A disturbance or variation that transfers energy progressively from point to point in a medium and that may take the form of an elastic deformation or of a variation of pressure, electric or
magnetic intensity, electric potential, or temperature
Wave motion: A stone thrown into a quiet pool of water causes waves to spread out in ever widening circles whose center is the point where the water was first disturbed. Small objects floating on the pool move up and when wave passes through their positions; they do not move outwards as the waves do. This shows that disturbance caused in the water is an up and down movement of the water. Although the waves on the water travel outwards from the centre of disturbance, the water itself does not move outwards. Such up and down movement or vibration of the water which causes waves is an example of wave motion.
“A disturbance or variation that transfers energy progressively from point to point in a medium and that may take the form of an elastic deformation or of a variation of pressure, electric or magnetic intensity, electric potential, or temperature”.
All waves have similar characteristics, and since all forms of wave motion follow the same laws and principles, knowing the fundamentals of wave motion is important in understanding sound, light, and other types of waves.
There are two categories of waves:
¾ Mechanical
¾ Non-mechanical
Mechanical waves
Mechanical waves require a medium for the transfer of energy to occur. For example, water waves are mechanical. Tsunami waves released after an earthquake transfer the energy of the quake to distant shorelines. Sound waves are another type of mechanical wave. They are compression waves that have a frequency between 20-20000 hertz and travel at a speed of approximately 340 m/sec at room temperature. Different substances carry compression waves at various speeds; metals carry it faster than water which transfers it faster than air. As a mechanical wave travels through a medium, it loses energy to the medium. The molecules in the medium are forced to vibrate back and forth, generating heat. Consequently, the wave can only propagate through a limited distance. When this event happens, we say that the wave has been damped. Damping can be observed by the fact that the wave's amplitude has decreased.
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An indication of amount of energy a wave possesses is given by the amplitude of the wave.
The greater the amplitude, the greater the energy carried, but measurements of wave energy, that is ‘wave intensity’ are calculated from both frequency and amplitude and is the energy deposited on a unit area every second.
Interference is the addition (superposition) of two or more waves that result in a new wave pattern. The principle of superposition of waves states that the resultant displacement at a point is equal to the vector sum of the displacements of different waves at that point.
If a crest of a wave meets a crest of another wave at the same point then the crests interfere constructively and the resultant wave amplitude is greater. If a crest of a wave meets a trough of another wave then they interfere destructively, and the overall amplitude is decreased.
Combined
Standing waves are produced whenever two waves of identical frequency interfere with one another while travelling opposite directions along the same medium. Standing wave patterns are characterized by certain fixed points along the medium which undergo no displacement.
These points of no displacement are called nodes (nodes can be remembered as points of no displacement). The nodal positions are labeled by an N in the animation above.
The nodes are always located at the same location along the medium, giving the entire pattern an
appearance of standing still (thus the name "standing waves"). Nodes are the result of the destructive
interference of the two interfering waves.
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Sound Waves
Sound is a series of compression waves that moves through air or other materials. These sound waves are created by the vibration of some object, like a radio loudspeaker. The waves are detected when they cause a detector to vibrate. Your eardrum vibrates from sound waves to allow you to sense them. Sound has the standard characteristics of any waveform.
Sound is a waveform that travels through matter. Although it is commonly associated in air, sound will readily travel through many materials such as water and steel. Some insulating materials (Sound proofing) absorb much of the sound waves, preventing the waves from penetrating the material.
Because sound is the vibration of matter, it does not travel through a vacuum or in outer space. Also note that light and radio waves are electromagnetic waves. They are completely different than sound, which is vibration of matter. Electromagnetic waves are related to electrical and magnetic fields and readily travel through space.
The back-and-forth vibration of an object creates the compression and rarefaction of sound.
The motions of a loudspeaker cone, drumhead and guitar string are good examples of vibration that cause compression waves.
Transverse Wave (water wave)
Characteristics of sound:
A sound wave has characteristics just like any other type of wave, including amplitude, velocity, wavelength and frequency.
1). Amplitude: The amplitude of a sound wave is the same thing as its loudness. Since sound is a compression wave, its loudness or amplitude would correspond to how much the wave is compressed. It is sometimes called pressure amplitude.
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A sound intensity of 0 dB represents the threshold of human hearing. This corresponds to an intensity of 10-12 W/m2(Io) . (The threshold of pain is somewhere between 120 dB and 130 dB, depending on an individual's sensitivity to pain.)
Minimum sound intensity perception in humans varies with frequency. A frequency of about 4 000 Hz yields the lowest threshold for people with acute hearing ability. The human ear is most sensitive to frequencies between 1 000 Hz and 5 000 Hz. ich is being measured.
Sound intensity decreases as distance increases from the source. An inverse square law relationship is observed.
I α 1/d2
Extremely loud sounds, as from a sudden explosion, may cause the eardrum to rupture.
It is recommended (or required in certain circumstances) that ear protectors be used when one is exposed to noisy environments. Hearing protectors cannot protect adequately against intense sounds which travel through the skeleton into the bones in the middle ear. (i.e., with vibrating power tools, some heavy machinery, etc.)
5). Frequency: Since sound is a compression waveform, its frequency is the rate that the maximum compressions pass a given point in a second. The unit of frequency is the hertz (Hz), which means cycles per second. Infrasound consists of very low frequency sound, beyond the range of hearing of humans. Audio frequencies are those that can be heard.
Ultrasound consists of very high frequencies, again beyond the human range of hearing.
Knowing the speed of sound in air, you can convert frequencies into wavelength.
It is also the rate at which a guitar string or a loud speaker vibrates. Frequency is also called the pitch of a sound. It is called the note in musical sounds.
The pitch of a sound that we experience is determined by its wavelength or its frequency.
The shorter the wavelength, the higher the frequency becomes, and the higher the pitch that we hear.
Since the speed of sound in air at room temperature is about 344 meters/second, the following chart shows the wavelengths for typical infrasonic frequencies.
Frequency (f) Wavelength (λ)
10 Hz 34.4 meters
0.1 Hz 3400 m
0.001 Hz 344 kilometers
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Examples
If TC = 0° C, then v = 331.4 + 0 = 331.4 m/s
If TC = 20° C, then v = 331.4 + 0.6*20 = 331.4 + 12 = 343.4 m/s
This also shows that as the temperature of air goes up, the speed of sound goes up.
Doppler Effect for Sound
The pitch of the sound an observer listens from a moving source will be either higher or lower than the emitted frequency, depending on the direction the source is moving. This is called the Doppler effect.
When the source of sound is moving toward listener, the pitch observer hear is higher than what was emitted from the source and the wavelength is shorter than emitted.
Note that the speed of the source must be less than the speed of sound. An aircraft flying at the speed of sound or greater creates a sonic boom, which is a different effect.
Frequency: The equation for the observed frequency of sound when the source is traveling toward observer is
Note that if the speed of the source is equal to the speed of sound, then dividing by 0 is impossible.
Wavelength: Also, since the velocity of the wave equals the frequency times the wavelength (v = fλ or f = v/λ), the equation for the observed wavelength when the source is traveling toward observer is:
λo = λ(1 − vt/v) where
• λo is the observed wavelength
• λ is the emitted wavelength (Greek symbol lambda)
• v > vt (vt less than v)