Lower imedance and sens = same output?

Discussion in 'Archived Threads 2001-2004' started by John Royster, May 24, 2002.

  1. John Royster

    John Royster Screenwriter

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    Provided I have a very capable stereo power amp that provides realistic stronger power ratings into 4 ohms, say 200w at 8 ohm and 350w at 4 ohm...

    Can a speaker with 3 db lower sensitivity but nominal impedance of 4 ohms play as loud as a speaker with impedance of 8 ohm?

    Say - 88 db, 4 ohm compared to 91 db, 8 ohm speaker.

    Seems to make sense to me, but maybe there are other factors I'm not taking into consideration?

    Thanks in advance,
    John
     
  2. Gregory F

    Gregory F Auditioning

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    No, I do not believe so.

    As I understand it, the sensitivity ratings of speakers are really at a 2.83 volt reference (1 watt at 8 ohms). A 200 watt @8 ohms amp will drive 23dB above reference so 91dB sensitivity + 23 dB = 114dB assuming the speaker can handle the power without breakup (distortion induced by non-uniform movement over the surface of the cone). A 400 watt @ 4 ohms amp will drive the same 23 dB above reference (i.e. the same voltage at twice the current as the 8 ohm example). So it would play at 88+23dB = 111dB (again assuming no breakup).

    I am sure someone will correct me if I am wrong

    Greg
     
  3. Norm Strong

    Norm Strong Stunt Coordinator

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    Quite the contrary. The 88db 4 ohm speaker will draw twice as much power and STILL be 3 db lower in output. Sensitivity is rated with an input of 2.83v--regardless of the impedance of the speaker.

    So actually you are 4 times worse off with the 4 ohm speaker. Half the output and twice the power, all at the same time.
     
  4. Mark Austin

    Mark Austin Supporting Actor

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    While the two previous posts are scientifically correct, they may not be correct in reality. All things being equal? Sure. But, things are rarely equal. While the less sensitive speaker would theoretically not play as loud, that really can't be established by the info given. For instance, that 8 ohm speaker may start to compress at 95dbs., while that 4ohm may compress at 100dbs, or vice versa. So, in real terms, it could be either speaker that plays louder cleanly, and there's no way those numbers can tell us that.
     
  5. Mark Austin

    Mark Austin Supporting Actor

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    On top of that, there's no way a speaker starting out with 88db's or 91db's will ever come close to maxing out 200 watts in volume terms. I would highly doubt that there are many speakers that could handle 109 db's(88 db's at 128 watts) continuous.

    4 ohm speaker with available 350 watts

    88--1
    91--2
    94--4
    97--8
    100--16
    103--32
    106--64
    109--128
    112--256
    _________________
    ***115--512***

    8 ohm speaker with available 200 watts

    91--1
    94--2
    97--4
    100--8
    103--16
    106--32
    109--64
    112--128
    ___________________
    ***115--256***
     
  6. John Royster

    John Royster Screenwriter

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    Hmmm, very interesting.
    I get anywhere between 105 and 110 c-weighted with my rat shack meter on my current 91 sens speakers and a 150 watt HK amp. At this point the amp still has some power left, it is the speakers that are starting to reach their limit...the mid-bass drivers are traveling too far and overexcurting. (is that even a word?) But then again it depends on the recording and music. Bass heavy stuff overdrives the speakers much sooner in terms of volume.
    We'll see what happens. I'm buying speakers tomorrow and have some serious auditioning to do. Either the B&W CDM 7/9 NT or the polk LSI 15/25. That's a pretty good chunk of change, but I still need them to play very well at pretty insane volumes.
    Hence the B&W 90/8 Ohm vs Polk 88/4 ohm. For my listening habits I need a speaker that can play moderate to very loud and not wimp out. If only I could enjoy horns....*sigh*
    Thanks for the help!
    ps - i won 500 bucks tonight in a promotion at my local bar. I figured it had to be an omen because I was gonna buy speakers tomorrow anyway. Now I have an excuse if not a preordained destiny to go up a model. I'll be picking up a rotel 1080 as well. [​IMG]
     

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