The Science and Technology of Counterterrorism

Appendix D

The Decibel (dB)

The decibel is very useful in depicting relative changes in quantities related to security. Decibels can be intimidating, especially if logarithms are themselves scary since the logarithm is an inherent part of the definition of the decibel. Simply put, a decibel is merely one means of expressing a ratio between two quantities. The decibel indicates the magnitude of one quantity relative to another expressed in logarithmic units.

It makes no sense to say a signal is 30 dB, 40 dB, 100 dB, etc., although such statements are made all the time. A response to this statement must be, “30 dB compared to what?” You may have in fact seen sound pressure levels seemingly expressed without such a comparative reference.

For example, ...