dedwards96 Posted March 10, 2013 Share Posted March 10, 2013 Hi, I am beginning to learn about sound reinforcement systems and have come across the term ohms with regards to speakers. I know it has something to do with resistance but am not sure in what way it affects the setup of a system. Help!!Thanks, Dan Moderation: moved to the "next generation" forum Link to comment Share on other sites More sharing options...
JCC1996 Posted March 10, 2013 Share Posted March 10, 2013 If you type 'Speaker impedance' into google you'll find many sources that can explain it much better than a single reply on here could. Or even type it into youtube; there are plenty of really informative tutorials on there that are especially good if you learn better by watching something than by simply reading an article. Joe Link to comment Share on other sites More sharing options...
Oldradiohand Posted March 10, 2013 Share Posted March 10, 2013 I blame the schools - surely everyone should know about Georg and his law? Link to comment Share on other sites More sharing options...
kerry davies Posted March 10, 2013 Share Posted March 10, 2013 Daniel, from your posts thus far it might help BR members if they knew a little more about you such as age and what you are studying. The questions are O level stuff and it would help members to point you toward information that will enlighten and not baffle you. One other thing your posts make clear is that you have a school technician who really ought to be your first port of call for questions on school events. If I were in his/her position I might just feel a bit miffed were a student seek help from strangers rather than ask me. The last thing to mention is that BR has been going for a while now and many, many questions have been answered pretty effectively and the search function is your friend. There is a huge amount of information there and in the BR Wiki. If nothing else a little more information would allow you to pose "better" questions and us to provide "better" answers. Link to comment Share on other sites More sharing options...
Alec97 Posted March 10, 2013 Share Posted March 10, 2013 measurment of resistance of the voice coil Link to comment Share on other sites More sharing options...
dedwards96 Posted March 10, 2013 Author Share Posted March 10, 2013 Daniel, from your posts thus far it might help BR members if they knew a little more about you such as age and what you are studying. The questions are O level stuff and it would help members to point you toward information that will enlighten and not baffle you. One other thing your posts make clear is that you have a school technician who really ought to be your first port of call for questions on school events. If I were in his/her position I might just feel a bit miffed were a student seek help from strangers rather than ask me. The last thing to mention is that BR has been going for a while now and many, many questions have been answered pretty effectively and the search function is your friend. There is a huge amount of information there and in the BR Wiki. If nothing else a little more information would allow you to pose "better" questions and us to provide "better" answers. Hi, In response to your point about the technician - the previous technician was brilliant and could answer any questions I asked him in simple form that I can understand without much technical knowledge. However, another technician has taken his place recently and to be honest, he is a liability. His boss is debating whether or not he made the right choice so I don't feel confident asking him about stuff. I apologise if I have asked any previously answered questions. Dan Link to comment Share on other sites More sharing options...
cedd Posted March 10, 2013 Share Posted March 10, 2013 http://gsuryalss.files.wordpress.com/2012/02/volt-amp-ohm.jpg I believe this best sums it up!!!!! Link to comment Share on other sites More sharing options...
dedwards96 Posted March 10, 2013 Author Share Posted March 10, 2013 Brilliant! Link to comment Share on other sites More sharing options...
mackerr Posted March 10, 2013 Share Posted March 10, 2013 measurment of resistance of the voice coil That's part of it, but there are other components to the impedance of a speaker, like inductance and capacitance. The DC resistance of a voice coil will generally be less than the AC impedance, which is also effected by the box it's mounted in and the frequency of the test signal. The main point of knowing the impedance of the speaker, and the minimum impedance an amplifier will work into, is to give you enough information to avoid loading down an amplifier with more speakers than it can drive while meeting its specs. Mac Link to comment Share on other sites More sharing options...
kerry davies Posted March 10, 2013 Share Posted March 10, 2013 to be honest, he is a liability. Just hope that after posting that here, Daniel, he isn't a BR member. Link to comment Share on other sites More sharing options...
Wol Posted March 11, 2013 Share Posted March 11, 2013 I believe this best sums it up!!!!! Little bit rapey looking too for good measure? What happened to the old hosepipe analogy! Either way, it doesn't quite take into consideration the main point of the OPs question though about how it affects the system as a whole. Link to comment Share on other sites More sharing options...
