why do they build hard to drive speakers?

Discussion in 'Speakers & Subwoofers' started by felix_suwarno, Sep 27, 2006.

  1. felix_suwarno

    felix_suwarno Screenwriter

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    I have monitor audio gold gs10 and gs-lcr, driven by an old yammy rxv730 receiver. At first i thought i would need a bigger amp for the golds, since they are the flagship product from that british manufacturer. I mean, most high end speakers are either 4 or 6 ohm ( impedance ).

    Well, if 8 ohm speakers sound this good ( i love the speakers to death ), what is the point of creating 4 ohm speakers? Why make it hard to drive? Is there any advantage in making it hard to drive?

    About the golds...well, i think i already played them for about 100 hours ( i started to use them about 2-3 months ago ). And they are definetely bright... the treble knob is pointed to 8 o'clock to prevent my ears to bleed when listening them real loud. But i like the way they sound. Crisp, clear, smooth, and full.
     
  2. Arthur S

    Arthur S Cinematographer

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    There is a big advantage to amp makers who thrive on low impedance speakers. Some speakers are likely deliberately low impedance so you need an expensive amp to drive them. After all, if you buy expensive speakers, you should need an expensive amp to drive them, right?

    Relatively few speakers actually measure 8 ohms. This is a very complicated subject with many variables, most companies probably try to achieve close to 8 ohms, however, most come in between 5-7 ohms. Just the way it is.

    There have been speakers with true 1 ohm resistance. Get the fork lift ready for the amps to drive them.
     
  3. John Garcia

    John Garcia Executive Producer

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    Hard to say really, but impedance affects how the amp and speaker react to each other, and they are focusing on the way the speaker works first and the resulting impedance second most likely. I wouldn't say they intentionally design them low impedance speakers so you will buy a bigger amp, they simply design the speaker that they want to, and if it happens to be a 4 Ohm nominal load, oh well.

    Impedance varies with frequency, so what they are really saying when they say 8 Ohm speaker is that you can expect an average load on your amp vs a 4 Ohm speaker. The fact is too, even 8 Ohm speakers can dip quite low in impedance at certain frequencies.
     
  4. BrianTwig

    BrianTwig Second Unit

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    Can someone please explain, in plain english, what all this 4, 8, ohm, impedance, etc... means? Thanks.
     
  5. Seth=L

    Seth=L Screenwriter

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    Impedance is resistance to electrical current. Lower # impedance is a higher resistance, which seems backwards I know. A 4 ohm speaker will will be more difficult to drive than say a 8 ohm speaker, making the amplifier work harder. Weaker amplifiers not designed for lower impedance speakers could be damaged attempting to deliver power to such demands.

    I hope that was English enough, because that is about as English as I can make it.
     
  6. RichardH

    RichardH Supporting Actor

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    Umm, no. Higher impedance numbers mean more resistance. Resistance isn't what makes an amplifier work harder. It's low levels of resistance, like a speaker that dips to 1 or 2 Ohms, because that means more current will be flowing.

    Perhaps you're thinking of wire guages, where a smaller number does mean a larger wire?
     
  7. Andrew Pratt

    Andrew Pratt Producer

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    Not exactly. Think of impedance as the resistance to moving something. If you remove all resistance (i.e. Impendance is 0) then you have a 'short' and the current freely flows. So what happens in a speaker is that the impendance changes with the frequency so its a dynamic system. Speaker builders will quote you an average value that's typically between 4 and 8 Ohms but keep in mind this is just an average and you can expect that to fluctuate a lot depending on what you're playing. So lets say we have a fairly typical power amp that is rated at 100 watts into 8 ohms. We get that rating by testing the output from the amplifer when driven into an 8 ohm resistor (hopefully with a wide range of frequencys). Now if we were to cut the resistance in half to 4 Ohms we would expect the amp to be able to deliver twice the power or 200 watts. This almost never happens though for a variety of reasons but in quality amps it'll get close. If we were to cut the resistance again to 2 ohms we'd expect the wattage to double again and so forth. Now there's a lot going on in an amp when you keep asking for more and more current and eventually its going to overheat and shut down...hopefully tripping its thermal protection before it melts on you [​IMG]

    So why bother making a 4 ohm speaker? Well there's a number of reasons but one is simply becasue its not normally that big a deal for larger speakers as the assumption is that you're not going to be using a low end receiver to power it and most dedicated amplifers have no problem with 4 ohm loads. If we take some speakers I'm familar with as an example you'll see another reason why there's often 4 ohm speakers. Lets take a look at the Usher 6311 You'll notice this has one tweeter and one 8945A woofer and is rated as being 8 Ohms. Now the Usher 6371 is virtually identical to the other model but you'll notice it has two 8945A woofers and is a 4 ohm load. Basically Usher designed a woofer that's 8 ohm so that they can use it in a number of models...but when they combine two of them together in parallel the resistance is cut in half which results in a 4 ohm load.
     
  8. Holadem

    Holadem Lead Actor

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    Impedance and resistance are the same property. They just affect different types of current. Resistance is actually a component of impendance, but really, for the purpose of this discussion, consider them one and the same.

    Impedance, is really just what it sounds like: A measure of how much the flow of current is... impeded in the device. "impeded"... "resisted"... just making the point once again that they are the same property.

    So a low impedance speaker does not impede current flow as much as a high impedance device (talk about truism...). In other words, it draws more current, it needs more current to function... and that current comes from the amplifier, which consequently, needs to be beefy enough to supply it.

