Impedance, Ohms and Watts: A helpful Guide

[Mr. Analog]

Ok, so we have another thread going on that bandies about terms like impedance, resistance, Ohms and Watts, but what does it all mean?



First things first, "Impedance" is a general term that refers to many things, however the most common usage (and the one we care about) refers to:

Electrical impedance or simply impedance is a measure of opposition to a sinusoidal electric current.

or in other words...

Impedance: A measure of the impediment to the flow of alternating current, measured in ohms at a given frequency. Larger numbers mean higher resistance to current flow.

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Impedance (resistance to current) is measured in Ohms, so something like rubber would have a higher Ohm rating than copper because it conducts electricity poorly. So how does this apply to speakers?

When the electricity passes into the speaker, some of it is 'resisted.' The ohms rating of the speaker is how much is resisted, and an indication of how much energy it takes to drive it - the higher the ohms rating, the more difficult it is to drive.

So, okay, let's say you have two speakers, one rated at 8 Ohms and one rated at 16. Looking back at the last quote, which speaker will resist more? If you said the 16 Ohm speaker you'd be right!



Now, knowing the speaker resistance is important because it will eventually tell you how many watts are being sent to each speaker and ultimatly how well they perform.



Ok, so we know that the more Ohms we have the less output we have from the amplifier. On the same coin we know that the less Ohms the more output we have from the amplifier. All recievers have ratings that tell us how many watts and what the impedance (resistance in Ohms) is per channel. How can we figure out how many watts (driving power) is being sent to each speaker? Easy!



Lets say we have a reciever rated at 100 Watts @ 4 Ohms (and it only has one channel). Let's hook up two 8 Ohm speakers to it (in series). The total resistance is easy to figure out, you just have to add the numbers, so the total resistance is 16 Ohms. Knowing this we can plug out numbers into a simple little calculation to figure out how many watts each speaker is recieving:

Code Select Expand

Amplifier Output = Amplifier Watts x (Amplifier Rated at Ohms / Speaker Chain Ohms)

Amplifier Output = 100 watts x (4 ohms / 16 ohms)

Amplifier Output = 100 watts x 1/4

Amplifier Output = 25 watts


Knowing this, we can say that an amplifier designed to put 100 watts into 8 Ohms will put 200 watts into 4 Ohms. So if we had two 8 Ohm speakers wired in parallel (4 Ohms total) it would cause that same amplifier to produce 200 watts

Code Select Expand

200 watts = 100 watts x (8 ohms / 4 ohms)


Most amplifiers are rated between 4 and 16 Ohms anything outside those numbers can cause problems. You must be sure that your speakers are capable of handling the wattage that they're receiving. As always, observe your equipment manuals and observe manufacturer's safety reccomendations.



Of ocurse, all this is more eloquently put (and of course I am paraphraising most of what I read from) here. a very concise, accurate definition of what impedance means to the home audio enthusiast.



But wait! There's more!



You might wonder what Cable Impedance is and whether it's important when assembling a sound system. In a nutshell:

The basic idea is that a conductor at RF frequencies no longer behaves like a regular old wire. As the length of the conductor (wire) approaches about 1/10 the wavelength of the signal it is carrying - good ol' fashioned circuit analysis rules don't apply anymore. This is the point where things like cable impedance and transmission line theory enter the picture.



The key tenet of all transmission line theory is that the source impedance must be equal to the load impedance in order to achieve maximum power transfer and minimum signal reflection at the destination. In real world case this generally means that the source impedance is the same as cable impedance and the value of the receiver in another end of the cable has also the same impedance.

Huh? For us, all this means is make sure your cable impedance is rated the same as your source and destination for maximum efficiancy. Incedentally if you are worried about your cable length causing delay; don't.

Most wires will have a speed of travel for AC current of 60 to 70 percent of the speed of light, or about 195 million meters per second. An audio frequency of 20,000 Hz has a wavelength of 9,750 meters, so a cable would have to be four or five *kilometers* long before it even began to have an effect on an audio frequency. That's why the characteristic impedance of audio interconnect cables is not something most of us have anything to worry about.

That's not to say that you won't get signal interferance, but that's why they sell properly shielded audio cable.



A Brief Note About Acoustic Impedance

The acoustic impedance Z (or sound impedance) is the ratio of sound pressure p to particle velocity v in a medium or acoustic component.

Acoustic impedance is related to the density of the air and the amount of force used to velocitate particles. When discussing the audible quality of sound in an obstructed field you are really talking about sound pressure.



If a trumpet player is playing a note the acoustic impedance is determined by how hard he's blowing and the resonance of his trumpet, if you put a pillow in front of your face, you have obstructed the field of the sound waves your ear drums are recieving and therefore reduce the sound pressure you experience, but, the acoustic impedance is the same.



Summary



-Impedance: A measure of the impediment to the flow of alternating current.

-Ohms: The units of measure for impedance.

-Acoustic Impedance: Ratio of sound pressure to particle velocity.

-Sound Pressure: Measurement of pressure deviation of a sound wave moving through a fixed point



References



Impedance:

http://en.wikipedia.org/wiki/Impedence

http://www.aventisav.com/AV%20Glossary.htm



Ohms, Amps & Speakers:

http://www.marktaw.com/recording/Electronics/OhmsAmpsandSpeakers.html



Cable Impedance:

http://www.epanorama.net/documents/wiring/cable_impedance.html



Accoustic Impedance:

http://en.wikipedia.org/wiki/Acoustic_impedance



Sound Pressure:

http://en.wikipedia.org/wiki/Sound_pressure



I hope this was helpful and clears up some mysteries about the world of sound