What’s proximity effect?

If you used microphone before, especially those handheld models, did you hear more and more bass sound, like the pops and plosives when you put the mic closer to your mouth. Or if you are learning some podcast skills, there must be a piece of advice that you seen or hear somewhere, which tells you to eat into the microphone to have a deep and warm radio sound.
 
Parts of both situations above can be explained in proximity effect. That is the low frequency response is boosted when the microphone gets closer to the sound sources. But you will hear the proximity effect only from a directional microphone. The most prominent proximity effect is from a microphone with figure-8 patter, then a cardioid microphone, and almost no proximity effect from an omni-directional mic. The closer you speak to either of those mics, the more noticeable this effect becomes. Your voice will become warmer but on the other hand will be more boomy and muffle.     
 
“But why?” A query from an audioguy wannabe. Yep, you must understand the the reason behind to be an actual audiophile, even if you are just on the way to become one. The proximity effect are the result of the differences of phase and amplitude in the sound waves reaching two sides of the diaphragm of a directional microphone. And those two variations are decided by the frequency and distance from the sound source.
 
First off the phase difference, it increases with frequency. But at the same time, it is offset by damping the diaphragm 6 dB per octave to achieve a flat frequency response. Basically that means the low frequencies still have the smallest phase difference. So what really matters is the amplitude difference. The high frequencies arrives in parallel waves at either a large or a small distance. But the low frequencies are consider as a point source when the mic are brought close. And because of the inverse square law that every doubling distance away from the source results in a 6dB loss in level, the intensity of low frequencies drop off remarkably when the audio arrive at the back of the mic. That amplitude changes cause the difference in sound pressure level of the diaphragm, which translates into a high intensity signal.