Sound waves are pressure waves caused by vibrating objects in a conductive medium, such as air. They travel at different speeds through different mediums and can be used for communication, imaging, and surveys. Sound can be described in terms of wavelength, frequency, and amplitude, with higher frequencies producing higher pitches and higher amplitudes producing higher volumes. Sound waves can be reflected, diffracted, refracted, and interfere with each other. Applications of sound waves include ultrasound for medical imaging and sonar for mapping the ocean floor. In seismology, sound waves can be used to map the internal structure of the Earth.
A sound wave is a type of pressure wave caused by the vibration of an object in a conductive medium such as air. When the object vibrates, it emits a series of waves which can be interpreted as sound. For example, when someone strikes a drum, it causes the drum membrane to vibrate, and the vibration is transmitted through the air, where it can reach a listener’s ear. The vibrations travel at different speeds through different mediums, but they cannot travel in a vacuum. In addition to being used for communication, sound waves are used to provide images of inaccessible objects and structures, in ocean surveys, geology and seismology.
Wave types
Sound travels through gases, liquids and solids as longitudinal waves. This means that the compression of the medium is in the same direction that sound travels. In solids and on liquid surfaces, vibrations can also travel as shear waves. In these, the compression is perpendicular to the direction of motion.
The speed of sound
The speed of sound propagation depends on the density of the medium through which it travels. It travels faster through denser media and is therefore faster in solids than in liquids and faster in liquids than in gases. Under familiar, earthly conditions, the speed of sound is always vastly slower than the speed of light, but in the super-dense material of a neutron star, it can approach very close to the speed of light. The difference in speed in air is demonstrated by the delay between a flash of lightning and the sound of thunder for a distant observer: the light arrives almost instantaneously, but the sound takes a considerable time.
The speed of sound in air varies with pressure and temperature, with higher pressures and temperatures giving higher speeds. For example, at 68°F (20°C) and standard sea level pressure, it is 1,126 feet per second (343.3 meters per second). In water, speed is again dependent on temperature; at 68°F (20°C) it is 4.859 ft/sec (1.481 m/s). Velocity in solids is highly variable, but some typical values are 13,700 ft/sec (4,176 m/s) in brick, 20,000 ft/sec (6,100 m/s) in steel, and 39,400 ft/sec (12,000 m/s) in the diamond.
Wavelength, frequency and amplitude
Sound can be described in terms of wavelength, frequency and amplitude. Wavelength is defined as the distance required to complete one full cycle. A complete cycle moves from peak to peak or valley to valley.
Frequency is a term used to describe the number of complete cycles in a given period of time, so shorter wavelengths have higher frequencies. It is measured in hertz (Hz), where one hertz equals one cycle per second, and kilohertz (kHz), where one kHz equals 1,000 Hz. Humans can hear sounds ranging from 20 Hz to about 20 kHz, but vibrations they can have much lower or higher frequencies. The hearing of many animals extends beyond human range. Vibrations that are below the range of human hearing are called infrasound, while those above that range are known as ultrasound.
The pitch of a sound depends on frequency, with higher pitches having higher frequencies. The amplitude is the height of the waves and describes the amount of energy carried. High amplitudes have higher volumes.
Phenomena d’onda
Sound waves are subject to many of the phenomena associated with light waves. For example, they can be reflected from surfaces, can experience diffraction around obstacles, and can experience refraction when they pass between two different mediums, such as air and water, all similar to light. Another shared phenomenon is interference. When sound waves from two different sources meet, they can reinforce each other where the peaks and troughs meet, and cancel each other out where the peak meets the minimum, creating an interference pattern, with loud and quiet areas. If the vibrations are of different frequencies, this can create a pulse or “beat” effect in the combined sound.
Applications
Sound waves have many applications in science and medicine. Ultrasound can be used to investigate medical problems and perform important checkups. A well-known application is ultrasound, used to produce an image of an unborn baby to check its health where an x-ray would be unsafe. Sound pulses, known as sonar, can be used to map the ocean floor by precisely measuring the time it takes to receive an echo.
In seismology, the internal structure of the Earth can be studied by observing the propagation of sound waves. Since shear waves cannot travel through liquids, this technique can be used to map areas of molten rock below the surface. Typically, sound is generated by an explosion and the vibrations are picked up at various distant points, after traveling through the Earth. By examining the pattern of transverse waves – known as “s-waves” in this context – and longitudinal waves – known as “p-waves” – an accurate three-dimensional map can be constructed, showing the distribution of solid and molten rock.
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