In intensity units:
In my experience, the ``threshold of pain'' is most often defined as 120 dB.
The relationship between sound amplitude and actual loudness is complex . Loudness is a perceptual dimension while sound amplitude is physical. Since loudness sensitivity is closer to logarithmic than linear in amplitude (especially at moderate to high loudnesses), we typically use decibels to represent sound amplitude, especially in spectral displays.
The sone amplitude scale is defined in terms of actual loudness perception experiments . At 1kHz and above, loudness perception is approximately logarithmic above 50 dB SPL or so. Below that, it tends toward being more linear.
The phon amplitude scale is simply the dB scale at 1kHz [76, p. 111]. At other frequencies, the amplitude in phons is defined by following the equal-loudness curve over to 1 kHz and reading off the level there in dB SPL. In other words, all pure tones have the same loudness at the same phon level, and 1 kHz is used to set the reference in dB SPL. Just remember that one phon is one dB-SPL at 1 kHz. Looking at the Fletcher-Munson equal-loudness curves [76, p. 124], loudness in phons can be read off along the vertical line at 1 kHz.
Classically, the intensity level of a sound wave is its dB SPL level, measuring the peak time-domain pressure-wave amplitude relative to watts per centimeter squared (i.e., there is no consideration of the frequency domain here at all).
Another classical term still encountered is the sensation level of pure tones: The sensation level is the number of dB SPL above the hearing threshold at that frequency [76, p. 110].
For further information on ``doing it right,'' see, for example,
DBA (A-Weighted DB)