Re: 3 dB bandwidth

Kevin Aylward <see_website@xxxxxxxxxxxxx> wrote:

> Adrian Tuddenham wrote:
> > 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 did, in just the same way as your "bad" dude, maans your good.

I'm sorry, but I really don't know what that means.


> > 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.
>
> None of this power stuff means *anything* today. Its simply irrelevant
> what book, committee etc attempt to define dbs. The dB is still a
> general term that is universally used as a means of expressing the log
> of a ratio. As I said, that's it. Period. No amount of discussion is
> going to change the way dBs (or db) are used as a general term. Its
> been redifined by common usage. No one has the bloody right to claim
> that their definition is the "correct" one.
>
> There is simply no bloody law that says Bell labs original definitions
> mean anything at all. "I don't need to stinking Bell labs" to tell me
> what is a correct definition. Today, Bell labs are wrong.

That's an interesting approach to a scientific/engineering subject.

But you should save all that vehemence for when you will really need it:
.... to convince an irate customer and the Trading Standards authorities
that your product is fine because it meets your definition of electrical
units, even though it obviously doesn't meet theirs.


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