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.
  9. HDOM Enthusiast

    HDOM
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    ok but then what about if you made a speaker that is identical the only difference is that one is 8 ohm and the other is 4 ohm.

    but if you want you can say some one is 16 ohm and the other is 6 ohm

    now imagine that we had 2 amplifiers that are identical the only difference is one is 100watt 8 ohm and the other 190 watts 4 ohm

    what do will sound better do you think? or maybe it doesnt matter?

    ok but then what about if WE COMPARE IT TO A RIVER?, we had the 100 ohm aplifier it doesnt mean then that listen to 4 ohm the current is more natural? is it not the 8 ohm something like a tree branch stoping the "river" of "flow"
    HDOM,
  10. j_garcia Audioholic Jedi

    j_garcia
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    An amp that is 4 Ohm capable will have no issue driving an 8 Ohm load, so the amp side is not so much a factor unless it is insufficient for the load.

    So back to the speaker itself, a 4 Ohm of the same sensitivity will draw more current from an amp than an 8 Ohm one. It will not sound better or worse IMO, just require the more power to achieve the same SPL. I currently own 8 and 4 Ohm variants of the same family of speakers and they do not sound noticeably different.
  11. markw Audioholic Overlord

    markw
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    Bad analogy. A water tank has limited capacity and then it will run out and can supply no more water. A river does not and is essentially is an unlimited supply of water.
    Last edited: Mar 15, 2016
  12. highfigh Audioholic Warlord

    highfigh
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    Changing the impedance doesn't leave everything else the same- the voice coil's inductance will usually change, the magnetic force and other parameters change and because the impedance is different, the crossover values must change.
  13. HDOM Enthusiast

    HDOM
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    thanks for the info but i guess more power means that is more easy for the amp to clip
    HDOM,
  14. j_garcia Audioholic Jedi

    j_garcia
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    Most definitely.
  15. highfigh Audioholic Warlord

    highfigh
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    Why? Clipping means the amp is operating at/beyond its limit and if the needed SPL is lower than the amplifier's power limit would provide with a given speaker, the amplifier should never clip unless the input stage is where the clipping occurs and that can happen with an amplifier developing .001W or 10 Million Watts.

    In a properly designed system, amplifier power demand should be determined before making a choice and that choice is driven by the bandwidth, SPL, speaker sensitivity and the source material that will be used. If there's no way to be absolutely certain about the source material and it's levels, it's necessary to make sure distortion won't be a common occurrence, so an amplifier that's more powerful should be used but if the level of the music/speech or whatever will never exceed some moderate SPL, there's no reason to choose an extremely powerful amp unless the speakers are terribly insensitive. Because a budget always accompanies a design, these speakers would be a bad choice.

    How loud do you need/want the system to be? If you find the speakers you like, base the amplifier's purchase on that. If you have an amplifier, choose speakers that won't stress it too much.
  16. j_garcia Audioholic Jedi

    j_garcia
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    I think it was meant in the context of having an amp that wasn't necessarily up to the task of lower impedance. A 200W amp isn't likely to have that issue relative to the original question, though if one were using something that is fine with an 8 Ohm speaker but might be pushing its limits with a 4 Ohm then clipping would be more likely. A situation that is likely to occur when the amp is already in place and then speakers that exceed that capability are chosen after the fact.
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  17. HDOM Enthusiast

    HDOM
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    i see the thing is amps over lets say 150 watt in 8 ohm are very expensive my friend, maybe this may even be look at that point that speakers of 4 ohm are rather for people with good money in the pocket

    in my cheap experience the ohm speakers use to sound more sweeted than 4 ohm with a 100 watt ohm amplifier
    HDOM,
  18. HDOM Enthusiast

    HDOM
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    YEA :)
    HDOM,
  19. PENG Audioholic Warlord

    PENG
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    As you know now, it depends on other factors. The significant factors are the SPL you need in your listening environment and the sensitivity of the speakers. If you compare two speakers that has exactly the same sensitivity (dB/1M/2.83V) then in theory the 8 ohm speaker should give you 3 dB more headroom under certain conditions.

    As highfigh alluded to, if your need is well below the power you need then the 4 ohms vs 8 ohms issue will a moot point. You really should not let the impedance influence your choice of speakers too much. Just get the one that sounds the best to you and chances are your amp will be fine because most decent amps can output much more (say, close to 2X) for short duration, some of them, e.g. NAD, Yamaha do list their dynamic output ratings. If you do get in trouble, you still have the option to the volume down a little and pretend you have instantly upgraded to an amp with twice as much power output.:D
    PENG,
  20. 3db Audioholic Overlord

    3db
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    There is far more to this then 4 ohm verses 8 ohm and everyone keeps forgetting the affects of phase angle. A 4 ohm speaker with a low phase angle is by far an easier load to drive than a 8 ohm speaker with a high phase angle. Talking about speaker impedance without including phase angle is meaningless.
    3db,

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