need help in building a sub woofer

[jojomeu]

can any one help me with how to build a mini sub woofer?

[audioguru]

Hi Jojo,
Have you ever seen a mini-subwoofer? Most are pretty big so that they can move a lot of air at low frequencies with a lot of power.

How "mini" do you want? The lower the frequency, then more excursion is needed from the subwoofer's cone. A little speaker driver doesn't have much excursion so it won't play bass very loud before it distorts. Also, little speaker drivers can't handle much power so can't be very loud at low frequencies.

Most speaker systems are designed to have a flat frequency response down the the resonance of the speaker driver in its box. Below the resonant frequency the response drops off.
There are ways to extend the bass response of speaker systems:

1) Make an electronic bass-boost circuit that begins boosting at the frequency that the speaker's response drops off. For years I have these circuits making my clock radio and home stereo speakers sounding like they have a subwoofer. The speakers and amplifier must have a high enough power rating for the boost, or the volume must be reduced so that only the bass is at full power. The circuit is here:
http://www.electronics-lab.com/forum/index.php?topic=4961.0

2) If the speaker's box is sealed and is big enough, then add a port to it and design the port to give its sound output at a frequency below the resonance of the speaker in the box if it was sealed. Then the speaker's cone excursion is reduced at frequencies near where the port is producing the sound. Most subwoofers are made this way.

3) A combination of the above: adding a port to a speaker, tuning it to a frequency too low for the small size of the box then using a bass-boost circuit to make it sound correct. I built some speakers this way and they sound great.

4) Find a speaker driver that isn't too small, has a low resonant frequency, high excursion capability and high power rating. Then mount it in a sealed box that is much too small for it which causes the resonant frequency to be much too high. Then use a bass-boost circuit to make it sound correct. I haven't tried it but it is an excellent idea for a mini-subwoofer. An article about it and a circuit project for it are here:
http://www.sound.westhost.com/linkwitz-transform.htm
http://www.sound.westhost.com/project71.htm

Here are some articles about subwoofers:
http://www.sound.westhost.com/subcon.htm
http://www.sound.westhost.com/qb5align.htm