Document LJq3eoVXMYDVzmR1QogxG2Y0q

t94 CHAPTER 14 1965 Guide And Data Book Table 3 .... Summary of Sound Sources, Sound Paths, and Usual Noise Reduction Methods Soeed Scores Path No*.*- No. Sotmd Path> Oeicnption DiSusera, Griltea, Registers 1 Circulating Fans . Unitary Equipment (located entirely within rDom). - ~- 1 Sound reflected by walls, ceiling and Room absorption and location of sound source floor have a meat effect bn received level, but are usually fixed by room layout. - ' Direct sound radiation from Source to Ear Only practical, way to reduce noise is to select sufficiently quiet.equipment. Induction Coil - Units and High-Velocity Miring Units (These axe not only noise. sources, but also attenua' tore for fan noise), 1, 2 SOLID BORNE SOUND TRANSMISSION - . 2 . Noise radiated from eatings and Locate high velocity ducts and terminals, as veil through 1oolls of. ducts and plenums as noisy parts of low velocity ducts, in nonis transmitted through walls and- critical areas. ceilings into dir conditioned rooms Design ducts and fittings for low turbulence. Unitary Equipment (located outside of rooih served).. Remotely Located Air Han dling Equipment: Fans ana Blowers Dampers and Fitting Air Washers 2,3,4, . 3 Through supply and return ducts to terminals in air conditioned rooms (and hence to listener as in Path 1) Select fans for minimum noise generation. Provide sound attenuation devices in supply and return ducts. Compressors Pumps Cooling Towers Air-Cooled Condensers' SOLID BORNE SOUND TRANSMISSION 4,A 6* 4 Noise is transmitted through equip ment room walls and floor .4,5, 6,7 _5 ' Building structure transmits vibra tions which turn into noise 1 6- Transmission along pipe and duct walls Locate equipment room away from critical areas. Providesound absorption in equipment room and sound-isolating wall and floor construction. . Design equipment room floor for dynamic loads.: Balance fans both st&ticafly and dynamically. Mount ail maehmea on proper vibration isolators.. Install flexible connectors in ducts and pipes. Exhausters 7,8 ' Window Air Conditioners .' 7-. Noise radiated to outside enters re Locate sound sources away from critical areas. ceiver's room through windows Reduce sound radiation with shields and covere. Select equipment for low enough source level 8 Noise radiated to inside follows Path 1 Select sufficiently quiet equipment. >r Cross Transmission between1' 9,10Rooms 9 Through air outlet of one room into Design and install duct attenuators to match ductwork and out through next air transmission loss of partition (Path 10) outlet . ' v SOLID BORNE SOUND TRANSMISSION . w - Through room partitions or over and - Partitions should be extended to ceiling slab and around them be sealed tightly all around. - 1 Also seal around all duct arid pipe openings. * Sound from *'particular Nm'w'Sb tay of the piths Qsted in this bornesoundtnnawni (seetest). - It should be noted.that Paths 2. 4. S, fl, end 10 is itelie type involve sefid the sound output of equipment'containing electric motors; fans' and. compressors/ To', indicate the generally undesirable character of noise containing pure.tones,1 one.approach which has beep suggested is to rate the source level up to 10 db higher than the'measured value in any octave band which contains a noticeable pure-tone.11-". ' Various specifications.based on octave band levels are used to1express'the1needs of diff^ng'atusitions: -l; For spaces m'Wluch'eaae af epeecb'communication ia most important* .a .maaamum; limit-is,sometimes specified for the speech interference level. ,Thiais defined as the arithmetic aver age of the' sound pressure levels'in the three octave bands of-the speech frequency range (600-4fi00 cps). 1 2. For spaces in' which speech privacy must be assured, such as private.offices, hotel rooBW.'etci; minimum sound levels may. alsobe specified in.the speech frequency bands-.These limits must be 'selected in accordance .with tne.sound isolating value of the room construction.**'**'v;' ' ' " 3. For general comfort specifications, limits are often set by reference to. the noise criterion (NC) curves1* shown in Fig. 2. It is often required, for example, that a sound not project above a specified NC contour in any frequency band. ' The contours of the-NC curves were, derived to express the background-Bound levels observed in loudness.and speech interference' studies in a large number of offices. These studis indicate thatthe background is least annoying if its spectrum, like these contours, is such that .the loudness in phons is approximately:equal to the speech,interference level plus-22.. If it is not feasible to provide enough attenuation to'reach the desired NC level in the low-frequency range, a somewhat higher loudness level may sometimes be tolerated, without compromising on the speech interference level Experience has shown that the loudness level should not exceed the speech interference' level plus 30. The: NCA curves shown with broken lines in Fig. 2 are based on this limit. ! Sowid'Control-. 195 loudness or Loudness Level. Loudness of a sound, in units of sones, is a mngla number rating based on statistical evidence of the response of humans to sound.*-*7-1* The loud- nes scale is a linear representation of_the relative loudness of rounds to humans. Using equal loudness contours, each octave rand level is assigned its loudness value. Total loudness is then calculated using Equation 7: uiftae. 8 - 5 + 0.3(25 - 8m) (7) 5 total loudness. * 8m " loudness of the loudest octave band. 25 sum of the kmdneases of all octave hands The logarithmic equivalent to loudness in wonaq is the loud- level in phons. One: sone equals forty phons, and the level increases ten phons for each doubling of loud ness. _ * ' ------- ----------------- -----------r- - - - Any interpretation of loudness.data should.recogriire that ""ferent authors have reported values based on'differing loud- contours. " 1 ,- AScale Sound LedeL This measuring method has the advan tage of simplicity, since the A sound level is obtamed;from one reading on a single instrument The A-scale.of-astendard' sound level meter includes an electronic weighting network which de-emphasizes the low frequency portions of the'noise spectrum, thus automatically compensating for the lower sen sitivity of the human ear to-low frequency sounds;^The re sponse1curve of the A-ecale -weighting network is similar/over most of-the frequency range, to an equal loudhess oOiitour! The three systems of expressions described cannot besubject to conversion charts orCexpression3. Each describes a sound in a different frame of reference; sounds,of1differing spectra will therefore involve different comparisons between the systems. For a given spectrum shape, however, some ap proximate comparisons can be made.1 For'sounds such'as air flow noise, which have a broad spectrum and im strong pure tones, a rough estimate of either'loudness level or NU level can be made from the A-ecale sound level-" ; .,: A Ur!/. IS loudness level (phonfl) - Arscnle.level.(db)'2 V 1(8) NC level * A-ecale level (dbjj--! ..6i-2`','(9)