ABSTRACT HEADING
An increase in population and limited land availability in modern cities means that concrete and steel frame high-rise buildings are being constructed more than ever. These buildings are technologically advanced, offering occupants comfortable environments, which is achieved by the use of building services installations. Mechanical rooms form part of these services and with multi-storey buildings they are inevitably located on intermediate floors, close to occupied areas. Noise and vibration generated from equipment and other installations in these rooms is emitted to surrounding spaces in the form of airborne and structure borne noise. Installing the correct type of vibration isolation can eliminate structure borne noise from machinery that is transmitted as impact sound and reduce the vibration to acceptable limits. To overcome the air borne noise, floating systems can be implemented, which are based on creating an air gap with a resilient surface. These floating systems are very effective in eliminating air borne noise by acting as barriers, which can be installed on walls, floors and ceilings in any room that requires acoustical treatment. Since MEP equipment and installations differ in size and shape and have their own operating characteristics, acoustical treatment and vibration isolation requires a technical and specialized approach. To achieve higher performance levels each piece of equipment will need individual attention besides investigating the whole installation. A general strategy together with key points for implementing acoustic insulation for mechanical spaces are evaluated in this paper.
INTRODUCTION
The most effective noise control measure is to locate plant rooms as far away as possible from noise-sensitive areas. This is not the case for high-rise multistory buildings in which the plant rooms are typically located on intermediate floors, close to the occupied areas they serve. In such a case, appropriate constructive layers should be selected for walls, ceilings, and floors once the amount of noise is assessed within the plant room. It is required that engineers and architects work in an interdisciplinary manner since it is essential to restrict equipment in the plant room to protect them against seismic loads whereas staying within the projects architectural design criteria, insulating the plant room noise, and isolating the equipment vibration. Also, economic costs of noise pollution results in devaluation in house prices, productivity losses from health related impacts. Therefore, general noise and vibration control strategies which are based on international codes and practical calculations are explained below.
Mete Oguc is a noise control engineer in Ulus Yapı Tesisat Malzemeleri A.Ş., İstanbul, Turkey. Okan Sever is the managing director of Ulus Yapı Tesisat Malzemeleri A.Ş., İstanbul, Turkey. Eren Kalafat is the president of Ulus Yapı Tesisat Malzemeleri A.Ş., İstanbul, Turkey.
EQUIPMENT NOISE LEVELS
Meaningful sound level measurements require a comprehensive measurement plan. This requires the selection of proper instrumentation and measurement procedures which addresses the source of the noise problem and helps out to draw a solution map. Unfortunately, it is usually possible to have such measurements when noise problem and helps out However, noise and vibration control design should begin durinfg a project’s schematic design phase and continue be used during the design phase, industry standards have been developed for manufacturers to declare the acoustical performance of various types of HVAC equipment. Although most of the test standards require performance data in octave bands, manufacturers usually give a single value to represent the total sound power level of their products’noise. Furthermore, it is generally very difficult to predict sound power level generated by equipmentor machinery in generated also depends upon the enviroenment presented to the noise source. Therefore, current noise-prediction can be found in literature. To name a few, procedures for estimating the noise radiated by refrigeration machines and chillers, air compressors and vacuum pumps, cooling towers, pumps, boilers, fans, transformers and generators are available to designers(Bees et.al 2009, Ver et al. 2006)
SOUND INSULATION PREFORMANCE OF SEPARATING STRUCTURES
Once the location of the plant room is set and the preliminary equipment selection is completed, the acoustic insulation for the plant room shall be designed according to the equipment type, expected noise levels and adjacent occupied spaces’ purpose of use. Sound generated in a room can be transferred to neighboring rooms via a number of transmission paths, such as through walls, floors, building frameworks and interconnecting ducts. Noise can either be evaluated as airborne or impact noise. Evaluating the impact sound insulation performance is often discarded in practice since building services that are found in plant rooms do not cause impact noise. The working machinary causes vibrations, not impact noise unless it is a metal stamping press or similar as can be found in manufacturing facilities.
Airborne Sound Insulation
The simplest case of sound transmission is a plane sound wave normally incident on a uniform homogeneous, isotropic, flat plate. Based on the normal-incidence mass law, airborne sound transmission loss, R, for such cases can be calculated from:
where m is the mass per unit area of the separating structure, is the radian frequency, and c are the density of the gas and speed of sound inside this gas which is assumed to be the same on both sides of the separating structure.
Since the highest sound transmission loss obtainable by a single partition is limited by the mass law, use of multilayered lighter partitions are inevitable. Though, the calculation of double leaf partitions are a bit more complicated, Goesele has proposed a simplified method to predict the sound transmission loss of double partitions (Goesele 1980). According to this method, for a double partition with a gap distance / the formula is:
where RI is the measured sound transmission loss for the first partition, RII is the measured sound transmission loss for the second partition, f is the frequency and s is the dynamic stiffness per unit area of the gap which can be calculated from:
Predicting models presented above are useful for the calculation of sound reduction performance of simple structures. However, advanced models such as finite element model, transfer matrix model and improved impedance model are usually preferred for the calculation of sound insulation performance of complex multilayered structures. (Mak et al. 2015; Kurra 2012).
Figure 1 Floating floor system built around a housekeeping pad to insulate airborne noise generated inside a plant room. Flanking sound transmission paths are avoided by the use of vibration isolating wall supports, insulation strips and rubber mounts.
