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Impedance?

Discussion in 'Sound, Music, and Intercom' started by geezer, May 28, 2007.

  1. geezer

    geezer Member

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    I feel very uneducated about many things, impedance is what baffles me now.
    What exactly is it? How does it gett bigger and smaller with different speakers/ quantity of speakers? I knew I had to ask the experts on this one so I could really under stand it. Thank you all!
     
  2. stantonsound

    stantonsound Active Member

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  3. mbenonis

    mbenonis Wireless Guy Administrator

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    Impedance is a topic which can be very confusing until you get it. I'll do my best to explain it in understandable terms.

    Impedance is composed of two parts, Resistance and Reactance. Resistance limits the flow of direct current, and impedance limits the flow of alternating current.

    Resistance is the easier of the two to understand, because a resistor does not change value depending on what voltage is placed across it and what current flows through it. In a purely resistive device, these two quantities are related by the equation V = I*R, where V is the voltage, I is the current, and R is the resistance.

    Reactance is the analog of resistance for alternating current. However, to complicate things reactance is also composed of two parts, Capacitive reactance and Inductive reactance. Inductive reactance is taken as a positive value of reactance, and capacitive reactance as a negative value. Indeed, you can add the values of reactance together and obtain what looks like an inductor or a capacitor, even if the network is composed of both types of components. You can even cancel out the reactance entirely by matching the inductive and capacitive reactances.

    It is important to note that the capacitive reactance is not the same thing as the capacitance, and the same is true for inductive reactance and inductance. They are, however, related by the equations Xc = 1/(w*C) and XL = w*L, where Xc and XL are the capacitive and inductive reactances, w is the frequency of the ac signal (in radians per second), and C and L are the capacitance and inductance, respectively.

    In order to fully understand impedance, we need to look at combinations of circuit elements (resistors, capacitors, inductors, etc). There are two basic ways to connect elements - in series, and in parallel. When items are connected in series, they're connected end-to-end. E.g., the end of a resistor is connected to the end of the capacitor like two strings of christmas lights. When items are in parallel, they are stacked, with all of the terminals on one side connected together, and the other side connected as well.

    Impedance is the combination of inductance and reactance. So, if you had an inductor and a resistor in series, then you would simply add their individual impedances - for example, (R + j*w*L) where R is the resistance, j is the imaginary unit, w is the frequency of the ac signal in radians, and L is the inductance of the inductor. When items are in parallel, a special formula is used. You take each individual impedance, and flip it (raise it to the -1 power). Then add them all together, and flip the final sum again (raise to the -1 again).

    The final impedance can also be expressed in polar form, where the magnitude is the square root of the square of the resistance added to the square of the reactance - sqrt(R^2 + (w*L)^2), and the angle the arctan of the reactance over the resistance - arctan(w*L/R).

    Now, how does this relate to the real world of audio equipment? For this, we need to understand the idea of a Thevenin equivalent circuit. Essentially, this is the idea that a real-world source of voltage (technically, a source of current as well) can be modeled by an ideal voltage source inline with an impedance (although this is often just modeled by a resistor). Thus, a low-impedance microphone can be modeled as an ac voltage source inline with a 150-ohm resistor (give or take).

    The input to your sound desk can also be modeled as an impedance - once again, often as a resistor. In the case of most desks, this is somewhere between 1k? and 10k?, although it can be higher and occasionally lower. The last piece of the puzzle here is why these two impedances are wildly different in value. This is because we don't want to draw much current from the microphone, but just have it produce a voltage waveform. Since maximum power is transferred when the two impedances are equal, we want to have one much higher than the other.

    Speakers are similar but work a bit differently. Earlier, we discussed how impedance varies based on frequency. In a speaker (we'll only look at cone drivers) there is a large coil of wire which is essentially a big inductor. Since it is made out of real-world wire, it also has resistance - thus making its impedance of the form (R + j*w*L). There's a bit more to it as well (stray capacitance, etc), but this basically holds true. This explains why the impedance of a speaker can vary based on the frequency fed to it.

    In the words of Bill Sapsis, ‘Zat help?

