Chapter4 While It Is not your responsibility to Barrier Noise Reduction provide detailed design assistance to Concepts Noise Attenuation the sponsor or developer, you should know enough about the attenuation (The following, with some editing and options to give him or her a basic with some additional graphics, Is understanding of what must be done. taken from the Federal H•ghway In many cases, you may be able to lntroducton Administration's Noise Barrier Design reassure the sponsor or developer Handbook.') that the necessary attenuation can be When no obstacles are present HUD's noise policy (24 CFR 51 B) achieved through the use of common between (a source) and adJoining clearly requires that noise attenuation construction techniques or materials. areas, sound travels by direct path measures be provided when proposed a Or you may be able to point out how a from the "sources"... to(the) pro,ects are to be located In high simple site design change can "receivers"... , as shown In Figure 1. noise areas. The requirements set out achieve the desired result without Introduction ot a barrier between the In Section51.104{a)aredesigned to additional cost. source and receiver redtstt1butes the insure that interior levels do not The following sections are sound energy into several(lndtrect) exceed the 45 t..oo level established as designed to provide you with the paths: a diffracted path, over the top a goal In Section 51.101(a)(9). Thus, In information you will need to fulfill of the barrier; a transmitted path, elfect,llthe exterior noise level Is 65 your responsibilities. Each through the barrier; and a reflected l<tn to 70 L<f0 , 25 db of noise attenuation approach is discussed path, dtrected away from the receiver attenuation must be provided; lithe both In terms of basic concepts and in These paths are also illustrated In exterior noise level Is between 70 and terms of what to look for In reviewing Figure 1. 75 L<f0 , I hen 30 db of attenuation Is attenuation proposals. The required. Likewise, for projects discussion does assume that you proposed for areas where noise levels have a working knowledge of the exceed 75 L<fn. sulflclent attenualon Noise Assessment Guiidellnes. II you must be provided to bt'lng Interior have not worked with the Guidelines levels down to 45 L<fn or below. before or not recently you may want to go back and review them, particularly 1Nol$• Blrrl« Ckltlgn H•~ US f)eptrtmenl o4 There are three basic ways to provide the section on calculating the effects Tranapor1atl01\, Ftdeftl Hlghwty Admlnllirttlon, the noise attenuation required: of barriers. Ff!brua.y 1976. (F><WA-RD-76·581 1. the use of barriers or berms 2. site design 3. acoustical construction F~t AltorotiO<> of Nol.. 01 these, only the first two provide Path• by • Bomer any Improvement in the exterior environment. Because HUD considers a quiet exterior environment to be Reflec1ed "" impor1ant, we prefer the use of those / measures that reduce exterior levels ' x as well as Interior levels. The use of / ' / acoustical construction by Itself Is, / ' :::./ -­ therefore, the least preferred /--__-­__ _ alternative since It only affects the Interior levels. While we recognize that in many cases barriers or site design cannot provide all the attenualton necessary, you should combine them with acoustical construction whenever possible. Your responsibility as a HUD staff member is to: • make sure the project sponsor or Direct Path developer is aware of the attenuation ~u ~-- requirements lor the project. ---- • make the sponsor aware of the options available 111 111 111 111 111 and • review attenuation proposals to make sure they are adequate. 21 Berrier Diffraction and Attenuation Flguro2 ._Dlfhc:llon Consider an Infinitely long, Infinitely massive noise ban1er placed between a highway and the receiver. Figure 2 Illustrates a cross-secllon through such a configuration. Pnl this example, the only way that sound can reach the receiver Is by bending over the top of the barrier; as shown in the figure. The bending of sound waves In this manner over an obstacle is known as dlffracllon. The area in which diffraction occurs behind the barrier Is known as the "shadow zone." The straight path from the source over the top of the barrier forms the boundary of this zone. All receivers located In the shadow zone will experience some sound attenuation; the amount of attenuation Is directly related to the magnitude of the diffraction angle o. As .plncreases, the barrier attenuation Increases. The angle 6 will increase If the barrier height Increases, or if the source or receiver are placed closer to Figures the barrier. Clearly then the barrier PolhUnglh attenuation Is a function of the DIH-fa A+B - d geometrical relationship between the source, receiver, and barrier. One way of relating these parameters to the barrier attenuation Is to define the path·length difference as shown In Figure 3. This