Re: 3 dB bandwidth

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

Kevin Aylward
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http://www.anasoft.co.uk
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