I have been dragooned into teaching a STEAM music class for 9-11 year olds this summer and am working out the science part of the curriculum. (The is my second time posting a related question, and I just want to say that the recommendation I got from this board about WaveWindow was AMAZING - it’s really terrific.)
Anyway…in one class we will play saron (see photo in link), which is a Javanese xylophone with metal keys (bronze or occasionally iron). I have both bronze and iron ones for the kids to play, tuned the same way.
When I bring out the saron, the kids will have been introduced to the idea of sound waves already, with a basic understanding that the faster the wave cycle, the higher the pitch.
But…what additional scientific principles can we learn (in simplified fashion) from the saron? Perhaps we can do a little experiential learning to arrive at answers to these questions:
The higher keys are shorter and fatter than the lower keys. Why do these differences in size and shape affect pitch?
The pitch of the equivalent bronze and iron keys is the same, but each bronze key is much thicker and heavier than its iron equivalent. Why are the bronze and iron keys not the same?
We can change the pitch by putting putty on the keys. What’s happening?
If we take the keys off of the resonating chamber and strike them, they don’t make nearly as big a sound as the do over the chamber. Why?
Sadly, I do not know the answers to the above questions myself, and the person I was relying on to produce the science part of the curriculum gave me only this: “The saron creates sound by vibrating itself compared to other instruments that vibrate off each other, the idiophones vibrate themselves to create sound.” Yeah, that’s pretty useless.
If anyone can provide age-appropriate answers in the next few days, I would be EXTREMELY grateful. Thanks!
OK, here is an illustration from Principles of Musical Acoustics, p. 270:
This is supposed to be a free bar vibraring transverse to the length, with mounting points that encourage the first mode and damp the others. They give a basic approximation
f = 0.113 \frac{v_L\, t}{L^2}(2n+1)^2
where v_L is the speed of sound in the bar, t the thickness of the bar, L the length of the bar, and 2n+1 = 3.011, \, 5, \, 7, \, 9. The bar is undercut to lower the frequencies of all the modes and make the second mode harmonic with the first, and mounted as shown where the first mode has a node. The resonators are used to emphasize the first mode frequency.
It’s just a schematic illustration of a bar vibrating in different modes. Nobody is going to write any equations on the board (probably!)
You could say that a thicker bar is stiffer, a longer bar will vibrate more slowly, the speed of sound waves in different materials affects the frequency, etc without getting into details.
When a stiff object is pulled or stretched or bent or otherwise distorted from where it normally wants to be – i.e., away from it’s resting shape – there is some force trying to pull it back to where it belongs. A tight guitar string fights against being pulled aside by a finger, and when the finger is removed, the tension in the string quickly tries to restraighten the string. That force will pull the string back to center, but the string will overshoot the center! Now it’s on the other side of center and the cycle repeats, back and forth, until all the energy is lost to heat and sound and the string stops vibrating.
As to what sets the frequency of this cycling back and forth: The tension in and the stretchiness of the guitar string determine how strong the “restoring” force is, and the mass of the string is what sets how easy it is for that force to affect the string. Have you heard of Newton’s 2nd Law? F=ma? More force means more acceleration, and more mass means less acceleration.
For a guitar, you can also shorten the string by pressing down on the fretboard. If you press on the 12th fret – cutting the string in half – it takes twice the force to pull the string sideways the same amount as before, so the restoring force is twice as much. Equivalently, you can displace the string less so that the restoring force is the same, but now the physical distance the string has to go to get back to center is less, so the cycle takes less time. It turns out for simple vibrating systems, these effects are perfectly matched, so that in the end the frequency doesn’t depend on how much you displace the string.
The xylophone case is the same, only now the object is a stiff bar that is being bent when struck, and the tension in the bar pulls the bar straight; it overshoots; and the cycle repeats. Longer bars give lower frequency. Heavier bars give lower frequency. Stiffer bars (due to being made of different material) give higher frequency.
If you can get your hands on a Slinky, you can have two kids hold the ends so that the Slinky is stretched out along a smooth floor, and then you can “pluck” it in the middle to watch the bulk vibration. Then, have them tighten the Slinky by bunching up the ends into their hands some (keeping the distance between the kids the same) and then pluck the now tighter Slinky again to see the frequency increase. Then, tape something heavy but not too heavy to the center of the Slinky and watch how the extra mass slows down the frequency.
Different stiffness of material ==> different restoring force when bent ==> different frequencies. So, the size/shape is tuned appropriately for the material.
See above.
A guitar is a good example case here, too. The string is very thin and doesn’t have much “bite” on the air to make sound waves. But when it’s attached to a giant piece of wood, the vibrating string vibrates the wood, which has a huge surface for pushing the air around. Same with this instrument.
There are many more subtle factors, like resonant amplification and holes to let the inside air connect to the outside air, but that’s not critical.
You can demonstrate the basic effect by making your phone vibrate when held in the air versus when pressed against a wooden table. The vibration transfers to the much larger surface of the table and makes things much louder.
