What's with 4 ohm speakers?

It's easier to design a speakers with a steady low impedance than a steady high impedance, something that some amps seem to have an easier time with. Historically there have been more speakers that can produce wave form fidelity with a lower impedance than a higher impedance. IME, I typically prefer systems that have speakers with a low impedance rather than a high impedance, YMMV.

It's easier to design a speakers with a steady low impedance than a steady high impedance, something that some amps seem to have an easier time with. Historically there have been more speakers that can produce wave form fidelity with a lower impedance than a higher impedance.

This is entirely false, plain and simple.

To answer the OP question, I suspect the reason is that many designers don't know how amplifiers work, so we often see crazy loads that are 'hard' to drive.

Folks, there is a reason such speakers are considered hard to drive- the amp has to work harder to do the job. You can always see it in the specs of any amplifier- the harder you make it work, the more distortion it makes. Unfortunately the distortion we are talking about is the kind that makes a system harsher and brighter- the odd ordered harmonics, to which the human ear/brain system is very sensitive.

Part of the problem is that speaker designers often confuse Sensitivity with Efficiency. I can point to examples if anyone is interested. You don't get something for nothing in this world. In electronics, this idea is known as the Law of Energy Conservation, or Kirchoff's Law.

But many speaker designers don't understand this. They think that if they put two drivers in parallel, that the speaker gets easier to drive (sensitivity increases). It does not! It gets *harder* to drive, and the amount of power to make it play a certain sound pressure does not change at all!

If lowering the impedance was actually helpful, why not 1 ohm instead of four? Then the sensitivity would be increased by 9 db! Nearly a 10:1 improvement... but of course that would violate Kirchoff's Law. If you are able to violate Kirchoff's Law, FWIW, you will have created a Free Energy Device which, as far as we know, does not exist.

Yet many speaker designers persist in trying to do exactly that, and many audiophiles that don't understand how this works (its really just math when you boil it down, FWIW) follow along in the fantasy.

So here is the bottom line: higher impedance speakers cause amps to make less distortion (smoother, more detailed). Increasing the Sensitivity of a speaker by decreasing its impedance does not affect Efficiency, but it does make the speaker harder to drive (amp will sound harsher, less detailed).

Put another way: If Sound *quality* is your goal, your amplifier investment dollar will be best served by a speaker of higher impedance, all other things being equal. If sound **pressure** is your goal and you have a transistor amp up to the task, then there is an argument for lower impedances.

Using solid state amplifiers without output transformers a 4 Ohm nominal impedance allows peaks 3dB louder than 8 Ohm speakers with the same cabinet size and low frequency cut-off which ultimately limit efficiency per Hoffman's Iron Law. This is generally a better engineering choice than doubling cabinet size (thus halving the spousal acceptance factor ) or choosing a low frequency cut-off 1/3 octave higher.

People don't go too over-board with lower impedance in the home market because of

1. How the FTC requires manufacturers to rate stereo and mono home amplifiers - they must be "pre conditioned" at 1/3 of rated output power and the power dissipated into low impedance loads would make the numbers look bad so the capability to run 2 Ohm loads isn't usually advertised and consumers would be leery of buying such speakers to go with their "4 and 8 Ohm compatible" electronics.

2. Some amplifiers are unstable (they start to oscillate) driving low impedances; and starting with a 2Ohm nominal impedance minimums of 1 Ohm aren't unreasonable.

3. Some audiophile amplifiers have silly high output impedances which interact with the speaker's varying impedance to change the frequency response and this is exacerbated with low load impedances. Output Transformer Less Tube amps are especially bad although single ended triodes without global feedback can also have problems.

For instance an Atmasphere M-60 Mk.II.2 has a 4.1 Ohm output impedance.

Driving a 3-way speaker with impedance varying from 16 to 64 Ohms this would cause a 1.4dB output difference between the minimum and maximum impedances.

With 4 to 16 Ohm impedance the difference would be 4dB. This is not atypical for a 3-way - the reactive components for a Zobel network to counter the bass driver's resonant peak would be too big and expensive so the best you can do is bring it down with a resistor in parallel.

At 2 to 8 Ohms it'd be 6dB.

Apart from this edge case the effects on distortion aren't interesting compared to what the speaker is adding to the sound.

If you do want to run such an amplifier you'll do well going out of your way to buy speakers with high (16 Ohm nominal) and intentionally flat impedance.

I wrote:
Apart from this edge case the effects on distortion aren't interesting compared to what the speaker is adding to the sound.

That's not quite right - my power amplifier biases run towards push-pull solid state class AB for practical reasons although I like building with tubes where I can actually see how a negative charge on the control grid surrounding the cathode limits electron flow to the plate on the outside.

There are output stage device, topology, and biasing combinations which won't play nice with low load impedance.

Point:

Just because a loudspeaker says 4 ohms on it's back panel does not mean it's actually 4 ohms..Perhaps it's 5 or 6 ohms or 2.5 ohms?? Or an 8 ohms loudspeaker is actually 6 ohms simply stating that 4 ohms is hard to drive is not the answer as the answer is all over the board. Speaker builders don't want to post 5 ohms in there stats they use 4 or 8 ohms.

Sony's new reference speaker is considered very hard to drive and it's rated at 4 ohms but my bet is that it's 3 ohms or less..again it depends on the Frequency as the ohm loads change by the frequency. 16 ohms loudspeaker? good luck... perhaps Avante-garde acoustic: several of there models are rated 16 ohms but they have there own problems such as sounding quacky and having poor bass... no simple answer here.