If reference level is 85 db (?) then I am not even using a watt of power

Discussion in 'Archived Threads 2001-2004' started by Aslam Imran, Jul 17, 2002.

  1. Aslam Imran

    Aslam Imran Second Unit

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    If reference level is 85 db(?) and my speakers are 90 db efficient then I am not even using a watt of power to listen at reference levels. Does that sound right?
     
  2. Andrew Pratt

    Andrew Pratt Producer

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    Well sort of remember DD specs are for peaks to 105dB so during those moments you're calling for A LOT more power which is where your headroom comes into play.
     
  3. Aslam Imran

    Aslam Imran Second Unit

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    So the peaks are 20 db above reference so if I am using 1 watt at reference all I need is 128W (20db headroom = 2^7 W = 128) to reproduce those peaks faithfully. So with 90 db speakers and a 125 x 5 amp I should be able to reproduce a movie at reference levels? of course that would be if I am sitting one meter away from all speakers. Whats the attenuation of sound as a function of distance from the source i.e., how many db/feet?
     
  4. Matt Jesty

    Matt Jesty Second Unit

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    Movie theaters used to balance their systems so that normal conversation on screen would be ,to the audiance, at the apparant same volume as real live normal conversation (@75 db) , not a problem for any speaker-amp combop, the problem comes when you want to realistically recreate (instintaniously) the realistic volume level of a Howitzer round landing next to those people having that normal conversation ! Not many speaker-amp combos like going from producing 75 db one second to 150db the next...efficiency in a speaker(all other things equal) never hurts...the real world(and the make believe one) are VERY dynamic
     
  5. Jason Wilcox

    Jason Wilcox Supporting Actor

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    1w-90db
    2w-93db
    4w-96db
    8w-99db
    16w-102db
    32w-105db
     
  6. Patrick Sun

    Patrick Sun Studio Mogul

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    If you sit 1 meter from your speakers, sure, you're using less an a watt to get to 85dB. [​IMG]
    Remember, SPLs measured diminish with the same amount of input power as distance increases.
     
  7. Jason Wilcox

    Jason Wilcox Supporting Actor

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  8. Aslam Imran

    Aslam Imran Second Unit

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  9. Tony Lai

    Tony Lai Stunt Coordinator

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    THX reference is 75dB for average levels (ie. conversation, ambient/environment). You also need to be able to instantaneously up the volume to ~100dB for transients.

    However this is a tough ask in a dynamic and perhaps sound absorbent living room with inefficient speakers.

    T.
     
  10. ThomasL

    ThomasL Supporting Actor

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    Aslam,

    90db is the sensitivity rating at 1meter distance with 1 watt of power. There are other factors involved into how much power your receiver is putting out to reach certain db levels from your listening position. Using an SPL meter you can stand about 1 meter from the speakers and then calibrate the Avia test tones at 75db. Then go back to your listening position and see how much the decibel level drops off. Due to room acoustics, distance from listening position etc., it will be different for everyone. As others have pointed out, in order to get an increase in 3db of loudness, power must be doubled.

    cheers,


    --tom
     
  11. Saurav

    Saurav Cinematographer

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  12. Andrew Pratt

    Andrew Pratt Producer

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    6dB/meter are the same numbers I've seen Saurav. Also if the calculations say you need 120 watts and your amp is rated at 125 I wouldn't feel to comfortable...amp ratings are often inaccurate and even if they were right that doens't leave a lot of head room left which means more compression at higher volumes and a great chance of clipping.
     
  13. Michael R Price

    Michael R Price Screenwriter

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    It's not exactly 6db per meter, but rather 6db per doubling of distance. So at 4 meters sensitivity is decreased 12db.

    Also, I think it's "okay" for your amp to be clipping at 105db per channel. As I understand it, the 105db level is only used in momentary peaks, and the continuous output per channel is usually in the 95-100db range even in the loudest passages. So it probably won't hurt if your maximum clean output capability is, say, 103db.

    Slew rate, which is an amplifier's transient "speed" capability, is usually not an issue in solid state amplifiers. It's usually on the order of several volts per microsecond. I don't know about tube amps though.
     
  14. Eric T

    Eric T Second Unit

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  15. Saurav

    Saurav Cinematographer

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  16. chung_sotheby

    chung_sotheby Supporting Actor

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    Actually, 150 dB is louder than a horwitzer. A horwitzer is roughly about 130 db, while the sound when standing directly under a 747 taking off is around 140 db, which is the threshold of the human hearing barrier. I seriously doubt theaters would ever crank anything at 150 db, as they would numerous lawsuit pressed against them for negligence. As I remember, the loudest Who concert ever recorded was around 128 dB, and look where it got Pete Townesend.[​IMG]
    As for amp power, many decent amps can handle momentary peaks higher than their RMS wattage. For a good amp, you can usually have momentary peaks of up to 25% above the RMS rating, but if you have any sustained passage of higher than RMS wattage, you can damage your amp and your speakers.
     
  17. Aslam Imran

    Aslam Imran Second Unit

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  18. Saurav

    Saurav Cinematographer

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  19. TylerN

    TylerN Stunt Coordinator

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    I think the whole 'instantly deaf' thing is a bit dramatic. The frequency of the sound has alot to do with it.. for example, SPL competitions for car stereos frequently exceed 160db!! I'm sure that's a note under 100hz though.
    http://www.soundoff.org/00finspl.htm
     
  20. Jeremy Hegna

    Jeremy Hegna Supporting Actor

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    A little off topic here...
    I read an article in our local paper this past weekend about the Navy using low frequency sonar. They are limited to within 15 miles of the shoreline in Alaska, Hawaii, eastern and western sea-boards because of damage to the fish and mammals in the sea.
    The sonar will hit 230-250 decibles in the low hz (sub 20) range[​IMG]
    Thought that was pretty interesting, being that you guys are talking about SPL's.
    Jeremy
     

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