Acoustics for Libraries

Transcription

1 Acoustics for Libraries Acoustics for Libraries This material has been created by Charles M. Salter, P.E., and provided through the Libris Design Project [ supported by the U.S. Institute of Museum and Library Services under the provisions of the Library Services and Technology Act, administered in California by the State Librarian. Any use of this material should credit the author and funding source.

2 1. INTRODUCTION 3 2. SOUND AND NOISE 3 3. ROOM ACOUSTICS 7 4. SOUND ABSORPTION 8 5. SOUND INSULATION Principles of Sound Insulation Sound Insulation Construction MECHANICAL AND ELECTRICAL SYSTEMS NOISE CONTROL Space Planning Noise Control for Main Building Equipment AUDIO-VISUAL (A/V) Auditoriums, Theaters and Large Meeting Rooms Teleconference Rooms Electronic Classrooms and Training Rooms Lighting in A/V Spaces 28 Page 2

3 1. INTRODUCTION The acoustical design issues for libraries involve the following principal issues: Site noise considerations, including the control of noise transfer to a project s neighbors, particularly if they are residential. Establishing noise standards for each use space, including limitation of excessive ventilation noise. Room acoustics considerations. Sound isolation between various use spaces. Vibration control for mechanical equipment. Audio/visual system considerations. Library planners should develop objective acoustical standards for library projects as an important component of the project program. The information contained in this article about library acoustics is intended as a source for these standards. As the architectural and engineering design of the project evolves, the design should be reviewed in light of the agreed upon acoustical programmatic requirements for the library project. Since acoustics is typically not a code requirement, a city or county building official cannot be expected to comment on the correctness of the acoustical design in the contract documents. Therefore, it is the responsibility of the facility s planners, user groups, architects, engineers, and others involved with the project to assure that the project acoustical needs are delineated and that there is follow-through, particularly for verification testing after the ventilation system has been installed and balanced. 2. SOUND AND NOISE Sound waves in air result from a physical disturbance of air molecules, such as when a truck drives by a building or when guitar strings are plucked. Sound waves combine and reach a listener via numerous direct and indirect pathways. The listener s inner ear contains organs that vibrate in response to these molecular disturbances, converting the vibrations into changing electrical potentials that are sensed by the brain, allowing hearing to occur. Acoustical analysis involves not only the sound source but also the listener and everything in between on the path of the sound. The perception of the receiver can be influenced by the Page 3

4 treatment of either the path or the source. Some source sound is desirable, for example a lecturer s voice, and some source sound is undesirable, such as the sound output from an idling truck outside a window. Undesirable sound is usually called noise. Unless it is a pure tone, a sound wave is typically made up of vibrations at different frequencies. Like the impact of a stone in a lake, ripples in the water are created that are analogous to sound in the air. The frequency is basically the number of waves that pass a single point in one second, moving at the speed of sound in air. One wave per second is a frequency of one hertz (Hz). A frequency of 1,000 hertz is a kilohertz (khz). Human speech contains frequencies between 200 Hz and 5 khz, while the human ear can actually hear sound generally between 25 Hz and 13 khz, a wider range. Frequencies below 20 Hz can be sensed as a vibration, though not audible to most people. Conversational Speech Window Air Conditioner Wind in the Trees Car Horn Large Truck Barking Dogs Birds Frequency (hertz) k 2k 4k 8k 16k Figure. 1. Frequency Range of Typical Sound Sources. Sound and noise are described using a metric called the decibel. The decibel scale is logarithmic, similar to the Richter scale used to describe seismic events, and translates a wide range of sound pressure levels that affect the human ear to a logarithmic scale. The range of decibels most commonly encountered in acoustics extends from 0 to 140 db. Figure 2 correlates the sound pressure levels of common sound sources to the logarithmic decibel scale. Page 4

