Biomedical Engineering Reference
In-Depth Information
external force. The mass of the object will
influence how much force is required to
affect its motion (or rest). We know that
a great deal of force is needed to make
a resting object with great mass, like a
freight train, begin to roll. When that
train is rolling, we know that we will need
a lot of force to bring it to a stop. A child's
ball will need much less force to cause it to
roll or to stop it from rolling. When com-
paring the train with the ball, it is easy to
see that mass plays a role in inertia. The
elastic force of an object is its tendency
to return to its original shape. When we
stretch a rubber band, we are distorting
its shape. When we release it, the rubber
band's tendency to return to its original
shape is known as elasticity. Sometimes
elasticity is referred to as the stiffness of
an object. We can think about the allegori-
cal comparison between the rigid oak tree
and the limber willow tree and conclude
that both trees are elastic but differ in the
degree of elasticity. Although we may not
think of solid objects, like glass, tile, wood,
and metal as being elastic, we can appreci-
ate that they can be subtly distorted before
they return back to their original shape.
With a vibrating object, amplitude
refers to the amount of displacement or
distortion in shape as the object vacillates
between the influence of inertia and elas-
ticity. In acoustics, amplitude is reflected
by the degree of pressure change between
the condensation phase and the rarefac-
tion phase, the pattern which is super-
imposed on the air particles. The unit of
measure for this change in amplitude, or
the intensity of a sound is the decibel for
sound pressure level (dB SPL). The inten-
sity unit,
dB SPL
, is commonly used with
hearing aids, hearing assistive and access
technology, and noise. The term
cycle
can be thought of as a complete series
of occurrences (e.g., the transition from
condensation to rarefaction and back to
condensation). The term
period
tells us
the time that it takes to complete one full
cycle, while
wavelength
(λ) tells us the
distance required to complete a cycle.
Figure 3-2 illustrates the comparison
between the tuning fork vibrating, ampli-
tude, period, and wavelength.
Period and wavelength both share
an inverse, proportional relationship
with frequency. Frequency, in hertz (Hz),
can be thought of as the number of cycles
completed in 1 second. As wavelength and
period increase in magnitude, frequency
will decrease proportionally, or with the
same magnitude. As Figure 3-3 shows,
an example of an inversely proportional
relationship is that of a lever, in which the
two ends move the same distance, but in
opposite directions.
While vibrations can travel through
and originate from any object with mass
and elasticity, including material such as
water and steel, among others; the speed
at which sound travels thorough an object
or a medium varies based on the object's
inertial (i.e., mass) and elastic proper-
ties. To illustrate this, we can compare
the approximate speed of sound travel-
ing through air (350 m/s), through water
(1,500 m/s), and through steel (5,000
m/s). Sound travels faster through solid
objects than through air. Even though
solid objects are denser than air, they hold
much greater elastic properties than air.
