So many times we still hear that doubling the power gets us 3dB and doubling the cone area gets us another 3dB.

It is a MYTH.  
It is untrue!

 Electrons flow through our wires to our speakers.
The electrons are changed to electromagnetic energy.
The electromagnetic energy works with permanent magnet energy to make a speaker cone move.

The movement vibrates air and we have sound..

Doubling the power into the speaker motor may get us 3dB more sound to our ears, but again it may not!
Just like dumping twice the gasoline and twice the oxygen into our cars motor may not make us go twice as fast.

It may not, because there are so many variables between the point of the energy being electrons and the energy making it to our ears. There are several variables that may not always have the same value and thus it is impossible to guarantee that a 3dB increase will occur. For instance a loudspeaker motor is not entirely linear, that means if we double the power to it then it may give us 3dB or it may give us less than 3dB depending on things like is the voice coil in the gap at the most efficient spot? Or is the speaker suspension restricting the speaker movement when you add this extra power (it always does!)..?   Or does the acoustic suspension of our box restrict the cone movement (it always does!)..?   

Along this path from electrons to sound, there is also a lot of our energy that is turned to heat and the heat does not translate to more sound, so there is some loss, quite a bit as a matter of fact.. 

 So, to say doubling the power will always get you 3dB is just a guess at best!
Defiantly not a law of physics! Definitely not always true.

But unfortunately, a lot of folks seem to spread this MYTH as if it were fact.  

Now,  Cone area!

Cone area is another MYTH, although like power it is very important, it is still just one variable out of many that have to do with getting sound to our ears.

Many say doubling your cone area will get you 3dB.

This is UNTRUE as well.

It is true that the cone vibrates the air to make sound,  and if we had double the cone area ALSO vibrating the air making the same sound then we might have 3dB more sound!

 But we did more here than to simply change cone area!

We added cone area that was already playing the same sound!

So that means that in addition to cone area we ALSO added a motor and the power to drive that motor so it would move the cone the same amount as the first cone!

We cant easily just ADD CONE AREA!

And if we did by just gluing more cone area to an existing cone then we would have a problem in that the cone area would add mass, thus needing more power to move the extra weight!   So it cant be JUST cone area!

PLUS!

We can play a 15 inch speaker at 100dB and then play a 10inch speaker at 100dB and we will achieve 3dB more and we can see we DID NOT double the cone area!

Or we can use a 15 inch speaker and have a fat lady sing the same song at the same amplitude and we would still get a 3dB increase, and we have to ask, where is her cone area? And/or where is her power and it is double that used by the 15?  Almost certainly not!

So you see, the POWER MYTH and the CONE AREA MYTH  although based in facts are often misused in the car audio world and are easily proven to be untrue..

But don’t loose all hope!  there is some truth!

The truth to the physics of sound is actually much simpler!

1. Adding one sound to another sound of the same amplitude (volume) will gain you 3dB.

2. Adding one sound to another sound of the same amplitude and correlated will gain you 6dB.

(correlated means exactly the same phase, and likely to occur with the long wavelength of bass notes in our cars)…

How can things be simpler than that?
Blame these things on the laws of physics of how two or more sounds work together, not
on cone area or power alone.