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  • 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 teeththe 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 melodyThis 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 upthe

  • spring will unwind quickly and the music will play too fast.

  • This piececalled the governorsolves 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 rapidlyover 3,000 revolutions per minuteair

  • 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 gearthis 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?

Automated music boxes began as cumbersome sets of bells struck by

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B2 US gear comb drum teeth governor music

How a Wind Up Music Box Works

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    Rain posted on 2018/07/20
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