4 ohm vs 8 ohm speakers

Discussion in 'Beginners and Audiophytes' started by Halla, Jan 9, 2008.

  1. Halla Audioholic Intern

    Halla
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    Hi,
    will you guys please explain to me what the difference is between a 200 watt 4 ohm speaker and a 200 watt 8 ohm speaker and what one needs a more powerfull amplifier than the other?


    It all has to do with an amplifiers ability to source current to a load. With all things equal, a 4 ohm speaker requires twice the current draw than a similar 8 ohm version.

    [ADMIN] Edited to add relevant topic info:

    Loudspeaker Impedance:
    is the sum of DC resistance and complex AC reactance comprising inductance and capacitance, the value of which changes with frequency.

    If your stuck on jargon like Ohm's Law, Impedance and Electrical Phase and wondered what makes a loudspeaker “difficult to drive”, you will want toreview our article on Understanding Ohm's Law

    Of course Speaker Sensitivity is also important b/c it tells you how much power you will need to drive the speaker with to hit a certain loudness level.

    We discuss the math of speaker impedance and how it changes depending on if you wire two in parallel or series in Loudspeaker Impedance: Series vs Parallel Connections
    Last edited by a moderator: Oct 28, 2014
  2. jagxtype Audioholic

    jagxtype
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    generally speaking you need a more robust amplifier to handle 4 ohms. Something to take into consideration that amplifiers usually output more the lower impedence you go. For example. @8ohms it might output 100 watts but at 4 ohms it might output 190 watts. As long as your amp is capable, you can use a 4ohm speaker to get more out of your amp so to speak. That statement must be taken with a grain of salt though. Accoding to ohms law Volts X Amps = Watts and is so derived in other ways such as Volts(squared)/ohms = watts. Hopefully someone on more knowledge of this can chime in:D I could figure this out in electron flow, but that is useless to most:p
  3. Rickster71 Audioholic Spartan

    Rickster71
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    Picture it this way: My imperfect analogy.

    For sake of discussion you have two 100 gallon / liter water tanks.
    Each tank will represent a 100 watt amplifier.

    One tank has a garden hose attached to it.
    The Other tank has a two inch diameter hose.

    The hoses represent the speakers; garden hose = 8ohm
    The bigger 2 inch hose = 4ohm speaker.

    The 8ohm speaker, like the garden hose, is a resistance to flow.
    The bigger hose, or 4ohm speaker offers 50% less resistance to flow; so it will require more current / water.
    The bigger requirement can only be met by a bigger water tank, or an amplifier that is rated for 4ohm speakers.
  4. mtrycrafts Audioholic Slumlord

    mtrycrafts
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    Well, the 200 watt 4 Ohm speaker needs more current than the 8 Ohms at the same power of 200 watts. Amps don't like to deliver lots of current.
    In this case, most likely that 200 watt amp may deliver 200 watts into 4 ohms so there is no difference in that case.

    But, comparing speakers is not that simple as your example, unless it is only about the power.
  5. TLS Guy Audioholic Overlord

    TLS Guy
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    The issue is that the amp does not necessarily have to be more powerful to drive the 4 ohm speaker. 200 watts is 200 watts. However the amp has to provide more current to the four ohm load so the output transistors have to be more robust.

    Now the power delivered is the square of the current X the resistance.

    So if we have the 8 ohm load the square of the current is 200 watts/8 ohms which is 25. The square root of 25 is 5 amps. So the amp has to deliver 5 amps to produce 200 watts into 8 ohms. Now the voltage from ohms law is the current X the resistance which is 5 amps X 8 ohms which is 40 volts

    Now lets take the four ohm load. The square of the current is 200 watts/4 ohms. Which is 50, the square root gives a current of 7.071067 amps
    Now the voltage from ohms law is 7.071067 amps X 4 ohms which is 28.284 volts.

    Now if the amp did not drop its voltage when driving the 4 ohm load then the power delivered is the square of the voltage divided by the resistance. So we would have 40 volts squared/4 ohms this is 1600/4 which is 400 watts. Now you might say what is the difference. Well if the amp drops voltage supplying the four ohm load it is current limited. The transistors will be stressed and the amp will likely hit a hard clip and in the worst case scenario will clip at five amps, if that is its maximum current output. If you do the above calculations that would be 100 watts into 4 ohms.

    The point is that amplifiers that have to deliver more current require larger power supplies and bigger output devices. It is the current output of the amp that has a bearing on cost. The maximum current output determines how much power an amp can deliver to a given load. The voltage an amp can deliver is determined by the operating voltage of the power transistors. Obviously the output voltage can not exceed that.

    I hope that has answered your question.
  6. jagxtype Audioholic

    jagxtype
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    woo...i was hoping i wasnt wrong. Thanks for adding more depth TLS guy. I was going to do all of the math, but it was late...
  7. Halla Audioholic Intern

    Halla
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    87db vs 90db speakers

    THANKS!!!!!!!!

    And the db rating of a speaker? - if I may?
  8. TLS Guy Audioholic Overlord

    TLS Guy
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    I don't know exactly what you mean by the db rating. I will assume you are talking about loudspeaker sensitivity.

    There are two ways of expressing it in common usage, which makes it confusing.

    The first is the sound pressure level driving a speaker with 1 Watt of power and measuring spl at 1 meter. This is spl in db at 1 watt 1 meter.

    The other is to apply 2.8 volts and measure spl. Now I think you can see that a 4 ohm speaker will have a higher rating with the later method as it will be taking more power. Power is voltage squared divided by resistance.

    Now the db scale is log. So under the same conditions of test a speaker with a 3db increase in sensitivity has the same effect as doubling the amp power to the less sensitive one.

    The other thing is max spl, which is not usually quoted, and varies with frequency usually. For instance power is often limited in the bass because of limitations of cone movement (xmax).

    Speakers seemingly so simple are in fact complex. There are a lot of specs that are hidden by manufacturers. One of the most useful pieces of information is the waterfall plot. Only the best manufacturers publish those. Specs are easily and often manipulated by manufacturers to their advantage.

    Even then you can not judge a speaker by the specs. You have to listen. A good spec. sheet however can give you a good idea which speakers not to waste your time with.

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