Document mq2xYD8jqo5r8q2r1m9JZe5bQ

r> _ j iOr x\o:s< ja o: BRUCE KADKIl Technical Services, Lead industries Association, Inc. 292 Madison Avenue, New York, New York I Reprinted for: rn Iced industries Association 292 iYiedison Ave. New York, New York\ZZ\7 PNYC00001045 N10157 Preprinted from AMERICAN INDUSTRIAL HYGIENE ASSOCIATION JOURNAL Vol. 27, Novcmbcr-Dcccnibcr. 1966 TT*"U2 j?.r i' a ip o Tiers 3afeed oa Lead BRUCE FADER Technical Services, Lead industries Association, inc. 292 Madison Avenue, Ncte York, New York Acoustical chcory predicts chat sheet lead and soft plastic materials loaded with ftnely divided lead should be excellent sound barriers. Seven case histories are given to show how this "limp mass" has been applied in practical problems. introduction NE OF THE most common solutions 'v-// to industrial noise problems is the en closure o: the noise source to reduce the noise level to bearable limits. This situation is easiiy recognized. The difficulties attendant to enclosure are easily recognized, too. Usu ally openings are required for feed of material or parts to and from the noisy operation. Al most- always there must be provision for access or at least visual inspection. These and other requirements often indicate that a complex shape will be required of the enclosure. And this, in turn, increases the difficulty of achieving monolithic, leak-free construction in practice. In addition to these practical difficulties, which apply to the construction of even a single noise enclosure, there may well be a question of optimizing performance of the noise barrier in terms of noise reduction versus weight, thickness, or cost. This is par ticularly important where noise reduction requirements are unusually demanding, or where a large number of the enclosures will be needed. Sheet lead and lead-loaded flexible plastic sheet recommend themselves in both par ticulars cited above. First, from a practical point of view, consider sheet lead used as a barrier in the plenum above hung ceilings. The lead sheet, approximately 1/64-inch thick in a typical case, is allowed to hang freely between the roof slab above and the hung ceiling below. Perforations for pipe, conduit, air ducts, etc., can be tightly sealed by the simple expedient of hand-crimping the surplus lead to the surface of the pipe and taping it there with duct-scaling tape. The lead surplus is created by folding a single 1-inch tuck vertically to coincide with the pipe. The significant fact about these installations is that the measured perform ance is excellent and usually in close agree ment with theoretical predictions. This hap py result is all too uncommon in noise con trol work and may be attributed, in large part, to the ease with which leak-free con struction is attained on the job. Second, acoustic theory and practical ex perience alike show lead (and lead-loaded plastic) sheet to be excellent barriers to noise. This fact has its foundation, in the intrinsic lack of stiffness of the sheet. The so-called "limp mass" thus furnished blocks noise transmission without any detrimental coincidence effects. Another theoretical pre diction borne out in practice is that, where the lead is correctly used with other build ing materials in a composite noise barrier, it can suppress or help suppress the unfavor able coincidence dip of the other materials. Types of Construction At one extreme the simple hung sheet of lead or leaded plastic mentioned above has .proved effective. Even at one pound per square foo< (psf) this simple expedient pro duces about 25 db as a nine-frequency aver age transmission loss. Other schemes of in creasing complexity have been used; there is no point in going into great detail concerning 520 * >. . \ S PNYC00001046 American Industrial Hygiene Association Journal 521 ilium. inn (ypic.ii atKinjciuaiis and perform,iua> may serve as an index of suitability: Typical Noise Barriers Based on Lead ircNtiiiniccn-cy, average Rarricr Description Notes 25 db Shoci iciid or Ictulcd vinyl plastic weigh ing 1 psf 30 db 0.9 psf leaded vinyl over 2 inches of low-density glass fiber as pipe en closure 30 db 0.5 lo L0 psf leaded vinyl with sheet metal skin and glass liber blanket retained by screen or perforated sheet 35 db Light sheet metal clamped to cither side of 3 psf leaded vinyl sheet 40 db 3 psf lead lamin ated to 18-gage sted 40 db 3 psf lead lamin ated to V% inch plywood 40 db . 