    Now, why make a speaker which requires more current than your average speaker, and presents such a large load to an amp? Well, engineering is compromise, while I have never designed a speaker, it's clear to me that all things being equal, the low impedance device represents less of a design challenge. A speaker which draws a lot of current consumes a lot of power. It's easer to do stuff (be loud & accurate) with more power than less, as long as the power is available (large amp).

    --
    H
     
  9. Seth=L

    Seth=L Screenwriter

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    Ok, I did get that a little mixed up, sorry.

    Point is lower ohms are harder to drive.
     
  10. Andrew Pratt

    Andrew Pratt Producer

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    Actually they are easier to drive its just that you need an amplifer that can handle the current demands that are being asked of it.
     
  11. Holadem

    Holadem Lead Actor

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    That doesn't make any sense.

    In the context of audio, "hard to drive" means "demands a lot of current" which lower end amplifiers will struggle to provide.

    How can the speakers be easier to drive if the current requirements are higher?

    Is there a different definition of "drive" that I am unaware of?

    --
    H
     
  12. Seth=L

    Seth=L Screenwriter

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    I concur.
     
  13. Arthur S

    Arthur S Cinematographer

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    As a matter of fact, a few speakers have been deliberately designed as 4 ohm, so the receiver COULD avail itself of its maximum output. You pick up a lot of tidbits when you have been involved with AV for 45 years.

    So, Andy is correct in the technical sense that a lower impedance allows more current to flow to the speaker and, it is also true that you need a more robust receiver/amp to drive a lower impedance speaker easily.

    Just to add another curiosity. Receivers are rated to drive up to 16 Ohm speakers. Yet, where are the 16 ohm speakers? What kind of output would a receiver have into:
    4 Ohms, 8 ohms, 16 ohms. If I remember correctly, in theory, the available current doubles every time the impedance drops by half. This is why you hear people asking about whether this or that amp will truly "double down".
     
  14. John Garcia

    John Garcia Executive Producer

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    "Double down" as I affectionatly refer to it too, or doubling power when impedance is halved, is actually not common. It is the theoretical ideal, but in real world output most receivers will NOT double output into lower impedance loads. External amps, which generally have far more robust power supplies are much more likely to achieve this, or at least come closer to it with more demanding speakers. That is where the benefit is IMO - get a demanding speaker (low impedance, low sensitivity) and you can throw a ton of power at it to yeild great results. Far better of an idea than getting ~100+dB sensitive speakers and using a lesser amp... A difficult to drive load isn't a bad thing in my book; you just need to be prepared to have enough power to do what you want to do.

    More to what Arthur mentioned, some manufacturers will use lower impedance speaker designs that are very sensitive so they can squeeze more power "on paper" out of their specs and so they can use a lesser amp - think HTiB all-in-one units. This pushes the amp harder, but likely still within specs, however the TOTAL current available from the power supply is the same and that means when you get peaks, if you are using 80% of your total available current during normal operation, you have NO headroom to handle dynamic peaks and the system will only be capable of a certain amount of SPL for a given room size.
     
  15. Holadem

    Holadem Lead Actor

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    Well, to nitpick here a bit, what I highlighted should be "the current DEMAND". Whether or not there is enough current available to meet the dramatic increase in demand, is the issue.

    --
    H
     
  16. Holadem

    Holadem Lead Actor

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    I just caught passage, it expresses very well what I have been trying to say wrt to engineering compromises.

    --
    H
     
  17. ChrisWiggles

    ChrisWiggles Producer

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    Also note that certain kinds of speaker designs, like planars, have lower impedance by the way they function as well, which is not an intentional thing at all. If you make planar speakers, it's not because you decided to try to make speakers that were more difficult to drive, it's because you think that's the best way to make a speaker sound good.

    On the other hand, others make large horn-loaded speakers which are extremely efficient and easy to drive so can be driven very loud with very low-powered amplifiers like tube amps etc.

    It's kind of just different philosophies on how to get the best sound, so different systems for different folks.
     
  18. Andrew Pratt

    Andrew Pratt Producer

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    I guess its really just a different way of viewing the matter...ie from the speaker end or the receiver end. From my vantage point I look at it as being a resistance issue...in a 4 ohm speaker there's little resistance vs a 16 ohm speaker. As was pointed out there's nothing stopping people from designing 16 or higher ohm rated speakers but you'll seldom see it (I've only ever seen one 16 ohm speaker) because they are IMO more difficult to drive as there's much more resistance...the fact that this is less draining on the amps power supplies doesn't change anything from my view point.

    That said I can understand why others don't view the situation the way I do and really it doesn't much matter so long as everyone's aware of what's really going on and what the implications are.
     
  19. ChrisWiggles

    ChrisWiggles Producer

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    I really don't see how one can characterize a higher impedance speaker as more difficult to drive. It's less strain on the amp.
     
  20. Holadem

    Holadem Lead Actor

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    Neither do I. This really isn't a matter of opinions Andrew, we're discussing facts that are as hard as they get.

    The only thing I can think of is that the word "resistance" is creating some confusion, as it seem to imply that the speaker which "resists" the most is "harder to drive". Based solely on the words, I could see how one would think that way. It is based on this mental model people have where they see the amplifier as PUSHING current through the speaker. But that is not at all what happens:

    From a stricly time domain POV (i.e., forget about frequencies), the amp establishes a voltage at the speaker terminal, effectively telling the speaker to play at a certain volume (more accurately, the driver to move a certain distance). In order to do that, the speaker will simultaneously demand a certain amount of current, based on it's impedance. A speaker with high impedance will need less current. A speaker with lower impedance will demand more current. When the amp cannot meet the speaker demands, funny things happen (distortion).

    --
    H
     

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