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13th April 2007, 01:19 AM  #1 
Speakerholic
diyAudio Moderator

Watts vs. decibels
I have oft wondered about this one.
I have been told that each increase of 3 dB requires 2X amplifier input. I have also been told that a doubling of apparent volume is an increase of 10dB and requires and increase of 10X from the amplifier. So translate that to my 96 dB PA system. In one instance it requires 1000 watts to achieve a 126 dB rating whereas with the other it takes 1024 watts. It's not a big deal, I'd just like to know which is correct. I have a feeling the 10X is correct because it all scaled logirithmically but I'd like to know for sure. Thanks. 
13th April 2007, 01:45 AM  #2 
diyAudio Member

Cal,
innit? Apples and pears, that's the problem. You double the amp output using the same signal in one case. You need ten machines (or motorbikes, or whatever) with different signal outputs to achieve the same nuisance in the other. Add the fact that noise is logarithmic and you get as confused as I usually am. Pit 
13th April 2007, 01:49 AM  #3 
diyAudio Member
Join Date: Oct 2004

In short, 6db is doubling of gain.
For loudspeaker it's half, 3db. The math is here: http://www.ac6v.com/db.htm Edit: I suck at math. I take the 3 and 6db at faith. I struggled like an idiot with Log in highschool. Poobah, I think, said something about db should be called Log. Something such anyway. Suddenly things made sense, only years too late. Maybe I hadn't hated math if I had had a better teacher. But then, maybe I would. 
13th April 2007, 01:54 AM  #4 
diyAudio Member

phn,
if I am not completely mistaken we have ~the same local time  another nightbird? Pit 
13th April 2007, 02:01 AM  #5 
diyAudio Member
Join Date: Jul 2005
Location: qc

I believe the +3dB>2xpower is an approximation of the other equation. Easier for most to comprehend.

13th April 2007, 02:02 AM  #6 
diyAudio Member
Join Date: Oct 2004

Indeed. I'm something of an insomnic. It's related to stress. I'm pretty sure of that. Not as bad as for my brother. He just returned to his job, different job, after a month. Doctor's order.

13th April 2007, 02:03 AM  #7 
diyAudio Member
Join Date: Dec 2001
Location: Melbourne, Australia

Re: Watts vs. decibels
Cal,
Twice the power is +3dB Twice the voltage (four times the power) is +6dB Twice the loudness is (unfortunately) anywhere between +6dB (4 time the power) and plus +10dB (approximately 10 times the power). I think this depends on the frequency and loudness of the original signal. Check out "Equal Loudness Contours" or "Fletcher–Munson curves", or Aweighting vs Bweighting vs Cweighting. On another forum, the question was posed "how much louder is the latest 600w amplifer than my 13w SET?" (close, anyway). The answer given was 16.5dB, or about 3 times as loud. And, assume you listen to 2 1KHz tones, one at 30dB, and the other at 80dB. A difference of 50dB. Now, listen to 2 40Hz tones at the same 'apparent loudness' as the 2 1KHz tones. A difference of 50Db, right? Wrong! You would only need to increase the 40Hz tone just over 25dB... To hear the 40Hz tone at the same apparent loudness as the 30dB 1KHz would be 69dB; to hear the 40Hz tone at the same apparent loudness as the 1KHz tone at 80dB, would need to be 95dB.
__________________
Jont. "It is impossible to build a fool proof system; because fools are so ingenious." 
13th April 2007, 02:23 AM  #8  
Speakerholic
diyAudio Moderator

Re: Re: Watts vs. decibels
Quote:
Quote:


13th April 2007, 02:42 AM  #9  
diyAudio Member
Join Date: Dec 2001
Location: Melbourne, Australia

Quote:
__________________
Jont. "It is impossible to build a fool proof system; because fools are so ingenious." 

13th April 2007, 03:06 AM  #10 
Speakerholic
diyAudio Moderator

Question officially answered. Thanks guys.

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