4 psf lead lamin ated to l0-gagc steel, glass fiber inside enclosure 45 db Conventional drywall with 4 psf lead furred of! V* inch and acoustical tile furred off 51 db 4 inch lightweight concrete block with 4 psf lead on V* inch fir plywood furred off one side 65-70 Two leaves of 8 db . psf lead on wood frames separated by 9 inch space con taining two 2-inch glass blankets on each leaf Standard tests Estimated (sec case 1) Estimated (see cases 2 and 3) Estimated (see case 4) Standard test Standard test Estimate (see case 5) Field test (see case 6) Standard test Field test (see case 7) Fig u r e 1. View of leaded vinyl-covered pipes in the gas pressure reduction station. the type of service, but 15 psi is typical. To maintain a set downstream pressure in the face of extremely wide variations in de mand, double V-port regulating valves arc used. With this high-pressure drop and cor respondingly high flow, a wide-spcctrum noise of 110 to 130 db is created. The enclosure (Figure 1) was a 2-inch blanket of fine yarn glass fiber (6 pounds per cubic foot) overwrapped with a single layer of reinforced leaded vinyl sheet. This fabric-backed, flexible material -weighs 0.87 psf and is 0.055 inch thick. Considerable care was exercised in taping all seams completely closed (Figure 2) with a tape of the same material. The result was to reduce the peak inten sity from 130 db to 100 db. While higher than desirable, this brings the noise into a range where conventional ear protectors can cope with it. Case 1--Fluid Noise and Pipe Line An arrangement pioneered at the Public Service Gas and Electric Company of New Jersey has now been employed all over the North American continent. The noise source was a set of pipes and valves in a pressure reduction station. Upstream of the valves the gas is at a pressure of about ISO psi. Pressure on the downstream side varies with Fio u m 2. Exploded view ihowe how leaded vinyl and class fiber are applied. PNYC00001047 & Novcmber-Dcccmbcr, 1966 vv-~. Fig u r e 3. "Before" and "after" curves show {he additional noise reduction achieved by adding glass fiber and leaded vinyl .sheet to the enclosure for an internal combustion turbine. Fig u r e 4. Overall view of the turbine unit. Smaller box at lower left is ducting for the air intake system. Fig u r e S. The flexible bellows (30 inches square) is used Co connect the intake duct to the air intake of the turbine. It provides a rea sonable transmission loss for noise without intro ducing a rigid structural tie between turbioe and ducting. Fig u r e 6. Noisy power sources constitute a real problem aboard a hydrofoil vessel like HS Deni* son. Since the vessel must lift itself out of the water to "fly,'' every pound counts, and sound barriers must have top efficiency. Cases 2 and 3--Gas Turbine Noise The gas turbine is an increasingly pop ular prime mover and, as is evident by Fig ure 3, it is a fairly noisy one. The "before treatment" curve in Figure 3 is the gas tur bine with an enclosure, but before the ad dition of leaded plastic. The before and after data shown in the curve arc based on tests carried out on a stationary gas turbine unit measuring 5 by 16 feet (Figure 4) and producing 750 kilo watts of electric power. Air for the combus tion is taken in at a rate of 11,000 cfm when the unit operates at rated load and the intake duct is 30 by 30 inches. To provide vibration isolation between the unit and the intake ducting, a flexible bel lows of leaded plastic was used. The plastic sheeting weighed 0.87 psf (Figure 5). The same material was used to line the interior of the metal cabinet, and the internal face of the cabinet was glass fiber batting re tained by screen cloth. Substantially the same scheme was used in a movile gas turbine--the power unit of the hydrofoil vessel HS Denison (Figure 6). Be cause weight was at a great premium in this foil-borne vehicle, a still lighter leaded plastic sheet was used. Its effect was enhanced by supporting it on 2-foot centers. The sheet weighed only 0.48 psf and was used, as be fore, in conjunction with glass fiber absorp tive material (Figure 7). Case 4--Sheet MewI Noise The wide-spcctnim noise produced by im pact in expanses of sheet metal is quite a PNYC000010A8 t r1 American industrial Hygiene Association Journal 523 li nouRE 8. One at (he power transformers before