Ohm Question

Discussion in 'Archived Threads 2001-2004' started by Daniel_M, Nov 19, 2001.

  1. Daniel_M

    Daniel_M Stunt Coordinator

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    I'm a little confused about

    the relationship between ohm ratings

    for amplifiers and speakers.

    Specs say an amp is rated for say,

    400 watts at 8 ohms and 800 for

    4 ohms etc. Are there 8 ohm and

    4 ohm speakers or can the same speaker

    be rated at both 4 and 8 ohms?

    Thanks
     
  2. Wayne A. Pflughaupt

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    Daniel,

    With speakers, the ohm rating you see is nominal, i.e., kind of an average. In reality, a speaker’s impedance will vary with different frequencies. For instance, it can drop as low as 1-ohm (or even less) at some frequencies, with a high well into the double digits at other frequencies. Typically though, the ohm variance is not as severe as that. A speaker’s nominal ohm rating is fixed, but it can be altered interally by modifying the crossover. A job for the design engineers, not you or I.

    With amplifiers, the ratings refer to how much power can be delivered with a certain load (ohms). In theory, halving the load (i.e., 8-ohms to 4-ohms) will draw twice the power from the amp, but in reality that is seldom the case. If you see an amp rated for 200 watts @ 8-ohms and 275 @ 4-ohms, that means it is not as “robust” as one that will deliver say, 375 watts @ 4-ohms. It would be safer to use 4-ohm speakers with the latter than the former.

    Hope this helps.

    Regards,

    Wayne A. Pflughaupt
     
  3. Saurav

    Saurav Cinematographer

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  4. Daniel_M

    Daniel_M Stunt Coordinator

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    Interesting........another question.

    You don't see alot of speakers (nominaly)

    rated at 4 ohms. Is there a reason most speakers

    are rated at 8? If you get a better punch

    from your amp with speakers rated at 4 ohms

    it seems this would be a better speaker to

    buy. Is it a price thing? Are the 4 ohm rated

    speakers more expensive?

    Dan
     
  5. Chu Gai

    Chu Gai Lead Actor

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    The best speaker to buy is the one that'll sound best to you in the environment you intend to play it in. To that end, nothing beats being able to audition the speaker in your own place. That said, it may be that you're mistaking the nominal ohm ratings of speakers with efficiency which basically states just how much sound pressure (loudness) a speaker will produce given one watt of power at a 1 meter distance. what is it that you're trying to achieve and what it is that you presently have?
     
  6. Saurav

    Saurav Cinematographer

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  7. BenS

    BenS Stunt Coordinator

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    I must be missing something here.

    Wayne states:
     
  8. Saurav

    Saurav Cinematographer

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  9. BenS

    BenS Stunt Coordinator

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    Yes, thank you a lot. I appreciate the information I get on this forum. Again thanks for the info.
     
  10. Chris PC

    Chris PC Producer

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    As far as ohm rating go, it works like this as far as I'm concerned. If your AMP can handle it, you do benefit from lower impedance, lower ohm rated speakers because an amplifier that puts out 100 watts into an 8 ohm speaker, can, if it has a good enough power supply, and can handle the current that low impedance demands, then you get 200 watts into the 4 ohm speaker. That is an over simplification, but it is generally true. I have 8 ohm speakers and 6 ohm speakers. My amp can handle the 6 ohm speakers, so I do get more dB's out of them because they are able to 'extract' more watts from my amp/receiver.

    What ohm ratings are good for is this. You must pay attention to them in order to match the speakers to the amp. You don't want to use 4 ohm speakers on a receiver that tells you "Do not connect speakers less than 6 ohms" for instance. There is some room to move with that, but generally, don't hook up 4 ohm speakers to an amp that you know is designed only for 8 ohm loads.

    Amplifiers can give you voltage for volume gain, but it is harder for them to put out that gain at higher current demands, those of lower impedances. It is also a bit more expensive to provide lots of current, so generally, unless the amp is designed for high current loads, you will find that it puts out its max power at 6-8 ohms. Going to lower ohm speakers won't necessarily give you more watts because the amp can't deliver the current.
     
  11. Daniel_M

    Daniel_M Stunt Coordinator

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    OK, so, if I have an amp and the specs say

    it's rated at 400 watts into 8 ohms and

    650 watts into 4 ohms, am I getting

    more bang for my buck if I buy speakers

    rated at 4 ohms?

    Are we talking more loudness

    at a given mark on the volume control for the

    4 ohm speakers as opposed the the 8

    or is the amp manufacturer just letting me

    know the power is there if I need it?

