Senate concurrent resolution 19
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SENATE CONCURRENT RESOLUTION 19 APPENDIX B CONTACT LIST Leaf Blower Report Contact List
APPENDIX C AMBIENT AIR QUALITY STANDARDS APPENDIX D CHEMICAL SPECIATION PROFILE FOR PAVED ROAD DUST APPENDIX E PHYSICAL PROPERTIES OF SOUND AND LOUDNESS MEASURES Physical Properties of Sound Sound is defined as vibrations in a medium, such as air or water, that stimulate the auditory nerve and produce the sensation of hearing. The vibrations propagate outward from the source of the sound in the form of pressure waves, traveling in straight lines in all directions outward from the source, as with the ripples in a pond resulting when one drops a rock into the water. Sound is a form of mechanical energy and is measured in energy-related units (WHO 1980). The speed of sound depends on the properties of the medium through which the sound wave moves. Sound travels more rapidly through air than through water, but may travel more rapidly through a solid than through air (Sataloff & Sataloff 1993). Sound waves, however, do not transmit through a vacuum. At sea level and 68o F, the speed of sound through air is 770 miles per hour, or 344 meters per second. A sonic boom is heard when an object is traveling through air faster than the speed of sound, which creates an impulse of sound from the leading and trailing edges of the object (Kryter 1994). Sounds are characterized by pitch, loudness, quality, and duration. Leaving aside duration, each of these is a psychological sensation, largely correlated to the physical attributes of frequency, intensity, and overtone structure, or timbre. Other physical factors, however, also influence the perception of sound. Sounds can be distorted by the wind, rendering them quieter or louder depending on the relative direction of the wind. Sound waves can bend around an obstacle, such as a wall, pass through the object unaffected, be reflected off the object, or be partially reflected and partially passed through or around the object. Two sound waves can also have the effect of canceling or amplifying each other at fixed distances from the source. Each of these behaviors depends on physical characteristics of the sound waves; frequency, amplitude, and wavelength; and physical characteristics of the environment (Sataloff & Sataloff 1993). The sensation of pitch is related to the number of vibrations per second of a sound wave, which is called the sounds frequency, and is measured in Hertz (Hz). A whistle and bird song, for example, are high frequency sound, and thunder and the bass line of a rock song are low frequency sound. The normal hearing range of a young, healthy person ranges from about 20 Hz to 20,000 Hz (20 kHz). Some animals can hear lower and higher frequencies than can humans; for example bats, moths, and dogs hear frequencies higher than the human hearing range. Loss of hearing acuity involves the inability to hear sounds of certain frequencies, usually at the upper and lower bounds of normal hearing. A sound that is made up of only one frequency is a pure tone. Most sound is made up of more than one tone, or several frequencies, sounding together. The quality, or timbre, of a sound is related to the presence and intensity of the additional tones contained in the sound; these overtones are the result of different frequencies sounding at the same time, resulting in a complex waveform. In addition, sound timbre includes the pattern of change over time of each of the tones. The relative intensity and pattern of change of each frequency in the sound is what allows us to describe sounds of the same fundamental frequency as tinny, flute-like, or brassy. One can thus discriminate between the human voice, a flute, a violin, and a french horn, each playing the same note. Industrial noises, on the other hand, consist of a wide mixture of frequencies, known as broad band noise. A sound composed of frequencies that are evenly distributed throughout the audible range is termed white noise and sounds somewhat like rushing water (Brüel & Kjær 1984). Sound duration can be described by the pattern of sound in time and intensity, or level, and can be described as continuous, fluctuating, impulsive, or intermittent (U.S. EPA 1979). Continuous sounds are those produced for a long period of time at a relatively constant level, such as the rushing of water in a river. Fluctuating sounds vary in level over time, such as traffic noise at an intersection. Impulse noises are those sounds with an extremely short sound pressure peak of less than a second in total duration. Impulse noises may be repetitive and occur close together, as in hammering or riveting; be spaced out in time, as in manual hammering; or occur as a single event, such as a single gun shot or explosion (Niedzielski 1991). Intermittent noises are those recurring noises lasting a relatively short period of time, such as the ringing of a phone, or aircraft take offs and landings. The intensity, or magnitude, of sound is described by the size or amplitude of the fluctuation in sound pressure. In general, the larger the amplitude, the louder the sound, although other factors also affect the perceived loudness of a sound. Over moderate distance, sound intensity decreases at a rate inversely proportional to the square of the distance from the source (Sataloff & Sataloff 1993). Thus, halving the distance from the source of the sound quadruples the sound intensity, assuming there are no interfering surfaces to reflect the sound waves. |
