This is continuation of the topic How to amplify sound, part 1. I am not allowed to add more than one picture, so I’m posting this question in two parts.
If I increase amplification from 12.149 dB to 12.2 dB and get this effect.
In fact, I don’t need to increase amplification at all. If I just click Apply in the first picture, I get this huge amplification. I cannot get any smaller amplification regardless of what I do.
What am I doing wrong?
You are misunderstanding something… You are increasing by 12dB which is an amplitude increase of about 4X, not to 12dB.
Also, 0dB is the “digital maximum” so digital dB levels are usually negative. 0dB SPL, (acoustic loudness in the air) is approximately the quietest sound than can be heard so SPL levels are positive.
Audacity has pre-scanned your file and its defaulting to whatever change is needed for "“maximized” 0dB peaks… It’s indirectly telling you that the peak is currently -12.149dB.
Change 12dB to something like 1dB and you’ll get a small (1dB) increase, making your peaks -11.149dB. Enter -1dB and you’ll get a small decrease.
You are right; thank you!
As you correctly observed, I’m misunderstanding the figures. Where can I read more about it?
What else do you want to know?
Decibels are relative and logarithmic. The fact that they are relative means you need a reference. There are 0dB references for acoustic, digital, and electrical levels. There are several different 0dB electrical references.
Like any measurement, you can have an absolute dB measurement (a loud rock band might be 100dB SPL) or it can be a difference (a boost or cut of 10dB, etc.)
Some handy numbers to remember: 6dB is an amplitude ratio of 2. +6dB is twice the digital value or twice the voltage and -6dB is half the value or voltage. 20dB is a factor of 10.
3dB is a power factor of 2 and 6db is a power factor of 4. i.e. If your amplifier is playing at 100 Watts and you cut the the voltage or digital value in half, you’ve got 25W. (The formulas are different but a 3dB change is always a 3dB change.)
There is no standard calibration between digital levels and SPL levels but there is a direct correlation. If you reduce the digital level by 3dB (a bigger negative number) the SPL level also goes down by 3dB (a smaller positive number).
Our hearing is also relative/logarithmic… +6 dB is twice the amplitude but it doesn’t sound twice as loud.
Also, digital levels are limited by the peaks. the “digital maximum” is 0dBFS (zero decibels full scale) and if you try to go over you can get clipping (distortion). Audacity uses floating-point internally so it virtually has no limits and it won’t clip but your analog-to-digital converter, digital-to-analog converter, and most audio file formats are limited to 0dB so that should be considered the maximum.
But… Peak levels don’t correlate well with perceived loudness. If you amplify or normalize all of your files for “maximized” 0dB peaks they won’t all sound equally loud.
@DVDdoug: Thank you, Doug, for taking your time and explaining this to me.
I assume that the values shown by Audacity are relative. Anyway, the 0 dB shown by Audacity cannot possibly be SPL 0 dB.
If I apply 12.149 dB amplification to my signal, the new amplitude after amplification will be 0 dB. What is 0 dB? As you are saying, in a relative measurement, you must have a reference to make values meaningful. So 0 dB for Audacity is some reference sound level. What is it?
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