    Dan
     
  12. Saurav

    Saurav Cinematographer

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  13. Wayne A. Pflughaupt

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    Ben,

    Saurav’s description of the crossover in speakers was very good, but for additional clarification:

    Actually the crossovers are comprised of coils, capacitors, and resistors. The resistors are there for a number of reasons, like “padding down” a tweeter to make its lever better match the woofer. The resistors used also affect the speaker’s overall impedance rating.

    For instance, consider some speakers I have: two way, woofer and tweeter each are 4-ohms. Yet, the binding posts measure 8-ohms. How can this be, since the speakers have parallel connections? Obviously the resistors in the crossover are doing it. Actually I removed the tweeter, and I thought that would give me 4 ohms at the binding posts. Wrong! It was still 8 ohms!

    That’s why I said that the design engineers can change a speaker’s impedance rating.

    Chris,

    The reason you “get more dBs” from the 6-ohm speakers has more to do with the speaker’s efficiency than the impedance rating. For instance, an 8-ohm speaker with an efficiency rating of 105dB will deliver higher SPL than a 4-ohm speaker with an 80dB rating.

    Daniel,

    The example I gave Chris should be convincing that a speaker’s impedance rating has little to do with the “bang for the buck” it delivers. Bottom line, a 4-ohm speaker is going to make an amp work harder. Just make sure your amp can handle it before you bring home 4-ohm speakers

    Regards,

    Wayne A. Pflughaupt
     
  14. Thomas_A

    Thomas_A Second Unit

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    One thing to consider...normaly an amp rated at 8ohms,6ohms and say 4ohms...the lower you drop the ohm rating..normaly you will see the distortion rate go up.
    say an amp is rated at 8ohms 100w @ .02% thd... at 4ohms.. it may be 200w but at .1% Thd... Now in reality... anything below 1% is not noticed by human ear..but imo felt by the speaker itself. One thing to do is to check out all aspects of th situation and understand before proceding[​IMG]
     
  15. Daniel_M

    Daniel_M Stunt Coordinator

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    Thanks Guys,

    Actually I'm not buying speakers.

    I have Paradigms which I am very happy

    with and they ARE rated at 8 ohms.

    I am amp shopping however, and I was

    just curious about what the ohm ratings

    in amp specs meant. Now I know. Thanks.

    Dan
     
  16. Bryan Acevedo

    Bryan Acevedo Second Unit

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    One thing that is interesting, is that most home speakers are rated at 8 ohms, while almost all car audio is rated at 4 ohms. I never understood why there is a difference - anyone know? My guess is because a car system is only 12V, and, therefore, has to have a lot of current to get the power out of an amp and a 4 ohm load is more appropriate and the amps in a car system can handle it. If I remember correctly, it takes more current (amperage) to drive a lower load, and that is why some amps (that aren't high current) have trouble driving a lower ohm load. And most of the time you are fine, it is only when you start cranking the volume, for long periods of time, and all speakers are set to large, that you will most likely have a problem. The size setting is important, because generally the dips in the ohm rating are at lower bass frequencies, and setting your speakers to small, cuts out the bass, and the hardest load the amp sees.

    Bryan
     
  17. Mark_R

    Mark_R Agent

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    Thanks to all for the questions and replies. I have found this thread very enlightening. [​IMG]
     
  18. Kevin Peak

    Kevin Peak Agent

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    I too would like to add a little nugget of knowledge (that is, if i am right, which i'm probably not... hah)

    When taking into consideration the sensitivity of a speaker, there are a few things to remember. If a speaker rated at say, 90db @1W that means that with one watt it is 90 db loud one meter away from the speaker. If one doubles the watts, you yield three more db headroom at the spot one meter away.

    1w - 90db

    2w - 93db

    4w - 96db

    8w - 99db

    16w - 102db

    32w - 105db

    64w - 108db

    128w - 111db

    256w - 114db...

    From the above, one could imagine a situation where if they have an amp that is still very robust and has lots of watts at 4 ohms, you could get a 4ohm speaker that has a sensitivity of say 86-88 and still have more db headroom than the 8 ohm speaker with sensitivity of 90db.

    Now, the db level when sitting on a coach across the room, the db levels are going to be lower than listed, but as you can see, without alot watts, one can get reasonable db levels... at least in smaller and medium size rooms like most of us have. There are always other factors like room size, wall coverings, etc. that can affect the situation.
     

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