Continuous vs Dynamic? Help?

Discussion in 'Speakers' started by Mark Shannon, Dec 11, 2003.

  1. Mark Shannon

    Mark Shannon Screenwriter

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    In my search for a new Subwoofer, I seem to have come across a rather nice find, the Polk Audio PSW202 for a reasonable price. listed in the website, it claims to have:
     
  2. DavidLW

    DavidLW Stunt Coordinator

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    Dynamic power is how much power it can pump out in a short burst. By short burst, we're usually talking in the milli-second range. It is usually refer to as "headroom" in High End audio gear.
     
  3. Jack Shappa

    Jack Shappa Second Unit

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    I got that subwoofer for free when I bought my Polk Speakers. I thought I'd be selling it quickly to get a "real" subwoofer, but damn if that thing doesn't rock. At only 1/3rd of the way up the thing pounds out the low end very nicely. It won't go way down to 20hz or anything, but I'm more than happy with it and feel no rush to replace it... yet [​IMG]

    - Jack
     
  4. Robert AG

    Robert AG Stunt Coordinator

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    Amplifiers with a very stiff power supply (which is actually more expensive to do) will have little or no "dynamic headroom".

    "Dyanamic headroom" is really a misnomer. It is really about "dynamic collapsing" of the amplifier's power supply. An amplifier with a perfect power supply would have no dynamic headroom. A lesser amplifer with a weaker power supply may be able to put out 50 watts continuously and 100 watts for a fraction of a second - which is called a "dynamic headroom" of 3db. In reality this is the amplifier is unable to sustain 100 watts for more than a brief period - it is really a 50 watt amplifier.

    The fallacy with the "dynamic headroom" arugment is that the "headroom" is only available for a brief period - but what happens when your power demand lasts for more than this period? What happens is that the amplifier collapses back to what it can sustain. This sustainable level is defined as it's continuous power.

    Continuous power is the only valid rating when it comes to what an amplifier will actually do. "Dynamic headroom" is really a song and dance that makes an amplifier seem more capable than it really is. This got out of hand in the 1970s with manufacturers using terms like "music power" and "peak power", all of which were just ways to inflate ratings on paper. The Federal Trade Comission finally stepped in and made manufacturers adhere to continuous power ratings.

    These rigid standards seem to be eroding away with home theater amplifiers. It's hard to make an amplifier with 7 channels of a solid 150 watts, and some manufacturers are trying to stretch the truth.

    So compare the continuous ratings of amplifiers (at the same load impedance, usually 8 ohms) to get a real idea of their capabilities.

    Also, a perfect amplifier should be able to double the power output for each halving of load impedance. For example, a 100 watt amplifier at 8 ohms should be able to put out 200 watts into 4 ohms and 400 watts into 2 ohms. Almost no consumer power amplifiers can actually do this, but it is a good measure of the amplifer's current capabilities and power supply stiffiness.
     
  5. Mark Shannon

    Mark Shannon Screenwriter

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    Well I'm either considering that one or the Axiom CTS-120 , which I found on ebay. Problem is, I can't seem to find any information on the CTS-120. Anyone who could direct me to some sort of website? The Axiom site doesn't seem to list the older subwoofers.
     
  6. Benihana

    Benihana Stunt Coordinator

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    Roberts brief about the amplifiers ratings seem accurate. Take a look at the old Nad 2200 amp. It is rated 100watts RMS x 2, but it has +6db of headroom. Now some people are saying this amplifier has as much power as some 500x2 rms amps, which could be true, but for how long? It seems that for a milisecond it can, but then its back to a 100x2rms. And I really don't get what dynamic headroom really means to us anyway. I mean, lets say the amp has 100 watts, and you are playing well under 20 watts, which is probably really loud by itself, wouldn't that extra 80watts be our headroom. Is dynamic headroom how much power can give over the rms rating??
     
  7. DavidLW

    DavidLW Stunt Coordinator

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    "Dynamic Headroom" is usually refer to the amount of power over the rated continuous output. "RMS" is another one of those measurement used by low end amps to make the amp appear more powerful on paper than it really is. Another factor not mentioned in low end amps is the %THD (Total Harmonic Distortion) at the rated output. Most high end amps rate thier continuous output at .001% THD. and still have a .001% THD at double the rated output (3db headroom). Most low end amps will have maybe .1% THD or more at rated continuous output and over 5% THD at double the rated output. If high end amps follow these guide lines, a 200w amp would be rated at 500w, easily. 6db of headroom is useless if it has a rating of over 10% THD.
     
  8. Edward J M

    Edward J M Cinematographer

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