Re: [Phys-l] [tap-l] more on sound

["Marc \"Zeke\" Kossover" <zeke_kossover@yahoo.com>]

Pretty much. On my amplifier, the dB scale is

output (in dBs) = 10 log [(actual power out)/(maximum
possible power out)]

So it reads 0 when I'm driving the 100 W amp at 100 w.

dB = 10 log (100 W/100 W) = 10 log (1) = 10 (0) = 0

On the other hand, at 10 W output it reads

dB = 10 log (10 W/ 100 w) = 10 log (0.1) = 10 (-1) =
-10

At no output, the dial reads negative infinity since
that's what log approaches if it's input is zero. 

Lots of devices use dB to represent changes in energy
or power. A connector that says it has a 3 dB loss
says that it has lost about half of its power.

Marc "Zeke" Kossover

--- Michael Porter <listmoe@gmail.com> wrote:

> I think the web site in question answers this -- the
> dB scale is  
> comparative, and on audio equipment it is used to
> compare signal  
> strength with some reference value, not the
> threshold of hearing. A  
> negative value just means that particular signal is
> a fraction of the  
> reference value.
>
> On Apr 25, 2006, at 9:21 AM, Anthony Lapinski wrote:
>
> > Interesting web site.
> >
> > I  have a related question. Why do some stereo
> systems (with digital
> > displays) show negative decibel readings? This
> seems to contradict the
> > fact that the threshold of hearing is defined as 0
> dB.
> > tap-l@lists.ncsu.edu writes:
> > > ...
> > > http://www.phys.unsw.edu.au/~jw/dB.html
>
> ---
> Michael Porter
> Colonel By Secondary School
> Ottawa, Ontario, Canada
>
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
>


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