5 Figure 2. Comparison of sound pressure and db SPL for typical sound sources. When designing new library buildings or correcting deficiencies of existing library spaces, materials and constructions are selected to control noise and other unwanted sound. The human ear does not perceive all frequencies of sound to the same degree, however, being less sensitive to lower frequency sound pressures than to middle or higher frequency sound pressures. People tend subjectively to measure their perception of the loudness of sounds based more on the SPL of these middle and higher frequency sounds. Design criteria and sound measurement devices are therefore weighted toward these upper frequencies in order to reflect the subjective perception of people in the space. The term dba, or A-weighted decibel, is often used to describe noise levels in spaces because this type of decibel measurement averaged over the range of frequencies within the range of hearing correlates well with people s subjective perception of the loudness of the noise. Sound level meters, which average the SPL across frequencies, usually have a setting for A- weighting, so that measured noise levels correlate to the human perception of the differences in noise level. The NC Rating is an acoustic design criterion for the target level of background noise in a room. This criterion is based on the fact that human hearing is less sensitive to lower frequencies than to higher frequencies, so that a specific criterion for the SPL of background noise in a space varies with the frequency of the noise spectrum. Figure 3 shows the Noise Criteria (NC) used in acoustic design. The loudest frequency region of the background noise Page 5

6 sets the NC-curve that applies to the space. Figure 3. Noise Criteria Curves. To meet the criteria of NC-25, for example, the measured loudness of all frequencies must fall at or below the NC-25 curve. A new building program for a library should list the acoustic criteria for each space. These criteria will usually include an NC Rating requirement, which depends on the appropriate level of background noise to the tasks and activities in the space. Some typical NC Ratings for library spaces are given in Figure 4. Page 6

7 Space Type NC Rating Open Public Areas (Circulation, Reference) Computer Work Areas 40 Private Offices Open Staff Work Areas Copy Rooms 40 Teleconference Rooms max 25 Reading Rooms Classrooms, Training Rooms Figure 4. Typical recommended background noise levels in library spaces. 3. ROOM ACOUSTICS Room acoustics pertains to the physical characteristics of a space for the hearing of direct and reflected sound. In libraries, the principal issue for room acoustics is speech intelligibility and control of background noise levels. Rooms with a high level of reflected sound may have poor room acoustics depending on the use of the room since the persistence of the sound creates unwanted background noise and interferes with the ability to understand speech. Such rooms are said to have a high reverberation time, the time required for the sound to be absorbed gradually and reduced below hearing levels. Therefore, design principles for room acoustics in library spaces typically focus on the locations and extent of sound absorbing material, to reduce reverberation and the interference with speech, as well as the shape of rooms to achieve acceptable acoustic characteristics in meeting and presentation rooms. Multi-purpose rooms require special room acoustics design since these spaces often must accommodate speech and musical activities at different times. For speech activities, the reverberation time should be low enough to allow syllables of parts of speech to be readily understood. Longer reverberation time is preferred for musical functions, since the musical sounds need to reverberate properly. A room having reverberation time of more than 1.5 Page 7

8 seconds may be acceptable for music listening but would probably create interference with speech intelligibility. A room having a reverberation time of less than 1 second would probably be judged acceptable for speech intelligibility but musicians may complain about the room being too dead. 4. SOUND ABSORPTION All materials have some sound-absorbing properties. Sound energy that is not absorbed must be reflected or transmitted. A material s sound-absorbing property is typically described as a sound absorption coefficient at a particular frequency range. Sound absorbing materials used in buildings are rated using the Noise Reduction Coefficient (NRC), which is basically a type of average of sound absorption coefficients from 250 Hz to 2 khz, the primary speech frequency range. The NRC theoretically can range from perfectly absorptive (NRC = 1.0) to perfectly reflective (NRC = 0.0). Adding sound-absorbing materials to a space usually becomes an interior design issue in the library. Many options are possible to provide sound absorption on walls and ceilings, which are attractive and maintainable. Absorptive materials are often covered with acoustically transparent surfaces such as fabric, perforated metal and spaced wood slats. These surfaces allow the sound energy to pass through and be absorbed by the material located behind. Figure 5 shows the example of a wood slat panel treatment that effectively screens the acoustic blanket and creates a handsome ceiling in a public area. Perforated metal panels, as shown in Figure 6, are commonly used to create a certain finish appearance. For best results, the material should be as thin as possible, with the smallest hole diameter and the greatest open area (the greatest number of holes). Some absorptive materials are attractively designed to be exposed to view, such as normal suspended ceiling tiles. Generally, thicker porous materials provide better sound absorption. 5/8-inch thick ceiling tiles have an NRC of 0.50 when mounted in a lay-in grid ceiling. A 1- inch thick glass fiber ceiling tile can have an NRC rating of 0.80 or greater. Figure 7 illustrates the appearance of a suspended acoustical tile ceiling. Another approach to adding acoustic absorption to the space is to suspend acoustic baffles as shown in Figure 8. Page 8

