Just how loud can an amp get in dB?

Discussion in 'Amps and Cabs' started by tonedaddy, Jun 4, 2005.

  1. tonedaddy

    tonedaddy Member

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    In a recent thread someone talked about measuring SPL levels on a Komet Constellation at the grillcloth.

    I'm not inclined to doubt the measurements (I've never heard a Connie, but my AC30 TBX could get painfully loud), but it got me thinking about charts you see of different loudness levels, and why guitar amps SEEM to be louder than some of the sound sources in those charts.

    My questions are:

    1. How loud can guitars amps get?

    2 How loud do guitar amps seem to you, when compared to other sound sources?

    AND

    3. Is there something else (Fletcher-Munson or anything else frequency dependent?) that makes guitar amps seem louder than other sound sources?


    Here's a typical chart of sounds and their relative loudness levels:
    http://www.sfu.ca/sonic-studio/handbook/Decibel.html

    Threshold of hearing - 0 dB
    Rustling leaves - 20 dB
    Quiet whisper (3 feet) - 30 dB
    Quiet home - 40 dB
    Quiet street - 50 dB
    Normal conversation - 60 dB
    Inside car - 70 dB
    Loud singing (3 feet) - 75 dB
    Automobile (25 feet) - 80 dB
    Motorcycle (30 feet) - 88 dB
    Foodblender (3 feet) - 90 dB
    Subway (inside) - 94 dB
    Diesel truck (30 feet) - 100 dB
    Power mower (3 feet) - 107 dB
    Pneumatic riveter (3 feet) - 115 dB
    Chainsaw (3 feet) - 117 dB
    Amplified Rock and Roll (6 feet) - 120 dB
    Jet plane (100 feet) - 130 dB



    To measure just how loud a guitar amp can get, I did this calculation:

    I'm using the Connie because it's a 4xEL84 amp, an amp type that hopefully most of us have had access to (feel free to comment with specs of any other amp type if you feel it helps).

    Since Komet measures the Connie at 33 watts before clipping, here's what SPL levels should be possible clean through 100 dB sensitivity speakers (that includes Celestion Blues, G12H 70th Anniversary and Heritage and Vintage 30):

    For a 100 dB sensitivity speaker, the loudness levels at 1 foot would be to:

    dB - Watts required
    100 - 1
    103 - 2
    106 - 4
    109 - 8
    112 - 16
    115 - 32
    118 - 64
    121 - 128
    124 - 256
    127 - 512
    130 - 1024


    Or that's why you see loudness levels written roughly on a 10x scale for every 10 dB increase in loudness:

    db - Watts required
    100 - 1
    110 - 10
    120 - 100
    130 - 1000


    So by my calculations, the answer to question 1 is:

    It would seem that a 32 watt amp clean should be able to create 115 dB at 1' distance using 100 dB sensitivity speakers.


    Since doubling the distance from a source drops the level by 6 dB (just like when you double the amplitude of a sound you increase it by 6 dB), moving away from speaker any distance would drop those levels substantially.

    Since a Connie at max power (clean) at 8 feet with 100 dB speakers should be about 33 watts (I'll substitute 32, just because the 32 watts is found in the chart above):

    Here's the loudness levels that should be found as you move away from the speaker

    Feet - dB
    1 - 115
    2 - 109
    4 - 103
    8 - 97
    16 - 93


    For question 2:

    Does a Connie (or assuming other 4xEL84 amps) at max clean power at 8 feet away with a 100 dB speaker sound like 97 dB?

    Remember, this is halfway between:
    Subway (inside) - 94 dB
    Diesel truck (30 feet) - 100 dB

    I think an AC30 with Blues at 8 feet away sounds louder than that clean.

    What do you think?



    For question 3:

    Are my calculations wrong?
    If it does seem louder, what is causing it? (Fletcher-Munson, maybe?)
    Are reflections adding to the perceived loudness?
    What's going on?
    Am I going deaf?
    :D
     
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  2. John Phillips

    John Phillips Member

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    Well, an AC30 will produce more like 64W fully distorted than 32. Two 100dB (which is measured at 1 meter, not one foot - significant difference) speakers working togther will give more like 102dB@1W/1m. And in a room, with an open-back cab especially, the SPL doesn't fall of with distance anywhere near as fast as in an anechoic space.

    So I'd guess that an AC30 will produce up to about 120dB anywhere close to it indoors.


    Just for a laugh I also worked out how much a Hiwatt 400 (fully distorted power in the 700W region) through a full stack of two EV-loaded 4x12"s would give... since at one time I had the parts to assemble this rig.

    ... about 136dB, standing right in front of it.

    I think that's how loud a guitar amp can get :).
     
  3. riverastoasters

    riverastoasters Member

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    Yeah. Usually it's measured at 1 meter. This means it's about 10 dB louder than you are figuring.

    Note that hearing loss is a concern any time you have continuous exposure over 85dB. Put on really excellent hearing protection (30dB) and that means 115 dB is still nasty even with earplugs. And you are betting that the earplugs aren't leaking much.

    Most guitar amps can get to 115 dB at one meter from the speaker. So they can all fry your ears. Getting your ears fried slowly at this sort of volume level is completely painless and you don't notice anything until it's gone.

