Sound waves are pressure waves that travel through Earth's crust, water bodies, and atmosphere. Natural sound frequencies specify the frequency attributes of sound waves that will efficiently induce vibration in a body (e.g., the tympanic membrane of the ear) or that naturally result from the vibration of that body.
Sound waves are created by a disturbance that then propagates through a medium (e.g., crust, water, air). Individual particles are not transmitted with the wave, but the propagation of the wave causes particles (e.g., individual air molecules) to oscillate about an equilibrium position.
Every object has a unique natural frequency of vibration. Vibration can be induced by the direct forcible disturbance of an object or by the forcible disturbance of the medium in contact with an object (e.g. the surrounding air or water). Once excited, all such vibrators (i.e., vibratory bodies) become generators of sound waves. For example, when a rock falls, the surrounding air and impacted crust undergo sinusoidal oscillations and generate a sound wave.
Vibratory bodies can also absorb sound waves. Vibrating bodies can, however, efficiently vibrate only at certain frequencies called the natural frequencies of oscillation. In the case of a tuning fork, if a traveling sinusoidal sound wave has the same frequency as the sound wave naturally produced by the oscillations of the tuning fork, the traveling pressure wave can induce vibration of the tuning fork at that particular frequency.
Mechanical resonance occurs with the application of a periodic force at the same frequency as the natural vibration frequency. Accordingly, as the pressure fluctuations in a resonant traveling sound wave strike the prongs of the fork, the prongs experience successive forces at appropriate intervals to produce sound generation at the natural vibrational or natural sound frequency. If the resonant traveling wave continues to exert force, the amplitude of oscillation of the tuning fork will increase and the sound wave emanating from the tuning fork will grow stronger. If the frequencies are within the range of human hearing, the sound will seem to grow louder. Singers are able to break glass by loudly singing a note at the natural vibrational frequency of the glass. Vibrations induced in the glass can become so strong that the glass exceeds its elastic limit and breaks. Similar phenomena occur in rock formations.
All objects have a natural frequency or set of frequencies at which they vibrate.
Sound waves can potentiate or cancel in accord with the principle of superposition and whether they are in phase or out of phase with each other. Waves of all forms can undergo constructive or destructive interference. Sound waves also exhibit Doppler shifts—an apparent change in frequency due to relative motion between the source of sound emission and the receiving point. When sound waves move toward an observer the Doppler effect shifts observed frequencies higher. When sound waves move away from an observer the Doppler effect shifted observed frequencies lower. The Doppler effect is commonly and easily observed in the passage of planes, trains, and automobiles.
The speed of propagation of a sound wave is dependent upon the density of the medium of transmission. Weather conditions (e.g., temperature , pressure, humidity , etc.) and certain geophysical and topographical features (e.g., mountains or hills) can obstruct sound transmission. The alteration of sound waves by commonly encountered meteorological conditions is generally negligible except when the sound waves propagate over long distances or emanate from a high frequency source. In the extreme cases, atmospheric conditions can bend or alter sound wave transmission.
The speed of sound through a fluid—inclusive in this definition of "fluid" are atmospheric gases—depends upon the temperature and density of the fluid. Sound waves travel faster at higher temperature and density of medium. As a result, in a standard atmosphere, the speed of sound (reflected in the Mach number) lowers with increasing altitude.
Meteorological conditions that create layers of air at dramatically different temperatures can refract sound waves.
The speed of sound in water is approximately four times faster than the speed of sound in air. SONAR sounding of ocean terrain is a common tool of oceanographers. Properties such as pressure, temperature, and salinity also affect the speed of sound in water.
Because sound travels so well under water, many marine biologists argue that the introduction of man-made noise (e.g., engine noise, propeller cavitation, etc) into the oceans within the last two centuries interferes with previously evolutionarily well-adapted methods of sound communication between marine animals. For example, man-made noise has been demonstrated to interfere with long-range communications of whales. Although the long term implications of this interference are not fully understood, many marine biologists fear that this interference could impact whale mating and lead to further population reductions or extinction.
See also Aerodynamics; Atmospheric composition and structure; Atmospheric inversion layers; Electromagnetic spectrum; Energy transformations; Seismograph; Seismology