Re: 3 dB bandwidth

Kevin Aylward <see_website@xxxxxxxxxxxxx> wrote:

> Adrian Tuddenham wrote:
> > keith <krw@xxxxxxxxxx> wrote:
> >
> >> On Fri, 24 Jun 2005 22:41:12 +0100, Adrian Tuddenham wrote:
> >>
> >>> Keith Williams <krw@xxxxxxxxxx> wrote:
> >>>
> >>>> In article <11boec4bl5ab1df@xxxxxxxxxxxxxxxxxx>,
> >>>> _see.web.page_@xxxxxxxxxxxxxxxxxx says...
> >>>>>
> >>>>>
> >>>>>
> >>>>> Keith Williams wrote:
> >>>>>
> >>> ...
> >>>>>
> >>>>> Decibels are units of power, not voltage.
> >>>>
> >>>> Nonsense! Decibels are dimensionless. Watts/watts is
> >>>> dimensionless, as is Volts/volts, or SPL/SPL. Decibels is simply
> >>>> a log-scaled ratio of two quantities. It's useful because many
> >>>> things in nature are log- scaled.
> >>>
> >>> Decibels are, as you say, dimensionless; but they are specifically
> >>> a way of expressing the ratio of two power levels - only power
> >>> levels, not voltages or currents or anything else.
> >>
> >> You're as pig-ignorant as Guy! Decibels are a log ratio of
> >> *anything*.
> >
> > Have a look at "Radio Designer's Handbook" by F. Langford-Smith (first
> > published 1934). In my 4th Edition copy (1953) the whole of Chapter 9
> > is devoted to the subject of decibels.
>
> We have moved on from there. The book is dated, and simply not relevant
> to the modern world on this issue.
>
> >
> > It is quite clear that only the power ratio is directly described by
> > decibels, if you wish to express the ratio of other electrical
> > quantities in decibels you "must involve the resistance". (Page 807)
>
> Again, in short, quite nonsense today. ....

Now you have started me on a very interesting quest, to find out when
the definition of the decibel changed - if it actually ever did.


It appears that the decibel originated in line telephony in 1923 when
AT&T introduced the "Transmission Unit". By 1924 it was agreed to
standardise on the "Bel" (10 TUs) and the "Neper", based on Naperian
logarithms. By 1929 the TU had been renamed the "Decibel" and was
firmly established as the unit of gain or attenuation.

All these units were defined as power ratios but could be used to
describe related quantities such as current and voltage if the impedance
levels were specified.

I have found a number of reference sources (listed below in order of
first publication date) and all of them unequivocally define the decibel
as a power ratio except the one I have named xxx (to spare any
embarrassment).

1931 The Admiralty Handbook of Wireless Telegraphy (HMSO)

1934 Radio Designer's Handbook (Langford-Smith)

1936 Foundations of Wireless (Scroggie)

1938 Radio Communication Handbook (RSGB)

1956 Programme Operations Handbook (BBC)

1960 A to Z in Audio (Briggs)

1972 Woods Practical Guide to Noise Control (Woods Acoustics)

1980 xxx

2003 The Decibel Revisited (AES)
<http://www.aes.org/sections/pnw/pnwrecaps/2003/mrogers_db/>


>.... dBs, today, are a general term.
> Its irrelevant how they may have been first used.

If you search on the Web, you will find a bewildering variety of
so-called definitions for the dB, many relating to absolute sound
levels, which I am sure the majority of knowlegeable people would agree
are utterly wrong.
(OK, I found the AES definition on the web too; but I like to think that
they, of all people, should know what they are talking about)


The decibel does not appear to me to have been formally re-defined at
any time since its inception - and its informal abuse by large numbers
of ignorant punters does not alter that fact.


--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
.

Relevant Pages

  • Re: 3 dB bandwidth
    ... >>> It is quite clear that only the power ratio is directly described by ... > the definition of the decibel changed - if it actually ever did. ... > 1931 The Admiralty Handbook of Wireless Telegraphy ... dBu, dBV, dB-SPL, dB-A and others. ...
    (sci.electronics.design)
  • Re: db Question
    ... Decibels are always used for comparing power levels. ... If you're comparing current or voltage levels, you first have to convert them into a power ratio, and that is the reason why you have to use a different formula. ... but the definition of the decibel remains the same. ... Actually what I was taught was the standard unit is the BEL, named in honor of Alexander Graham Bell. ...
    (rec.radio.amateur.antenna)
  • Re: 3 dB bandwidth
    ... > 1gysddc.8jsl0emwpw1kN%poppy.uk@xxxxxxxxxxxxxxxxxxxxxxxx, "Adrian Tuddenham" ... >> the definition of the decibel changed - if it actually ever did. ... > dBu, dBV, dB-SPL, dB-A and others. ... It is one of my pet grouses that the reference levels often aren't ...
    (sci.electronics.design)
  • Re: db Question
    ... Decibels are always used for comparing power levels. ... If you're comparing current or voltage levels, you first have to convert them into a power ratio, and that is the reason why you have to use a different formula. ... but the definition of the decibel remains the same. ... There is a similar-looking term, sometimes written as either "dBV", "dBmV" or "dBuV", but that has a different meaning. ...
    (rec.radio.amateur.antenna)
  • Re: 3 dB bandwidth
    ... >>> It is quite clear that only the power ratio is directly described by ... > the definition of the decibel changed - if it actually ever did. ... There is simply no bloody law that says Bell labs original definitions ... what is a correct definition. ...
    (sci.electronics.design)
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