    PS - please forgive any spelling errors - it's late, and I probably misspelled at least one word or wrote a few very awkward sentences.
     
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  4. geezer

    geezer Member

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    There is so much more to it than i thought there would be. Mostly math, but also the lovely cpacitors etc. that i don't know anything about. It sounds like I need to take a class just to figure out how all of this works. If there is a simpler version I would love to read it, although I do really appreciate the depth of mbenonis's answer.
     
  5. SHARYNF

    SHARYNF Well-Known Member

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    So let me give you the over simplified, what it all means to you on a very basic level

    Speakers, in essence based on the word impedance, it is how much the speaker impedes the flow of power to the speaker, so again not totally correct but to give you the concept, a speaker with a higher impedance say 16 ohms will take less power from the amplifier. So why is this important, mainly again over simplified and more from a practical standpoint if the impedance gets to low (approaching 0) you are in danger of destroying the amp. This is why you will see some amps rated for 8 or 16 ohms impedance, with the power rating increasing as the impedance goes down, have an even higher rating at 4 ohm IF the amp will allow it, and typically NOT be rated for 2 ohms.

    So couple of things to also remember, as the above post outlined the impedance varies with audio frequency, So from a practical stand point the ratings tend to be done and the warnings issued since in a real world audio situation the impedance of the speaker will vary with frequency, and there is a level below which from a practical stantpoint you are in effect shorting out the outputs of the amp.

    IF you were to take an OHM meter and measure the impedance, you will see that you are only measuring resistance, and so the readings will be lower, for instance a 16 ohm speaker might only show 11 ohms and an 8 ohm speaker might show 5 ohms so don't expect them to measure what the speaker is listed at (again due to the mathematical relationship outlined in the previous post.

    Typically with speakers and amps you are concerned about, the minimal rating for the amp that you can safely run. The reason for this is that depending on how you hook up your speakers you will vary the impedance

    SERIES CONNECTION

    For instance if you take two speakers, and connect the positive on the first speaker to the positive on the amp, the negative ON THE FIRST SPEAKER TO THE POSITIVE ON THE SECOND speaker, and then the negative on the second speaker to the negative on the amp you will have put them in series, and you will have doubled the impedance of the system. SO two 8 ohm speakers in series looks to the amp as a 16 ohm load, each speaker will get equal power, and of course like with christmas lights wired in series, if you blow the first speaker , you will not be able to get a signal to the second.

    If you put an 8 ohm speaker in series with a 16 ohm speaker, you will get a 24 ohm load, and the 8 ohm speaker will be taking twice the power from the amp as the 16 ohm. this is why usually you combine speakers with the same ohm rating.


    PARALLEL CONNECTION
    If on the other hand you take two speakers and connect the positive of the first speaker to the positive on the amp and also to the positive on the second speaker, and connect the negative on the first speaker to the negative on the amp and the negative on the second speaker, you will get a 4 ohm load from two 8 ohm speakers, since in parallel you divide by the number of speakers.

    THIS Is where you need to be careful, since paralleling speakers is reducing the impedance load on the amp. For instance if you have speaker cabinets with in and out jacks and you connect the amp to the first with a cable and then jack in the second, and then someone decided to jack in a third etc, you can very easily reduce the load on the amp to the point where you can destroy the amp. This is probably one of the major causes of failure in a non informed user environment. I have seen this happen many times, you have the basic system connected, someone shows up with more speakers and someone just plugs them into the jacks on the speakers OR plugs them in to the amp. It is VERY RARE to have a jack setup that will allow you to series wire the additional speakers, SO in most cases if you take the rating of the speakers and divide it by the number of speakers on the channel you will get an idea of what load you are putting on the amp and MAKE sure it is safe.

    For mics and line level gear it is not quite as critical, typically you have a mic with a low impedance and plug it into a moderate or higher impedance connection. same as with line level connections. there are some factors to consider but in most cases it will be safe.

