| Lecture
4A Wave Basics Wave Measures Wave Collisions Interference Reflect,Refract, & Diffract Sound Waves Doppler Effect WaveSimulation 1 Infrasound Recording Processing Examples WaveSimulation 2 |
| Contacts Description Exercises Index Outline Q&A Resources StudentInfoForm Student Pages What's New |
| Measuring Waves |
Any point on a transverse wave moves up and down in a repeating pattern.
The shortest time that a point takes to return to the initial position (one vibration) is called period, T. In this example, every vibration is marked with a short pause.
The number of vibrations per second is called and is measured in hertz (Hz). Here's the equation for frequency:
f = 1 / T
The shortest distance between peaks, the highest points, and troughs, the lowest points, is the wavelength, .
By knowing the frequency of a wave and its wavelength, we can find its velocity. Here is the equation for the velocity of a wave:
The velocity of a wave is only affected by the properties of the medium.
It is not possible to increase the speed
of a wave by increasing its wavelength. By doing this, the number of vibrations
per second decreases and therefore the velocity remains the same.
The amplitude of a wave is the distance from a crest to where the wave is at equilibrium. The amplitude is used to measure the energy transferred by the wave. The bigger the distance, the greater the energy transferred.
Once a wave (incident wave) has reached the end of a medium, part of the energy is transferred to the medium that is immediately next to it (transmitted wave) and part is reflected backward (reflected wave).
The energy transferred depends on the difference between the mediums.
If there is a significant difference, almost all the energy will be reflected.
If the mediums are similar, most of the energy will be transferred.
However, the reflected
waves will be inverted if the medium that comes next is more dense or
it won't be inverted if the medium is less dense.
Collision of
waves
When two waves traveling in opposite
directions
through the same medium collide,
the amplitude of the resulting wave
will be
the sum of the two initial waves.
This is called interference
and there are of two types:
Constructive interference is when the amplitudes of the initial waves are in the same direction.
The resulting wave will be larger than the original waves.The highest point of a constructive interference is called an antinode.
Destructive interference is when the amplitudes of the initial waves are opposite.
The amplitude of the resulting wave will be zero.The point in the middle of a destructive interference is called a node and it never moves.
of waves of two dimensions
(waves on water surface)
When a wave hits a barrier,
it will be reflected depending on the direction of the barrier (normal).
The angle between the incident
wave and the normal is the same as the angle between the normal and the
reflected wave.
When a wave enters a different medium (more shallow region) at an angle, the direction of waves changes.
This change is called refraction.
When a wave travels through a small hole in a barrier, it bends around the edges. This is called Diffraction.
4B7
Sound waves are longitudinal
waves produced by variations in air pressure.
A vibrating source pushes molecules
in air back and forth, creating areas of compression and rarefaction.
When a molecule moves, it collides
with the next one and makes it move too.
The energy of a sound wave
travels away from the source trough a series of molecule collisions parallel
to the direction of the wave.
Sound cannot travel through
a vacuum.
Sound waves can also travel
trough liquids and solids.
The velocity of a sound wave
depends on the temperature of the medium and its elasticity (more elasticity
means that molecules will move easily).
Through air, sound waves travel
at 343 m/s.
Actually, sound waves move
faster through liquids and solids than through gases.
Measuring sound waves
The frequency of a sound wave is called pitch. In music, different pitches
(C, D, E, etc.) are
represented by notes.
The human ear is able to feel frequencies between 20 Hz to 15 000 Hz,
depending on the age
of the person. Sound waves with a frequency above 20 000 Hz are called
ultrasonic waves.
The amplitude or volume of a sound wave is the amount of pressure exerted
by a sound source
to air molecules. The higher the pressure, the harder the molecules will
collide and the farther
the wave will travel.
Scientists measure the amplitude in atmospheres. Humans can detect from
less than a billionth
of an atmosphere to values one million times higher. However, it is hard
to deal with this huge
range of different values. Instead, the pressure is measured by the intensity
of the sound. The
quietest sound corresponds to a value of zero decibels (unit of sound
intensity) and a value
above a hundred corresponds to annoying sounds.
Doppler effect
The sound produced by the
source is always the same, however, John is receiving more oscillations
per
second because the source
is moving toward him.
The sound that reaches John's
ear has a higher pitch than the sound that reaches Bob's ear.
This effect is called Doppler
effect.
Interference of waves
When two or more waves with the same frequency reach the ear, the ear
interprets these waves
as one wave with amplitude as big as the sum or difference of the initial
waves.
Constructive Destructive
When two waves with different frequencies reach the ear, the resulting sound will be pleasant if the frequencies have ratios that are small whole numbers (2:1, 3:2, 4:3). This is called
consonance.
(2:1) Octave
On the other hand, an unpleasant
sound or noise will be produced if the frequencies have ratios
other than small whole numbers.
This is called dissonance.
| Home | Lectures | Resources | Index | Contacts | Students |
|
|||||