Can Someone Explain Impedance?

[MBM135]

What is all the hubbub over impedance ratings on speakers? What does this mean in terms of efficiency etc?

For example I have older KG4s rated I believe at 4 ohms. I notice amp manufactures give their wattage ratings in comparison to ohms--for example "100W into 8 ohms." What if mine are 4 ohms?

Thanks in advance the explanation!

MM

[WMcD]

The short story is that impedance is the "load" presented to the amplifier by the speaker. It is technically voltage divided by current. This is the definition of an ohm, and impedance is measured in ohms.

However, speakers don't have a constant impedance. You'll see graphs of how it varies with frequency.

However, suppose your speaker presents a 4 ohm load at some frequency. If you crank up the output of the amplifier so it delivers 4 volts, the current is going to be 1 amperes. At 8 volts, it is two amperes.

Now, if the amplifier is a perfect "voltage source", it would be able to deliver the required current and voltage under all conditions. But amplifiers are not really perfect. They may run out of voltage supply when driven hard. This is clipping.

But they are also limited in the amount of current they can provide.

The latter is why you'll see ratings of amps like 100 watts max at 8 ohms, and 150 watts max at 4 ohms. Note, you'd expect the power to double to 200 watts at 4 ohms.

The result is that amps said to not "like" low impedances. Indeed, putting speakers in parallel reduces the impedance and you can run into problems.

On the other hand, unless you are driving the amp to high output, a 4 ohm load should not be a big problem. It is diffiuclt to generalize further.

Others on the bbs might add their comments.

Gil

[Ray Garrison]

Chirping up from the peanut gallery, here's a much simplicated analogy.

Your house wiring is underground water you can tap into with a well. You, furiously pumping up and down on the handle of an old hand operated well pump, are the amplifier. A hose connecting the pump to a water turbine powered siren is your speaker cable. The siren, of course, is your speaker. At the end of the hose, where the water shoots out and hits the blades of the turbine, there's this little constricting washer. The diameter of the opening in the washer is analogous to the impedense of your speakers.

Suppose there's a big opening on the end of the hose. Very little resistance to water flowing. The harder you pump, the more water flows through the hose. However, with nothing to work against, you'll find it very hard to keep pumping indefinately. You pump as fast as you can, but you never build up much pressure in the hose. Also, as there's not much pressure in the hose, the water doesn't shoot out of the end of the hose with much velocity, thus imparting little energy to the turbine. You'd have to be both very strong and in great shape to maintain sufficient pressure to get much work done. This is the state of affairs with a speaker with LOW impedence (ie, low resistance to current flow.)

Now, put a constricting washer on the end of the hose. You have resistance to work against, so it's easier to build up some pressure in the hose. You don't have to pump as hard. Likewise, the constricted water jet will have a higher velocity exiting the hose, and transfer power to the turbine more efficiently. Of course, due to the constriction, there's not gonna be as much water hitting the turbine blades, so even though the water that does shoot out is moving faster, the lower quantity might mean less work is actually being done. Will sure be easier on the guy manning the pump, though. This is analogous to a HIGH impedence speaker, one with greater resistance to current flow. Pressure in the hose is analogous to voltage, and the amount of water moving through to current.

An amplifier rated to operate safely into some resistance, say 8 ohms, will be working harder and generating more heat into lower resistance. Depending upon how low the resistance, and how well built the amplifier, it may not be able to maintain voltage as impedence drops, and it's ability to push power into the load will decrease as the load decreases.

Boy has it been a long day.

[Al Klappenberger]

MM,

Ok.. Now you have read the simplified analogies to get the idea of what impedance

is all about, here's the "meat"!

There are two main types of opposition to the flow of electric current.

These are DC opposition as provided by a "resistor", and AC opposition as

provided by a capacitor or inductor. Impedance is simply "total opposition"

which combines the two. If you connect a resistor in series with an inductor

you have "resistance" in series with "reactance", which is "impedance". They

add together by the pythagorean theorem. R = DC resistance, X = reactance

and Z = impedance. So: Z squared = R squared + X squared. All of these are

measured in Ohms. This is for a series connection. If you put a capacitor

and resistor in parallel, the formula a different, but you get the idea (I

hope!).

By the way, Xc is the type of reactance created by a capacitor and Xl is the

type created by and inductor. The formulas are the same for both.

Al K

[Malcolm]

The impedance you refer to is the nominal impedance of the speaker. It is not the full story by a long shot. The actual impedance of a speaker varies considerably above and below this rating depending on the frequency, reaching a maximum at the resonant frequency.

Impedance does not have anything to do with with speakers. A speaker with 8 or 16 ohms nominal impedance can be just as efficient as one with 4 ohms nominal impedance. For example, they could both produce 96dB @ 1 watt @ 1 meter.

Impedance does affect the maximum amount of power that a given amplifier can produce into the load. So, if you had two speakers of identical efficiency, one 4 ohms and one 8 ohms. You might be able to play the 4 ohm one louder with that amplifier, if the speaker could handle it.

Why should you care about all this? You shouldn't. Just pick the speakers you like the sound of, then make sure your amplifier can driver them as loud as you can stand without clipping.

Others have more or less covered the technical explanation.

[Ray Garrison]

Malcolm,

Don't really follow what you meant by "...Impedance does not have anything to do with with speakers..."

Yes, the impedence of a speaker varies with frequency, sometimes drastically. In the case of a two way system with a ported woofer and a ribbon tweeter, you could get an impedence maximum of, say, 40 ohms or higher at the local maxmima in the bass, while the impedence in the high treble could be as low as an ohm, or even less. This kind of varying impedence load drives some amplifiers nuts. If you hook up something like a Cary 300B, with its high output impedence (4 ohms or thereabouts as I recollect) to that speaker, even if it can drive it (speaker would have an awful damn high sensitivity), the frequency response is going to be all over the map. Speaker impedence is extrEMEly important factor with some amps.

Your other point, about two different speakers, one 16 ohm and one 4 ohm, having the same rating (say 96dB/1 watt/1 meter), my point would be that to push 1 watt into a "16 ohm" speaker (let's assume we're talking about a 1 kHz sine wave so we don't have to worry about the impedence varying with frequency for the moment...) anyway, to push 1 watt into that speaker, an amp is going to output 4 volts across the 16 ohm load, and needs to provide 1/4 amp. With the 4 ohm speaker, we're outputing 2 volts across the 4 ohm load, and providing 1/2 amp current. At 1 watt, this is no big deal, but this 2 to 1 ratio will continue to be true. At, say, 20 watts, we'd be holding about 18 volts across the 16 ohm load, pushing about an amp. Into the 4 ohm load, we're looking at about 9 volts, and almost 2 amps. Many amps, particulary tube amps, don't have massive amounts of current reserves, and at some point we're going to run out of current. Going back to my 1 ohm ribbon tweeter, if we try to push 50 watts into THAT load, we're talking about 7 volts and 7 amps of current, whereas with the 16 ohm load 'twould be about 28 volts and a bit less than 2 amps. A lot of tube amps would find it much easier to output 28 volts at 2 amps than they would 7 volts at 7 amps.

Which means, of course, that your point that "Just pick the speakers you like the sound of, then make sure your amplifier can driver them as loud as you can stand without clipping." is exacly correct.

[Malcolm]

Oops! Can't edit worth beans. Proof read even worse. I meant that nominal impedance doesn't have anything to do with the EFFICIENCY of a speaker.

[MBM135]

Thanks guys--that helps!

Mike