How a Wind Up Music Box Works

How a Wind Up Music Box Works


Automated music boxes began as cumbersome
sets of bells struck by hammers, but over the course of several hundred years they’ve evolved into compact devices like this one.
I’m sure you know how it works: wind it up and it plays a tune. The
melody is programmed on this rotating drum. The drum has protrusions,
called pins, that pluck the teeth on the comb. The comb is a piece
of steel with eighteen teeth. Each tooth is a note. Longer teeth
are lower notes and shorter teeth are higher notes. The comb works like
a multi-pronged tuning fork. In this high speed video — slowed by
250 times — the drum appears to not move, but you can see the teeth
vibrate. The shorter tooth is vibrating faster than this longer
one. These vibrations produce the sound. The teeth are like this
saw blade, when it is longer, it produces a lower note when plucked,
and when shorter it produces a higher note. Notice when you turn
the comb over, the teeth don’t have the same thickness. The longer
teeth — the lower notes — are weighted more on the ends. This added
weight lowers their resonant frequency even farther. Here I taped a lead
weight to the end of the saw blade, and it produces a lower note than
without the weight. Because of this weighting the comb is more
compact. For this particular design, if the comb were unweighted
it would have to be roughly 40 percent longer to produce the same
range of frequencies. Another advantage of the weighting is that
the combs can be mass manufactured in a single size, you just cut
away the proper amount of material to produce a unique set of notes.
For example, although each comb has eighteen notes, the specific notes
vary for a particular song. Here’s a music box playing London
Bridge with a comb specifically designed for this melody. And now, here it is with the comb cut for a different melody — This Old Man. The timing is the same but the notes are different and it sounds odd. The difference in weighting is so subtle that these two combs
are indistinguishable by eye. Inside the casing of the music box is
a clockspring. It’s a coiled strip of steel that is 40 centimeters
long unwound. The outer end of the spring has a T-shape which affixes
to the casing of the music box and so holds it in place. The inner
end of the spring has a slot. This slot hooks onto a notch on a metal
shaft. This shaft is attached to the winding key. The shaft also
has an angled six-tooth ratchet gear. This gear fits inside this larger
plastic gear. On the inside there are four flexible pawls so the
axle turns independently from the plastic gear. This happens when the music box is wound. When the spring unwinds, the axle turns in the opposite direction and the six tooth gear catches the pawls, which rotates
the larger plastic gear with it. This rotation drives the music
box. As the spring unwinds, it rotates this bevel gear. Which
engages a second bevel gear affixed to the drum. But there’s a problem
with this set up — the spring will unwind quickly and the music will
play too fast. This piece — called the governor — solves this
problem. It’s connected to the drum by a gear train. The gear train is
compactly built into the music box. The rotation of the governor controls
the speed: stop the governor and the drum stops. The governor
uses air resistance to control the release of energy from the spring.
Air resistance is proportional to the velocity squared of the
object. When started from rest, the governor encounters little resistance
and speeds up readily, but when it spins rapidly — over 3,000 revolutions
per minute — air resistance swiftly increases which prevents
it from moving much faster. This action limits the speed of the
governor and limits the rotational speed of the drum. To spin the
governor so fast, the music box uses a multiplying gear train. It starts
with the bevel gear driven by a spring, which engages a smaller gear on the drum. This multiples the rotational rate by the ratio of the number of teeth on the larger gear to the number of teeth of
the smaller gear — here 2.75 times. The drum is also affixed to a
larger gear, which engages another smaller gear — this time multiplying
the rotational speed by 5.75 times. The larger gear on this piece
engages the smaller end of another spur gear, further multiplying the
rate by 6.3 times. Lastly, this spur gear engages a worm screw on the
shaft of the governor. It moves so fast it’s blurred — here, slowed
down by a factor of thirty, the movement is visible. The gear that engages
the worm screw differs from the other gears: it has curled teeth.
The shape of these teeth allow it to better engage the screw. The worm
screw turns once for every tooth on the gear, and, since there
are twenty-four teeth, it multiplies the rotational rate by twenty-four
times. This means that for every single revolution of the first bevel
gear, the governor rotates 2,400 times. Since the first gear
rotates roughly one and a half times a minute, the governor spins at
3,600 revolutions per minute. As I noted this music box evolved
from devices that used bells struck by hammers. The replacement of these
bells with a comb was the technical breakthrough that catalyzed a music
box industry that blossomed in the nineteenth century. The compact
comb movements were built into snuff boxes, clocks and large pieces
of furniture. As the industry flourished, music boxes grew more
complex: some, for example, sported dual barrels and combs, which played
simultaneously to produce rich harmonies. The first music boxes used
cylinders, but were superseded by boxes that used disks, which
could be easily changed. Here the melodies were punched into
a metal disk. With this innovation, music boxes shrunk and their cost
declined. For a hundred years music boxes where the way a family listened
to music in the home, but by the turn of the twentieth century
the phonograph and radio had displaced them. Music boxes were
shoved into attics or, more often, left to rot in junk yards. These
modern music boxes, then, are a charming vestige of a past filled
with brilliant engineering and craftsmanship. One last thing,
if you hold a music box in your hand, it’s not very loud, but if you place it on a hollow container, it’s much louder and richer.
The vibrations of the comb are transferred through the metal base, into the
container where they resonate. This resonance amplifies the sound.
Also, if you rest the music box gently against your teeth, the music
will resonate inside your skull. So, the next time you listen to
a music box appreciate its sound, but also think of the centuries
of innovation and design that lead to it. I’m Bill Hammack, the engineer guy. The drum has protrusions, called pens, that pluck the teeth of the comb. The comb. pins, pens, right? Pins. Ins. Pens.
Ok, I’ll get it. The drum has protrusions, called pens. The drum has
protrusions, called pens. Pens, pin. Ok. The drum has protrusions, called pens that plunk the teeth. Did I get it right? In, in, pen.
Pen. The drum . . . . Now, I can’t say the “i-n” one because when I say it I say pan.
Pin. Pin. Pin. Okay. The The drum has protrusions, called pens, that pluck the teeth of the comb. The comb is a piece of . . . Did I not
get it? Pen. Pin. Pin. Pen. Pen. Is that right?


