Related Links
|
NOISE CONTROL: BASIC CONCEPTS AND TERMSThere are a number of words and concepts which must be understood before beginning a discussion of noise control methods.
SoundSound is produced when a sound source sets the air nearest to it in wave motion. The motion spreads to air particles far from the sound source. Sound travels in air at a speed of about 340 meters per second. The rate of travel is greater in liquids and solids; for example, 1,500 m/s in water and 5,000 m/s in steel. (Note: Measurements in this book are generally given in the metric system. To convert, one meter equals about 39.4 inches, one millimeter equals 0.04 inches, and one kilogram equals about 2.2 pounds.)
Frequency (M)The frequency of a sound wave refers to the number of vibrations per second, measured in units of hertz (Hz). Sound is found within a large frequency range; audible sound for young persons is between about 20Hz and 20,000Hz. The boundary between high and low frequencies is generally established at 1,000Hz. Sound may consist of a single pure tone, but in general it is made up of several tones of varying intensities.
NoiseIt is customary to call any undesirable sound "noise." The disturbing effects of noise depend both on the intensity and the frequency of the tones. For example, higher frequencies are more disturbing than low ones. Pure tones are more disturbing than a sound made up of many tones. Infrasound and ultrasoundSound with frequencies below 201-Hz is called infrasound, and sound with more than 20,000Hz is called ultrasound. There is some evidence that these sounds which cannot be heard can under certain conditions be hazardous to workers' health. This book deals only with noise which can be heard.
Figure 6 At the same intensity, the noise from a truck is less disturbing than the sound of air blowing or suction because it is at a lower frequency. Decibel (dB)Sound levels are measured in units of decibels (dB). If sound is intensified by 10 dB, it seems to the ears approximately as if the sound intensity has doubled. A reduction by 10 dB makes it seem as if the intensity has been reduced by half. Noise level measurementIn, measuring sound levels, instruments are used which resemble the human ear in sensitivity to noise composed of varying frequencies. The instruments measure the "A-weighted sound level" in units called dB(A). Workplace noise measurements indicate the combined sound levels of tool noise from a number of sources (machinery and materials handling) and background noise (from ventilation systems, cooling compressors, circulation pumps, etc.).
In order to accurately identify all workplace noise problems, the noise from each source should also be measured separately. Measurements at various production rates may be useful in considering possible control measures. A number of manuals for noise measurements are commercially available. Adding noise levelsDecibel levels for two or more sounds cannot simply be added. Figure 8 shows how the combined effect of two sounds depends on the difference in their levels. Two or more sounds of the same level combine to make a higher noise level. Figure 8. A fan produces a sound level of 50 dB(A). Another fan produces 56 dB(A). The difference is 6 dB(A), and according to the diagram, 1 dB(A) will be added to the highest level. Operating together, the fans will result in a level of 57 dB(A). Octave bandIt is common practice to divide the range of frequencies we can hear into eight octave bands. The sound level is then listed for each octave band. The top frequency in an octave band is always twice the bottom one. The octave band may be referred to by a center frequency. For example, 500Hz is the center frequency for the octave band 354-708Hz. Sound transmissionThe word "sound" usually means sound waves traveling in air. However, sound waves also travel in solids and liquids. These sound waves may be transmitted to air to make sound we can hear. ResonanceEach object or volume of air will "resonate," or strengthen a sound, at one or more particular frequencies. The frequency depends on the size and construction of the object or air volume. Sound reduction by distanceSound spreading in open air and measured at a certain distance from the source is reduced by about 6 dB for each doubling of that distance. Sound is reduced less when spreading inside a room. (See Figure 9.)
Sound transmission loss (TIL)When a wall is struck by sound, only a small portion of the sound is transmitted through the wall, while most of it is reflected. The wall's ability to block transmission is indicated by its transmission loss (TL) rating, measured in decibels. The TL of a wall does not vary regardless of how it is used. Figure 9. If a small sound source produces a sound level of 90 dB at a distance of 1 meter, the sound level at a 2 meter distance is 84 dB, at 4 meters 78dB, etc.
Noise reduction (NR)Noise reduction is the number of decibels of sound reduction actually achieved by a particular enclosure or barrier. This can be measured by comparing the noise level before and after installing an enclosure over a noise source. NR and TL are not necessarily the same. Sound absorptionSound is absorbed when it strikes a porous material. Commercial sound-absorbing materials usually absorb 70 percent or more of the sound that strikes them. Figure 10. Part of the sound that strikes a wall is reflected, part is absorbed, and part is transmitted. The transmission loss (TL) of the wall is determined bv the portion of the noise which is not transmitted through the wall. [To Table of Content] [To Previous Page] [To Next Page]
| |||||||||||||||||||||||||||||||||||
