# Decibel FAQs:

Q: Are decibels ten times the logarithm to base 10 of the ratio of two power quantities?

A: Yes.

Q: Is a decibel often used to express power or amplitude ratios?

A: Yes, and in preference to arithmetic ratios or percentages.

Q: Was a decibel the proposed working unit?

A: Yes.

Q: Is a decibel based on the measurement of power in telephony of the early 20th century in the Bell System in the United States?

A: Yes.

Q: Is a decibel one tenth of one bel, named in honor of Alexander Graham Bell?

A: Yes, however, the bel is seldom used.

Q: Are decibels ten times the number of bels?

A: Yes, P and P0 must measure the same type of quantity, and have the same units before calculating the ratio.

Q: Are decibels written with additional significant figures?

A: Yes.

Q: Is a decibel accepted for use alongside SI units?

A: Yes, and the practice of attaching a suffix to the basic dB unit, forming compound units such as dBm, dBu, dBA, etc.

Q: Is a decibel defined with respect to power?

A: Yes, and not amplitude, conversions of voltage ratios to decibels must square the amplitude, or use the factor of 20 instead of 10, as discussed above.

Q: Is a decibel defined as a unit of measurement for quantities of type level or level difference?

A: Yes, and which are defined as the logarithm of the ratio of power- or field-type quantities.

Q: Is a decibel commonly used in acoustics as a unit of sound pressure level?

A: Yes.

Q: Is a decibel recognized by other international bodies such as the International Electrotechnical Commission and International Organization for Standardization?

A: Yes, The IEC permits the use of the decibel with field quantities as well as power and this recommendation is followed by many national standards bodies, such as NIST, which justifies the use of the decibel for voltage ratios.

Q: Is a decibel used for a wide variety of measurements in science and engineering?

A: Yes, and most prominently in acoustics, electronics, and control theory.

Q: Are decibels awkward in inherently additive operations: "if two machines each individually produce a [sound pressure] level of"?

A: Yes, and say, 90 dB at a certain point, then when both are operating together we should expect the combined sound pressure level to increase to 93 dB, but certainly not to 180 dB!" "suppose that the noise from a machine is measured and found to be 87 dBA but when the machine is switched off the background noise alone is measured as 83 dBA.