100 thoughts on “How a Wind Up Music Box Works

  1. This is so interesting…o be learned so much on you tube…in a year…But in school I learnt very little….

  2. I want to try to make a music box. A few days ago I was at my great grandparents house. As me, my mom, sister and grandmother were looking though paper work, I told my sister about a music box that scared me out of my wits a while back. We looked around for a few minutes and found an old 18 note music box that was made in Switzerland and played “Annie’s song” by John Denver and now I want to make one (sorry for the paragraph and thank you so much if you read to the end 😊)

  3. Oh my gosh! 45 years, and one time several years ago I repaired one of these by replacing the spring, I think, and I still never knew until now that the combs were actually matched to their drums because I never thought to venture off and swap any! Very interesting!

  4. Yeah, so the governor is basically a little fan. Some of them are designed to use the blades as a stopping device.

  5. Yeah, in the Christmas of 2011 (I think), my parents bought me a modern version of one of these disc-based music boxes! The discs are the same size as CDs-Blu-rays; even the same size of hub (just don't get them mixed up, haha)! The bells are actuated electromechanically. The brand is Mr. Christmas. Have you seen them?

  6. I always wondered what “the spinny thing” was! I just thought it was FOR stopping the music at will. I didn’t realize it was for controlling the speed! Wow! PHYSICS.

  7. I got such a giggle out of the "pens" bit at the end. It is so humanizing to see that occasionally, even simple tasks can sometimes confound us ALL. Thank you for sharing that – you are so clever and intelligent that it can be intimidating; the ability to laugh at one's self is quite charming.

  8. The part that starts at 5:49 is so poetic. I wonder how can him make everything sounds so great.

  9. It looks to me like the governor uses a different method of limiting the speed to the one you described. There are those "big" weights on each side so that when the governor spins faster and faster, the weights are forced upwards (observable at 03:33) by the centrifugal force because they are attached eccentrically, and they rub against the rim of the surface above it at a set rpm, slowing it down as the rpm and frictional force increases. The critical rotational speed depends on the stiffness of the rubbery material, which functions as a spring, and the density of it in the weights, which changes it's centrifugal force.
    Over a long period of time, the speed of the music runs away as the frictional surfaces wear down and the "hinge" rubber stiffens naturally.
    If it was designed to interact with air, I would imagine it to be very light and to have some sort of aerodynamic surfaces on and/or around it, as is more obviously the case with the music box at 05:15.

    Thanks for all of your phenomenal videos Bill! They feel like the German series "Die Maus" but more interesting and sciency!

  10. I really love this channel, wish he would make more videos to electronics and things like electric motor (rotator) or how the cooling system of a fridge works.

  11. First, this was way more interesting than I could have imagined.

    Second, I can't distinguish pen/pin at all…. Not when I say it, not when I hear it, it's a complete homonym to me.

  12. "These modern music boxes then,are a charming vestige of a past filled with brilliant engineering,and craftsmanship."

  13. Trying to fix a music box and stumbled upon this video…just loved it! Nothing like actually finding out how something works! So intricate! Thank you!

  14. For modern, beautiful music boxes handmade in the USA by people with a love of the art. Take a look at bettermusic boxes.com

  15. When you said touching it to your teeth made the sound resonate through your skull this got me thinking of these bone conduction earphone technology I am starting to see. I assume those draw from the same concept?

  16. sooo i would never be able to build one for my gf cuz this engineering stuff is too hard for me to follow lol

  17. Actually wondering where I can have a drum version made. Wrote my own music box track, played with an artificial music box instrument in fl studio

  18. Excellent as always!

    Re the outtakes….I feel ya! I have my own series on armory for the sport of fencing (I can haz armory) and I rarely get it right on the first try….usually it takes me half a dozen before I'm happy with it.

    One day I'll actually have the money to redo all of my vids with something approaching actual production values….right now it's just me and my iPhone.

  19. Also note that the governor has weights and a spring such that the weights move outward at greater speed, increasing their angular momentum for more stable operation, and increasing their air resistance for better governing.

  20. Bill thank you for the entertainment at the end with the pin/pen fiasco. Your effort is appreciated.

  21. We have a few of the disc based music boxes and many discs. There is a company online seeking donations of discs that they will add to their library so they can make reproductions. I also have a paper based music box that you can punch your own holes to make original tunes.

  22. Thanks for this video! Easy for me and my kids to understand and a great jumping point for talking about sound.

  23. No technology was ever actually directly created by someone alone, it was passed by generations with little upgrades and who knows tomorrow our smartphones will be in junk yards

  24. JAJAJAJA the last part of right pronunciation was Hilarious!!! That happens to me too a looooot!!! JAJAJAJAJA

  25. That blooper shows the enormous effort put into the making of this very informative n concise video:-)
    Thank u!

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