Addenda for the decibel entry

The “db” symbol for the decibel

The symbol “db” was assigned in the paper that introduced the word “decibel.”

For convenience, the symbol “db” will be employed to indicate the name “decibel.”

W. H. Martin.
Decibel–The name for the transmission Unit.
Bell System Technical Journal, vol. 8, no. 1, January 1929. Page 2.
Available online at

Examples of authorities for the db symbol include:

Institute of Radio Engineers, Technical Committee on Symbols.
Standards on abbreviations, graphical symbols, letter symbols, and mathematical signs.
New York: IRE, 1948.
Page 4.

United States Government Printing Office.
Style Manual. Revised edition, January 1967.
Washington: U.S. G.P.O., 1967.
Page 161.

The transition to “dB” can be followed in the revisions of the well-known style manual published by the University of Chicago:
A Manual of Style, Twelfth Edition, 1969, page 334, “db.” Note the period.
The Chicago Manual of Style, Thirteenth Edition, 1982, page 395, “db” Period is gone.
The Chicago Manual of Style, Fourteenth Edition, 1993, page 482, “dB”

K. Diem and C. Lentner.
Scientific Tables. 7th edition.
Ardsley, NY: Geigy Pharmaceuticals, 1970.
Page 224: “in decibel (dB)”

In our opinion, the db symbol is still the best symbol to use for the decibel as originally defined, that is, as a power ratio with no reference level.

The magnitude of the transmission unit, and hence decibel, was chosen to approximate the magnitude of the mile of standard cable

The symbol TU in the selection stands for “transmission unit.”

The transmission standards in general use vary from 18 miles of standard cable to about 30 miles of standard cable, depending upon the locality and the class of service such as local and toll. It has become customary among telephone people interested in standards of service to associate certain figures for transmission standards with the corresponding standards of service which they represent. It is a distinct advantage, therefore, to retain the same figures for the same standards of service when changing to the new unit. The zero of reference was so selected, therefore, that 24 TU is equivalent to 24 miles of standard cable in volume reproduction. This means that if one talks with the same loudness over a circuit of 24 TU as over a circuit of 24 miles of standard cable, the volume received from each will be the same. As the attenuation corresponding to the TU is only about 6 per cent. less than the attenuation corresponding to the mile of standard cable and 24 miles represents the mean between the highest and lowest standards in common use, transmission standards on the new basis are very little different numerically from the same standards on the old basis. The former 18-mile standard is equivalent in transmission to 17.6 TU and the 30-mile standard is equivalent to 30.4 TU. The same numerical values can, therefore, generally be used for transmission standards in the new system, as in the old, since the greatest differences encoun tered will be 0.4 TU.

It is also true that a given transmission loss specified in miles will correspond very closely in numerical value to the same loss expressed in TU. People not directly engaged in transmission work, therefore, may generally disregard the slight difference which exists in considering transmission losses expressed in TU as compared with standard cable.

C. W. Smith.
Practical application of the recently adopted Transmission Unit.
Bell System Technical Journal, vol. 3, no. 3 page 409, (July 1924).
Available online at

Even in its first year, and even in the hands of its originators, the decibel began to be used for new purposes

In handling certain types of crosstalk problems, it has been found convenient to express crosstalk in terms of transmission units rather than crosstalk units. Miles of standard cable have previously been used in such problems. TU can be used for this purpose as well as miles and it is somewhat simpler to make the conversion from crosstalk units to TU than from crosstalk units to miles. Crosstalk may be converted from crosstalk units to TU as follows :

equation as a graphic

C. W. Smith.
Practical application of the recently adopted Transmission Unit.
Bell System Technical Journal, vol. 3, no. 3 page 412,413. (July 1924).
Available online at

An example of the kind of casual use of the decibel that drove its engineer originators crazy

How many questionable or incomplete assertions can you find?

Decibels may be written in lower case as we do in this text, or they may be written dB. Either is correct. Db's and VU’s are of relative (not absolute) size. (The db is an expression of a ratio between two voltages, two currents or two powers.) A reference level should be stated, in each case to which the db measurement is referred. Thus the db itself is referenced to six milliwatts of power in a 500 ohm line, which is an obsolete reference base. The dbm, a more current measurement, is referenced to 1 milliwatt of power in a 600 ohm line which has a sine wave RMS (root-mean-square) value of .774 volt across it. Both the db and the dbm are ratios of sinusoidal power such as that emanating from an audio tone oscillator. Other db references are: dba— referenced above noise level, dbv— referenced above 1 volt, dbw— referenced above 1 watt, dbx— referenced above cross-talk measurements. The formula for derivation of db of power is: db = 1010 log P1/P2 where P1 and P2 are input and output power.

Robert S. Oringel.
Audio Control Handbook for Radio and Television Broadcasting. 5th ed., rev. and expanded.
New York: Hastings House, 1983.
Page 5.

The neper-decibel contention

Consultative Committee for Units (CCU).
Report of the 15th meeting (17-18 April 2003) to the International Committee for Weights and Measures.

Reports of CCU meetings are now only published online, at  Retrieved 22 May 2007.

See Section 3, President's Report, which states in part:

“At the CIPM meeting in October 2001 (90th meeting) he had reported that the CCU recommended the neper should be recognized as the (only) coherent unit of logarithmic decay, and recommended recognizing the bel and the decibel as widely used non-coherent units. The reasons for this recommendation are summarized in the 2001 paper in Metrologia by Mills, Taylor and Thor. After a brief discussion this recommendation was approved, although without enthusiasm.

“Although there was no meeting of the CCU in 2003, the President discussed the situation with many users of these units, and found widespread dissatisfaction with the proposal that only the neper should be recognized as a coherent SI unit. This arises from the fact that the decibel is very widely used as a unit, but the neper almost never used, by workers in the field. In fact most of the community have difficulty in recalling the definition of the neper.

“After discussion with experts in the field, and with colleagues on the CCU, the President decided to present a modified proposal to the CIPM at its meeting in 2002 (91st meeting), recommending that we should recognize two coherent SI units of logarithmic decay for two slightly different quantities: the neper for logarithmic amplitude ratio, and the bel for ‘power-like quantities’. ... However, after a lengthy discussion, the CIPM decided that it did not yet wish to make any change in the present situation, in which neither the neper nor the bel are recognized as SI units.”



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