Re: [Phys-l] [tap-l] speakers

[brian whatcott <betwys1@sbcglobal.net>]

On 4/26/2011 9:52 AM, Brian Huang wrote:
> The impendance of the speakers has to do with matching the amplifier
> output.  If the impedance is different from that of the amp, you get
> reflections.  The power output of the speaker is reduced by the power
> reflected.  So...  sometimes  you will see an imepedance matching circuit at
> the output of an amplifier so that it is matched at 8 ohms.  If you look at
> the output of your stereo receiver, for example, it usually has a stated
> output impedance.
>
> For 100% power transmission, you want a perfect match.  Unfortunately, the
> match is frequency dependent - so, you try to get it as close as possible in
> the mid-band and build a broadband matching circuit to account for the rest.
Yes indeed.  About that impedance matching issue - although people don't 
tend to think
 of low frequency energy reflecting from a mismatch, there is no doubt 
that it does.
It is rather hard to observe however. It takes a more extreme case to 
bring it to light.
When a long distance AC transmission line system gets significantly 
mismatched at the far end,
the returned power has the effect of changing the amplitude and phase of 
the line,
so that on occasion,  measures have to be taken to prevent destructive 
1/4 wave resonances
 over the hundreds of miles of transmission path.

Or consider the source impedance of the domestic power outlet.
 You expect to be able to take 12 amps at 120 volts with a moderate 
volt drop of say 5 volts.
That indicates a source impedance of 5/12 ohms at most into a load of 
say 10 ohms .
Or your electric cooker - say 40 amps with a 7 volt drop? say 0.2 ohms 
source  loaded with 3 ohms.

Are you going to try for maximum power transfer with a matched load, in 
this case  0.2 ohms?
I hope not!

Brian W