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Technical Question my Arcam P-25 amps and M&K speaker SPL

Discussion in 'AV Receivers & Amplifiers' started by Brad_Porter, Feb 20, 2003.

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  1. Brad_Porter

    Brad_Porter
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    Here goes Brad again - lack of deep knowledge and a fairly irrelevant question but I just need to know the answer and how its all worked out.

    M&K have said that my S-85's are slightly higher spec in THX levels. The speakers will produce 106-107db (at 2 meters) which requires a 125Watts into 8Ohm or 250W into 4 Ohms amplifier.

    I believe that my Arcam P-25 will deliver around 140W into 4 Ohms all three channels running. So, what's my question?

    If 250W will get me speakers to play at 106/107db at 2 meters, what will 140w deliver in db's? Even more confusing is that fact that these speakers operate at 6Ohms nominal between the range of 87 - 20,000! :rolleyes:

    I just got to know how they work this stuff out. A worked example of this would be good.

    Somebody please explain!!!
     
  2. EvilMudge

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    Brad, you do ask the most difficult questions sometimes!;)

    About 104dB - can't be more accurate unless M&K have quoted the sensitivity in dBW.
    Effectivley you've got just over 125W which will produce 103dB - but since it's a logarithmic scale it'll be under 104dB.
     
  3. Brad_Porter

    Brad_Porter
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    OK, heres the thing - so my amp (into a 4Ohm load) will only produce around 140W of clean power. M&K say that you need a 250W amp (into 4Ohm loads) to get to the the 106/107db required.

    Are you saying that the 110Watt difference only accounts for a drop of around 3 or 4 dbs? :confused:

    Sorry EvilMudge, I am just trying to get my head around the mechanics around that?

    I know I am a pain. :(
     
  4. EvilMudge

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    Exactly. I'd revise my estimate of the output downwards since 140-125 is a small fraction of 250-125 - more likely you'll get about 103.5dB from your Arcam.
    Every time you double the power input the dB rating will increase by 3 - in a mathematical sense, not real world. So if you started at say 90dbW - that's 90dB at 1W - to get to 93dB you'd need 2W, 96dB 4W, 99dB 16W, 102dB 32W, 105dB 64W, 108dB 128W, 111dB 256W, and so on.
     
  5. Brad_Porter

    Brad_Porter
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    Ok - I understand that completely now. Thanks.

    So what about sound decay? :devil:

    If 64 Watts produces 108db of sound at 1 meter, then (all things being equals such as accoustics and no objects in the way) what would the same measurement be at 2, 3, 4, and 5 meters.

    Ah ha! Answer that one EvilMudge or any of your minions! :D
     
  6. John Dawson

    John Dawson
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    In free field conditions and with no special directivity in the speakers then the inverse square law would apply and the amplitude would halve (i.e. -6dB) for every doubling of distance. So at 2 metres it would be 102 dB, at 4 metres 96 dB and so on.

    In a real room you hit the reverberant sound field at probably 2 metres or so - at that point and beyond there is very little further fall with distance, as the SPL is maintained by all the reflections off the room walls, ceiling and floor.

    BTW a good example of a directional speaker is the line source or column loudspeaker often seen in churches, airports and so on. Here the outer speakers help maintain the sound field at greater distances and the fall off in SPL is more like 3 dB for every doubling of distance. That's why they are designed that way.

    HTH.

    John Dawson (Arcam)
     
  7. Brad_Porter

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    Thank you John and EvilMudge - I shall now refraim from asking difficult questions............

    ...........for now. :devil:
     

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