Document 85rbEvVv2oNoZrxMGJXo2GRzo

78 1955 National Safety Congress Experience in bis own field will enable the plant and manufacturing engineer to recognize and avoid the ttse of noisy ma- dunes and processes when possible. impact and the remedies are fairly obviou?. Proper inspection and maintenance, seem ing, stiffening or damping of loose panels, and replacement of worn parts, are effec Personal Protection The use of ear protectors may be indi cated under conditions where noise cannot be controlled by engineering. Nearly all ear protectors are effective in the high fre quency region, but at low frequencies, where a tight fit is important, there is less need for attenuation. Contrary to usual opinion, speech understanding in a high noise level is often improved by ear protectors. The noise is reduced as well as the speech, and the speecb-to-noise ratio is unchanged, but the levels at the ear are in a region where there is less distortion and better understanding. The principal difficulties in using ear pro tectors are sanitation and discomfort, but these handicaps are being overcome. En forcing the wearing of ear* protectors is usually more difficult than enforcing safety goggles or shoes, but alert management can popularize and enforce their acceptance. tive. Elimination of rattles prevents low fre quency energy from being transformed into more undesirable high frequency energy. Significant noise redaction can be obtained by these simple control means as great as ID decibels. Friction: Considerable noise can be made by frictional forces in general and in grind, ing, dutches or brakes. Part of the noise of cutting metal and wood can be classed as frictional noise. Motion along a contact surface creates force variation if the sur faces sue not well lubricated. The frequency characteristic is rich in high frequences, usually because of resonances in the part excited. Clutches and brakes tend to squeal grinding produces broad band noise, and other friction noises are of the squeaking and scraping type which have objectionable high frequency energy. Remedies include lubrication, substitution of materials, especially in brakes and Redaction at the Source Prevention of noise at its source appears to be a proemsing method of noise reduc tion. It is often difficult or impossible, since noise energy may be an unavoidable portion of the mechanical work done. Methods of reduction which follow are listed under types of noise sources: Impacts: Where a material is hammered, siteared, tumbled or pccned. the impact is a necessary part of die work and is always accompanied by noise. It is difficult to re duce this noise. However, if an impact can be made less sudden, the noise will tend to change from a sharp crack to a thud and will be less objectionable. In a punching operation, one portion of the die can be made to enter the work first and convert the process into a -shearing action. When punching involves a multiple die which makes several holes at one stroke; the die can be stepped to convert a single intense impact noise to several lesser noises. Prevention of unnecessary bounces from a hammer-type blow also will be effective. In tumbGng, impact noise on the containing cylinder can be decreased by suitable lining. Hatties, Lost Motion, end so forth: This type of impact noise can be classed as minor clutches, changes in shapes of culling tools and sharpening of tools which become noisy with wear. Gear noise is caused partly by frictional forces as well as variable forces at tooth* impact frequency. Scene improve ment can be obtained by lubrication, substi tution of helical or worm gears for direct gears, use of plastic or fibre gears, and reduction in speed. Fluctuations Gas Flow: Turbulence in the escape of compressed air or steam causes a loud hissing or roaring noise. When the air velocity is not being used, a simple muffler is effective. This, for outlets having small air flow, may consist of a can filled with steel wool or any other means of slowing down the escaping gas. Reductions of 10 to 15 decibels are possible. j When the air velocity* serves a purpose, i such as cleaning out chips, stripping or removing parts, noise appears to be unavoid- able at the present state of knowledge. The least that can be done is to keep the air flow at the minimum velocity necessary for the operation by tiring a reducer valve in the line. ' i Sound Absorption Total Enclosure: Noise in the air can be kept from spreading by almost any type of enclosure. Porous sound absorbing mate- Jndustrial Safety 79 rials are of direct use in reducing sound transmission through a wall, but of little value oa the inride of an enclosure to reduce the sound level which must be contained. Transmission loss can be greatly reduced by King double walls with an air space. In some cases, an extremely noisy machine or area can be totally enclosed to take advan decibels. Sound absorbing linings must al ways be used inride these partial enclosures, or there will be little noise attenuation. This type of enclosure is useful mainly in giving a shadow effect for workers who otherwise would be in the high level direct noise field. Basically, an operator is not protected in this type enclosure, bat nearby tage of large noise reductions of 20-4$ operators are. decibels. Space Absorption: The most common use Complete covers over a noisy part of a machine are another useful method in noise reduction. Whether a enclosure over part of a machine will do a good job of noise redaction should be checked by trial with an improvised cover first. If such a cover gives promise, a cover made of: (1) an impervious barrier such as sheet metal, 16 gauge or thicker, (2) rubber-like mate of sound absorbing material is a general or overall method of reducing noise in enclosed spaces. However, absorptive materials used on ceilings and walls of a general work area give a reduction of noise levels in the reverberant field at a distance from the noise source^ but absorption will not de crease the level for workers less than five to 10 feet from the source. rial to seal the edges and vibration isolated fasteners, and (3) absorbing material in side the cover to absorb sound and prevent baild-ttp of noise, should be deigned In most practical installations of areatreatment with absorbing walls or ceilings, there would be a naxnnum redoction of about 10 decibels. In addition, area treat Partial Enclosure: The term "partial en closure** is used for noise shields and odosnres which have openings greater than a ment will "push back" distant noises and allow an operator to bear his own machine better. few per cent of their area. Noise radiation from these openings is the principal path of noise flow from partial enclosures, since the transmission loss of the barrier is at least 20 Specific Noise Control Measures Following are a few examples of arise control attempts made in certain industries: HEAVY DUTY DRILLING OF TRANSMISSION HOUSINGS Noise Intensity in Decibels Frequency in cyder per second Machine A Before After Machine B Before After Both A&B Before After Overall ................................. 20-75 106 75-150 101 150-300 102 300-600 102 600-1200 ............................... 112 1200-2400 ............................... 105 2400-4800 ............................... 103 4800-10KC ............................. 98 Senes ................................... 1268 86 87 85 88 83 82 75 65 173 102 98 87 100 84 101 82 107 77 104 76 999 72 94 63 999 129 112-117 112 108 108 110 112 106 104 100 1485 8942 87 90 86 89 84 83 77 66 193 The before readings refer to a dry drilling of this east iron housing. The after readings refer to drilling with an oil lubricant This change from dry to oil-lubricated drilling has reduced the loudness by approximately 87 per cent.