Why Only hear the sound from one speaker in my iPhone 4 / 4S ?

When playing a music or sound in the iPhone 4 /4S, try to block the speaker hole (at the bottom of the iPhone) using your thumb or finger and you will know that only one speaker produces the sound, only the right side speaker. and the left side speaker does not work. why? is it a problem with your iPhone 4 /4S ?.

The answer is not a problem with your iPhone, because the left side is not a speaker, it is a microphone, for pick-up your voice when speaking.

Microphone and Speaker at iPhone 4
The microphone of course will not produces the sound :)
The iPhone 4 /4S only has one speaker.