Ohms?

Discussion in 'Speakers' started by Andrew O'Brien, Dec 21, 2003.

  1. Andrew O'Brien

    Andrew O'Brien Stunt Coordinator

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    In preparing for Santa's delivery of a new surround sound receiver I've been reading the FAQs and basic HT primer. I note that the section on understanding Ohms says...


    "First of all, I hope you bought 8 ohm speakers. Especially if that is the only speaker on that channel. Remember that ohms fluctuate widely during playback. However, a good built in speaker crossover network will keep you safe. If you use 4 ohm speakers, which some receivers allow, fluctuating ohms could easily be below 4 and even 2 ohms during playback of a signal. These momentary deviations are not a problem if the receiver is built to handle this and your built in crossover network in the speaker cabinet is a quality product. PS// Faulty speaker crossovers have been known to fry amplifiers to a well done state"

    Well, the Infinity speakers I plan on using for my L and R fronts have the follwoing specs.

    Frequency Response: 47Hz - 25kHz
    Crossover Frequency(ies): 400Hz and 3.5kHz
    Sensitivity: 90dB (1 watt/1 meter)
    Nominal Impedance: 6 ohms
    Power Rating: 20 - 125 watts
    Woofer: 8" (20.3cm) IMG
    Midrange Driver: 5" (12.7cm) IMG
    Tweeter: 1" polycell

    Since it says "nominal impedance is 6 Ohms", that is NOT 8, is this a problem ?
     
  2. Wayne Ernst

    Wayne Ernst Cinematographer

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    Which receiver will you use to power these speakers?
     
  3. reed

    reed Auditioning

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    fine, i think it's no problem when using multi-channels av receiver.

    you might notice ohms' adaption once you get a transistor amplifier. [​IMG]
     
  4. Chu Gai

    Chu Gai Lead Actor

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    And the above quote, punctuated by a grin means what specifically?

    To answer your question or make a reasonable attempt at doing so requires a bit of backing up.

    Speakers, can and do, have different nominal impedances and the way this nominal impedance is specified varies amongst manufacturers. For the purpose of this thread, let's simply call it the average impedance. Now if you ever peruse audio publications, you may find graphs that show the impedance of a speaker as a function of frequency. You'll see peaks, valleys, relatively straight portions, and a rather obvious sense that there is a lack of any kind of symmetry. Speakers you see are composed of drivers and a crossover network that forms a rather complex electrical (and acoustical) network. As such, if one were to measure the impedance (resistance if you will) of the system at a particular frequency, you'd find that depending upon the frequency, different values are obtained. For example, consider the following graph as an illustration of this.
    [​IMG]

    One of the specifications that you'll see with receivers is that of their power rating. What follows is a reader's digest version of this so if you want to nit-pick about what I'm saying, feel free. It's close enough for government work IMO. Now if the manufacturer is specifying stereo power (only 2 channels), then they are bound by FCC regulations to do so in a particular way. After a mandatory warm-up time, the power is measured across a frequency band, typically taken as 20-20kHz, into an 8 ohm load (a resistor) and the distortion is given. Hence, you'll see something like 110 watts/channel, 20-20kHz,
     
  5. John S

    John S Producer

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    In short, I think you will be ok.. I think 6ohm actually becomes an advantage with most systems.
     
  6. reed

    reed Auditioning

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    i was not sure about the right way to express my opinion.

    anyway, the curve tells me the concept of controllabilty of amps. thanks!
     
  7. Andrew O'Brien

    Andrew O'Brien Stunt Coordinator

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    Thanks Chu Gai for all the effort you put in to your reply. I read it a few times and "think" I comprehend most of it.

    One more dumb question...if 4 Ohm speakers require "beefier" receivers with better power supplies and/or capacitors, what is the advantage of buying 4 Ohm speakers?
     
  8. Chu Gai

    Chu Gai Lead Actor

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    Maybe you liked them?
     
  9. Andrew O'Brien

    Andrew O'Brien Stunt Coordinator

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    Maybe I should re-phrase the question. Why do they make 4 Ohm speakers if they are more demanding on a receiver?
     
  10. Chu Gai

    Chu Gai Lead Actor

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    Boy, it's gonna sound like I'm just some kind of smart ass. They make them because the designer, for whatever reason, felt that his/her particular selection of drivers best met the needs at hand. Those needs are intertwined with things like driver availability, price, performance capabilities, design goals, and so forth. 4 ohm speakers are nothing new and going back to the 70's we'll find that the AR-3's for example were a 4 ohm speaker. Not only that, but they had an efficiency of 80. Just the way things are. Buying a speaker that's 4 ohms means you'll need to either be more selective in what receiver you buy or you'll have to realize that if all you've got is a receiver that'll do 8 ohm loads then you won't be able to use the full capability of that receiver. For smaller rooms this is no big thing. For larger rooms it is. Further when you're on a budget you'll just have to be a bit more selective which speakers you choose especially in larger venues.
     
  11. Chuck Schilling

    Chuck Schilling Stunt Coordinator

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    Actually, I can also think of one VERY good reason for 4 ohm speakers, particularly for surrounds.

    If you wire two 4 ohm speakers in series, that will result in a total impedance of 8 ohms. You might want this in large HT's where you need multiple surrounds on each side wall.
     
  12. Cees Alons

    Cees Alons Moderator
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    Cees Alons
    Chu Gai already said it: power is voltage times current. In practice, the highest voltage to be used in specific standard circuits is pretty well defined. And the maximum voltage at a speaker in domestic environments cannot be really high.

    So to increase the power level, you need more current through the speaker coil. At a given max. voltage, that can only be achieved with a lower impedance.

    Cees
     

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