DB's vs. Watts?

Discussion in 'Amps and Cabs' started by slopeshoulder, Jan 27, 2008.


  1. slopeshoulder

    slopeshoulder Senior Member

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    I often wonder why watts are the standard way we talk about amp volume if it's true that wattage, voltage, rectifier, transformer and speaker efficiency all effect output volume. This is compunded by the "difference" in perceived volume when comparing solid state, tube class AB and tube class A "watts." Matchless watts are loud, Peavey watts are not. Mesa watts are loud, tweed not so much... etc ad nauseum.
    Wouldn't it be more usefull to speak about db's (decibals)?

    I'd like to know how many db's I need:
    - until i'm too loud in a living room/bedroom
    - to jam jazz and hang with drummer and sax
    - to hang with piano in small space
    - to play small club (no mic)
    - to play church
    - to hear myself from 10 feet with a good monitor mix, and also without ANY monitors
    - to blast fusion or rock with powerful drummer
    - to fill a big club (no mic)
    - to be heard on a big stage
    - before I hurt my ears
    - before I hurt the audience's ears
    - to have good tone

    I don't understand: is db a measure of sound pressure (at the ear), not output power (at the speaker), and if so, is distance and size of room a factor? Hmm...

    I know that we usually think this way:
    - bedroom: 1-5 watts
    - rehearsal in small room: 7-12 watts
    - small club, quiter drummer: 18-22 watts
    - medium club, average drummer and thumpin bass, or clean jazz: 30-40 watts
    - loud rock: 50 watts
    - deaf, no PA, trapped in '69: 100 watts
    - silly: 150-200 watts
    That works, when we also factor in solid state, tube, class, room, band members, headroom, speaker efficiency (and number), etc., then maybe db's are a more usefull way to approach it? I think I no longer care about wattage; I care about db's: how many db's before breakup? How many db's compared to other instruments? how many db's preferred listening in various rooms? etc.
    I know my mastering engineer recommends listening at 89db, no more (hurt ears) or less (lose fullness).
    Watts vs. db's.
    Please share your thoughts. Thanks.
     
  2. MikeMcK

    MikeMcK Member

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    Decibels are actually a way to express a ratio of something that grows or is perceived exponentially. But you need two numbers to have a ratio, right?

    When people talk about db to describe volume, the sound pressure is expressed in terms of SPL compared to a nominal, barely-audible sound. So if the SPL is 100 times larger, we say the sound is 10*log(100) = 20 db. It just makes sense to use db to describe volume, because a 20 db SPL sounds about twice as loud as a 10 db sound.

    Because it's a ratio, db are dimensionless. If someone says "db-Watt" or "db-milliwatt", they're comparing a measured power to a Watt or milliwatt. In other words, 8 milliwatts of power is 10*log(8) = 9 "db-milliwatt".

    The long story short is that db can be used when talking about volume (ratio of SPL to a barely-audible sound), gain (ratio of output to input), or power as a ratio to a fixed unit of power. Hope that helps a little bit.
     
  3. MikeMcK

    MikeMcK Member

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    Me again...if you're wondering why this is allowed to be so confusing, remember that most of the world who deal with dB only deal with sound volume...they only need to know that 50 dB sounds 2x as loud as 25.

    Some of the world deals with electronics and usually uses dB to express gain. Only in audio electronics do we deal with both. That's why it's so difficult to follow sometimes.
     
  4. somedude

    somedude Member

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    By db I'm assuming we're talking SPL....

    Anyway, plug any amp into an efficient speaker, then measure the SPL one meter out front of it.

    Plug the same amp into a less efficient speaker, then measure the SPL one meter out front of it.

    Toss in different speaker combos (1x12 vs 4x12), open back/closed back, frequency response of both the pickups and how you have your amps EQ dialed in.....

    db suffers the same problems as watts.
     
  5. AndrewSimon

    AndrewSimon Member

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    Marketing conspiracy!
    "100Watt vs 50Watt" simply looks better on paper then "108dB vs 114dB".