and after enclosure. The enclosure is 14 feet high. Fig u r e 7. Section through the sound harrier used in HS Denison. It is basically similar to that used in the stationary turbine (Figure 4) except that ligutcr leaded vinyl sheet is employed and it is point.suspcndcd for maximum limpness. common problem. In one case, an enclosed chute, or duct, was being used to convey roasted coffee beans from the upper floor of a plant to one of the lower floors for packaging. Although no actual sound mea surements were made, the noise levels--par ticularly the enervating high-frequency noises --were well beyond the annoyance level. Short tempers and worker inefficiency were directly traceable to the noise of millions of coffee beans bouncing through the chute. The solution was quite simple. A sleeve of flexible leaded vinyl sheet weighing 3 psf was hung inside the vertical chute. The sleeve was made with scaled seams so that noise could not leak through it. Because coffee is a foodstuff, a second sheet metal chute was inserted through the sleeve as a liner. The prime intent was to remove any possibility of contact of the food with a lead-bearing material. Actually, however, the inner liner probably made the arrange ment more effective because it forced the leaded plastic and sheet metal into loose contact, thus enhancing the damping effect. The result was completely satisfactory. The beans cause only a quiet murmur now, and frayed nerves have long since healed in that department. an unusually stubborn noise and typifies noise speccrums of low frequency, which sometimes occur in industrial work. Low-frequency per formance of most sound barriers is notori ously poor; thus the achieved improvement of 17 to 20 db achieved by Portland General Electric Company is especially noteworthy. The transformers in question (Figure 8) were each rated at 41.7 Mva, stood about 14 feet high, and required an enclosure mea suring approximately 5 by 10 by 14 feet. This enclosure was a built-up laminate of 10-gagc steel, 4-pound (1/16-inch) lead with an interfacing of glass fiber. Some noise es capes the enclosure via the radiator network. As an indication of the excellent detailing of the design, this loss was held to only 5 db. Tests of the completed enclosure with out the radiators in place showed a 22-db improvement over the unenclosed transform er. When the radiators were again installed, the level rose, cutting the noise reduction to 17 db. This 'construction is classed as approxi mately a 40-db noise barrier because of the many openings of this sort that had to be left in the transformer enclosure as it was assembled. Had it been complete or sub stantially complete, the reduction would cer tainly have been of the order of 40 db. Case 6--A "Quiet Room" in a Noisy Envi ronment Case 5--Transformer Noise The "hum" of big power transformers is Franklin Institute in Philadelphia is well known for iu excelicnt contract research work. One project at the institute was a study \ i It m PNYC00001Q*9 .! 524 O 'h* PLASTERBOARO November-December, 1966 w o o d f r ame 4 BATTEN Fig u k l 9. Section through the wall employed in the Franklin Institute tot room. The original drywall (gypsum board faces and wood stud tram* ing) existed and the lead and finish were added. 1ji : 60- NO'St REDUCTION - L ACTUALLY ACrtltVEO! 8Y WAU tLEADED / UJ *,v: CONSTRUCTION^"]- ! JJ 1 ! V X^1 -- !i ! ___ o ^ X: ; vf ' S' \' 1 ... s 2oi-- "a J !^ f y ii i < ' 1PLASTERBOARD WALL S"olf.. r1 ~ii. ~"il ii ii i. : t7Vn v7m n7vm x7aw/ /u h in7viua tw/oA/tm m/ytute OCTAVE BAND CPS. Fig u r e 10. Acoustical performance of the orig* ina! wall and the wall after sheet lead had been added clearly shows the improvement. (Original wall performance shown is estimated and prob ably quite conservative.) The curve for the leaded wall is not corrected for "flanking" or passage of sound around the wall. \"8 L9- $Q. FT. ORJVEN CALKING SHEET LEAO SECTION IN PLAN 70-db WALL Fig u r e 11. Section through the 70-db wall. ! _________ , i 1 ^ . .. .. .. r- - - V - - i^t / I i i i ____ i____ TEST OF A 70 db WALL J J, | 9 fRCQUtNCY . A7V0C.9RaAsCE[$TuCniicrpo.uted <MUt , % *%m *%. "%m OCTAVE BANO CPS Fig u r e 12. Actual performance of the 70-db wall at tested. of noise made by bearings. Good bearings are, of course, rather quiet in operation, and the only space available for testing was not! It had, in addition