Phon is the SI unit of loudness. How many microphons is one megaphon?

The phon is a logarithmic unit of the loudness level for tones and complex sounds.

Human sensitivity to sound is variable across different frequencies; therefore, although two different tones may present an identical sound pressure to a human ear, they may be psychoacoustically perceived as differing in loudness.

The purpose of the phon is to provide a logarithmic measurement (like decibels) for perceived sound magnitude, while the primary loudness standard methods result in a linear representation.

Phon is equivalent in quantity to deceibels, but the phon matches the sound pressure level (SPL) in decibels of a similarly perceived 1 kHz pure tone. For instance, if a sound is perceived to be equal in intensity to a 1 kHz tone with an SPL of 50 dB, then it has a loudness of 50 phons, regardless of its physical properties.

The SI prefix "micro-" means 'one millionth' while the SI prefix "mega-" implies 'millionfold'. Thus one megaphon equals million times million (one trillion) microphons = 1000,000,000,000