9 Figure 5. Wood slat ceiling. Figure 6. Acoustical perforated-metal deck. Page 9

10 Figure 7. Lay-in acoustical ceiling tile. Figure 8. Suspended acoustical baffles. Page 10

11 Open-cell foam panels are effective sound absorbers because they have increased surface area due to the contoured surface of the foam. Figure 9 illustrates an application near an open copy machine area. Figure 10 shows another type of fabric-covered absorptive material. Figure 9. Open-cell acoustical foam. Figure 10. Quilted sound screen. Page 11

12 5. SOUND INSULATION Everyone has experienced unwanted sound intrusion a television in the next room, a loud neighbor walking on the floor above, or a jet flying over. Measures are often required to reduce intrusive noise. One of the most essential techniques in acoustics is reducing the transmission of sound through solid barriers in buildings. This form of sound reduction is referred to as Sound Insulation. 5.1 Principles of Sound Insulation The reduction of sound energy from one building area to another by absorbing it or reflecting it with an intervening solid panel of material is called sound transmission loss (TL). Typically, building materials attenuate more high frequency noise than low frequency noise. The higher the mass or weight of a wall, the more force is required to make it vibrate. For this reason a massive wall has higher TL at all frequencies than a lighter panel. Another way to increase the transmission loss of a panel or construction, such as a wall, is by increasing its thickness and isolating one side of the construction from the other. This is commonly done by using two panels separated by an air cavity, and is known as a dual panel partition. Doubling the air space width increases the TL by about 5 db. Usually, the dual panel approach is more effective and lower cost than increasing wall mass. These sound reducing partitions are needed between spaces with different acoustic requirements or spaces that require acoustic privacy. They are also necessary in some cases as part of the exterior building envelope, if environmental noise at a site is a particular concern. Walls, floors and ceilings enclosing spaces where unwanted noise is generated, such as mechanical rooms, normally require a high standard of sound reduction. 5.2 Sound Insulation Construction In the U.S., the standard way of describing sound isolation of constructions is a metric called STC, or Sound Transmission Class. The STC rating of a wall, floor or ceiling is determined by the components of the construction and how they are assembled Wall Construction A standard partition used to separate rooms in a building is typically a single stud wall and one layer of gypsum board on each side, and it has an STC rating of 35. The acoustic performance of the standard wall can be improved by using light gauge (25 gauge) metal Page 12

13 studs instead of wood studs. There are some conditions in a library where more sound isolation will be required, which can be accomplished by adding insulation within the wall cavity, providing a second layer of gypsum board on each side of the partition, or possibly using staggered stud construction. These program areas include conference rooms and offices requiring confidential speech privacy, where STC ratings in the range of STC are recommended. To control noise transfer from rooms having amplified sound systems such as meeting rooms into other library spaces, the surrounding walls should have a minimum rating of STC These wall constructions are illustrated in Figure 11. Figure 11. Typical wall construction and sound insulation ratings. Wall Type Description STC Rating 2X4 wood studs with 5/8 gypsum board on each side. 35 2X4 metal studs (3-5/8 ) with 5/8 gypsum board on each side. 39 2X4 wood studs with 5/8 gypsum board on each side. Add 2 inches of fiberglass batt insulation. 40 2X4 wood studs staggered on either edge of a 2X6 wood plate, with two layers of ½ gypsum board on each side. Add 3-1/2 inches of fiberglass batt insulation. 55 Page 13

