, measurements in practice are almost always given in decibels (dB), where one decibel equals one-tenth of a bel.
Plain English
A bel is a way of comparing how strong one sound or signal is compared to another. Going up by one bel means the power is ten times greater. In real use, we usually measure in decibels, which are smaller and more practical.
Context Anchor
Seen in aircraft noise, hearing protection, sound measurement, and radio or electrical signal discussions, usually through the related unit decibel.
Derivation
Named after Alexander Graham Bell, the inventor of the telephone, in recognition of his work on sound and communication. The unit was originally created by engineers studying signal loss in telephone lines.
Why Pilots Care
Pilots rarely use the bel directly, but its smaller form, the decibel, appears constantly in aviation -- noise ratings for headsets, cabin sound levels, and radio signal strength are all measured in dB. Knowing the bel is the parent unit helps make sense of those numbers.
Grounding Statement
If one sound is 1 bel higher than another in sound intensity, it is ten times as intense, not just one small step louder.
Intuition Check
Do not confuse bel with a bell that rings. Here, bel is a measurement unit for comparing sound or signal strength.
Example Sentence 1
The bel is too large for everyday use, so headset noise reduction is rated in decibels instead.
Example Sentence 2
A change of one bel means the sound intensity increased by a factor of ten.