to normal traffic and con versation, machinery running continuously under test, occasional light sheet metal work, and the background rumble of downtown Philadelphia's heavy street traffic. Beginning with a room 13 feet 6 inches by 9 feet 6 inches by 10 feet high with plasterboard walls on wood studs, the insti tute constructed the wall shown in section in Figure 9. Note that the original wall was retained and that no new framing or dis mantling of existing structures was required. Other measures included use of sound-ab sorptive facing on the walls and ceiling and addition of mass to the ceiling by dump ing sand in it from above (there was no floor over the joisu} and careful sealing of cracks and openings throughout. The result, shown in Figure 10, was to increase the noise reduction from outside the room to inside from 32 to 45 db. While this wall would not meet really critical re quirements (for a broadcast studio, for ex ample), it was attained at a reasonable cost in a short time. Actually it was six weeks between the first inquiry about the use of lead for noise control and the successful use of the completed room. No outside contractor was required, the work being done by the regular building and maintenance staff. Case 7--A 70-DB Wall in an Acoustical Test facility Sometimes the need arises for a barrier with q rating of 60+ db. As a practical matter, this often means that some alterna tive solution to the problem should be sought, since it is not usually possible to attain n- PNYC00001050 S -V o Anwricuu Industrial Hygiene Association Journal 525 lings of tl-.ls sort in practice. One of the few pieces where such performance really is required and really can be attained is in test facilities. The wall described here forms the perma nent dividing waii between the source and the receiving rooms. As such it must be capable of dismantling and reassembly as lest panels of various sizes are substituted for it. Although it is hardly a thin or light weight wall, it is much thinner and lighter titan the solid concrete block that makes up the other walls of the test facility. And of course it is conveniently handled in sections so that test panels of several sizes can be accommodated--a problem of considerable magnitude had any other construction of similar acoustic rating been chosen. The wall (Figure 111 has faces of */6-inch (3-psf) sheet lead separated by 3 Vi-inches of space. The faces arc separately framed with wood. To eliminate structural bridg ing of the two faces, slots have been sawed in the concrete (loor, sidewalls, and ceiling of the test facility. These arc sealed with nonhardening mastic. Within the wall, each lead face is backed with 4 inches of fiber fill. A wooden retaining batten around the edge of each panel serves two purposes--it clamps the lead face to the frame and pro vides an opening into which calking material can be driven. The performance curve of this wall has been checked in the facility itself and, as shown in Figure 12, is a remarkable 70 db. It must be understood that this performance would not be achieved in any noise barrier whore it was not possible to use extreme care in hunting down and correcting small leaks through cracks, etc. Neither this wall nor any other should be considered a prac tical solution to the type of field problem where access is required through the bar rier, or where leakage or flanking paths or movement or mechanical wear is liable to be a factor. Conclusions Where noise must be blocked or cantaincd by a barrier, there arc three cardinal require ments lo be met in order to get the best performance: First, the more massive the barrier is. the higher the transmission loss it is capable of. Second, the less stiffness the barrier pos sesses, the better the performance that may be expected of it. Third, to achieve good transmission loss, the barrier must be free of leaks--either air leaks or structural features that constitute sound paths. Sheet lead and leaded plastic sheet have been shown to possess high mass relative to their thickness and sufficient "limpness" to produce excellent results in practical noise barriers. Although it cannot be demon strated as a separate effect in the ease his tories reviewed here, part of the effectiveness stems from th? ease with which the materials can be formed to seal or minimize leaks. Received June 26, 1966 PNYC00001051