14 It is important to note in general that the high STC rating of any wall construction can be compromised in a number of ways. Care should be taken during actual construction of these sound-rated partitions to ensure that common construction errors do not occur. These compromising circumstances could be the following: 1. Air or sound leaks through cracks. A small air gap can completely compromise the effectiveness of the wall construction. Long cracks, such as those that normally occur at the base and top of a wall are especially detrimental. For this reason, flexible acoustic caulking should be used at the perimeter of a sound partition to seal all edge cracks. 2. Air or sound leaks through normal openings in the wall. Electrical and data outlet boxes or other penetrations of the wall for plumbing or sprinkler piping must also be carefully sealed with flexible acoustic caulking. A common error is to place electrical outlet boxes for two rooms back-to-back in the intervening sound partition. These boxes should be located in different stud spaces to prevent sound transfer between the rooms. 3. Structural connections between double stud partitions. The wood studs in each partition frame of a double stud wall must not be structurally coupled to the other frame in any way. No plumbing or electrical lines should be located in the open space of the air gap between the two partition frames. 4. It is important to seal both faces of a concrete masonry wall with paint or plaster in order to control possible sound leaks. In meeting rooms and classroom spaces in libraries, movable partitions are often considered as solutions for flexibility in space utilization. The sound insulation properties of these walls are always an important issue to ensure that sound from a resulting adjacent area is not distracting. There are two types of operable partitions; accordion and folding panel, and these are illustrated in Figure 12. Most accordion walls are not tested for sound isolation, and are intended for visual rather than acoustic privacy. However, some manufacturers have made modifications to their standard products and can achieve sound isolation ratings of 30 to 37 when installed in a building, which is still a marginal performance. (This number represents the equivalent of an STC metric, but accounts also for field conditions of the building and space, not just the laboratory-tested properties of the partition itself.) Panel operable walls provide better sound insulation than accordion partitions because they are heavier and their perimeter seals are more effective. However, even the best models have only moderate sound isolation ratings (42 installed in the building is a typical rating). Page 14

15 Operable partitions can be electrically or manually operated. Electrically operated doors move into position and back into storage automatically with the flip of a switch, while manually operated doors may take twenty minutes or more to move into place. Manually operated walls are more reliable for sound insulation than electrically operated doors, because they have special hardware for compressing the perimeter seals. Figure 12. Accordion-type operable wall (left) and Panel-type operable wall (right). Note in the illustrations of Figure 12 that a plenum barrier is installed above the operable wall, extending from the top of the wall to the underside of the structure above. This plenum barrier is required in order that the sound insulation value of the wall is maintained, and not short-circuited through sound travelling over the top of the partition Floor Construction Floor and ceiling assemblies perform two acoustical functions. Like walls, they provide acoustical separation between adjacent spaces (airborne sound insulation), but they also reduce the sound of footfalls and other impact sounds from an upper floor (impact insulation). Impact insulation and airborne insulation can be upgraded by decoupling ceilings from the structure and by altering floor finishes. A base assembly consisting of plywood subfloor, joists and gypsum board can be upgraded from STC 37 to STC 58 by adding a lightweight concrete topping slab, fiberglass batt insulation, resilient channels and a second layer of gypsum board, as illustrated in Figure 13. Page 15

16 The concrete topping slab reduces impact noise from footsteps heard in the space below. Using a carpet and pad or a resilient floor underlayment improves the impact insulation. Figure 13. Wood framed floor and ceiling construction having an STC rating of MECHANICAL AND ELECTRICAL SYSTEMS NOISE CONTROL When designing a building, it is important to control the noise and vibration of its mechanical and electrical equipment. Without adequate consideration during design, the very equipment that provides thermal comfort and electrical power can generate annoying noise and vibration. Proven techniques are available for mitigating noise and vibration from this equipment. The recommended acoustical design sequence for a building project is: Select noise criteria for each space in the building. Organize spaces to avoid adverse adjacencies of noisy equipment with quiet spaces. Provide adequate noise and vibration control for equipment. 6.1 Space Planning Space planning can be the most cost-effective noise control technique. Avoid locating mechanical equipment rooms and electrical transformer rooms near spaces (either vertically or horizontally) that require low background noise levels. If this location is unavoidable, it will be necessary to introduce costly sound isolation methods such as a floating floor as shown in Figure 14 or heavy masonry walls, if proper sound insulation is to be achieved. A floating floor consists of a second concrete slab installed on neoprene pads and a layer of insulation. Page 16

