PAUL MCGOWAN ...
For those with a clear understanding, this is a good post to skip. For those still unclear about what it means when we speak of high and low impedances, this might be right up your alley.
The simplest way to think about impedance is to consider it an opposer. The higher the impedance the greater the opposition to alternating current. A wire has almost no impedance and thus offers little to no obstruction to the passage of a musical AC signal. On the opposite side of the scale, air has very high impedance (resistance) to passing electrical signals. Basically, not much electrical energy passes through air (its resistivity is approximately 2×1016Ω⋅m).
Let’s now take a simple and practical example of what this means. A speaker cable with big heavy gauge wire has nearly no impedance and thus offers little to no resistance (opposition) to delivering the output power of your amplifier to your speaker. A very thin wire might have relatively high impedance and thus offers much more opposition (resistance) to the passage of power from amp to speaker.
If our goal is to deliver power without hindrance, we want as low an impedance as possible.
Now let’s take a look at another aspect of understanding impedance in the practical world, using musical signal power to make sound.
Imagine for a moment a speaker driver like a woofer. It has a magnetic motor used to move the woofer cone back and forth to generate sound. That motor needs fuel to work. Its fuel is electric current. The more fuel we feed it the harder it works and the louder the sound.
Remembering that impedance restricts (impedes, resists, opposes) the flow of electric fuel, it’s perhaps easy to see how the higher the impedance in the path of powering the speaker equates to less output.
Thus, if my power amplifier has an 8Ω output impedance feeding a 4Ω speaker driver, the higher output impedance of the amplifier (a higher Ohm number means higher impedance) will restrict power flow and the resulting musical output will be cut in half.
If however, my power amplifier has a wicked low output impedance (well below an Ohm) then power will flow without restriction and my speaker will enjoy all the power fed to it without opposition.
If we do not want to lose power or signal level, we always want to have a low output impedance feeding a high input impedance.
Which is why preamps have low output impedance while power amplifiers have high input impedance.
Hope that helps clear the mud.