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- Beats are the pulsations resulting from a combination of two waves of slightly different frequency. For waves of almost equal frequencies beat frequency is the difference between the frequencies of the combining waves. When the interfering frequencies are in the audible range, the beats are heard as alternating soft and loud pulses. The human ear can detect beats with frequencies up to 10 hertz, or 10 beats per second. Piano tuners listen for beats when comparing the pitch of a tuning fork to that of a vibrating string; when no beats are heard, the fork and string are at the same frequency. Ultrasonic or inaudible frequencies can be superimposed to produce audible beats.
- Light Waves include all electromagnetic radiation, but especially that portion of the spectrum visible to the human eye. It is a form of energy that travels through empty space at a speed of about 186,000 miles per second (300,000 km/s). In the early 19th century, light was described in terms of waves, but experiments later showed that it exhibits properties of particles as well. In modern times light has been described in terms of wave particle duality. Light is the basis for the sensation of sight, and for the perception of color. The eye distinguishes the color of an object as the color of light which the object reflects or transmits.
- Resonance is the selective response of an object or a system that vibrates in step with an externally applied vibration. Acoustical resonance is the vibration induced in a string of a given pitch when a note of the same pitch is produced nearby, in the sound box of an instrument such as a guitar, or in the mouth or nasal cavity when speaking. Mechanical resonance, such as that produced in a bridge by wind or by marching soldiers, can eventually produce wide swings great enough to cause the bridge's destruction. Resonance in frequency-sensitive electrical circuits makes it possible for certain communication devices to accept signals of some frequencies while rejecting others. Magnetic resonance occurs when electrons or atomic nuclei respond to the application of magnetic fields by emitting or absorbing electromagnetic radiation.
- Vibration generated by an earthquake, explosion, or similar phenomenon and propagated within the earth or along its surface. Earthquakes generate two principal types of waves,
1. Body waves, which travel within the earth, and
2. Surface waves, which travel along the surface.
Seismograms yield information about the earth and its subsurface structure. Artificially generated seismic waves of short duration are used in oil and gas prospecting.
- Mechanical disturbance that propagates as a longitudinal wave through a solid, liquid, or gas is known as sound. A sound wave is generated by a vibrating object placed in a suitable medium. The vibrations cause alternating compressions (regions of high density) and rarefactions (regions of low density) in the particles of the given medium. The particles move back and forth in the direction of propagation of the wave excecuting Simple Harmonic Motion. The speed of sound through a medium depends on the medium's elasticity, density, and temperature. The frequency of a sound wave, perceived as pitch, is the number of compressions (or rarefactions) that pass a fixed point per unit time. The audible frequencies range from approximately 20 hertz to 20 kilohertz. Intensity is the average flow of energy per unit time through a given area of the medium and is related to loudness.
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