- February 2004 -
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Speaker Impedance, Your Amplifier And You.
In the audio world, simple spec numbers rarely adequately describe real-world performance. When a speaker is specified as being "8-Ohms," that is really an oversimplification of a complex issue. A speaker's impedance is different at different frequencies!
Have you ever looked at speaker specs and wondered, "How many Ohms are in this speaker?" Or, have you ever thought, "My receiver says 'use with 8-Ohm speakers only,' so is it safe to use with my speakers?" Ah, the mystery of The Ohm. Let me try to simplify what is one of the most complex issues in speaker and amplifier techno-speak: impedance.
First, A Simplified Definition.
Impedance is the electrical characteristic of a speaker that restricts ("impedes") the flow of power from your receiver or amplifier. Impedance is the combination of the resistance of a speaker plus its reactance. (Reactance describes the electrical effect of the inductors and capacitors typically found in a speaker's crossover network. The drive elements themselves also have a "reactive" element.) The impedance value of a loudspeaker is expressed in Ohms. So much for "simple." Impedance attempts to quantify how difficult a speaker is to "drive," and thus describe its compatibility with various amplifiers.
Unfortunately, the impedance issue is usually so mishandled by manufacturers and misunderstood by consumers that for most folks, it fails to be a meaningful specification. (We'll try to change that here.)
Water, Water Everywhere
A useful analogy for the understanding of impedance is that of a water pipe and pump. The pipe diameter represents the impedance of your loudspeaker; the water flowing through it is power and the water pump is the amplifier (or receiver) itself. The pump pumps water through the pipe. A large diameter pipe allows a large amount of water to flow. This is a low impedance situation: the large pipe does not impede the large flow of water. A small diameter pipe allows less water to flow. This is a high impedance situation: the small pipe impedes the flow of water, keeping it small. Low impedance, large flow of power; high impedance, smaller flow of power. I'm trying to keep it simple, folks. Stick with me. Ultimately, this will help you better choose your electronics for optimum performance.
Now that we can think of impedance in terms of water flowing through a pipe, it's helpful to think of amplifier power (voltage and current) as water pressure and water flow respectively. Voltage (pressure) and current (flow) together create power. As an aside, "current" is measured in amperes and "voltage" is measured in volts. If 5 amperes of current flow with 40 volts of pressure the result would be 200 watts of power.
So, back to "simple." We have an impedance (the pipe) in which a given amount of power (water) flows, and the receiver (the pump) producing the voltage (pressure) and current (flow). As the pipe gets larger and the pressure stays the same, the flow drops and so you get less water. To keep the water the same, the pump must provide more flow. Thus, lower impedance flows must have higher power pumps (amplifiers). If the pipe is smaller, impedance to the flow is higher. Pressure can build up and flow becomes more difficult. Thus, lower impedance demands higher power "pumps"; higher power receivers and amplifiers.
And now, kind reader, we will be leaving "simple" behind. Please put your seats in their upright position and stow your tray tables.
In the world of speakers and amplifiers, the speaker's impedance influences the ratio of voltage and current. In my example up there, I gave you 5 amperes and 40 volts multiplied together to get 200 watts. If we reduce the impedance of a speaker load by half, from 8-Ohms to 4-Ohms, but kept the power the same, we would find that the amount of current would multiply by the square root of 2 to 7.07 amperes and the voltage would reduce by the square root of 2 to 28.28 volts. Multiply 4 (the Ohms; smaller number, larger pipe) times 28.28 (volts) and you get the same 200 watts, but from the amplifier's point of view the job has gotten harder. Pressure has gone down because the pipe is larger, but the flow has gone up! So you see how the speaker's impedance changes the task of the amp, asking it to produce more (in this case) or less current flow.
Higher Or Lower?
Impedance restricts the flow of power from your receiver or amplifier. So it stands to reason that less impedance would be better, right? More flowing power is always better, right? RIGHT?! Ha. Truth is a low impedance load-large pipe-stresses a receiver or amp by asking it to put out more current, and that can be bad especially if your amplifier is incapable of putting out the amount of current the low impedance speaker demands.
Using our water pipe analogy, increasing the pipe diameter (lowering the impedance) increases the water flow (current) but causes the water pump (amplifier) to work harder to maintain the desired amount of water pressure (voltage). In cases where a low quality amplifier attempts to "drive" (pump) its small amount of power into an impedance that is too low (too large a pipe), it may overheat and shut down. In extreme cases, the receiver can break. Some receivers and amps are built to put out "high current" flows and can drive virtually any real-world speaker load without breaking a sweat. But many, particularly in the lower-cost range, simply cannot. Trying to get more power out of an amp that's not built for it is a sure way to wreak destruction. (Stay tuned for some practical receiver and amplifier evaluation/ shopping tips later in the program.)
So, higher impedance is better? No. Impedance that is too high restricts the flow of current, vital current that a speaker needs in order to play loudly for next weekend's house party.
For most low to mid-grade electronics, and for most average speaker owners, a speaker impedance specification in the 6-8 Ohm range (the most common kind) represents a good compromise between current and voltage flow. That is to say, most amplifiers and receivers can safely drive speakers with 6-8 Ohm specs to enjoyable levels.
Entering The World Of "Difficult"
But of course, in the audio world, simple spec numbers rarely adequately describe real-world performance. When a speaker is specified as being "8-Ohms," that is really an oversimplification of a complex issue. A speaker's impedance is different at different frequencies!
Back to the pipe: A speaker's impedance actually behaves like a water pipe that is constantly changing its diameter! For a fraction of a millisecond it's 4" in diameter, the next millisecond it's 6" in diameter, then back to 4", up to 8", and so on and on. Add to this the fact that the "water pressure and flow," the audio signal, is constantly changing its own intensity. Things get louder and softer. Frequencies change. Nothing is constant.
The blue line in the graph above shows the actual impedance of a speaker that is rated as "8-Ohms." As you can plainly see, at 75Hz the speaker's impedance is 40 Ohms. At around 100Hz the impedance drops to just over 5-Ohms. Simply averaging the plot doesn't begin to describe the impedance of the speaker! There is one last thing technical thing you should know, as we pull right into the station at "Difficult." The impedance load goes down (the pipe gets wider) when you run two speakers on the same amplifier channel. If you connect one pair of 8-Ohm speakers to the "A" outputs of your amp, another pair on the "B" outputs and run them at the same time (A+B), the receiver will "see" a 4-Ohm load. The red line in the graph shows the combined impedance of two of the same "8-Ohm" speakers hooked up in parallel. At around 100 Hz the impedance drops to about 3-Ohms and is 4-Ohms or less from just about 1kHz to 20kHz. If you're gonna do this, make sure your amp or receiver can drive a 4-Ohm load before even attempting such a hookup. For the three or four of you who don't remember High School math, the formula for calculating the total impedance of parallel loads is:
Net Impedance = Product ÷ Sum
Still here? Now that your head is filled with techno-babble, you're asking yourself, "How do I practically apply all this incredible information?" So here's some good ole Polk Audio real-world advice:
Questions? We've Got Answers.
If you are ever in doubt about amp/speaker compatibility issues, Polk Audio has a staff of knowledgeable Customer Service Representatives who can help you. Call us with your questions Monday through Friday during normal East Coast business hours, 800-377-7655, or email us anytime at firstname.lastname@example.org .