I'm confused with watt ratings, sensitivity, and overall response. Can we discuss...?

Discussion in 'Archived Threads 2001-2004' started by Javier_Huerta, Jan 29, 2003.

  1. Javier_Huerta

    Javier_Huerta Supporting Actor

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    Say, for example.

    I have a set of speakers with 91 dB sensitivity, and a receiver with 130 watts max power. This particular receiver was rated at the SMR forums as having 82 watts of real RMS power.

    So...

    I'd think I will be able to get around 109 dB continuos (64 watts RMS power) and 112 dB peak (128 watts peak) - considering every 3 dB require double in power. So far so good.

    But these ratings are measured 1m away from the speaker, right? If SPL's on a piston speaker drop 6 dB every doubling of the distance, I'd get -6 dB at 1 m, -12 at 2 m, -18 at 4 m?

    So I'd be getting around 91 dB continuous at 4 meters away from the speakers? That's not even *near* reference, right?

    Furthermore, the subs I was designing can deliver 120 dB at max RMS power. So they are, to say the least, overkill for my speaker system.

    My question is... am I forgetting something? I can't believe my system can only provide 95 dB of continuous SPLs with 80 watts RMS. Something must be wrong (I hope).

    And then, if first, second and third order reflections are the reason the drop in SPL's is not that brutal, if I treated my room as if it were an IMAX theatre (dead silent), would I really only be getting those 91 dBs?


    Thanks for your help!
     
  2. John Garcia

    John Garcia Executive Producer

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    Calibrated reference is 85dB with 105dB peaks. Reference is measured from the listening position. So if your system is theoretically capable of more than that, you do not have a problem.

    I say theoretically because it doesn't sound like you have actually done any readings with this gear in this room yet.
     
  3. Tim Morton

    Tim Morton Stunt Coordinator

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    Keep in mind just how LOUD reference really is. Plus since you have the speakers and the amplifier already the point is sort of mute...it is what it is, so to speak. And no its not over kill to build a subcapable of over powering your mains, because that gives yo the ability to dial it back a bit and get cleaner signal to the sub.
     
  4. Michael R Price

    Michael R Price Screenwriter

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    Javier,

    You have 82 watts RMS, or about 19 dbW. You are 4 meters away, so there will be a 12db drop due to distance (it's 6db for each *doubling* of distance). There will probably be at least 3db of boundary gain from your room, depending on its size and your placement. With 91db speaker sensitivity a good output estimate would thus be 101-104db peak per channel at the listening position... slightly short of reference. It is likely that room gain, along with your subwoofer taking over the bass load, will bring this level up to the point where the amplifier does not clip on reference level peaks, or it will be subtle enough that it won't sound too bad. So, don't worry about it. Reference is quite loud in the first place and your system can go plenty loud for most movies.

    Don't worry about having too much output capability in your subwoofer though - recall that the LFE channel contains peaks up to 115db and the bass content from other channels can add up to make this 121db.
     
  5. Javier_Huerta

    Javier_Huerta Supporting Actor

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    Thank you very much for your input! I wasn't considering the fact that the speakers will run on "small", and the fact that "reference" is actually pretty loud [​IMG]

    I was wondering about this, though, since my receiver is THX Select approved, so I guess you can only hit reference levels with THX approved speaker systems.

    John, you are right - I haven't tested this in my room (the room doesn't exist - yet). I'm trying to design everything beforehand, just to know whether I might want to get some monoblocks for the front channel later on. Although I could only gain another 3 dB by doubling the input power, and I'd be very close to the speakers' rated output, so I guess it's not a good idea after all.

    Thank you very much for all your input!

    Javier.
     
  6. Lee Carbray

    Lee Carbray Second Unit

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    Javier, if you really want to make your brain explode by thinking about this stuff extend you comments to include surround speakers. If yo are like most people you have different and smaller surround speakers then the mains. Take a popular set up like the Paradigm Monitor series. You have 7 and a CC370 up front both have 90 Db sensitivity. In the back you go with the smaller mini monitor for the rear. It has a sensitivity of 87 Db. You have you system all calibrated to reference, but it is too loud so you turn it down. Wouldn't the surround drop faster then the front, and your calibration would be out of whack? Of course it is just another reason to get the same speakers all around, but for some that is not an option.
     
  7. John Garcia

    John Garcia Executive Producer

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  8. Javier_Huerta

    Javier_Huerta Supporting Actor

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    Lee, I have given a lot of thought to this.

    I'd suppose if you calibrated your surround system, you'd be boosting the surround level by 3dB, and then it wouldn't really matter the volume you were using.

    Then again, I actually tested my idea with an SPL meter, and lo and behold - you are right. The levels do vary according to your volume level. And that's with a Definitive Technology setup, with bipolar speakers all around (anyone would assume their sensitivity would stay the same at all times since they use the same drivers).

    John, what I meant was that my particular receiver can only hit reference when used with THX speakers because the wattage rating isn't sufficient for my speakers - and I'd assume you'd need some pretty efficient speakers to hit ref levels from 80 watts RMS. I wonder how they go about certifying amps.

    Javier.
     
  9. Lee Carbray

    Lee Carbray Second Unit

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