Brian Posted March 11, 2013 Share Posted March 11, 2013 Something I wrote years ago... Volts, Ohms and Amps A refresher for people who didn’t pay attention in physics class Instead of thinking about electricity moving down a piece of wire let’s imagine water flowing through a pipe. We are all familiar with the idea of water pressure. It’s the pressure that determines how much water moves through a pipe. Well, in electrical circuits the pressure is measured in Volts and it’s the voltage we have which determines how much electricity moves down a piece of wire. The symbol for volts is ‘V’. The next thing to think about is the size of our pipe. A small pipe will limit the amount of water we can get out. To get more water out we could increase the size of our pipe or we could increase the pressure. We say that the smaller sized pipe ‘resists’ the flow of water. Well, in electricity it’s exactly the same; a wire resists the flow of electricity and, just like a pipe, the smaller the wire the more it resists the flow. We measure resistance in Ohms which have the symbol Ω or ‘R’. We can measure the amount of water we get out of a pipe in a given period of time. We can measure this in units such as gallons per minute or litres per fortnight. In our electrical circuit we measure the amount of electricity flowing in a given time in Amps (or Amperes to give the full name). The symbol for Amps is ‘A’ but, just to confuse things, when it is mentioned in a formula it uses the letter ‘I’. So now we need to work out how much electricity we can get flowing in a piece of wire (the amps) if we know the size of wire (its resistance) and the pressure of the electricity (its voltage). Time for an equation… I = V/R ...that is, Current = Voltage divided by Resistance Where I is our electrical current flow, V is our voltage and R is the resistance of our wire. This equation is called Ohm’s Law and is more usually written… V = IR ...that is, Voltage = Current multiplied by Resistance So let’s look at a simple example. We already know that a bulb has a resistance of 1.6 Ω. What happens when we connect this across a 9 V battery? Well, using ohm’s law we can put the numbers into the equation and see what we get… I = V/R = 9/1.6 = 5.625A So a current of 5.625 Amps will flow through the bulb. Link to comment Share on other sites More sharing options...
Bobbsy Posted March 12, 2013 Share Posted March 12, 2013 All the above is totally accurate but the practical application when setting up a PA system is as follows: When you connect a single 8 ohm speaker to an amp, the amp is seeing an impedance of 8 ohms and all is good. However, if you want two speaker cabinets per side, you need to be careful. You have two ways to wire up the two cabinets--series and parallel. In series, the signal loops into, say, the positive input on one speaker, out of the negative side and into the next speaker's positive side. This has the effect of raising the impedance to 16 ohms and a similar effect of cutting the sound level from each speaker. It also means that if one speaker fails, all the ones on that side die. The other way to connect multiple speakers (and the one most often used) is parallel. It's handled in the wiring but basically the feed from one side of the amp goes to all the positive sides of the speakers--and the other to all the negative sides. The effect this has is that every additional speaker you add roughly halves the impedance. Two 8 ohm speakers in parallel present 4 ohms to the amp. Add a third speaker and it's around 2 ohms...and unless you have an excellent amp, there's a good chance that you'll get a display of magic smoke at the amp because 2 ohms is very near a short circuit. So...if you're just assuming one amp and 1 speaker per side, as long as they're both rated at 8 ohms, no problem. You may well find you get an amp rated down to 4 ohms which would mean you could have 2 speakers per side--which could be useful if you need to cover a spread out audience. Some very expensive pro amps can go down to 2 ohms but I suspect you won't get one of these on a high school budget. Hope this helps...now go back to the earlier replies and read the theory behind this! :) Link to comment Share on other sites More sharing options...
revbobuk Posted March 12, 2013 Share Posted March 12, 2013 The effect this has is that every additional speaker you add roughly halves the impedance. Two 8 ohm speakers in parallel present 4 ohms to the amp. Add a third speaker and it's around 2 ohms...and unless you have an excellent amp, there's a good chance that you'll get a display of magic smoke at the amp because 2 ohms is very near a short circuit. I have days like that - when I know what I meant to say, but what I actually say isn't quite the same. Two 8 ohm speakers do indeed (roughly) present a 4 ohm load. But to get down to 2 ohms takes two more, not just one more. 3 8 ohm speakers in parallel come out at nearer 3 ohms - a fairly trivial difference, but 'every additional speaker roughly halves the impedance' isn't really quite true. Link to comment Share on other sites More sharing options...
alistermorton Posted March 12, 2013 Share Posted March 12, 2013 In general, if you have resistors R1, R2, R3 ... Rn in parallel, and you want to know what Rtotal is 1/Rtotal = 1/R1 + 1/R2 + 1/R3 ... + 1/Rn So for 3 identical 8 Ohm loads 1/Rtotal = 1/8 + 1/8 + 1/8 1/Rtotal = 3/8 Rtotal = 8/3 = 2.66666 Ohms In the simple case most people treat the speakers as a resistor. In fact the impedance is reactive, but the same expression applies, it's just that taking reciprocals of impedances with real and imaginary parts is less trivial (most good scientific/engineering calculators can do complex maths these days, though). EDIT : spelling, layout. Link to comment Share on other sites More sharing options...
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