Impedance is a topic which can be very confusing until you get it. I'll do my best to explain it in understandable terms.
Impedance is composed of two parts, Resistance and Reactance. Resistance limits the flow of
direct current, and
impedance limits the flow of
alternating current.
Resistance is the easier of the two to understand, because a
resistor does not change value depending on what
voltage is placed across it and what
current flows through it. In a purely resistive device, these two quantities are related by the equation V = I*R, where V is the
voltage, I is the
current, and R is the resistance.
Reactance is the analog of resistance for
alternating current. However, to complicate things reactance is also composed of two parts, Capacitive reactance and Inductive reactance. Inductive reactance is taken as a positive value of reactance, and capacitive reactance as a negative value. Indeed, you can add the values of reactance together and obtain what looks like an inductor or a
capacitor, even if the
network is composed of both types of components. You can even cancel out the reactance entirely by matching the inductive and capacitive reactances.
It is important to note that the capacitive reactance is not the same thing as the
capacitance, and the same is true for inductive reactance and
inductance. They are, however, related by the equations Xc = 1/(w*C) and XL = w*L, where Xc and XL are the capacitive and inductive reactances, w is the frequency of the ac signal (in radians per second), and C and L are the
capacitance and
inductance, respectively.
In order to fully understand
impedance, we need to look at combinations of
circuit elements (resistors, capacitors, inductors,
etc). There are two basic ways to connect elements - in series, and in parallel. When items are connected in series, they're connected end-to-end. E.g., the end of a
resistor is connected to the end of the
capacitor like two strings of christmas lights. When items are in parallel, they are stacked, with all of the terminals on one side connected together, and the other side connected as well.
Impedance is the combination of
inductance and reactance. So, if you had an inductor and a
resistor in series, then you would simply add their individual impedances - for example, (R + j*w*L) where R is the resistance, j is the imaginary
unit, w is the frequency of the ac signal in radians, and L is the
inductance of the inductor. When items are in parallel, a special formula is used. You take each individual
impedance, and flip it (raise it to the -1
power). Then add them all together, and flip the final sum again (raise to the -1 again).
The final
impedance can also be expressed in polar form, where the magnitude is the square root of the square of the resistance added to the square of the reactance - sqrt(R^2 + (w*L)^2), and the angle the arctan of the reactance over the resistance - arctan(w*L/R).
Now, how does this relate to the real world of audio equipment? For this, we need to understand the idea of a Thevenin equivalent
circuit. Essentially, this is the idea that a real-world source of
voltage (technically, a source of
current as well) can be modeled by an ideal
voltage source inline with an
impedance (although this is often just modeled by a
resistor). Thus, a low-impedance
microphone can be modeled as an ac
voltage source inline with a 150-ohm
resistor (give or take).
The input to your sound
desk can also be modeled as an
impedance - once again, often as a
resistor. In the case of most desks, this is somewhere between 1k? and 10k?, although it can be higher and occasionally lower. The last piece of the puzzle here is why these two impedances are wildly different in value. This is because we don't want to draw much
current from the
microphone, but just have it produce a
voltage waveform. Since maximum
power is transferred when the two impedances are equal, we want to have one much higher than the other.
Speakers are similar but work a
bit differently. Earlier, we discussed how
impedance varies based on frequency. In a
speaker (we'll only look at
cone drivers) there is a large coil of
wire which is essentially a big inductor. Since it is made out of real-world
wire, it also has resistance - thus making its
impedance of the form (R + j*w*L). There's a
bit more to it as well (stray
capacitance,
etc), but this basically holds true. This explains why the
impedance of a
speaker can vary based on the frequency
fed to it.
In the words of Bill Sapsis, ‘Zat help?
PS - please forgive any spelling errors - it's late, and I probably misspelled at least one word or wrote a few very awkward sentences.