Document Mn1gNr9EBM4dXj058n946V97

76 3955 Xationa! Safety Congress useful when there is only a limited time to collect the data, or the particular type ot analyse* desired cannot be decided at the time. It is very convenient to "sample** the noise with the tape recorder and then "ana* lyzc** it in the laboratory with a graphic recorder. To be usctul. the tape recorder has to be portable; it probably should operate from the 115 volt lines. Its power requirements should be low enough so that a converter operating on a storage battery will operate it. This makes it possible to operate it from an automobile or in other locations remote from power lines. This becomes particularly important in neighborhood xnnoxancc prob lems. Fidelity ot recording, however, cannot be sacrificed for portability. High fidelity in the audible range is obviously of extreme importance. Moreover, the instrument must be so arranged, or modified, that it is cap able of reproducing the level of die original noise. You must he able to calibrate it-- and maintain the calibration. This is fre quently done by replacing the continuous attenuators with step attenuators. When recordings are made, it is desirable to play a known noise onto the tape for a reference. This can be done with a micro phone calibrator. It has been said that in many cases higher precision noise measure ments can be made by first recording on tape and then making the analyses in the laboratory, where time and interference is unimportant. Our objective in any noise investigation is a survey of the area nr the analysts of an industrial noise source. The measurement of noise is an art. We liave a lot of electronic instruments to aid us, but the results we get depends to a large extent on how we use the instruments. The development of a technic of measure ments is very important'--it is the art of noise measurement. Microphone m Use on Coble: The loca tion of the microphone is surely the most important single factor. In survey work and measurements made to determine per sonal exposure, the best location is at the operator's ear level. However, when meas urements are made the operator should be out of the way to prevent the shielding of the microphone. The exact location of the microphone is important when machines are being ana lyzed. and when measurements are made dose to the source of the noise. A small change in the location may give a measur able difference in level. The location be comes particularly important when changes arc to be made and you want to determine what reduction Juts been achieved. Hood: The calibration or sensitivity of the instruments change and measurements should never be made without checking the calibration of the instruments used. There is available a small hood, containing a loud speaker and attached oscillator with which the calibration can be checked in the field. The dh level of the noise, as indicated by the meter on the sound level meter or ana lyzer. is not simple to read. More often than not. the pointer is in constant, random motion, often over a range of several dh. Very* tow industrial noises are constant m IcveL - It is common to sec the readings vary three, five, or even JO db. When this happens, the best reading is often difficult to decide upon. One rule commonly used is to record the reading as three db below the peak indicated by the meter it the variation is over six dh. This peak reading depends to a considerable ex tent on whether the meter response is in the slow or fast position. Records 1 Itdicvc the most important thing 1 have to say concerns the keeping of records. AU too often records of measurements made some time ago are not clear because it is not known where the microphone was, how close it was. what was going on. how many machines were running, and so forth. The type of work being done varies, and this to a large extent determines the details desired on the record form. On the back of the form should be a place tor a sketch--and this is roost impor tant. Below this, you should have space for n number of analyses. The locations should l*c indicated on the sketch by letter or de scription, There should also be a note as to the particular operation going on when the*analysis was made. In survey work, a blue print of the floor plan is very useful to spot the location of the readings taken. Industrial Safety 77 The presentation of the results of the measurements is the meat of the investiga tion. The results are not much good if they are not seen, or, as more frequently hap pens, are not understood by the people who $ce them. It is important that they' be pre sented so their significance can be readily appreciated. To me this means a report with a concise summary and dor, simple sketches and charts. For example--the results of a general survey might be presented with figures showing the level in db throughout the area. so that the locations of high level are easily noted. Or you might draw contours, like weather maps, to indicate the variation in noise level. You can then tdl, for instance, when noise from a machine is bounding off a wall, and thus raising the general noise level in a whole area. Frequently you will have some land of band analysis taken of some of the noise when a genera] survey* is made. This can be indicated by letters on the plan, and the curve of the noise can be included in the report on separate sheets. Methods of Reducing Noise By J. C. RADCLIFFE Supvr. Industrial Hygiene Unit, Medical Section, Ford Mortor Co^ Dearborn, Mich 1 would like to be able to say that you can find in one textbook the methods of reducing noise. 1 know of no such textbook. There have been numerous articles written in many scientific journals regarding spe cific noise control jobs. They usually fol low certain prescribed avenues of control. But each one. even though following a cer tain avenue, must be tailor-made for the specific noise source. If your company is planning a new build ing or manufacturing site, you are fortunate in the opportunity presented. Many dollars can be saved by building-in noise control features. The laying out of work processes, the location of noisy machines and the seg regation of as many workers as possible from noisy areas will do much co minimize your future problems. (The article by Bonvallet & Karplus, in the Deconber, 1953. AIHA Quarterly, gives general bands of noise levels to be expected in different types of industries, and specifically those bring produced by different types of machines and processes.) Operations involving impacts will be noisy, especially on large metal plates. Balancing noise reduction against costs of changes in work-processing must depend on particular applications. The following list of changes is given as a few obvious examples of alter nate processes which cause noise reduction. without regard to economic factors. This is a guide for the engineer: 1. Welding instead of riveting. 2. Compression riveting instead of com mon pneumatic riveting. 3. Pressing or rolling instead of forging. 4. Crinding instead of chipping castings and welded joints. 5. Electrically-operated tools instead of unquieted pneumatic tools. o. Selection of less noisy types of con veyors. 7. Avoidance of dropping hard materials during handling. 8. Mechanical stripping of work from dies instead of air blast stripping. 9. Low speed process machinery instead ot high speed machinery. 10. Substitution of materials; for example, fabrication of plastics is usually less noisy than sheet metal fabrication. 11. Wide spacing banks or rows of noisy machines instead of close spacing. 12. Hot working of metals instead of cold working. 13. Belt drives instead of noisy gars. 14. Substitution of less noisy materials used is friction processes, as Is brakes and dutches.