    :confused:
     
  6. Free

    Free Member

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    First, maybe I'm not following your equation symbols, Old Tele, but I'm getting (0.750512)*100 = 75.1%

    Plus it doesn't add up regarding some conventional notions: All SPL and speaker factors being the same, 10 dbs is about equal to DOUBLING volume intensity. For example, a 100 watt amp is generally considered about twice or 10 dbs louder than a 10 watt amp, so that would equate to a 100 watt amp being 200% the loudness of a 10 watt amp. Your equation comes to about 300%.

    -Mike
     
  7. The Pup

    The Pup Supporting Member

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  8. GenoBluzGtr

    GenoBluzGtr Supporting Member

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    The appropriate formula for measuring differences in radiated SPL would be:

    10log(P2/P1) or in this case ten times the logarithm of 50/100. That would net you 10X the log of .5. The log of .5 is .30103, the dB reduction in output level between a 50 watt amp and a 100 watt amp is 3.0103 dB.

    As has been mentioned earlier, dB alone is a useless term. dB is ALWAYS referenced to a stable factor such as volts, watts, or pressure. Since volulme level is generally what is being measured then the reference is most normally used when compared with or referenced to pressure (micropascal, microbar, dynes, etc...) so as long as all measurements are made using the same reference, it's a useful tool.

    Don't forget about "Psycho-Acoustics" as well. this is related to how differences in frequency spectrum makes some things SOUND louder when they actually measure out differently.

    here's a link: http://www.pa.msu.edu/acoustics/

    So it is never as easy as it looks. THe environment (reflectivity, refraction, absorbtion, propagation, phase angels, grazing angles, etc... ) all affect what shape that sound wave is in when it gets to our ears.

    BTW, my "day" job is measuring acoustic signals from underwater sources for the Dep of Defense. It's very interesting to me how often this topic comes up in the music afficianado's realm. Makes me feel more 'relevant' to real life!
     
  9. Free

    Free Member

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    Great information and points to consider written here in your post, GenoBluz. I appreciate it. I just bookmarked that link you pointed out too - great info there too.

    Thanks,

    -Mike
     
  10. Tone_Terrific

    Tone_Terrific Member

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    So, we are back where we started-
    -you want louder, you add watts until it's too loud. easy:jo
     
  11. µ¿ z3®ø™

    µ¿ z3®ø™ Member

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    just a wee bit off there...
    all things being equal, doubling the power results in a 3 dB increase in SPL.
     
  12. fr8_trane

    fr8_trane Member

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    That's my (admittedly limited) understanding as well. Also I am under the impression that a 50 watt amp, all things being equal, is twice as loud as a 5 watt amp- not 25 watts. IOW double power = +3db volume. Double volume = 10x power.
     
  13. Teahead

    Teahead Member

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    This made me chuckle.
     
  14. Johnnythunders

    Johnnythunders Member

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    I use a radioshack meter set in the back of my practice room aimed straight down the center. I have 2 amps and two sepaker cabinets one in each corner aimed towards the center of the room about 18 foot from the speakers. I use this to tune and balance my sound trying for the following:

    1) Best overall sound regarldess of volume
    2) Best overall sound at the lowest volume
    3) Best overall sound between the two.

    I use this as reference point to determine which setting I can use when my wife is home. I usually use number 3, sometime number 2. I use number 1 when I have the house too myself.
     
  15. BIGGERSTAFF

    BIGGERSTAFF Member

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    50dB is considerably more than twice as loud as 25dB. Volume doubles with every 6dB increase.
     
  16. Free

    Free Member

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    Right he is way off, but 6dB is wrong too. You need to read the previous thread history here. 10dB is doubling volume intensity and a significant increase. And, when one gets around 20 dB plus increases it's really a huge volume difference in general. A 25dB increase has the potential to blow your ears off.
     
