Help with power needs...

Discussion in 'Beginners, General Questions' started by Tom Jr, Feb 24, 2004.

  1. Tom Jr

    Tom Jr Extra

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    I'm planning a basement HT with about 350-400 ft^2. With a 7'6" ceiling, this comes to about 2500-3000 ft^3. I'm in the very early planning stages, and want to get a feel for an appropriate amount of power needed to fill the room.

    I'm interested in volume levels equivalent to a modern cineplex, maybe a bit less loud. I haven't selected speakers, but let's assume a sensetivity of 90dB. I can scale the numbers later for actual selected speakers.

    Finally, how does one figure out how much subwoofer power is needed, relative to the other speakers.

    Thanks
     
  2. Greg_R

    Greg_R Screenwriter

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    I assume you're asking because you are running electrical in the basement and want to know the total electrical load?

    Modern Dolby Digital soundtracks have a max peak of 105dB for the mains and 115dB for the LFE channel (if you are listening at reference levels). So, assuming a 90dB/W/m sensitivity, we get the following:

    1W : 90dB
    2W : 93dB
    4W : 96dB
    8W : 99dB
    16W : 102dB
    32W : 105dB

    Since a doubling of amp power results in a 3dB increase (doubling of SPL), you need 32W of power to drive reference levels in your HT. HOWEVER, this is a 1 meter and doesn't account for room interaction. I believe the rule of thumb is a doubling of power for each meter (or ~3ft) so plan on 128W per channel of power to acheive clean reference levels. Note that some speakers can't hit these levels due to the limitations of their drivers.

    Most subwoofers come with a built in amplifier. I would plan on 1000W of electrical load. Most sub amps are Class D and are very efficient (i.e. more output power with less electrical load). You will choose your sub based on your performance needs and room size (which is medium).

    Overall, 2 20amp circuits (on the same phase) would be more than enough for 99% of the systems out there.
     
  3. Tom Jr

    Tom Jr Extra

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    Sorry about the confusion. My intent is to determine my equipment needs. For example, do I need 100W/channel or 200W/channel, etc?

    When people refer to reference levels (105dB), is it in reference to a single speaker, all speakers, or some subset? Also, is 105dB realistic?

    Here's why I ask:

    My first row will be about 11 feet from the screen (and mains). My second row will be about 5'6" back, so 16'6". The average of these is 13'9" (4.2m). I know that spl falls off at 1/(d^2) so thats 6dB for every doubling of distance. At 4 meters, there should be a 12dB drop, so if I want 105dB at the (average) listening position from a main speaker, I need 105+12=117dB at 1m. Given 90dB/w/m speakers, thats a 27dB power gain, or 500W per channel. Obviously something's wrong here. Can anyone help me find what it is?

    Thanks
     
  4. Greg_R

    Greg_R Screenwriter

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    105dB is the peak level each channel needs to reproduce in a properly calibrated DD/DTS system (115dB for the LFE channel). However, these levels are only seen maybe 1-2 times per movie (so designing your system around an event that happens 1/2 sec. per movie may not be the best allocation of funds). If this is crucial to you, consider more efficient speakers (horns, etc.). Read the FAQ for more on reference levels.
     

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