Ohms are the units which measure resistance or impedance. Resistance or impedance are two terms which define the tendency for a substance or electrical component to attenuate or reduce a voltage when current is passed through it. The relationships between resistance, current and voltage are described by Ohm's Law, a simple mathematical equation. With regard to speakers, generally, the lower the impedance rating the more inefficient the speaker is. That is, speakers rated at 16 ohms tend to be more efficient than 8 ohms which tend to be more efficient than speakers rated at 4 ohms. All speakers have an impedance which varies with respect to the frequency being produced. Most speakers tend to have large impedance dips and increases near their woofer resonance point, depending on the design, as well as at other frequencies throughout their operating range. Since amplifiers have to work harder in order to drive inefficient speakers as well as speakers which have low impedance somewhere within their frequency range, the impedance characteristics of any speaker are most properly expressed on a graph which has frequency response on the X-axis and impedance (Ohms) on the Y-axis. Often two impedance ratings are quoted for a speaker, when a graph is not used to indicate a speaker's impedance behavior. These are the nominal impedance rating and the minimum impedance rating. The nominal impedance rating is meant to express the "average" or "typical" impedance behavior of a speaker while the minimum rating is meant to indicate the minimum impedance at any frequency within the operating range.
bravo, rayhall! yours is an exceptionally literate and erudite answer to a seemingly simple question.
Here's my two cents. In order for electric current to flow it must be drawn by a load, which is like a hungry energy magnet. The resistance in Ohms is a measure of the load/hunger. The higher the resistance in Ohms the less energy flows. Conversely, as the resistance of the load drops, the magnet gets bigger, hungrier for more energy. Good amps can accomodate these load variations and spill more juice into the load. But as the resistance of the load approaches zero the amp goes nuts trying to fill the void and hopefully overheats and shuts down. Any speaker's ability to convert this energy into sound at a given frequency varies, so I wouldn't say offhand that the rating in Ohms will give you much of a clue as to efficiency. The rating does point towards the adequacy of the amp you'll require to feed it. The bottom line is that amp designers and speaker makers have to work from some kind of starting point to get these components to work together, which for some reason has a baseline of 8 ohms. Is it arbitrary? It's probably just a freak of nature stemming from the measurement of early design efforts that later became an agreed upon average design target.
Be careful! Speaker impedance and efficiency are two very different things. If a speaker is 8 ohms, all that means is that the speaker's voice coil has a nominal impedance of 8 ohms (Rayhall gave an excellent explanation of impedance). The efficiency is the measure of the speaker's ability to turn electrical energy in sound waves. This is represented in db (sound wave intensity) per watt (electrical energy). The higher the rating ie. 93 db/watt, the better the speaker is in converting electrical energy into sound waves. For example a speaker rated at 93 db/watt is more efficient than one rated at 89 db/watt and their impedances can both be 8 ohms nominal.
Great Question !!! My speakers have Sensitivity 90dB spl; Nominal Impedance 8 Ohm; Minimum 3.5 Ohm. The bass is on the light side (I use a REL sub) and very very slightly slower compared with the rest of the speaker. Is this reflected in the 3.5 minimum Ohms?? I've read an amplifier with a lower output impedence the better to drive the bass better. Is this true? Comments??
Simplified answer for the non-technical: As consumer stereo evolved, speakers manufacturers provided an "ohm" (electrical resistance) rating, so that consumers could match a speaker with their particular amplifier. This way done many, many years ago, before transistor amplifiers were even produced! Now what does that "ohm" rating mean to today's consumer , not really that much (unless he buys some exotic, old fashoned tube amp design of yore). If a speaker is rated at four ohms vs eight ohms, your amplifier may be able to produce more clean power into the lower ohm rated speaker, but this is a general statement at best. To reiterate: Unless you have some sort of exotic tube amplifier; this specification will not help you determine if one speaker is superior versus another. Four ohm speakers don't necessarily sound better than eight ohm speakers and visa versa.
Great thread and great explanations. However, ohm's are important even with modern solid state equipment and it's not so much to do with exotic tube equipment. Not nominal imedance, but IMEDANCE CURVES. In the real world, this is what drives amps (yes, solid state amps) nuts. I've been on the planar/ribbon/electrostatic road for quite a few years and what makes amp matching so critical for these puppies is the impedance swing (impedance minima at high frequencies combined with higher impedance in low frequecies). Lots of people fret impedance dips but this isn't such a big deal since these occur at high frequencies and there isn't a lot of energy going on there. It's how an amp responds to the dips combined with what goes on in the lower mids that really counts. The Newforms have a curve of 2ohms to 100ohms, I think. If you were an amp tring to play dynamic music with lots of fast changes in impendance you would be working your proverbial ass off to keep up with a curve like this. My speakers (CLS's) are tame by comparison... 1.5 ohms to 35 ohms in the mids. (One version of my speakers dropped down to .6 ohms!) Most speakers rated at a nominal 8ohms have far less swing -- usually less than 6ohms in either direction -- making the amp's job easier. The type of music (degree of dynamics and low frequency extension) also makes a big difference in an amp's ability to keep up with the swing. Again, great explanations of terminology that's thrown around and not fully understood in real world applications. Thanks for bringing it up.
Jim, I agree with your overall explaination...but....Many entry level audiophiles want to understand what are the pertinent specifications, of each component that they are considering for purchace. For an entry level audiophile to compare the "ohm" specifications of different speakers, and make a determination to which might be best for their modern amp, is darn near rediculous. Your explaination of the impedance curve of a speaker, and its potential implication on the amplifier chioce will intimidate the hell out of any newcomer to audio. Again, for those of you out there that are studying all of these "important" specifications, you don't have to become an undergrad in the understanding of electrical resistance (read: "ohms"), in order to choose good sounding electronics for your speakers. Don't let all of the above aforementioned explaninations intimidate you. They are just part of the "hobby". We seasoned audiophiles like to dig down deep into the electrical theories, even if they are not really needed in our purchace decisions.
I agree with you in general. But when I was an audio pup, I decided that I really liked those flat speakers with the ribbons on them. I'm not a technical person and picked up what was relevant along the way out of necessity. (Boy, did I make some mistakes.) Whe you get down to it, impedance curves (simply minimun and maximum) are not too complicated and might help in choosing an amp for speakers that don't have coventional loads. And today this doesn't seem to be limited to ribbons and electrostatics. But I agree that most people will go with speakers for which impedance swing is not an issue. No harm, no foul. Have a good one.
Thanks to all I had every thing backwards which was leading to much confusion! For me as a beginer the more info the better-I figure if I don't understand I just have more reading to do!!! Thanks Again Thanks to every one!!
What is the old analogy? Please correct me if I am wrong. Electricity is like a water faucet. Volts is the resevoir, amps are the amount of water coming out of the faucet, watts is the pressure and ohms is the lever that turns it on to full blast or off and everything inbetween. Is that right? Its been a while.
Close, I will give you an A for your effort. Try this: Voltage is the pressure at which the water is being pushed, current is the amount of water of water and resistance is the size of the pipe. Or maybe I am wrong?? :-)
How about this? Think of your speaker as a Schwinn 10-speed. A 4 ohm speaker is like riding with lower gears- easier effort, but lower speed; an 8 ohm speaker is akin to using taller gears, more effort but higher speed. Thus lower impedance speakers need amps that can pedal _fast_, i.e., high current designs while high impedance speakers that can pedal _hard_ - high voltage amps. Now think of the interconnects as ...