***Disclaimer*** The following is for informational purposes only, I take no responsibility for what you do with this knowledge.
Step 1: What You Need
I have less pictures of this project because i built it out of a active subwoofer and so it is a bit different. but its very easy and watching the final clip you will understand everything.
IMPORTANT: Before you search for the materials read through the whole tutorial else you will have problems to fit the parts together!
What you need is:
1) a old speaker from 40W upwards
2) a Hi-Fi with "line in" or a amplifier.
3) a 100 to 200g weight, if possible flat and round. (i made mine melting some lead into a steel cup.)
4) some wires
6) a office chair to use
1) sharp cutter,
2) soldering iron
4) glue suitable for the material of weight you use
I used this active subwoofer. Active means that it has a amplifier inside and does not need an additional one. The disadvantage of mine is that it uses a signal coming from the player to switch on and off so I had to bridge the circuit.
Step 2: Prepare the Speaker
1) (most common) you will see the membrane and at the center of it a tiny rounded membrane.
2) you will see the membrane and a smaller hi frequency speaker at the center of it (this one is not suited for our project)
3) (best for this project but most difficult to find, can be found inside woofers) you will see the membrane and a round flat 5cm disc at the center.
In both suitable cases you need to remove the membrane, letting the lower (usually brown) membrane intact. Be careful not to cut the upper membrane where the wires are attached. If you make it correct you are going to let only a 2 cm large peace of the upper membrane removing the rest. This step is important, else your chair is going to play music.. :D
Now to the second part of preparing the speaker: this is different for the case 1 and case 3 speakers (see above). for the case 1 speaker you need to cut away the little rounded membrane at the center. After removing it you will see a hole, it is a cylinder on which the coil is mounted. You can use this hole to mount your weight. It is important that you do NOT deform the cylinder else the coil touches the magnet and it could start to burn. the best way to do this is to put a little peace of garden tubing inside the tube and then screw a big screw inside using it as a weight.
For case 3 speakers everything is easier. if you can find one of these it is better for several reasons:
MUCH more powerful
easier to build
usually a low frequency speaker
you can find these inside subwoofers.
in this case you need to glue a weight centered on the disc (use the correct glue!). thats it.. :)
Step 3: Setting Everything Together
Solder the wires to the two speaker connectors.
Screw the speaker under your chair (IMPORTANT! IT MUST BE VERTICAL! ELSE THE WEIGHT WILL DECENTER THE COIL CAUSING A SHORTCOMING! IF IT IS VERTICAL THERE ARE NO PROBLEMS! used this rig for months now.)
connect the amplifier or the Hi-Fi to the pc/tv/player and the speaker to the hi-fi. sit down and play!
Step 4: Watch the Movie...