    Microphones on the other hand, are much tougher than the hair cells in our ears. If you close mike a guitar cab then even though you make the SPL maybe 20dB higher than one meter away, many microphones (although not all ribbon microphones) can handle it without much problem. It's really, really, hard to kill a good microphone capsule with too much SPL, although you can make the microphone's internal amplifier distort. So if you place an SM57 within inches of a speaker driven by a cranked 100 Watt amp then there isn't too much problem. And if there is one, you notice it right away (almost nobody likes microphone distortion).
     
  4. TieDyedDevil

    TieDyedDevil Member

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    For reference, here's a table of safe exposure times. Note that even a cranked one-watt amp through a speaker with 100 dB sensitivity is only safe for 2-hours listening per day. A ten-watt amp through the same speaker cuts your safe exposure time to 30 minutes. A hundred-watt amp through that speaker is only safe for 7 minutes or less without ear protection.

    Code:
    Safe exposure times vs. SPL 
    
    Duration per day (hours)	Sound level (dBA slow)
    8				90
    6				92
    4				95
    3				97
    2				100
    1.5				102
    1.0				105
    0.5				110
    0.25 [15 minutes] or less	115
    0.125 [7 minutes] or less	120
    0.0625 [3 minutes] or less	125
    
    Ref: OSHA 1910.95
     
  5. SteveVHT

    SteveVHT Member

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    I don't know how accurate my meter is...They say it's +/- 1 db accurate(radioshack)...
    But I hit 162db with my rig...:D And it wasn't nearly dimed.
    Steve
     
  6. Bajan

    Bajan Member

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    Geez 162db, what were you playing?
    [​IMG]
     
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  7. LSchefman

    LSchefman Member

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    There are lots of factors that haven't been mentioned.

    F'rinstance, X number of watts at one meter for a speaker's sensitivity should really be referenced to a frequency and distortion. As the frequency changes, the resistance of the speaker in ohms will vary. No speaker is truly linear, an 8 or 16 ohm rating is not an absolute. The speaker could be as low as a couple of ohms at a given frequency. And a speaker will usually play louder than its rated sensitivity at higher distortion ratings.

    The effect of the cabinet will also play a role.

    The way the amp is rated, and even its rise time, will play a role in how loud it seems or gets; remember that an amp's power rating is referenced to a certain percent of distortion, and whether the rating is RMS or IHF "peak" or some other measurement system will make a difference in how loud it will go.

    I've had amps that seemed louder when I've changed tubes (I didn't measure them, so this could be something else).

    So here's how loud an amp will play:

    TOO F*CKING LOUD ;)
     
  8. tonedaddy

    tonedaddy Member

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    Well, DUH!
    [​IMG]

    [​IMG]

    No wonder my calcs were off!
    My brain went dead on the one meter calcs and I substituted one foot!

    That explains it guys, thanks for jumping in to straighten me out AND to add all the other good info.

    And my wife thinks I'm deaf, but I'm actually just ignoring her....
    :D
     
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  9. SteveVHT

    SteveVHT Member

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    My main rig in quad stereo with four 4x12's...
    Pictured here with 2- 4x12's and 2- 2x12's....
    It's powered by a pair of VHT 2/90/2 power amps....
    [​IMG]
     
  10. riverastoasters

    riverastoasters Member

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    It's not that unusual for a real 8 Ohm rated speaker to be 6 Ohms at some frequencies and 60 Ohms at some other frequencies.
     
  11. malabarmusic

    malabarmusic Member

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    Check your meter. A single blast at 162dB could literally blow out your ear drums, rearrange your internal organs, or possibly even kill you.

    Your rig is indeed fearsome, but doing some basic math suggests that it probably tops out at "only" 140dB -- enough to knock some birds out of the sky, perhaps, but you'd need to up the power level (or number of cabinets) by a factor of about 7 to hit 162.

    FWIW, my Radio Shack meter (maybe even the same one) measured my cranked Marshall Super Bass through an Orange 4x12 at 127dB from ~1 meter away.

    In rehearsals, I find the optimum SPL to lie between 110-115, depending on how many beers the drummer has had.

    - DB
     
  12. spikeRI

    spikeRI Supporting Member

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    :eek:
    is there going to be a quiz?
     
  13. John Phillips

    John Phillips Member

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    :)

    Actually that would be nowhere near enough. You'd need more than 150 times as much power to gain that 22dB - 10 times as much power is needed to increase the level by 10dB, and 100 times to gain 20dB. That's why the dB scale is so deceptive and often misunderstood...

    I'm not sure you could achieve enough concentration of audio power in a given space (via speakers, anyway) to reach 162dB. The meter is off for sure - that's the sort of level you'd find literally at the back end of a jet engine.
     
  14. SteveVHT

    SteveVHT Member

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    I'm sure it's not 100% accurate, but I would give it a +/- 5 Db accuracy.
    I use to build car audio systems, and my personal system in my Camaro SS hits a perfect SQ at 155db.
    My old competition van for SPL registered 172 Db at the car audio drags 5 years ago. But that was also with 8 amps, 12 sub woofers, and a bunch of componenet speakers...
    I can definately tell you the rig is retardedly loud, but I'd have to get a real accurate meter to see what it's really doing.
    Trust me, you walk a little funny after an audio SPL drag cometition.....:D
     
  15. malabarmusic

    malabarmusic Member

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    Somewhere, the ghost of Douglas Adams is laughing ... :D

    - DB
     
  16. GasMask

    GasMask Member

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    [​IMG]


    :dude :dude :dude
     

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