3.3 Waves
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heat up. A candle taken from the freezer will sound like a wood or metal bar
when dropped, compared to the dull thud of a warm softened candle. Other
thermosetting
materials harden and get more elastic with temperature.
Structure and Strength
Materials may have structure at many levels. We may see regular crystalline
structure in metals and glass, or a chaotic and dispersed structure in graphite
or clay. Complex structures exist, such as those found in wood where fibrous
strands of cellulose are woven into vessels containing air or moisture. At the
atomic and microstructural level metal is quite uniform in a way that wood is
not. Glass or china ceramics fall between these two. Homogeneous structures,
where each bit of the material is largely the same as all the other bits, tend to
give purer tones than heterogeneous structures, where the material is composed
of many different bits mixed up. Cellulose as corn starch in water, like dough
or custard, is a lot less elastic than the same amount of cellulose arranged
as wood. The difference here is material
strength
. Elasticity only works if the
bonds aren’t stretched too far, governed by two things. One is a constant factor
for each material. The constant has no units, being a ratio, a nonfundamental
unit made up of elasticity and plasticity in two modes, compression and ten-
sion. The other is temperature, which changes the strength. Since most things
are thermoplastic they lose strength as they get hotter. A material that can
crack, splinter, or deform changes its structure permanently. Like plasticity,
if the material breaks any elastic energy is lost along with the potential for
vibration. If a material is
hard
, like diamond or beryllium, the bonds may be so
strong that sound is transmitted very fast; it is as if the whole structure moves
together.
SECTION 3.3
Waves
We have talked about sound in an abstract way for some time now without ever
alluding to its real nature. Now it’s time to take a closer look at what sound
really is. The things that carry energy from one place to another are called
waves
and they move by propagation through a
medium
. The medium, which
is made to
vibrate
, is any intervening material between two points in space.
Waves are imaginary things. Watch the surface of a pool as waves move on it;
the water does not move along at the speed of the wave, it merely rises and
falls. The local or instantaneous velocity of the medium is different from the
speed of the wave. The wave is something else, less tangible; it is a pattern of
change that spreads outwards.
Wave Models
If a wave is imaginary, how do we sense it? We can’t directly see forces or accel-
erations; they exist to help us understand things. However, we can clearly see
and measure displacement by the position of something, such as when water
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