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The added 3dB of a twice as powerful amp

Discussion in 'Amps and Cabs [BG]' started by Muhwi, Jun 9, 2004.


  1. Muhwi

    Muhwi

    Jun 8, 2004
    Finland
    Often you hear about the 3dB you get from doubling amp power. I was wondering how this 3dB is actually measured. Is a 400w amp 3dB louder than a 200w at any given setting of gain/volume? Or is the 3dB a kind of reserve you get to use when the 200w runs out of noise, like the 200w producing a maximum of 125 db and the 400 a max of 128 dB?
     
  2. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Yes.

    Alex
     
  3. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    There is a formula that illustrates the 3dB pirinciple:-

    Max SPL = Sensitivity + 10*Log(Watts)

    Lets assume 100dB sensitivity:-

    Max SPL at 200w = 100 + 10*Log200 = 123dB.
    Max SPL at 400w = 100 + 10*Log400 = 126dB.

    As you can see the formula relies fairly heavily on the cabinet's sensitivity, and therefore we need to ask, how is sensitivity measured?

    Usually it's a case of running a 1kHz sine wave through the cab at 1/w, then measuring the SPL in dB from 1 meter away.

    So to relate this to your original question, no it doesn't represent 3dB across the board, only at 1K. How much variation you get at other frequencies depends on a lot of things, which will explain why 2 speakers with the same frequency response curve will still sound different in the voicing.

    What's my point. The specs don't tell the whole story. You're ears can tell you more in 20 seconds than you can observe from pages and pages of maths and specs. Equipment auditions are essential.
     
  4. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    You might be surprised to know that most of the time when you are playing (even with high wattage amps) the amp is putting out 10 or 20 watts. It's only on the dynamic peaks that the amp puts out higher power, like during the attack of a note (a fraction of a second).

    A wattage ratio of 10:1 yields a 10dB increase which is to our ears "twice as loud". So while you may be using all your amp's power during the very loudest passages you play, a lot of the time you much, much power. That means when you upgrade to a higher powered amp it may not seem any louder at all, especially if you use compression to limit your dynamic range.

    Because 3dB is not a HUGE increase in volume, my rule of thumb has always been that if I feel underpowered I should go for at least FOUR times the wattage (an increase of 6dB) to get a noticeable change. This gets to be a problem quickly if you're already using an amp in the 300-500 watt range because you would have to go to 1200-2000 watts :eek: and speakers that can handle that much.

    Anyway, about 15 years ago I went from using 50-100 watt amps to a 400 watt amp (using the same cabinets) and since then I have not felt a need for higher wattage.
     
  5. Muhwi

    Muhwi

    Jun 8, 2004
    Finland
    IMO if a bassist feels he can't get enough sound from a 400w amp there must be something wrong, either with the bassist or his/her band's overall volume. :rolleyes:
     
  6. geshel

    geshel

    Oct 2, 2001
    Seattle
    It completely depends on the efficiency of the speakers. Bass cabs range in sensitivity from 92dB (Acme 1x10) to, realistically, probably 106dB (some 8x10s). That's 14dB, or roughly a 30x power ratio to get the same volume. EG, 300w into the first cab is only as loud as 10w into the second one. In reality it doesn't quite work exactly like that, however it is close, so "400w should be plenty" is not a very useful thing to say.
     
  7. Muhwi

    Muhwi

    Jun 8, 2004
    Finland
    Phew, I knew there was this thing called speaker sensitivity, but what I didn't know was that it has such a dramatic impact on how loud an amp sounds. It's good to go to bed knowing you've learned something new during the day, thanks a bunch geshel :)
     
  8. One more thing.

    The larger the cab the more eficient they are. It is no exact rule but back in the day the old folded horns could make alot of bass with a 50 tube watt head. They are big and heavey and sound best only in the back of the room but very efficient. We didn't have a 1200 watt Bassmen.

    With todays powerful and light SS heads its a lot cheaper and lighter to get lots of watts. The smaller speakers are much more portable. I also think efficiency is sacrificed to get a more even frequency responce.

    Peace,
    Benton
     
  9. Ericman197

    Ericman197

    Feb 23, 2004
    Iowa
    Yeah... speaker sensitivites really only tell you the efficiency at 1 watt. That doesn't necessarilly mean they're going to be as efficient at 100 watts.
     