  17. BIGGERSTAFF

    BIGGERSTAFF Member

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    I think you're mixing watts up with dBs. 100w is twice as loud as 10w. That's why when your comparing speaker efficiency, there's a huge difference in perceived volume from a 96dB speaker and a 102dB speaker.
    That's why some lower wattage amps can sound louder than higher wattage amps(there are other reasons as well, but that's a huge part of it).
     
  18. Free

    Free Member

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    No. Actually I'm not. I'm well aware of all those variables. Your assertion that 6dB is doubling volume is simply wrong - 10dB is equal to doubling volume intensity. A 6dB increase is around 52% more loudness.
     
  19. BIGGERSTAFF

    BIGGERSTAFF Member

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  20. eru

    eru Member

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    There's a lot of BS in this thread, most of it I've heard before from people who know just enough of what they're talking about to get things wrong.

    I look at it like this - Output power (watts) roughly translates to clean headroom (preamp differences aside). Volume is a product of how hot you run the amplifier, how the cab is wired, how efficient the speakers are, how many speakers there are, how the cab is designed, how it's placed in the room, what the room is made out of, how full the room is, etc..

    If you're going for a clean tone, more Watts=more headroom, which means the more powerful the amp, the louder you can make it before you start to get breakup, which is either good or bad.

    If you want it completely clean, design the cab first, figure out it's RMS power handling (taking into account impedance factors like series/parallel wiring) and buy an amp with a clean RMS rating of at least that amount, aiming for a peak power of about twice what the speakers can handle. This ensures you've got enough headroom to stay clean until you're going to blow speakers anyway and gives you the most clean volume you'll get with a given cab. If you need it to be quieter, just don't turn it up as high.

    If you're going for a crunch/distortion tone from the amp, the same adage is true (wattage translates to headroom) but since the amp will be clipping (operating well above it's clean RMS power rating) you need speakers that can handle its peak power without distorting. Again, start with the cab first. Decide on a design and figure out (if you can find the numbers published) what kind of volume (dB SPL @ 1m) to expect from a specific cab and do math accordingly to your desired volume level. The closer your amp gets to the total power handling of the cab, the faster the speakers will wear out. Then, the important numbers are 1) the absolute limit of the cab and 2) the efficiency rating (in dB SPL @ 1m / W) of the speakers in a given cab design. Use this to decide on the proper power of your amp, choose tube type and preamp design to suit your taste. Done.

    It's interesting that people make fun of, among others, Orange for making 200W heads, when this seems perfectly reasonable given the above criteria. The thing is that people buying that amp who know what they're doing would never expect it to clip, they're using it for loads of clean headroom, probably for country or jazz. Keep in mind that power ratings for PA amp systems are routinely upwards of 2000W, again, because of the need for an amp that does not clip.

    It's also interesting that more metal/hard rock players don't use lower-power amps. If your amp is rated at 5W Clean RMS into 8 Ohms, it probably peaks around 10 or 12W @ 8 Ohms, which for a full-on distortion tone isn't bad. Run into a fairly efficient 4x12 wired in parallel, this would be closer to 20W actual power produced, would not risk any current speaker I'm familiar with, and would easily keep up with a quiet drummer or jam band. If you decide you want it to be a bit louder, going up to 10W Clean RMS into 8 Ohms (40W peak into 4 ohms) and you're fine.

    If you want to do both, aim for somewhere in between.

    My amp is 5W RMS Clean, and I'm running it into an 8 Ohm cab with an efficiency of about 10 dB SPL @ 1 m/W, making its crunch/distortion tone somewhere in the area of 80-100 dB SPL @ 1m, which decreases with the inverse square of the distance from the speaker. Or, about enough to keep up with a fairly quiet drummer without too much risk to my hearing.

    Honestly, I don't know why metal amps don't have lower power ratings than amps designed for country except that metalheads tend to like a single, concrete reason why their amp is better than another person's and amp companies, having adequately deciphered this, started promoting power as the only measurement necessary even though it's far from true.
     

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