Re: Perception of volume change at higher/lower levels
There are two reference grids here. The phon levels of equal loudness makes a comparison of what is perceived to be equal at different frequencies, while the actuel level is not subject to any subjectivity on the listeners part, reference is 1KHz where phon by definition equals dB.
The equal loudness curves doesn't say anything about perceived difference between levels, that part is already defined and locked away from any discussion by the definition of Bel and dB.
I reality however, "double" is a very subjective and unprecise term when our senses are at play. If a rock is twice as big as another rock, how much bigger is it really? If it is twice as high, wide and deep, then it is in reality eight times bigger and heavier, although many might perceive it as twice as big. To the average person, 20lbs is twice as heavy as 10 lbs, but they might find that 150 lbs is twice as heavy as 120 lbs because the effort to lift it gets closer to the limit of their strength.
Perception of speed is another area where what feels twice as fast is very subjective. Same with light, a ten fold increase from 1 to 10 lux feels rather significant because you can see a lot better, then the increase up to 100,000 lux doesn't seem quite as steep in any given 10 fold interval, but the increase from 100,000 to a million is overwhelming unless you have sunglasses.
In addition to the non-linearity of our senses, you have the time factor that JR pointed out, most of our senses adjust in various manners, a slow shift might go totally unnoticed until you reach a threshold. Alertness/tiredness comes into play etc. etc.
A 1dB change is supposed to be the limit of what we can perceive, but we can clearly perceive the qualitative change of a 0.5 dB cut on vocals at 1.2 KHz.
Anyway, it is all subjective. When is music twice as good? What tastes twice as good as chocolate?
