PART VI: FUNDAMENTAS OF NOISE CONTRO Elden F. Ray June 10, 013 TABE OF CONTENTS Nose Path Modelng Nose Reducton and Crtera 3 Sound From Openngs 3 Sound From Structures 4 Transmsson oss 5 Nose Mtgaton 5
NOISE PATH MODEING Nose control or mtgaton nvolves several steps and the amount of nose reducton s drven by havng to meet some regulatory lmt. The followng fgure llustrates the classcal approach to nose control and by developng ths smple model the optons for effectvely and economcally reducng nose may be examned. Source of Nose Path of Nose Receved Nose Fgure 1 - Classc Nose Control Modelng Method Applyng nose control nvolves affectng one of these three elements and most often t s the Path of Nose that s controlled by use of acoustcal enclosures, barrer walls, duct slencers, and other smlar nose control treatments that are nstalled near the source to effectvely reduce the sound level. Ths method s the most wdely used as the degree of nose control can be vared dependng upon the nose requrements. Reducng the nose at the Source of Nose can be expensve because most equpment manufacturers assemble ther products usng commodty parts that are economcally produced for the ndustral market and reducng nose at the source may requre a complete redesgn and retoolng process whch takes tme and money. Ths model can be further demonstrated by the followng tradtonal equaton used for calculatng the sound level at a receptor (receved nose). p Q 4πr = W + 10og A db re: 0µPa (1) Where p s the receved sound level, W s the source of nose and the remanng terms descrbe the path of the sound energy. Refer to Part IV for a descrpton of Equaton (1). Agan, ths equaton s appled for each frequency band and the overall sound level determned from all the bands. Remember, the acoustcal characterstcs n each frequency band s unque thus all the frequency bands must be consdered when determnng nose control needs; refer to Part I. Frequently, more slencng s needed than what can be descrbed as the bare mnmum n order to account for nose from other equpment or sources that all combne to create a total sound level; thus, a balance of plant or total nose analyss must be performed to adequately account for all possble sources of nose, ncludng those out-of-scope. The reducton of sgnfcant sources of nose frequently results n what were once obscure sources of nose now becomng mportant when havng to meet a low nose requrement. Groupng smaller sources together can be benefcal such that a common nose barrer or enclosure can solve a lot of small problems.
NOISE REDUCTION AND CRITERIA Nose control and the prncples of acoustcal engneerng apply unversally to vrtually every type of faclty; the approach s the same: dentfy the need, evaluate mtgaton optons and specfy the mtgaton selected. The amount of nose reducton s drven by meetng a regulatory lmt, ether establshed by a governmental body or by the developer/owner of the property. For nstance, say the sound level at a receptor 100 meters away from the nose source cannot be n excess of 55 decbels (db). Applyng the standard model for outdoor sound propagaton and for smplcty, assume only the ground plane s present and s zero, the allowable sound power level (note the algebra) s determned by A p Q 4πr = W + 10og A db re: 0µPa (a) 55 10og 4π100 = W db re: one pco-watt (b) W =103 db re: one pco-watt (c) If the source of nose s an exhaust that has a sound power level of 140 db, then the amount of nose reducton needed s 37 db and addng 3 db for safety margn results n 40 db reducton beng needed. Ths s a smple llustraton of the process. If there was an octave band crteron, then ths calculaton would be performed for each frequency band of nterest and the necessary nose reducton for each frequency band determned. Now, n examnng the above equatons we can dentfy at least two varables that affect uncertanty n calculatng the sound level, the accuracy of the sound power level and we are assumng perfect hemsphercal dvergence. Thus, the reason why a lttle desgn margn s desred, especally n crtcal applcatons. Sophstcated computer models can help refne the calculaton process but are stll dependent upon the accuracy of the sound power level of the nose source, the dstrbuton of sound energy across each bandwdth, dscrete frequency tones versus broadband nose and the correspondng performance of nose control devces that typcally consst of slencers, nose barrers, enclosures, etc. SOUND FROM OPENINGS Sound radaton from openngs ncludes any type of openngs allowng equpment nose to drectly enter the envronment by what s commonly called, the gas path; the sound level s reduced by nstallng slencers n the gas path (ductng). See the above example. 3
