This device is designed to play a song by rolling a golf ball down a ramp with a series of xylophone bars. It was built to promote a summer day camp at a local church, and it quickly became the star attraction for the entire town. It's location near a sidewalk encouraged passers-by to play with it.
The user places a ball in a PVC tube ball elevator, and slides a lever to lift the ball to the top, where it drops onto the xylophone ramp.
Step 1: Tuning and Cutting the Bars.
The bars can be made of any wood, but hardwood is best: the harder the better. If it's going to be outdoors, it should be either rot resistant, or painted with exterior paint or varnish, or both. Maple would sound better but cedar will last longer. Ours was only going to be up for a few weeks, so we used painted poplar, which is a cheap hardwood that every big box store sells. Our bars were 1/2 inch thick and about 2 inches wide, but the exact dimensions aren't critical.
Pick a simple song, with a simple melody around 20 seconds long. Find the sheet music and make a list of how many of each note you will need. You will need to cut a xylophone bar for each note. we color coded the notes to make it easier to assemble, and because it looked nice.
The pitch of the notes is determined by the length of the xylophone bars. longer bars have lower/deeper notes. For simplicity, let's just use a basic chromatic musical scale with 12 notes per octave, like you would find on a piano. A musical scale can be thought of as a circle, beginning at a note, and ending one octave above at the same note. I've provided an example of a scale starting and ending at C, but you could start the scale at any note and use the same length ratios to get a scale.
Start with the lowest, deepest note of the song, and cut a bar that sounds right. This may take some trial and error. Tap the bar, listen, and if necessary, trim a little off to raise the pitch. Go slowly, and dont take too much off at once. Using a sander to slowly shave down the ends might be the best way to fine tune the pitch. If you really want this tuned perfectly, you can use a pitch pipe or guitar tuner.
Once you have the lowest note figured out, the length of the other notes can easily be calculated relative to that note. Use the chart above to determine the relative length of each bar.
For example: Let's say our lowest note is G, and we've decided a 10 inch bar sounds right. If we want to get an A, (which is 2 half notes above G) we would cut a bar that is 9.43 inches long. because 10 x 0.0.943= 9.43.
Note: Expect the pitch to change when you add paint, varnish, or any kind of finish to the wood bars. It will also change pitch when it rains, or during any change in humidity.
Step 2: Building the Ramp.
The basic framework consists a sloping trough made of two parallel rails, made of 2 2x6 boards, with a third board connecting them below. The trough functions as a resonating chamber to amplify the sound. If the structure is long, you may need to join several lengths of this trough end to end. We attached the trough to trees, using various improvised arrangements of boards, ropes, and bungee cords. In some places, the distance between supports was quite long, and we added additional supporting boards below the trough to keep it from sagging, although it is actually quite a stiff and sturdy structure, so that wasn't probably necessary.
The overall angle of the ramp is about 12 degrees. The xylophone bars are supported on small angled wedges cut at 11 degrees. That means that each step is actually almost (but not quite) perfectly level. There is just enough slant to keep the ball rolling. I used a mitre saw to cut these wedges. Once the angle is set, it's easy to make all of them in a few minutes.
The xylophone bars do not rest directly on the wedges. Instead, they are attached by a velcro pad. This keeps the bars in place, but allows them to vibrate. They should be rest centered on the wedges so that the rails are about 1/4 of the way in from each end of the bar.
Directly after each bar is a track made of two pieces of wood molding. The molding is the type used around doors, or for the baseboards of old houses. The v-shape keeps the ball in the middle. The length of each piece of track detemines the timing of the notes. You will have to experiment a bit to get the timing right.
Step 3: The Ball Elevator
The top of the ramp will probably be too high to reach, especially if it is a long melody. We rigged up a ball elevator out of PVC pipe.
The elevator consists of a 2 inch PVC pipe, with a 1.5 inch pipe inside it which lifts the ball up. the inner pipe is half the length of the outer pipe. A slot is cut in the outer pipe, extending from a few inches above the ground, up to the midpoint of the the outer pipe. The slot allows a handle connected to the bottom of the inner pipe to slide up and down, lifting the inner pipe. We made the handle out of a bolt and a golf ball.
Two holes are cut in the outer pipe The first hole is at the top of the slot, at the mid point of the outer pipe, we drilled a hole in the outer pipe to insert the ball. The second hole is the exit hole, which is at the top. When the ball is lifted up by the inner pipe, it rolls out of the exit hole, onto the top of the xylophone.