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Hi Mom! Please turn off The Price Is Right so that I can explain speaker impedance to you. You need to learn why those new speakers which you got for Dad and which you hooked up yourself burned up Dad's amplifier.
Resistance is how hard it is for direct current (DC) to pass through a wire. The DC voltage doesn't matter since the DC resistance will always be the same for a wire. For DC voltage, the impedance is also equal to the resistance. The same is true for direct current (DC) passing through a speaker even though the speaker has a magnet and a voice coil. The measured resistance will be constant, and the impedance will be equal to the resistance.
Things get more complicated when alternating current (AC) is used instead of direct current (DC). Another variable is the frequency of the AC. If you pass a very low frequency (perhaps 1 or 2 Hz) AC through the speaker, the impedance will still be almost exactly equal to the measured DC resistance of the speaker. If you pass progressively higher frequencies of AC through the speaker, then the impedance will rise or fall in comparison to the speaker's measured DC resistance. Why the impedance rises and falls depends on the mechanical design of the speaker. You can create a graph of the rise and fall of the impedance versus the frequency of the alternating current (AC).
For example, let's say that you have an 8 Ohm speaker. The measured DC resistance usually will be noticeably less than 8 Ohms, yet the DC resistance will always be constant. Yet when you send AC through the speaker, the impedance at various frequencies can rise above 8 Ohms or drop below 8 Ohms. In other words, the graph of the speaker's AC impedance versus the frequency will be a wavy line when measured at AC frequencies from 20 Hz to 20,000 Hz. How wavy the line is will depend on the mechanical design of the speaker.
The DC resistance of a speaker is always constant. Think of impedance as being the equivalent the DC resistance of the speaker for AC current at a specific frequency. If the AC frequency is increased or decreased, then the equivalent DC resistance of the speaker can either increase or decrease.
We now have a wavy line for the speaker's measured AC impedance versus frequency. What is the average impedance of the speaker for AC frequencies from 20 Hz to 20,000 Hz? It is the average of the wavy line. The average of the wavy line is the impedance rating for the speaker. If the average of the wavy line is 8 Ohms, then the speaker gets a stamp which says "8 Ohm" on the back of the speaker's magnet. As I mentioned, the average AC impedance of the speaker usually is a good bit higher than the measured DC resistance of the speaker. And as I mentioned, the actual AC impedance of the speaker can be higher or less than the average AC impedance, depending on the frequency of the alternating current (AC).
Mom, you burned up Dad's amplifier since you hooked up the additional pair of speakers into the B channel of his amplifier. The minimum combined impedance for the A and B channels of Dad's amplifier is 4 Ohms. Dad's original speakers have a rated impedance of 4 Ohms. When you hooked up this new set of 4 Ohm speakers to the B channel, the combined impedance of both pairs of speakers became only 2 Ohms. This combined impedance was too low, and this is how you burned up Dad's amplifier since the combined pairs of speakers drew way too much current from Dad's amplifier.
Hey Mom, don't worry. I can replace the burned up output transistors on Dad's amplifier with new transistors from either Mouser or DigiKey since nothing else in Dad's amplifier was burned up.
Mom, it seems that you now understand about impedance. Impedance is simply the resistance at a particular alternating current frequency. Cool. You seem to totally get it.
Lets look at some other stuff inside Dad's amplifier. Do you see these things which look like little cans inside Dad's amplifier? Those are capacitors. Do you see these coiled wire things? Those are chokes. Do you see these things with three legs coming out of them? Those are transistors. Transistors are like the pedals and the control column and yolk in a passenger airliner. Movements of these by the pilot get greatly amplified in order to move the huge control surfaces on the wings and the tail of the airliner. In other words, transistors amplify small signals in order to turn the signal into much greater signals. Hey Mom, how cool is that? The power assisted brakes in your car do the same thing when you press the brake pedal. The power assisted steering in your car also does the same thing. You can think of both systems in your car as being the mechanical equivalents of transistors! This kind of signal amplification stuff has been around since the 1920s.
Yes Mom, transistors simply amplify things. Let's look again at those capacitors which look like tiny cans and look at those chokes which look like small coils of wire around small magnets.
Now things get even more fun. All capacitors totally block DC current, yet will pass AC current. How much AC current flows through a capacitor depends of the capacitance of the capacitor (basically the physical size of the capacitor) and on the AC frequency! Cool, isn't it? All chokes and coils pass DC current, yet progressively block AC current at progressively higher frequencies! Again, the physical size of the choke, and whether or not the choke has an internal magnet, determines how progressively the choke blocks higher frequencies. More cool stuff, isn't it?
Now lets think about Dad's 3-way speakers. In a nutshell, capacitors and chokes are used to limit what range of frequencies go to each speaker in a 2-way or 3-way speaker system. The combination of chokes and capacitors in a speaker are usually mounted on a crossover board inside the speaker.
Dad really loves his expensive IEMs. Amazingly, miniature crossover boards are used in Dad's expensive multi-driver IEMs to send specific frequency ranges to the specific drivers. This is very similar to how crossover boards are used in Dad's speakers to control what frequency ranges are sent to each speaker in Dad's 3-way speakers. Now that is really cool, isn't it? I think that it is really amazing that many expensive IEMs truly implement resistance (R) and choke (L) and capacitor (C) or RLC crossover boards inside these tiny IEMs.
Hey Mom, do you remember a few years ago when your cat Precious decided to use one of the midrange speakers on Dad's favorite speakers as a scratching post and that Precious destroyed that midrange speaker. Do you remember that Dad replaced that speaker with a generic speaker which had the correct stamped impedance and power rating? And do you remember that this replacement speaker sounded muddy at frequencies around 600 Hz? The reason that this replacement speaker sounded muddy around this frequency which was in the crossover range in Dad's 3-way speakers is that this replacement speaker's impedance around the crossover range was significantly different than the original speaker. This resulted in a significant phase shift for the replacement speaker versus the original speaker. In other words, the replacement speaker's sound, moving out of sync due to the phase shift, was somewhat cancelling the sound from the woofer.
Mom, I need to teach you more about phase shift and how varying impedance affects phase shift. Yet at least you now have an understanding about impedance and some understanding about other stuff such as capacitors, chokes and transistors.