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Power/decibels - do I have this right? (a little long)

Discussion in 'Archived Threads 2001-2004' started by ScottAndrew, Dec 30, 2001.

  1. ScottAndrew

    ScottAndrew Stunt Coordinator

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    I recently ordered some new JBL speakers (N38s, N24s, N-Center) to replace the speakers from my Kenwood HTB-503. So I'm using a Kenwood VR-407. With all the recent talk about receivers not supplying as much power as advertised, I was concerned that I might underpower the new speakers, and I can't find anything concrete I can understand on how much power the VR-407 really produces. So I tried the following experiment. I'm wondering if anyone can poke some holes in this and tell me what I might have done wrong or what I could have missed.

    [*]I bought a Radio Shack sound meter like everyone keeps saying to.[*]I put Steely Dan's Bohdisattva in my DVD player, turned off the power to my subwoofer, sat in my listening position, and cranked it up as loud as I could take it, louder than I would ever listen to music in real life. That's about -20 on the volume display. My room is about 18x14, and I sit about 10 feet from the main speakers (set to "normal")[*]With the volume at this level, I paused the CD, and disconnected my left speaker so that I could measure the sound from just one speaker.[*]I moved to position one meter in front of the remaining speaker, resumed play on the CD, and measured the sound. The music mostly peaked at 96db, with one brief burst to 100db. My ears still hurt.[/list]My initial conclusion is that playing music at a level louder than I ever normally would, the maximum level I produced was 100db. My current front speakers have a sensitivity of 88db. That means that at most I was using about 16 watts. Here I am worrying about whether my obviously overrated 100 watts is really delivering 75 or 60 or 45, and in reality I'm only using 16 watts?

    I occasionally, but rarely, turn the volume to -28 or so if I'm watching a "talking" movie. Action films never go above -30.

    My JBLs will be, on average, much more efficient than my Kenwoods. (Fronts/center 90db, surrounds 86db vs fronts 88db, center 86db, surrounds 87db.)

    So am I missing anything? Is there a critical flaw in my experiment? Are their other factors that cause larger speakers to require more power? Or am I just blessed with the hearing of Lord Usher?



    Thanks,

    Scott
     
  2. Todd Terwilliger

    Todd Terwilliger Screenwriter

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    I think you want to use the test tones on your receiver to measure the sound levels. The pink noise of the tones will give you a flat level as opposed to the jagged peaks and valleys of a sound track.
     
  3. ScottAndrew

    ScottAndrew Stunt Coordinator

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    True, I agree that the test tones would produce a more level sound. But part of what I wanted to measure against was my own tolerance for loud noise so I could determine whether I'm straining the receiver when I play it as loud as I do. I can tolerate the test tones much louder than music or movies. I wanted to find out how much power I was using when I listened to something as loud as I could. Does that seem right?
     
  4. Patrick Sun

    Patrick Sun Studio Mogul

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    Most of the power is soaked up on the low end of the frequency spectrum.

    Also, wear earplugs when you do these experiments.
     
  5. David Judah

    David Judah Screenwriter

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  6. ScottAndrew

    ScottAndrew Stunt Coordinator

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    So is it there anything I'm leaving out that indicates I might be using more than 16-32 watts per channel?

    Thanks,

    Scott
     
  7. ColinM

    ColinM Cinematographer

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    Sounds right to me. That 16 - 32 would increase quickly to 64 wpc, which I'll bet is the limit of the Kenwood's power supply. Especially in DD.

    -CM
     
  8. Jim_Stu

    Jim_Stu Stunt Coordinator

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    Scott,

    I've had a pair of N38s for some time now. They are located

    in an 18'W x 28'L x 16'H room. I could not 'under drive'

    them with an old 60W RCA receiver. Furthermore, for music

    CDs, they do sound better, at low volume (what ever that means) with the

    newer 100W Yam.

    JRS
     
  9. Ron Alcasid

    Ron Alcasid Stunt Coordinator

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    It takes takes 10 times more power to achieve a 10dB gain. If the sensitivity of your speakers are 88dB then...

    88dB @ 1 Watt

    98 dB @ 10 Watts

    108 dB @ 100 Watts

    Doubling the power yields only a 3dB gain...

    111 dB @ 200 Watts

    As you can see 100 Watts/channel should be sufficient for most people. When I watch a movie, I set the volume on my reciever so that dialouge falls between 60 to 70 dB, within range of a normal conversation. If find this setting to be very comfortable without compromising the soundtrack's dynamics too much.
     
  10. ScottAndrew

    ScottAndrew Stunt Coordinator

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    I'm pretty sure 100 watts per channel isn't going to happen. What I was trying to determine was not so much how loud my receiver can play, (that may be impossible unless I could get real-life specs) but rather how much power I actually use. If playing the receiver way louder than I prefer only requires a maximum of 16 watts, than it wouldn't seem to matter if my maximum continuous output is 45 or 75 or whatever. If music playing at 96db is way too loud, than even a burst of 106db, which is twice as loud, would only take 64 watts.

    After reading all the Onkyo vs. Denon vs. Kenwood posts I was fretting that if didn't have 75 watts of continuous output I would be missing something. But when people review a receiver they usually don't mention exactly how loud they play it or how far they sit from the speakers. I didn't even know how loud I played mine.

    I just thought that the results might be interesting to others, but at the same time I had wondered if there was anything obvious I had not taken into account, as this was not the most scientifically performed experiment. I'm certainly no expert, just some guy armed with all these figures and calculations I got off the internet.
     
  11. Ron Alcasid

    Ron Alcasid Stunt Coordinator

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    Scott, I was actually trying to support your results. If your speakers are reasonably efficient, it won't take a lot of power to drive them to uncomfortable levels, so it isn't suprising at all you would only use 16 -32 watts. As long as the amplifier doesn't clip then you should be OK.
     
  12. ColinM

    ColinM Cinematographer

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    For instance, I use the HK AVR110, rated notoriously low at 50 x 2.
    I will never say I wish it had more power, and I have 86db NHT SuperOnes.
    I listen to The WHO at loud levels, and since I have a powered sub helping, the HK will never break a sweat.
    Even if I'm not using the sub, it's loud. At about 1 meter with pink noise in my room at 86db the thing is barely idling.
    100 wpc is not necessary in real life small room HT.
    IMHO![​IMG]
     
  13. ScottAndrew

    ScottAndrew Stunt Coordinator

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    That's good to know. I should have figured, because many people seem to like their music and movies a whole lot louder than I do. I keep reading how prolonged exposure above 85db can damage hearing, and I don't need that!

    Thanks Ron, Colin, all.
     

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