    Again this is overly simplified, not totally corrrect but a quick safe way to look at impedance especially from a speaker stand point that will work

    Sharyn
     
    Last edited: Jun 3, 2007
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  6. geezer

    geezer Member

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    Doe your divide by the number of speakers rule work if you daisy chain the speakers or does it only apply to parallel, if not how does # of speakers affect the impedance load on the amp?

    thank you
     
  7. SHARYNF

    SHARYNF Well-Known Member

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    That was the whole point of my post ;-( you need to reread it, and explain what you mean by daisy chain and parallel. You have two options with speakers either you connect them in series or parallel. All wiring is based on that

    Sharyn
     
  8. geezer

    geezer Member

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    I'm sorry I was not clear enough. If you were to use 1/4" jacks or Speakon jacks on speaker A and used the paralel jack on speaker A to go to speaker B would this have a different load than the example that you gave for series wiring. Because this would now have the (+) of speaker A connected to the (+) of speaker A and the (-) of speaker A connected to the (-) of Speaker B. So would this created a different Ohm Load?
     
  9. SHARYNF

    SHARYNF Well-Known Member

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    That is why I talked about a parallel connection in addition to describing a series connection, and that the parallel connection takes the load and in the case you described if the individual speakers were 8 ohms the amp would see 4 ohms which could be a problem

    Sharyn
     
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  10. mbenonis

    mbenonis Wireless Guy Administrator

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    Yes. In this case, you would effectively half the impedance presented to the amplifier, since they are in parallel. If they were in series -- that is, + to -, + to -, and + to -, in one big loop, you would effectively double the impedance presented to the amplifier. That make any more sense?
     
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  11. geezer

    geezer Member

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    ok, ok, I think that I had misunderstood the way you were wiring. Thanks to all of you, sorry I was a difficult person to explain to. I think I at least understand the basic parts of it now.
     
  12. Chris15

    Chris15 CBMod CB Mods Premium Member

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    A couple of notes I'd like to add...

    Where Mike has used j, it has the same meaning as i. This is complex numbers and i = j = sqrt (-1). It's just that we use I for current and other things, so we choose to use j to lessen the confusion. Outside of electrical engineering, it's i.

    Sharyn, it's been my experience that there are plenty of amps out there that are happy down to 2 ohms, especially the newer stuff. And any amp that'll take a 4 ohm load in bridge mode will take a 2 ohm load unbridged. That's also something to be aware of. If you are running your amps in bridge mode, then you need to double the minimum impedance because in essence what happens is channel A sees half the load and channel B the other half, though it's more complex than that.

    Hope it helps.
     
  13. SHARYNF

    SHARYNF Well-Known Member

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    In the context of professional sound reinforcement amps many of the modern high end units do support 4 ohms bridged, and 2 ohms. There are a few additional points to keep in mind. As you approach 2 ohms the current requirement to run the amp rises rapidly, so there is the obvious issue of having the mains supply to support it, but more subtly is the problem that many of the tests that are performed on these amps at 2 ohms have a mains feed that can accomodate the current draw, and IF you cannot accommodate the current requirement you can run into problems in terms of distortion and the subsequent potential to blow the drivers.

    So there are many schools of thought, and still most sound folks are very careful when running 4 ohms bridged or 2 ohms. The issue is that it is also very easy to drop considerably below the 2 ohms load based on the signal

    Based one the nature of the question, and the environment perhaps I was taking a bit more conservative view. So I'd proceed with caution unless you really know exactly what you are doing.

    Sharyn
     
  14. Chris15

    Chris15 CBMod CB Mods Premium Member

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    Good points there. The other thing to consider is that at those lower impedances, the resistance and I guess impedance of the speaker cables matters more. Whereas say half an ohm of cable resistance ain't that much at 8 ohms, it is a MUCH greater proportion of the total load at 2 ohms...
     
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  15. David Ashton

    David Ashton Active Member

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    No-one has mentioned the major increase in the resistance of the speaker with heat, the resistance can rise in excess of 60% at working temperature, it is strange that there is an irrational belief among lighting people that lamp filaments need "pre-heating" even though they cool down again in seconds, but one never hears anyone suggest pre-heating the speaker cones, which makes much more sense as they have quite a lot of thermal mass.
    Why is this not discussed?
     

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