Since it is required to properly support heavy equipment such as chillers and generators to account for additional loads such as seismic loads, housekeeping pads that are rigidly attached to the buildings structure are usually utilized as shown in Figure 1. This application causes another problem to arise. When evaluating the airborne sound insulation performance of such a system within a plant room which is adjacent to a noise sensitive space, the contribution of airborne sound insulation performance of rigidly attached housekeeping pads are questioned since they create a short cut of airborne noise transmission even if a floating floor system is built around the housekeeping pad. This system which consists of a floating floor surrounding a housekeeping pad which is rigidly attached to the load bearing slab beneath the floating floor can be treated as a composit structure. It is possible to calculate sound transmission loss from this composite system from:
where SFF is the surface area of the floating floor, SHP is the surface area of the housekeeping pad, RFF is the sound transmission loss of floating floor and RHP is the sound transmission loss of housekeeping pad.
Impact Sound Insulation
Impact isolation quantifies the ability of a construction to prevent transmission of impact noise which can be measured using a standard tapping machine. Whether a floor is excited by impact sound or by an airborne sound field in the source room, it will in both cases radiate sound into the receiving room. The distinction between airborne sound and impact sound insulation performance of a floating floor can be made by making both airborne sound insulation performance measurement for R, and impact sound insulation performance Ln. The flanking paths can be immediately identifined by measuring the acceleration level on the wall surfaces in the source room and receiving rooms during acoustical and impact excitation. The floating floors in plant rooms are generally not designed as part of a vibration isolation or impact noise insulation scheme (ASHRAE 2015). Vibration isolation products should be used when the foundation or base of a vibrating machine is to be protected against large unbalanced forces or impulsive forces.
Vibration Isolation
The perception of vibration involves a large range of sensory systems. When people are asked about they often describe the associated noise and visual sensations alongside the physical sensation of feeling vibration. Vibration is often accompanied by noise so that the residents are usually uncertain of the differences between noise and vibration (Whittle et al. 2015). Therefore, it is significant to examine and solve the vibration problems step by step.
First step is to determine the source, in this case any equipment with a rotating element. Second step is to find out the path which the way vibration is transferred. Almost all building structural and installation components, such as beams, columns, walls, etc. transmits vibration. Third step is to define the receiver and the criteria for the disturbancy. The vibration isolation requires the use of relatively resilient elements to reduce the vibrotary forces or motions that are transmitted from one structure or mechanical component to another to acceptable limits.
The procedure to reduce the undesirable effects of vibration is called vibration isolation. Vibration isolation aims to reduce the dynamic response of the system by the insertion of a vibration isolator between the vibrating equipment and the source of vibration. Vibration isolation efficiency I of vibration isolators that are installed to a vibration source equiment can be accounted by:
where T is transmissibility which indicates the fraction of the disturbing motion or force that is transmitted. So that the isolation efficiency indicates the fraction by which the disturbance is less than the excitation (Rao 2004). Many aspects of vibration isolation can be understood from analysis of an ideal, linear, one-dimensional, spring-mass system. For such a single-degree-of-freedom model, the transmissilibilty of the system for a disturbing frequency fd is:
where fn is the natural frequency for a system with an isolator stiffness k which can be calculated from:
The structure or foundation to which the isolator is connected cannot be assumed rigid when an equipment is mounted on a floating floor system. Eventhough this sort of a system is not ideal from a noise control perspective, it is requested by architects and investors for its ease of application. For such cases, the impedance of the supporting structure can no longer be ignored. Transmissibility of this system should be calculated from:
The two-degree-of-freedom system can model a system where an equipment with mass m1 is mounted to an isolator with a stiffness k1 which is rigidly attached to a floating floor with an equivalent mass m2 installed on rubber mounts with stiffness k2. However, caution should be exercised to ensure adequate damping is provided in the floating floor and that there are no common natural frequencies present between the floating floor system and equipment isolation mounted to it (ASHRAE 2013, ASHRAE 2015).
EVALUATING THE NOISE TRANSMISSION TO AN ADJACENT SPACE
There is an ordinal relationship between noise exposure and annoyance which in turn causes social and health problems. Introducing exposure-response relations which are being used in strategic or health impact assesments for estimating long-term annoyance to these insulation performance criteria help designers to develop lower cost and highly efficient insulation systems. Directives in many countries and codes written by independent technical institutes lay down and define specific noise emission limits mostly for environmental noise problems (Nathan 2015). Despite these valuable efforts, there has been little interest on improving exposure-response relationships considering people living in occupied residential and office spaces.
Once the transmitted noise levels are calculated from the total noise produced within the plant room, the noise transmitted can be calculated by substructing the sound reduction of the separating structures. The noise levels that are obtained in the adjacent occupied space should be lower than the legal requirements (or at least from the guideline values given by WHO in 1999). However, in most cases, these limit levels are also given as single numbered equivalent noise levels that give no indication of which frequency components may be the source of non-compliance. Therefore, single-number weighting curves should be used.
The human response to vibration inside the occupied space can be evaluated by making measurements and following ANSI S2.71 and ISO Standard 2631-2. From the designers perspective, vibration isolators transmissibility values can be used eliminate vibration induced noise in the design phase. The transmissibility of the isolators can be selected as 10% for nonsensitive, %5 for sensitive installations and 3% for critical installations. The static deflection of the isolation system can be calculated once the maximum allowable transmissibility has been decided (Simmons 2007).
CONCLUSION
Properly accomplished acoustic insulation and vibration isolation measures within the design phase is the most inexpensive way to prevent objectionable vibration and noise. The general scheme described above is based on following up international codes and common engineering principles. Eventhough decoupling structural components of a plant room to avoid flanking sound and vibration transmission while properly mounting the equipment to rigid housekeeping pads and leaving clearances for service installations for seismic safety considerations creates a dilemma, it has been shown that both sound and vibration transmission values can be taken into account within the design phase.
ACKNOWLEDGMENTS
This project has been funded by Ulus Yapı Tesisat Malzemeleri A.Ş.
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REFERENCES
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