Sound Waves

Sound waves are pressure waves that travel through Earth's crust, water bodies, and atmosphere

Sound waves induce vibration in a body (e.g., the tympanic membrane of the ear) or are produced as a result of vibration of that body. Sound waves can be detected and interpreted by instrumentation (e.g., by a seismograph), or by variety of pressure sensitive organs in living beings (e.g., the lateral line system in sharks, the human ear). In humans, conversion of the mechanical energy of the sound waveform into nervous stimulation results in electrical and chemical nervous impulse transmission that is propagated through the human auditory nerve to the brain. The brains then interprets these neural signals as "sound.")

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 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.