Transcript Interference & Standing Waves

April 14 – April 15
What is the definition of a wave? A disturbance that carries energy
without transferring matter
Example: Sound from a speaker
traveling through air to my ear.
All waves ‘travel’ or carry energy
from one location to another
until the wave encounters some
sort of barrier or new medium.
What happens then?
Depending on the barrier / new
medium, some of the wave
energy may transmit through.
But some of the wave energy
will be reflected.
A great example of wave reflection and the interesting patterns that
result from it are waves on a string/slinky.
Because the string has a short length, the wave quickly runs into a
barrier and reflects backward.
.
Slinky Demo!
What would happen if I increased the rate
of the pulses?
The original and reflected waves would
interfere, creating odd patterns.
Example: Rope attached to
a fixed point. A wave pulse
travels to the end and
reflects back.
Are the original and reflected waves still
traveling through the medium?
Yes! It just doesn’t look like it because we
see the sum of their behavior.
If we vibrate the string at exactly the right frequency, we can create
standing waves. Standing waves have points – called nodes – that
appear to ‘stand still’.
Standing waves result from the interference of two identical waves
with the same frequency and the same amplitude traveling in
opposite direction – such as we get with reflected waves (of certain
frequencies) along a string.
Antinode – maximal displacement
.
node –zero displacement
Slinky Demo!
By adjusting the frequency of disturbance, standing waves of
different wavelengths can be generated.
The production of standing waves is how tones are generated in
most instruments (strings, winds, brass, etc.)!
.
Fundamental tone (first harmonic)
Second harmonic
Third harmonic
What wavelengths can be produced?
Only regular divisions of the string …
.
Note that the
fundamental
frequency has half a
wavelength (one loop)
across the string.
In other words, it has
1 * ½ wave lengths.
As we go up
harmonics, we add ½
wavelengths.