It didn’t take a ton of research to figure out that the xylophone bars had to be shaped, not just cut to length. In this case, tuning means carving the bottom of the bar to yield the desired tonal characteristics.
Here is a picture of the underside of the bars for a Kori xylophone that I checked out at the University of New Mexico. Thanks to Scott Ney, I was able to dissect this instrument!
While I don’t have the skill (or patience) to provide a full-on description of the physics related to vibrating objects (like a xylophone bar,) I will describe a few basics to get you started. Nerds: you probably already know this stuff and can safely skip to the next section.
We all know that when something, like a bar of wood, is whacked, it vibrates for a while and then the sound eventually dies out. The duration of the sound is referred to as the “decay” of the signal, and there is a precise definition that quantifies the decay duration in seconds. What many people don’t know is that pretty much every object has a preferred frequency at which it generally vibrates. Think of whacking a frying pan versus big pot; you can hear the difference in the tones between these two objects. The frying pan “wants” to vibrate at a higher frequency than the big pot. Further, each object will typically vibrate at a series of discrete frequencies called modes.
For those without science knowledge, what you may not know is that what you are really hearing is one “fundamental” tone, and then a series of different, higher tones that are related to the fundamental tone in complex ways. It is the relationship of this fundamental tone and the “overtones” (and how quickly each tone dies off) that creates the overall tonal characteristics (i.e., the timbre) of the object.
It turns out that it is possible, by choosing the shape of the object, to control the fundamental and overtones.
WARNING: You’re entering a zone of extreme nerdiness. If you don’t care about physics and math, you may want to skip the next few sections. Although, be warned – nerd-speak is sprinkled throughout the rest of this blog.
Our xylo-adventure started when Jack asked for a xylophone for Christmas. He wasn’t asking for a Fisher Price, but rather a full-on concert grade instrument.
Jack plays piano and percussion in his school bands and wanted an instrument to practice at home. My knee-jerk response was to say “sure, dude!” When I did my research, however, I almost had a heart attack- I was not going to pay $2000-$4000 for a series of wooden blocks! So I, an electrical engineer and a weekend woodworker, set out to build one. “How hard could it possibly be to build a one?” I asked myself. (Answer: pretty damn hard!)
So last December, I jumped in with both feet. Like most people, I assumed that Google would do the majority of the work for me. Over Christmas break, I begin scouring the web. My laptop kept me warm for countless winter evenings while I searched for a canned xylophone recipe. It quickly became apparent that if anyone like me had built a high-end xylophone, they had not published their efforts on the web. I found lots of “toy” efforts built from PVC pipe and the likes, but got exactly zero hits on building a concert xylophone. Along the way, I did find one amazing site called La Favre that had tons of info about building a marimba (basically a giant xylophone). The La Favre site was pivotal in that it was the first source that I found that addressed tuning of the bars. It doesn’t take more than high school physics to know that the longer the bar, the lower the pitch; I, therefore, assumed that tuning just meant cutting the bars to the correct length. Au contraire – the tuning of idiophone bars (the fancy, scientific name for musical instruments that you whack with a stick) is performed by shaping the underside of the bars in just the right way.
Getting shape right is really the secret sauce to building an instrument that sounds great. Stick with me and you’ll hear a lot more about this!
As a teaser, below is a one-page summary of the xylophone that Jack and I built. Read the subsequent posts if you want to learn more about our journey.
Here is what it sounds like. These are just a couple of clips of Jack noodling around.
Here is a photo of the completed instrument.
May 21, 2016 update: Here is a sound file of Jack playing a song he learned. The recording is so-so, but hey, I’m not a recording engineer!