parameter is the difference In distance that the sound must travel in diffracting over I he lop of the ban1er rather than passing d1rect1y through It In the preceding discussion it was assumed that the barrier was "infinite"; I.e., long enough to shield the receiver from all sound sources up and down the highway. For short barriers, the attenuation can be seriously limited by the sound from sections of highway beyond the barrler"s ends, which are unshielded from the receiver, as shown In Figure 4. Similarly, when there are large gaps In the barrier (to permit access, lor example), sound from the unshielded section of highway adjacent to the gap can greatly compromise barrier attenuation, especially for those receivers close to the opening. I -:~"'".l.-L...J:Z~L~Z~/~ 1 ' ReceiYer 22 a.rierTrensmlsslon Is at least 10 dB above the attenuation Barrier Reflections value resulting from diffraction over In addition to the sound I hal travels the top of the barrier, the barrier noise As shown In Figure 1, soond energy aver I he top of the barrier to reach the reduction will not be significantly can be reflected by a barrier wall. For receiver, soond can travel through the affected by transmission through the the configuration shown In that barrier Itself. The amount of sound barrier (decreased by less than 0.5 figure, the reflected energy does not " transmission" through the barrier dB). For many common materials aIfact the receiver, but may affect depends upon factors relating to the used In barrier construction, such as receivers located to the left of the barrier material (such as Its weight concrete and masonry blocks, TL highway. However the Increase In and stiffness), the angle of incidence values are usually more than noise level for these receivers would ofthe sound, and the frequency adequate. For less massive materials be less than 3 dB, because this single spectrum of the sound. One way of such as steel, aluminum and wood, TL reflection can at most dooble the rating a material's ability to transmit values may not be adequate, sound energy. (Remember how you noise Is by the use of a quantity partlculat1y lor those cases where combine noise levels? The most you known as the transmission loss, TL large attenuations are required. (See add Is 3 db when levels are the same.) The TL is related to I he ratio of the Table 1 for a list of typical TL values.) The situation Is entirely different, Incident noise energy to the Even If a barrier material Is massive however, when a double barrier transmitted noise energy. enoogh to prevent significant sound situation Is involved (refer to Figure 5). Transmission loss values are transmission, the barrier noise In addition to the energy that reaches normally expressed In decibels and reduction can be severely the receiver by diffraction over the top represent the amount noise levels will coml)(omlsed If there are holes or of the barrier, if the barrier walls are be reduced when the sound waves openings In the barrier. For large reflective, additional sound energy pass through the material. The higher openings, sound energy Incident on can reach the receiver by a reflection the TL value the less noise the barrier will be directly transmitted from the left wall as Illustrated In the transmitted through the material. through the opening to the receiver. figure. The same principles apply Typically, the TL value lml)(aves with When the opening Is small an when there is a vertical retaining wall Increasing surface weight of the additional phenomenon occurs: upon opposite a noise barrier; similarly, In a material. striking the barrier wall the sound deep vertical cut the opposite walls The noise reduction provided by a pressure wllllncrNM, resulting In an will create multiple reflections. barrier can be severely compromised amplification of the transmitted II the barrier walls are not perfectly If the TL value of the material permits sound to the receiver. Thus, the reflecting but absorb some or the too much noise to pass through the presence of openings or holes may sound energy, the contrlbuton of each barrier. This Is due to the fact that seriously degrade the noise reduction reflection Is decreased by an amount when attenuation is a function of two l)(a<tided by otherwise effective that depends upon the absorptive or more factors, the noise level at the barriers. characteristics of the barrier. For very measurement point Is actually the hard, reflective surfaces,the combination of the reduced noise absorption characteristics are very levels resulting from each attenuation poor. Although a serious degradation factor. For example, with atypical In barrier performance may result lor barrier the noise levels are reduced by the dooble barrier situation, use of (1) sound waves being diffracted over materials with good absorption values the barrier and (2) sound waves will usually recover all of the lost passing through the barrier.