  10. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Sure, but let's get real. Let's say I have an Acme 1-10 cab and an SVT 8-10 available to use. Why would ANYONE expect the two to be just as loud when driven by the same head?

    Since sound is just vibrations in the air, it's pretty intuitive that a cabinet with eight times as much speaker cone area would be a lot louder because it can vibrate more air.

    That 92 db sensitivity of the Acme 1-10 is for ONE WATT at a distance of one meter. For ten watts you'd get 102 db, 100 watts 112 db, 400 watts 118 db. That's pretty darn loud! Literally deafening in fact if your ear was only 1 meter away from the cabinet ;) Sure, the 8-10 cab would be louder still, 132 dB which is like standing next to a jet engine :eek: Of course, if you upgraded that Acme 1-10 to a pair of Acme 4-10s, that same 400 watts would now get you to 127 db...not quite as loud as the SVT cab but still pretty punishing.

    So in that sense one might indeed wonder how much more than 400 watts is really needed with large, efficient cabinets. A full SVT head/2 cabs setup after all only puts 150 watts into each 8-10 cabinet. I'm still amazed at people who are running 2400 and 3000 watt power amps in their rigs.
     
  11. Ericman197

    Ericman197

    Feb 23, 2004
    Iowa
    But that's assuming that the cab is 100% efficient. The problem with the 10*Log(wattage) calculation is that it does not take into account the efficiency of the cabinet, only the input sensitivity. In reality, only about a few percent of the wattage put in is going to actually be turned into sound... that's not even taking into account the resistance of cables and the room's dampening factor. I too believed that the calculation worked, until I realized that the 121dB I was creating with my Low B-2 was hardly even 111dB.
     
  12. geshel

    geshel

    Oct 2, 2001
    Seattle
    You're confused. How, exactly, could putting a certain power signal into a speaker and measuring the output not take its efficiency into account? Speaker sensitivity and efficiency are not exactly the same thing, but they are very closely related. A speaker with efficiency of 100% would produce 114dB with one electrical watt input (edit: at 1m I believe, radiated into half-space)
     
  13. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    No, it doesn't assume 100% efficiency.

    The efficiency and the sensitivity are interrelated, especially at bass frequencies.

    Hopefully, the cable resistance will be very small compared to the impedance of the loudspeaker. Rooms don't have "dampening factor" or "damping factor."

    If 1 watt produces 95 dB measured at a certain point in front of the speaker, then 10 watts will produce 105 dB. 20 watts will produce 108 dB. And so on.

    If another speaker produces 97 dB, measured at the same place, with 1 watt going into it, then it is more sensitive than the other, probably due to better efficiency.

    What the dB calculation doesn't account for is power compression. If you push hundreds of watts into the speaker for a length of time, its voice coil will start getting pretty hot, and as a result its resistance will increase. It will draw less current for a given output voltage, and therefore the amount of power going into the speaker will decrease.
     
  14. geshel

    geshel

    Oct 2, 2001
    Seattle
    Of course, for anyone a few meters away, the sound level drops off rapidly. Especially at low frequencies. I can attest to the fact that an Acme B1 driven by 120watts is "not loud enough to be heard by the drummer in a room with a guitarist and keyboard player, using small combo amps, when the Acme cabinet is 10 feet away from and aimed directly at the drummer's head". :D
     
  15. Fuzzbass

    Fuzzbass P5 with overdrive Supporting Member

    I don't bridge my 2100 watt amp. I typically use it stereo, 400 watts per channel into 8 ohms, 650 watts from one channel into a 4 ohm cabinet, or 900 (?) watts into a 2.67 ohm load (single channel). "Better to have it and not need it...." With 5 string it's nice having no worries about amp distortion, especially on the low notes. It's also nice to have so many options for speaker wiring, rather than being forced to bridge as I might with a less powerful amp.

    Also, my amp always runs cool... my understanding is that this is a good thing!
     
  16. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    You've just described the rig I've been using for 10+ years, so I agree. If my bass ig isn't loud enough for a bar gig, the band is unnecessarily loud and I think twice about playing with them again.

    However, I'm not a "flat to 30Hz" type of guy. If I was, 400w wouldn't cut it.