Let’s take look at the history of low impedance speaker loads.  Just about everyone knows that home audio speakers are usually 8 ohm, while car audio speakers are most commonly 4 ohm.  Why?  Well, the most basic proposition, when it comes to power to run a speaker is that you want to get it (and use it) as efficiently as possible.  In  order to understand how to do that, knowledge of what “power” is, is helpful:  It is the expression of work done (or that can be done) by something like an engine, and amplifier, or even a horse.  In fact, perhaps the oldest and best known example of a definition of “work” or capacity to do work, is “horsepower”.  Engines, animals, and yes, amplifiers, can all be rated in horsepower.  Even something like an electrical switch can be rated for the amount of horsepower it can pass on to an electrical circuit.  Usually, though, power in an electrical circuit is defined a little differently, and the simplest definition is that power in watts (note 1) is voltage multiplied by current, or commonly (an extension of Ohm’s Law):

P = V x I, where:

P = Power in watts (work capacity)
I = Current in Amperes (basically the number of electrons flowing in the circuit.)
V = Voltage in volts (the amount of “push” on the electrons.)

What all this basically means is that one can get more power by increasing either voltage or current (or both).  In the case of home speakers, amplifiers run off of 120 volt (in the U.S.) AC supplied by the power company, so, plenty of voltage is at hand.  In fact, it is usually stepped down in the power supply, and one ends up with (for cone type speakers) a moderate and reasonably safe voltage, also at moderate current (note 2), running into an 8 ohm load.  8 ohms turns out to be a hany design target for most dynamic / cone speakers, and the moderate current requirement keeps wire size requirements (from amp to speaker) moderate as well.  (We are talking about the “average stereo” setup here, not $20,000 and higher systems.)

However, when people got the notion to put radios, and especially music systems in automobiles, they ran into a bit of a problem:  There, the “power supply” is only running at ~12-14 volts, so you run into trouble quickly — In order to get much power, you have to increase I (current).  Ohm’s Law tells us that you do this by decreasing the impedance (think “resistance”, if impedance is not a familiar concept to you) to the voltage applied:  Imagine using a long skinny garden hose, and the spigot is already wide open.  One way of getting more water by simply increasing the diameter of the hose.

This is per another extension of “Ohm’s Law:

P = (I²) x R, where:

P = Power in watts (work capacity)
I = Current in Amperes, (”I” is basically the number of electrons flowing in the circuit.)
R = Resistance (the “skinnyness of the hose”, if you want to think of it that way.)

Also handy to remember:

P = (V²) / R, where:

P = Power in watts (work capacity)
V = Voltage in volts, (”V” is essentially the amount of “push” on the electrons flowing in the circuit.)
R = Resistance (the “skinnyness” of the hose, if you want to think of it that way.)

AND (the original expression of Ohm’s Law):

I = V / R, where:

I = Current in Amperes (”I” is basically the number of electrons flowing in the circuit.)
V = Voltage in volts (”V” is essentially the amount of “push” on the electrons flowing in the circuit.)
R = Resistance (the “skinnyness of the hose”, if you want to think of it that way.)
Back in the “old days”, the first thing people did for higher power car audio was to reduce the speaker resistance to 4 ohms.  Voila:  Power has been doubled!  However, this was not then taken further to get even more power, because most people did not want to put drastically heavier wire in their cars, 4 ohms for car audio had become sort of a standard, etc. (Note 3)  For the most part, customers were stuck on 4 ohms, and technology (switching power supplies) came along that made it more practical to design inexpensive power supplies in car audio amps that stepped up the voltage available.  The breed of high power car audio amplifiers was born…

However, car audio power requirements kept going up and up, as with most other things, simply increasing the voltage of the car audio amp’s power supply ran into some practical limits.  At the same time, interesting things were going on over in “low” (or at least moderate) power categories of SPL competitions.  (See our discussion here.)  Essentially, there is a sort of optimum balance between high voltage and high current capability, if one is trying to maximize car audio amplifier power output.  With present technology, all factors considered, that balance ends up resulting in voltage and current capabilities that essentially define the optimal load.  At present, at very high power levels, that load usually works out to be 1 to 2 ohms speaker (total) impedance (per amplifier).  (See test results and further discussion, here.)

Notes:

1)  For watts to horsepower conversion, and lots of information, simply Google “watts to horsepower conversion”.

2)  By “reasonable current”, we mean amounts of current that can be easily carried by 16 or 18 ga. copper “zip cord” / speaker wire, with fairly low losses.  We are talking about “average” home stereos here — keep in mind that a “good” by most standards(Yamaha / Denon / Pioneer / Rotel / HK / etc.) surround sound receiver might be putting out 80 - 100 watts per channel, which comes out to around 3.5 amps per wire (maybe 5 amps max. on peaks) with an 8 ohm load.  Long runs, impedance variations, audiophile concerns, high power, etc., can push that requirement up, but relatively few home systems really NEED to go bigger than 12 ga. wire.  See also our discussion on ’Wire and Connectors (Tips)‘. 

3)  For many years, about the only people who seriously bucked the 4 ohm speaker / amplifier car audio standard was Bose, who had developed < 1 ohm speakers for their 901 home speakers (which used 9 of the <1 ohm speakers in series to make an 8 ohm load.)  Since Bose already had low ohm speaker technology (at least in a 5″ speaker), it made sense for them to push such, for high $$ OEM audio systems in upscale GM vehicles, when they got that business.  This never got much further, though:  How many 0.9 ohm aftermarket component speakers do you find, for example?