Information AboutSound |
|
Sound is a disturbance of Mechanical Energy that propagates through Matter as a Wave . Sound is characterized by the Properties Of Waves , which are Frequency , Wavelength , Period , Amplitude , and Speed . Humans perceive sound by the Sense of Hearing . By sound, we commonly mean the vibrations that travel through air and are audible to people. However, scientists and engineers use a wider definition of sound that includes low and high Frequency Vibrations in the air that cannot be heard by humans, and vibrations that travel through all forms of matter, Gas es, Liquid s, Solid s, and Plasma s. The matter that supports the sound is called the Medium . Sound propagates as Waves of alternating Pressure , causing local regions of Compression and Rarefaction . Particles in the medium are displaced by the wave and oscillate. The scientific study of the absorption and reflection of sound waves is called Acoustics . Noise is often used to refer to an unwanted sound. In science and engineering, noise is an undesirable component that obscures a wanted signal. PERCEPTION OF SOUND . Yellow: )]] Sound is perceived through the Sense of Hearing . Humans and many animals use their Ear s to hear sound, but loud sounds and low-frequency sounds can be perceived as vibrations by other parts of the body via the Sense Of Touch . Sounds are used in several ways, notably for communication through Speech and Music . They can also be used to acquire information about properties of the surrounding environment such as spatial characteristics and presence of other animals or objects. For example, Bat s use Echolocation , ships and submarines use Sonar and most humans acquire some spatial information by the way in which they perceive sounds. Elephant s and Alligator s use very low frequency sounds to communicate, and mice, bats, cetaceans, and some insects use high frequency sounds, both outside the human hearing range. Humans can generally hear sounds with frequencies between 20 Hz and 20 KHz (the audio range) although this range varies significantly with age, occupational hearing damage, and gender; nearly all people in the developed world can no longer hear 20,000 Hz by the time they are teenagers, and progressively lose the ability to hear both higher frequencies and low level sounds as they get older. Most human speech communication takes place between 200 and 8,000 Hz and the human ear is most sensitive to frequencies around 1000-3,500 Hz. Sound above the hearing range is known as Ultrasound , and that below the hearing range as Infrasound . The amplitude of a sound wave is specified in terms of its Pressure . The human ear can detect sounds with a very wide range of amplitudes and so a Logarithm ic Decibel amplitude scale is used. The quietest sounds that humans can hear have an amplitude of approximately 20 µPa ( Micropascal s) or a sound pressure level (SPL) of 0 dB re 20 µPa (often incorrectly abbreviated as 0 dB SPL). Prolonged exposure to sound pressure levels exceeding 85 dB can permanently damage the ear, resulting in Tinnitus and Hearing Impairment . Sound levels in excess of 130 dB are more than the human ear can safely withstand and can result in serious pain and permanent damage. At very high amplitudes, sound waves exhibit Nonlinear effects, including Shock . Just how sound travels, or propagates, is difficult to imagine for many, as sound is invisible. Sound is an oscillating pressure wave, in which air is compressed, then decompressed, as sound moves away from its origin. Imagine a tube exposed to air whereby sound travels longitudinally through it. The air acts rather like a Slinky spring would if confined to the tube. As sound is generated at one end, a pressure wave will begin to travel through the air in the tube. Watching an earth worm move by pulsating its long body may help the imagination. The cycle length (i.e., the distance between successive 'bunched up parts of the slinky') is a particular sound's wave length, though most real world sounds are a mixture of many wave lengths. Low frequency sounds (eg, low organ or piano notes, bass guitars, etc) have large wave lengths, on the order of 10-50 feet long. High frequency sounds (eg, some parts of the noise associated with transient sounds as in many percussion instruments), have wave lengths as small as 1/2 inch. SPEED OF SOUND See Also: Speed of sound The speed at which sound travels depends on the medium through which the waves are passing, and is often quoted as a fundamental property of the material. In general, the speed of sound is proportional to the square root of the ratio of the ). SOUND PRESSURE See Also: Sound pressure Sound pressure is the of the instantaneous sound pressure averaged over a given interval of time. In a sound wave, the complementary variable to sound pressure is the Acoustic Particle Velocity . For small amplitudes, sound pressure and particle velocity are linearly related and their ratio is the Acoustic Impedance . The acoustic impedance depends on both the characteristics of the wave and the Medium . The local instantaneous Sound Intensity is the product of the sound pressure and the acoustic particle velocity and is, therefore, a vector quantity in time. The loudest sound ever in air reported was the 1883 volcanic eruption of Krakatoa , whereby sound levels reached 180 dBSPL at a distance of 100 miles (160 km). SOUND PRESSURE LEVEL As the human ear can detect sounds with a very wide range of amplitudes, sound pressure is often measured as a level on a logarithmic Decibel scale. The sound pressure level (SPL) or ''L''p is defined as : |