Acoustic short circuit
When a cone moves forward, it compresses the air in front of it while, at the rear, there is rarefaction of the medium. This creates a phase difference of 180°. At low frequencies, the cone moves so slowly that the air can move from one side to the other and balance out the difference in pressure. This will produce a low-frequency air flow but no sound. To prevent this happening, a baffle or enclosure is required.