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?
