Output impedance is the resistance to an AC or DC signal.

A modern audio amplifier has negative feedback which reduces its output impedance to about 0.04 ohms or less so that it damps resonances of a speaker very well.
When a speaker resonates it becomes a generator. The very low output impedance of an amplifier shorts the generation which stops the resonance.

Try it. Tap the cone of a woofer speaker when it is not connected to anything. It will resonate with a boooom sound.
Then short its two wires and tap it again. It will make a short duration thud or pop sound without resonating.

With such a low output impedance then the output voltage is very well regulated and therefore it barely changes when the amplifier is loaded or is without a load.
Then the speaker does exactly what the amplifier is doing.

__________________
Uncle $crooge

 

hi rags,
You have 3 or 4 posts running with specific questions, is this homework/classwork...

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

An Ideal Voltage Source has zero output impedance [approximated by a wall outlet or a car battery], and its dual, an Ideal Current Source [approximated by a flourescent lamp ballast], has infinite output impedance.

Amplifier inputs can be voltage controlled or current controlled and their outputs can simulate voltage or current sources.

E.g., a bipolar junction transistor is a current controlled current source.