What is impedance?


I’m currently trying to get the hang of electronics and sound production and I come acrosse “impedance issues” from time to time… For instance, “a preamplifier might load the microphone with low impedance, forcing the microphone to work harder and so change its tone quality” or a contact mike having a low amount of bass because its impedance doesn’t suit the amplifier…

Can anyone help me?

In: Physics

It essentially means “resistance,” in the electrical engineering sense ot the word. It has to do with how electricity flows through the circuit. They’re designed to have a certain amount of resistance to make the circuits work correctly. I don’t understand the text you quoted entirely but it seems like the preamplifier might have a different design resistance than the other parts and therefore mess with the circuitry?

To impede something is to get in its way or delay it.

Sound and electrical current both resonate or travel in waves. i.e. they have a frequency. When you try to push sound or current through at a similar frequency, it’s easy because there’s nothing pushing back. If you try to push something through at a different frequency, you get impedence – the sound or electricity is trying to move one way, you’re trying to push something the other way.

Imagine pushing a child on a swing – it’s a lot easier if you push at the same frequency as they are swinging. If you try to push forward as they are swinging backwards, a lot less of your energy ends up moving them forwards.

This is an electrical engineering term. Impedance is the amount the circuit will fight the current flow running through it.

There are basically 3 main components we use when designing circuits: resistors, inductors, and capacitors. Any combination of these in a circuit cause a certain total impedance, or amount of “friction” the current flow will see.

In your case the microphone has some small electric current carrying the signal of your voice, so impedance will cause higher “friction” of that voice signal.

Electricity for audio signals is a tradeoff, you want the desired sound with good clarity while minimizing noise. High impedance is one solution to this, where higher voltages and larger currents are used. This tends to overpower electrical noises, but it requires more power. That’s particularly a problem with microphones. Until very recently, batteries sufficient to generate good signals for adequate times were heavy. That’s why you often see tiny microphones tethered to a battery pack clipped to the person’s belt.

Low impedance is a different solution, very small signals, very low resistances, and not batteries. The energy in the sound waves makes the electrical signals, what’s called a sound powered solution. This takes excellent connections, frequent cleaning, and every wire much be shielded to protect from induced currents.

Both solutions can work well, but they don’t mix and match. Or, more realistically, they only mix and match if you have even more expensive gear that can operate in both modes.

Impedance is in general, the electrical resistance, but the why can vary a bit.

For what you want to know, it’s two things, with microphones you need to understand that they typically put out very little power, therefore you want a high impedance pre-amplifier (one that draws very little current from the microphone). This is because with things like a microphone it may physically stop working under high load. Typically that’s what a pre-amplifier is for, they have a high impedance input and low impedance output, they don’t draw significant power from your microphone but can provide moderate power to drive your amp and normal equipment (so you can plug lots of equipment into your pre-amp without reducing the signal out of your microphone).

With speakers it’s a bit different, amplifiers are designed with an output impedance that needs to be matched to the speakers. Basically they have a certain max voltage, and a max current, and thus to get the rated power the speaker needs to have the right impedance such that at the max voltage it draws the max current. Failure to do that means the rated power doesn’t go into the speaker. With some amplifier designs it may also mean the amplifier is damaged as that missing power actually goes into the amplifier and may blow the amplifier.

Now as what impedance is, it’s how we measure the resistance to current flow when an AC signal is flowing. Many things have frequency dependent resistance, so impedance is the resistance at a specific frequency. For speakers, typically the nominal impedance (the number on the sticker) is the lowest impedance over the entire audio range, but it varies. For low frequency this is important because the speaker may drop below the nominal impedance below the “audio range”, and audio range may be arbitrarily defined.