Acoustics III Lecture Topics PDF

ACOUSTICS: LECTURE TOPICS III Sound Reinforcement Systems, Noise Control, Air-Borne Noise Reduction, Solid-Borne Sound Reduction, Fundamentals of Lighting, Daylighting Sound Reinforcement Systems Objectives and Criteria Components and Specifications - Input Devices - Amplifier and Controls - Loudspe...

ACOUSTICS: LECTURE TOPICS III Sound Reinforcement Systems, Noise Control, Air-Borne Noise Reduction, Solid-Borne Sound Reduction, Fundamentals of Lighting, Daylighting Sound Reinforcement Systems Objectives and Criteria Components and Specifications - Input Devices - Amplifier and Controls - Loudspeakers Loudspeaker Considerations Objectives and Criteria Purpose of sound reinforcement system is just what the name indicates to reinforce the sound, which would otherwise be inadequate. An ideal sound system will give the listener the same loudness, quality adjacent to him – a distance of 2 to 3 ft. for speech and further for music depending on type and number of instruments. Quality means that frequency response should be linear so that reproduced sound bears the same relation between its frequency components as the original sound. Directivity is the characteristic whereby the sound appears to be coming from the originating source, that is, the loudspeakers should be directionally “invisible” and the listener must have the impression of actually hearing the source. Generally, sound system will be required in spaces larger than 50,000 cu. ft. or 550 persons in terms of population. Components and Specifications All sound systems consist of three basic elements: Input devices, amplifier(s), and loudspeaker systems (a) Input Devices – Microphone, Recorded materials from phonograph, tape deck, cassette deck, and possibly commercial music source, Radio source – AM/FM tuner. (b) Amplifier and Controls - to produce intensity levels of 80 db for speech, 95 db for light music, and 105 db for symphonic music. Components and Specifications Another control frequently required in theatre systems is a delay mechanism or circuit that can introduce a time delay into a signal being fed to a loudspeaker. Noise Control Introduction Noise in Buildings Outdoor Noise Acceptable Noise Levels in Buildings Siting and Planning Against Noise Grading and Landscaping Building Layout Noise Control Introduction An intelligent approach to the problem of constructing quiet buildings must be based upon a knowledge of: 1. The magnitude, nature and distribution of noise in buildings and out-of-doors. 2. Acceptance noise levels in various types of buildings. 3. The propagation, and especially the attenuation (reduction of the energy or intensity of sound) of sound through the free air, through openings and ducts, and through or around obstacles, embankments, and landscaping. 4. The reduction of sound, and the suppression of vibration by varied types of partitions and flexible connectors. 5. The reduction of machinery noise at its source by appropriate selection of equipment from a noise-producing standpoint; 6. The reduction of noise by the proper use of sound-absorptive treatment. Noise Control Noise in Buildings The principal sources of room noise may be grouped into 3 broad classifications: People Machinery, and Outdoor sources Outdoor Noise Sounds of outside origin are often the principal contributors to noise in offices, churches, and residences. The largest source of outdoor noise is generally automobile traffic. For this reason it is desirable that all buildings in which quietness is an important factor, including churches, auditoriums, and hospitals, be not constructed near a busy or potentially busy, street. Noise Control Acceptable Noise Levels in Buildings Noise Control The highest level of noise within a building that neither disturbs its occupants nor impairs its acoustics is called the acceptable noise level. Noise Control Siting and Planning Against Noise The selection of the site of building, the layout of the building itself, and the grading and landscaping of the site are indispensable parts of good planning against noise in buildings. The existence and persistence of quiet sites is dependent on zoning ordinances and their enforcement. Architects in every community should cooperate with the civic authorities in the segregation of noxious activities including noisy industries, power stations, airports, traffic arteries. From buildings where quiet is an absolute necessity, such as schools, churches, hospitals, and residences. These buildings should be protected by civic planning. Noise Control Grading and Landscaping An earth embankment or a masonry garden wall often can be used to reduce the noise that impinges on a building and aid in the establishment of quiet conditions within the building without resorting to costly measures of sound insulation. Hedges of trees with dense foliage act as sound absorbers and reflectors, and their effectiveness increases with the extent (thickness, height, and density) of growth. Noise Control Building Layout The location of a building on its site, the arrangement of rooms, corridors and vestibules, and the location of doors and windows, all have a bearing on the control of noise; they require careful consideration. It is advantageous to place the speakers platform at the end of the room adjacent to the street, which is the primary source of noise. Air-Borne Noise Reduction Principle of Noise Reduction How Sound Is Transmitted Transmission Through Openings Rigid Partitions Compound Wall Construction (Cavity Walls) Windows and Doors Noise Insulation Requirements Noise Reduction By Sound Absorptive Treatment Absorption Recommendations. Principle of Noise Reduction Noise reduction is essentially the science of converting acoustical energy into another less disturbing form of energy-heat. This conversion is by absorption, by the room contents and wall coverings, and by the structure itself. Noise control treatment in a room will affect the reverberant noise level within that room but will have minimal effect on the noise level in adjoining spaces. Air-Borne Noise Reduction The best that can be accomplished with acoustic room treatment is elimination of the reverberant field, that is, to make the intensity at the room boundaries what it would have been in free space. How Sound Is Transmitted Most sounds that are communicated to a room, either from the outdoor or from elsewhere in the building, are included