Bass shakers are like the rumble packs in a Playstation or Xbox game controller. They make whatever they’re attached to rumble when they get a signal from the LFE (low frequency effects) channel of your audio system. That means when the T-Rex puts its foot down in Jurassic Park, you don’t just hear it… you feel it.

You can buy an $800 shaker system from a commercial outfit like Buttkicker or Clark, or you can put the components together yourself for as little as $30. I’ve done it both ways, and I like the $30 results better. That cost assumes you have some speaker wire and an old receiver or amp around. If you don’t, plan on spending another $50-$100 for a used amp or receiver, plus $10 on speaker wire at RadioShack.

You can get the shakers at on eBay. When I bought mine, a pair of Aura Bass Shakers was going for $30, and the pro model was about twice that. People whole used both report little difference, so don’t worry about buying the cheap pair. (I happen to have bought the pros before I found this out, so the ones shown here are the pros.)

For the receiver, make sure it puts out the necessary watts per channel (25 for the Aura, 50 for the pro) and you’re set. Any speaker wire should do. You’ll also need an RCA“Y” splitter and cable, which together run about $6.

Once you’ve got your parts in hand, start by installing the shakers in your seating. One per chair is a good rule of thumb. If you have a couch, you may want more, but I’d start with one and see how much shaking you get. I didn’t notice much difference in my three-seater couch whether I had one or four installed. I settled on two because a) I had extra shakers and b) I thought it might even out the shaking effect more.

You can attach the shakers to any surface you can get four screws into, but you’ll get the best shaking if you can fit them onto the longest part of your seating that’s suspended between two points. On my couch, that was the piece of wood that ran the length of the seating, between the legs on either side. I had to pull off the underlining of the couch to do it, and then I used a staple gun to put the underlining back on after running my wiring.

If you’re only using one shaker, make sure to center it, otherwise you might feel the shaking more on one side than the other (in other words, don’t put them in the arms of your couch or chair). With multiple shakers, spread them out as evenly as possible. If you really can’t find a place to screw them in, you can also use zip ties and attach them to the seating springs, but that won’t transmit the shaking effect throughout the whole seat as efficiently. Still, in a pinch, it’s a quick and easy solution.

Next, hook the shakers up. Start by splitting the RCA wire going to your subwoofer with the RCA “Y» splitter, and run the split signal into your

Here’s the subwoofer out line on my receiver. Sometimes this output is called the “LFE out,” which stands for «low frequency effects.» amp/receiver with the RCA cable. Then, hook up the shakers to the amp/receiver’s speaker outs with the speaker wire. The only trick here is whether to wire them in series or in parallel, which depends on how many shakers you’re using. I’d recommend reading that thread in any case, as it’s full of good tips and information for trouble¬shooting and modifications.

The rest you know how to do: Stick a DVD in your player, press play, and start watching a movie. When the explosions begin, your seat should respond with a perfectly synchronized rumble. You can control how much rumble you get with the volume control of your amp/receiver. I find the effect is best when it’s subtle, so you can’t quite tell if you’re feeling your subwoofer or the shakers. But if you want your teeth to rattle, just pump up the volume.

To demo your shakers, I recommend the lobby and helicopter scenes from The Matrix, just about any scene from Black Hawk Down, the beach landing in Saving Private Ryan, the battle scene at the end of Lord of the Rings: The Two Towers, and my personal favorite, Das Boot. Also, make sure you don’t tell your friends about your shakers before you invite them over. I find it’s best to let them discover the shakers on their own.

Irock transmitters let you play audio from your iPod (or CD player) through your car stereo by «broadcasting” it via FM to the antenna just a few feet away. It’s great when you’re in a car, but what if you want to transmit over a larger area?

Using common electronics components and an old ammo box, I turned my irock into a portable micro radio station with a range of about 200 feet. Instead of drawing power from a dashboard’s 12VDC power jack, it uses a modified flashlight and two D-cell batteries, which run the rig for about 18-24 hours.

The main trick I found was to start by mounting the irock’s printed circuit board onto a metal plate, which I later secured inside the case at the end of the project. This way, I could do almost all of the delicate assembly out in the open. With all the drilling, it’s also important to keep metal flakes off of the circuit board.

First, disassemble the irock by removing the three screws underneath the battery cover. Lift the board out, cut off the battery wires, and it’s ready to mount. For the pi ate, I used a 3″x4″ piece of 0.06-inch-thick aluminum, cut with tin snips, and then filed the edges smooth. Using the existing holes on the PC board as a template, I drilled holes through the plate and bolted them together, separating the two with small plastic standoffs to avoid massive short-circuiting. Then I drilled a matching pair of holes in the ammo box. I also drilled two more matching pairs of holes through the mounting plate and ammo box.

Remove the board and plate, and drill another hole in the case, big enough for an F-type connector. This will be the jack for a 75-ohm coaxial cable that will carry the signal out to the station’s external antenna. Locate the hole less than one inch from where the narrow front edge of the board will sit, right next to the irock’s ground and antenna connections. Then solder a short pair of wires to the ground and antenna connections of the irock board.

Mount the irock board assembly back inside the box, and install an F-type connector in the hole, using a lock washer and a ground lug. My F connector from RadioShack did not come with a solder lug, so I took one from a connector. Solder the wires from the board’s ground connection to the ground lug and from the antenna connection to the F connector’s center pin.

You can power the irock with any 3VDC power supply, but I used a rewired flashlight (see sidebar) and split the leads to supply juice to both the irock and the CD player, my audio source. To keep this conveniently switchable power supply out¬side the case during broadcast hours, I bolted a terminal strip to the side of the box. This became the power supply for both irock and player.

Inside, I soldered wire pairs from the back of the strip that were long enough to connect to the two devices, taking care to retain the correct polarity.

I soldered the other ends to a 3VDC adapter plug and insulated it with heat-shrink tubing.

To prevent the box’s guts from rattling around, the insides needed some structure. I used a metal divider, cut from an old piece of galvanized sheet metal. I shielded the sharp edges with duct tape, and duct-taped the partition to the inside of the box.

A styrofoam spacer underneath the CD player has a cutout for the flashlight, and the space can also hold CDs. The other side of the divider stows the antenna when the station is not in use.

To start broadcasting, connect a regular indoor FM antenna to a standard 75- to 300-ohm matching transformer, and plug it into the box’s antenna jack.

Matching transformer connected to FM antenna.

Hook up the player to the irock, and arrange them in the box so that you can still reach the irock’s power and frequency selection switches. Finally, fire them up and see how far you’re transmitting by moving around with a portable radio.

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