The SOUND of Ohms?

Discussion in 'Amps and Cabs [BG]' started by Arranger, Apr 20, 2004.

  1. Arranger


    Mar 9, 2003
    What would you assume be the general difference in sound between running one 4 Ohm cabinet with an amp rated at say, 400w@4ohms vs. running one identical cabinet rated at 8 Ohms off the identical amp?
  2. In general, the 8 ohm cab wouldn't be quite as loud.

  3. I don't know if this has anything to do w/ the question, but I've often wondered if cabs of different impedances drivin by the same amount of power sound - does a 4ohm 4x10 drivin by 400watts sound identical to an 8ohm 4x10 driven by 400 watts.... And where would a 2ohm 4x10 driven by 400watts fall? I dunno - I've thought about it a few - does an amp producing 400 watts into a 4 ohm load have a different sound then an amp producing 400 watts into an 8ohm load? It makes sense to me that they would have a different wound as the amp would be working harder to produce the 4 ohm output....
  4. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    That brings up a very similar question:
    How long is a piece of string?

    But seriously, would a 4-ohm loudspeaker sound different from an 8-ohm one?

    Maybe. All else being equal, it'll tend to be a little louder, because the power into the speaker will be double what it would be into an 8-ohm speaker. But with loudspeaker systems there are a lot of other variables involved, too, so there are no firm "this sounds better" rules.
  5. any thoughts on my post?
  6. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    You're wondering if the amp itself would sound different driving a 4-ohm load versus an 8-ohm load? It would depend on the amp, but most wouldn't, particularly solid state ones. Tube amps would be most likely to sound different.
  7. 8 vs. 4 ohm in tone? I've never heard any consensus in the audio community.
  8. Ohms? Well, they sound like this:

    [Settles into mild meditative trance.]
    [Comes out of trance state.]

    Oh, sorry, that was "Om's," wasn't it?

    Seriously, though. Ohms is simply an electronics term. The ohm rating of a cabinet shouldn't affect it's tone in any way whatsoever. If you can hear a difference in tone between two similar cabinets with different ohm ratings, I would say that this has to do with how that particular amplifier reacts differently to the different loads. It has nothing to do with the "sound" of the different ohm rating of the cabs. That would be my guess, anyway.
  9. One way to think of this is that any given amp would have to drive harder to achieve the same volume with an 8 ohm cab as apposed to a 4 ohm cab. That may result in a change of sound comming from the amp itself, not the speakers.

    If the amp likes being cranked up to sound good who knows, this might be of interest.
  10. bassmanjones


    Feb 23, 2002
    Boston, MA
    I always thought that as the ohm rating increases (decreases?) from 8 ohms to 4 ohms to 2 ohms etc. that the THD increases creating a loss of clarity. Whether or not this is actually audible to anyone without golden ears, who knows...I've never sat down to try it.

    Bob Lee, am I right or no?
  11. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Yeah, if you want to be nit-picky about it, in technical terms, no amp will sound the same under different load conditions. But if the amp is of reasonable quality, there's no way the listener could tell the difference, the variation in the actual sound would probably be so minute relative to whatever signal was being applied, that it would be below the psychophysical threshold of "noticeable difference". The question as I understand it pertains to both the amp and the cab (or let's say, the system as a whole). If you simply swapped the speaker with an identical speaker of lower impedance, there's no way you could do that without altering the conditions in the amp. The speaker is a reactive load and the amp is designed to accommodate it. On a tube amp you'd have to change taps on the output transformer, and if you didn't do that you'd have an impedance mismatch and your amp would definitely sound noticeably different (if you listen carefully, and especially at full volume). In a solid state amp (an ordinary one), there's usually a big capacitor hanging right on the output, in series with the speaker, so if you change the load impedance you're definitely changing the sound of the amp. Whether or not that change is significant is probably one of the differences between a well designed amp and a bad one. I'm not sure how it works in digital amps, I'm guessing that the same considerations would apply, but Bob please correct me if I'm wrong.
  12. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    If you go to a speaker manufacturer's site and look at the Thiele-Small parameters of a 4 ohm and 8 ohm version of the "same" speaker, they are slightly different.

    This would suggest that when placing each one or several of each in a given cabinet they could sound different. How different...who knows.

    Would your audience notice or even care? Nope.
  13. Petebass


    Dec 22, 2002
    QLD Australia
    True but most of them can't tell the difference between a bass and a guitar either :)
  14. One of the differences that you might hear would be down to the quality of the capacitors used in the amp's power supply.

    Cheaper capacitors do not release high currents as freely as audio quality ones, or banks of smaller ones if the hifi nuts have got it right.

    On that basis, demanding higher peak currents from the amp will change the character of the sound.
  15. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Two out of three ain't bad. ;) The first part about minuscule changes is spot on, and so is the part about taps on a tube amp, but the part about a big capacitor in series with the output isn't correct. There have been some amps like that, and maybe some of those are still on the market, but thanks to better and more experienced design engineers, the vast majority of power amps don't have capacitor-coupled outputs.

    The actual quality of the audio from the amp shouldn't change with the load impedance, as long as the amp isn't clipping as a result of the load. Below clipping, there shouldn't be any audible difference between "working harder" and "not working as hard" in an adequately designed amp.
  16. ... and there endeth the reading for today... :D

    It doesn't get much more authoritative than that.
  18. The only caveat in there is "in an adequately designed amp."

    Otherwise, what else can you say?
  19. Yeah. I caught that, too. Sounds like Bob Lee (aka, "The Voice of God") is referring to some kind of ideal, strictly hypothetical "perfect" amplifier. Which makes me wonder if you would, in fact, hear a difference using "real world" gear.

    If nothing else, the amplifier would produce much more power at 4 ohms than 8, so you'd have to turn it way down to match levels for a 4 ohm load. Wouldn't that mean you'd have more headroom with 4 ohms than at 8? And, wouldn't you be able to hear that?

    I must say, it certainly is a thought-provoking question.
  20. 7flat5


    Nov 28, 2003
    Upstate NY
    I don't think "the voice of QSC, which is close to God" was referring to amp perfection at all. When you are the voice of something, you choose your words carefully, and he said "in an adequately designed amp." Adequate and perfect are very different things, and I think Bob was referring quite rightly to adequate. There should be no difference in sound.

    I do wish Bob or someone with some tech knowledge could address the loudness issue a little more fully, though. Speaker efficiency (and I know that strictly speaking it's not efficiency precisely) is expressed as dB at 2.83 volts because that represents 1 watt into 8 ohms. It represents 2 watts into 4 ohms, though, for the same dB output. So, twice as much power into half the impedence is more watts, but not more dB. And, doubling the power into an identical speaker, assuming no dynamic compression, is still only 3 dB more output. So, when bass players with partial understanding of the electronics involved want a 4 ohm cabinet because their amp will "put out twice the watts" it results in exactly no more sound pressure. And, it stresses the current capacity of the amp's power supply. So, maybe the higher impedence option is usually better. Am I off base on this?