in one of the following classifications: Sounds originating in the air which are transmitted (a) along a continuous air path through openings, (b) by means of diaphragmatic action of partitions. Sounds originating from direct impacts. Air-Borne Noise Reduction Transmission Through Openings Air-Borne Noise Reduction Noise can be readily communicated from one portion of a building to another through openings such as windows or open doors, frequently these openings limit the total amount of insulation which can be attained. Thus if it is necessary to open windows for ventilation, the sound insulation between two adjacent rooms may be limited by the open windows to 20 db or even less. Under such circumstances, it would be profitless to provide separating partitions of relatively high insulation. Even very small openings, such as cracks around doors or windows, are effective in transmitting sound. Air-Borne Noise Reduction Rigid Partitions The transmission of sound through a “rigid” partition, such as a brick, concrete, or solid plaster wall, is accomplished principally by the forced vibrations of the wall Air-Borne Noise Reduction Compound Wall Construction (Cavity Walls) Double-wall construction frequently offers the most practical means of obtaining high insulation at moderate cost and reasonable dead load. Air-Borne Noise Reduction Windows and Doors Windows and doors are usually the “paths of least resistance” in the overall sound insulation of a room. The positions of windows ought to be carefully planned so that, without sacrificing utility or beauty, they are removed as far as possible from other windows in adjacent noisy rooms or from street noise. The average transmission loss of a window depends primarily on the thickness of the pane. The transmission loss of a door increases with increased weight. Air-Borne Noise Reduction Noise Insulation Requirements Satisfactory Rule: Subtract the acceptable noise level from the average level (averaged over time) of the outside noise; to this difference add 10 db. The result is the noise-insulation factor required to furnish adequate sound insulation. The additive 10 db is included to: Provide some protection against disturbance from the usual surges of outside noise that are above the average level, and To allow for unavoidable differences between the sound insulation provided by the actual structures and those determined by laboratory tests on model partitions. Air-Borne Noise Reduction Noise Insulation Requirements Air-Borne Noise Reduction Noise Reduction By Sound Absorptive Treatment The installation of acoustical materials in a room has the following beneficial effects: It reduces the reverberation time, usually several fold; It reduces the overall noise level; It tends to localize noise to the region of its origin (a distant source is attenuated relatively more than one nearby). Air-Borne Noise Reduction Absorption Recommendations. To summarize, absorption techniques are useful and effective: To change room reverberation characteristics. In spaces with distributed noise sources such as offices, schools, restaurants and machine shops. In spaces with a hard surface and little absorptive content. Where listeners are in the reverberant field. (no amount absorptive material can reduce intensity levels in the direct field.) Solid-Borne Sound Reduction Reduction of Solid-Borne Noise Floors and Ceilings Floating Floors Control of Impact Noise Mechanical System Noise Control Quieting of Machines Building Siting Room Assignment Solid-Borne Sound Reduction Reduction of Solid-Borne Noise The means of transmission and suppression of solid-borne sound are considerably different from those of air-borne sound. Solid-borne noise usually originates from impacts or machinery vibration. Once vibration is transferred to a solid building structure, such as concrete slab, it travels through the structure with a speed of about ten times that of sound in air. Measures for accomplishing this include the use of heavy carpeting, cork tile or linoleum on felt to reduce impact transmission to the floor; a segment of flexible metallic or rubber hose, in a pipe to lessen the propagation of impulses along. Solid-Borne Sound Reduction Floors and Ceilings The table gives the ratings of a number of floor constructions laid on a concrete slab, compared with the rating of the concrete slab itself. Floors and Ceilings Solid-Borne Sound Reduction The table gives the ratings of a number of floor constructions laid on a concrete slab, compared with the rating of the concrete slab itself. Solid-Borne Sound Reduction Floating Floors The isolation provided by a floor system against mechanical impact can be greatly improved by the use of a “floating” floor which rests on the structural floor but is separated from it by a resilient support or quilt. Control of Impact Noise Solid-Borne Sound Reduction Impact noise problems can be controlled in two ways. 1. By preventing or minimizing the impact, and 2. By attenuating it once it has occurred. Solid-Borne Sound Reduction Mechanical System Noise Control The more power mechanical devices consume, the more noise they make. About 40% of the total building cost is spent on mechanical systems. In most buildings, the primary sources of mechanical noise are the components of the air conditioning and air handling systems such as fans, compressors, cooling towers, condensers, ductwork, dampers, mixing boxes, induction units and diffusers. Pumps are another sources of mechanical noise. Solid-Borne Sound Reduction Quieting of Machines There are 3 ways to reduce noise from machines: 1. Reduce the vibration itself; 2. Reduce the air-borne noise by decouping the vibration from efficient radiating sources. 3. Decouple the vibrating source from the structure. Quieting of Machines Solid-Borne Sound Reduction Solid-Borne Sound Reduction Building Siting Buildings should be sited, with respect to noise sources: To use natural terrain noise barriers. With respect to trees as noise barriers, rely only on thick wooded areas. To avoid naturally poor sites To avoid sound reflection from other buildings. Building Siting Solid-Borne Sound Reduction Building Siting Building Siting Solid-Borne Sound Reduction Building Siting Solid-Borne Sound Reduction Room Assignment Solid-Borne Sound Reduction Where avoidance of an exterior source is impossible, quiet zones can be buffered from the noise by placing higher-noise areas on