Personally, I’d start by having the kids make standing waves on a jump-rope, and point out how they can only make patterns by shaking it at certain speeds. Then I’d use ropes of different lengths, and point out how the speeds change. Then I’d go from there to a string instrument like a dulcimer, zither, or harp (or a guitar if you don’t have access to any of those), and mention how the different speeds of vibration make different sounds. And then from there, now that they’ve got that short things make high sounds and long things make low sounds, go to the glockenspiel.
Thanks everyone. As @pjd says, these are 9-11 year olds (and products of the Hawaii education system which ranks pretty low) so simplicity is always good.
I did NOT know that that about the definition of xylophone! I had what I thought was a xylophone as a kid but the keys were metal. Everyone called it a xylophone, though. Live and learn …
ETA: also, it is 6 classes and the instruments we will use are:
Angklung
Saron
Theramin
Jaw Harp
Water ammonium
Kazoo
So additional comments about the science of sound are very welcome, if they relate to any of the above instruments.
You could also use wine glasses (substitute the wine with something else for little kids), that’s always fun. Also blowing into bottles containing varying amounts of liquid (beer bottles can act as Helmholtz resonators)
I’m on your team (as a staunch linguistic descriptivist). “Xylophone” is used in practice much more fluidly in casual speaking than its original etymology. If I went to a toy shop and asked if they had any glockenspiels on offer, I would 100% expect to be given a funny look.
(To wit: a glockenspiel was originally an instrument of struck bells, and then the metal bar version began to be called that some time later. Deep-dive nitpickers could say that what you had as a kid was actually a metallophone. And I’d say they are just as wrong. )
Sorry, combination of stupid autocorrect and brain fart combined. I was trying to type “armonica” but that is actually not quite correct anyway even if autocorrect had not had its way with my typing. I meant
one of these, but its most common name seems to be “glass xylophone.” Which apparently is not correct either.
Fortunately, this is not strictly a science class - if it were I’d have no business going anywhere near teaching it. (Actually I did have someone from a science museum tasked to develop the science part of the curriculum for me, but his work was disappointing, which is why I’m looking elsewhere to buttress what he gave me). It’s a little taste of science plus a chance to play some weird instruments, compose for them, and put on a little performance for family members at the end of the course.
A theremin is supposedly difficult for beginners and really takes a lot of practice. It is very sensitive—you will need to re-tune it if people move around in the room, etc., and requires mastering several arm and finger positions and using the two hands independently.
It’s not just etymological, though. It also matters when dealing with musicians, as the two instruments sound different and thus may be called on for different uses.
It’s basically more like “That toy xylophone you had as a kid? A musician would say it’s a toy glockenspiel.” People who work in a field tend to have more precise names for things.
I also note that the group of said instruments are often just called “keyboards,” at least, in marching band. Marching band tends to be a bit more loose in its terminology.
I agree fully. Such distinctions are useful in-field. My point was just that calling a glockenspiel a xylophone out-of-field is not incorrect and is in fact the normal usage.
Most nit-picky terminology arguments are exactly of this type. Someone tries to insist that a niche (and useful) in-field usage must 100% apply in all other contexts. That’s what riles me up (clearly ). I call tomatoes and cucumbers vegetables in basically every context I encounter, but I recognize that there are more narrow contexts where they would be called fruits. Language need not be rigid in usage, is all I’m noting.
I once did a quickie experiment for a curious young friend - blow into a bottle (say, 16oz), get a certain pitch. Take a 2L bottle, fill it with water, pour out some to fill the smaller bottle. You now have an air cavity the same volume in the 2L, but not the same dimensions (depth or diameter). Yet the pitch seems to be the same.
You can also change pitch of glasses (glass ones, of course) by how full they are. If you can get identical glasses and tume them (another fun musical make-work) then compare depth or volume - mark the correct point on each, see how much water is in each note.(and by contract, how much is air).
You can demonstrate harmonics with the skipping rope too. )Or a long slinky). wave it back and forth and see the first resonant frequency. A bit faster, you get two lobes and the middle is almost stationary. Faster still, 3 lobes and two stationary points.
you can also hang the xylophone bars by one point instead of across two and see what happens. (Sometimes there are holes in the bars to keep them in place that you could use).
Yes - I’ve owned one for years and when I was “competent” I could recognizably play “Mary Had A Little Lamb.” The kids aren’t going to become theremin players in one session, but there is no harm in introducing the instrument - everyone agrees, the kids are going to have a blast trying it out. Heck, even our gallery director is excited, and he’s in his 50s.
Tomatoes and cucumbers are vegetables in every context. There is no definition of “vegetable”, at all, that does not include tomatoes and cucumbers.
In some contexts, they are also considered fruits. “Fruit” and “vegetable” are not mutually exclusive.
And I would recommend against getting too much into blowing across the tops of bottles-- Helmholtz resonators work very differently from other wind instruments, and are likely to be a confusing distraction, unless you spend way more time on them than it sounds like you have available here.
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