Working of a subwoofer

what is an woofer??--which responds to only low frequencies(150hz to down as far as 20hz). now we'll see how it works. *when low frequency is given to a speaker,due to vibration of membrane(a thin paper like material that vibrates in the speaker) there will be resonance that occurs inside the box.(here the box is made up of wood because to to absorb the sound waves and to give wooing effect)and so occurs harmonics.the first harmonic that hits the wooden plate,that small area or place is calculated using formulas(available in electronics sites)and a cutout is made at that small area. *this is the reason why woofer suddenly blows large amount of air and suck the same amount in and gives clarity even for the low frequencies in other Words to say Excellent bass effect.