We need to discuss this thing called "IMPEDANCE" (pronounced imm-PEA-dents). Why? Because besides their mechanical dimensions, the speakers connected to the radio in your Freedom Machine are also defined in terms their impedance rating in “ohms”.
First, we need to take a step back and define electrical RESISTANCE. Warning! You are entering the pocket protector zone. Electrical resistance is defined as an 'opposition' to the flow of electricity. This opposition is a reduction in the electrical 'pressure' (we call it voltage) as electrical current flows through a given resistance, or resistor. WHAT? Consider this simplified example to illustrate the concept:
When you connect a light bulb designed to operate on a 12 volt battery, to a 12 volt battery, all is well: electricity flows thru the bulb's filament which obviously creates light (and heat) and the bulb lives a full life. But if the light bulb is designed to operate on 6 volts and you connect a 12 volt battery to it, the light bulb will not be long for this world. The solution is to connect a resistor of the correct ohm-rating between one battery terminal and one terminal on the bulb (leaving the other connection as-is). This resistor will consume 6 of the 12 volts available from the battery, leaving the remaining 6 volts to correctly energize the 6 volt light bulb. LESSON: the resistor reduces the applied battery voltage in proportion to its resistance value, which is measured in OHMS. That is an example of how resistance can manipulate direct-current voltage (often called "volts D.C.").
Now if you have an alternating current, or A.C., the unavoidable influences of inductance and capacitance become a factor (I think we're gonna need a bigger pocket protector over here!). Exploring those two terms is outside the scope of this article; Just know that inductance and capacitance take resistance to a new level, and so we now introduce and use the term IMPEDANCE. Think of impedance as a measure of resistance to alternating-current voltages (often called "volts A.C."). An impedance also uses the word "OHM" to describe its rated value. ...And also realize that A.C. isn't just the 115 volts available in your home: music signals are also considered A.C. signals, but operate at much higher frequencies (and considerably lower voltages, LOL).
The factory AM-only radio in my '64 Tempest drives one speaker, which failed after only 50 years. I discovered when I removed the factory-original speaker that it had an impedance rating of 10 ohms. Searching for a new replacement, I wondered whether I could install ANY speaker that fits. Referring back to the light bulb example above, it turns out that the answer is NO (if I wanted the radio to live as long as possible).
However, finding a 10 ohm speaker for this girl can be tough nowadays. But given the somewhat loose design tolerances in these radios, the "20% rule" applies. Huh? Since 20% of 10 = 2, the radio will be OK with a speaker rated at 10 ohms plus or minus 2 ohms. This means I can safely use an 8 ohm speaker in its place. [As an aside, speakers commonly-available these days have impedance ratings of 8, 4 or 2 ohms.] But this ALSO means that deviating from the original speaker's impedance by more than 20% could be asking for trouble.
FAIR WARNING, dear reader: A speaker with a 4 ohm impedance is too low for a radio that was designed to drive a 10 ohm speaker load. The reason is because the audio amplifier with a 4 ohm load has to work 2 ½ times harder to get the same results than if it had the 10 ohm load. This added current flowing thru a lower-impedance load generates more HEAT that will shorten the remaining useful life of the amplifier's power transistor.
Hopefully this article helps clarify the concepts of resistance, impedance and ohms. By the way, the motivated reader can visit the Kahn Academy for more on Ohm's Law and resistance.