SOUND FROM STRUCTURES Sound radaton from structures s mportant for two man reasons; meetng a near feld or far feld sound lmt. The near feld s typcally three feet or one meter from the surface and can be problematc n calculatng because the sound feld s not well developed and tradtonal spreadng loss does not work. Instead of a 6 db reducton for a doublng of dstance, t s more lke a two or three decbels reductons per doublng of dstance untl free feld condtons are obtaned. The sound emtted from the structure s based on the sound power level ( W ) that s nsde the wall or duct that generates a sound pressure level. In the case of aerodynamc sources (behavng as a plane wave), the sound pressure level nsde a duct, ppe or smlar defnng contanment area (S) n square meters s, p = W 10og ( S) db re: 0µPa (3) The sound pressure nsde ducts s requred n order to determne the necessary transmsson loss (T) of the duct wall n order to meet any near feld sound level requrements or to reduce the breakout nose so as not to become a contrbutor to the far feld sound level. The sound level outsde the wall or duct s based on ts nose reducton (NR) whch s the dfference n sound pressure levels across the wall and s a functon of the wall s transmsson loss (T). The nearfeld sound level outsde the wall ( po ) s calculated based on the NR of the wall, not the T, po = p NR db re: 0µPa (4) The NR s the nose reducton of the wall and accounts for the acoustcal boundary condtons on the recever sde as explaned n Part V. The sound power that s radated by the structure ( Wo ) nto the envronment s based on the near feld sound level ( po ) and the surface area (A) n square meters as calculated by, Wo = po + 10og ( A) db re: one pco-watt (5) The sound level at some pont as caused by the structure s calculated by mportng Wo nto Equaton (1). Drectvty parameters may become a factor dependng on where the far feld recever s located relatve to the walls or f there s a barrer of some type. Substtutng the varous expressons above nto Equaton (5) results n, A Wo = W NR + 10og S db re: one pco-watt (6) 4
What s shown s the relatonshp between the duct surface area (A) and nsde cross sectonal area (S). A reducton n Wo s realzed when S > A, but ths s very rare. TRANSMISSION OSS The transmsson loss (T) of walls (ncludng duct walls) cannot be smply calculated. There s no closed formed equaton that adequately models all the varables nvolved n the wall desgn, tolerances, and constructon varances that affect the transmsson loss. The spacng of stffeners, weld spacng and varablty, torson of panels, the wall thckness and packng densty of nsulaton and the feld erecton are all too complcated to smply calculate or model. There are fundamental calculatons of wall T for homogeneous (nfnte) plates based on mass law that result n hgher T values than what s ordnarly obtaned and you cannot smply add T values together n complex wall assembles. NOISE MITIGATION The precedng sectons presented fundamental methods for modelng and determnng the necessary nose reducton to meet a crteron. The partcular nose reducton s of course talored to the partcular source of nose and how that nose enters the envronment or the space n queston. The vast span of applcatons makes any detaled treatment formdable to convey, but there are many excellent nose control texts avalable. 888.300.47 solutons@unversalaet.com www.unversalaet.com 015 Unversal AET. All rghts reserved. About B&W Unversal B&W Unversal delvers on one smple yet powerful promse to provde the hghest qualty, complete ar management solutons. Wth more than a half-century of ndustral and power generaton experence, we engneer solutons to our customers unque needs and back them wth unparalleled support, across the entre energy generaton lfecycle. Your comprehensve sngle-sourced soluton not only meets envronmental, regulatory and operatonal requrements, but also helps you reduce costs, mprove the effcency of your equpment and elmnate safety and complance rsk. We put our expertse to work solvng problems for your specfc needs. The result: Your world. Clean. Quet. Safe. NORTH AMERICA n ATIN AMERICA n EUROPE n ASIA PACIFIC n INDIA 5