17 Figure 14. Example of a Floating Floor construction. 6.2 Noise Control for Main Building Equipment Large fans used as part of the air conditioning system in a building are sources of a significant amount of unwanted noise. The quietest type of fan that will satisfy the operating requirement should be selected whenever possible to reduce the need for mitigation measures. The cost of mitigation may exceed the cost savings for a less expensive but noisier fan. A down discharge fan on the rooftop should be located only near spaces that allow a noise goal of NC 45 or higher, since noise is inevitably transmitted to the space below, as shown in Figure 15. For such a location, a side discharge fan with long lengths of rectangular ducts on the roof should be used so that as much noise as possible is dissipated before entering the space below. Figure 15. Noise paths for down discharge fan (left) and side discharge fan with long rectangular ducts, in a rooftop fan unit installation. Page 17

18 Fan noise transmitted into a room is generally either duct borne noise or breakout noise as shown in Figure 16. Duct borne noise can be described as fan noise that is carried within a duct and then transfers into a room through a register. Breakout noise is fan noise that passes through the walls of a duct and through the ceiling into a room. Figure 16. Duct borne noise (from the register) and breakout noise (through the walls of the duct). Absorption of fan-generated noise and mitigation of air turbulence are the strategies for reducing unwanted mechanical noise in a building. To reduce fan-generated noise, provide long duct lengths between fans and the nearest air register serving a room and treat the duct internally with duct liner. Fifteen feet of lined duct inserted after the fan can reduce fan noise by 10 db. Air turbulence can be minimized by using ducts with ample cross-sectional area and keeping duct runs as straight as possible. Round ductwork allows very little breakout noise in contrast to rectangular ductwork. Internal duct lining and external insulation do not significantly Page 18

19 reduce breakout noise. Noise between adjacent spaces served by common ducts is known as crosstalk. To reduce crosstalk, main duct runs should be located above corridors, with individual branches extending to each space. Return air transfer ducts to plenum spaces above ceilings should have duct liner installed, and there should be an elbow in the duct as shown in Figure 17. Figure 17. Internally lined return air transfer duct above ceiling (with one elbow) to control crosstalk. Silencers are also called sound attenuators, mufflers, or sound traps. As air flows through silencers, fan noise is reduced. They are usually placed between sections of ducts but can also be located inside an air-handling unit or adjacent to a louver. Duct lagging is usually specified as part of a design or as a retrofit to solve an existing breakout noise problem. As shown in Figure 18, duct lagging may include enclosing the duct in gypsum board or insulation wrapped in sheet lead. Page 19

20 Figure 18. Duct lagging using a gypsum board enclosure (left) or lead-wrapped around insulation (right). Variable speed drives adjust the fan speed to match the ventilation needs of a room. When the fan slows down, the noise level generally decreases. Mechanical variable speed drives can be noisy when speeds are being changed. Electrical variable speed drives and their cabinets are often noisy. The cabinets should be vibration isolated and never attached to a partition adjacent to an acoustically sensitive room. A chiller is the part of the HVAC system that cools the refrigerant, which in turn cools the air. Most of the noise and vibration is generated by the chiller compressors. The tonal noise produced can be intrusive. If chillers are installed adjacent to acoustically sensitive spaces, mitigating measures such as floating floors and double-stud or masonry wall constructions will be necessary. 7. AUDIO-VISUAL (A/V) In libraries, audio-visual design is important for meeting rooms, auditoriums, teleconference facilities, children s theaters, and for multimedia and electronic classroom spaces. There are images, both recorded and real-time, which are viewed by groups of people along with associated sound systems. Audio-visual design is concerned with the conditions and requirements for comfortable viewing, listening, and communicating. 7.1 Auditoriums, Theaters and Large Meeting Rooms Auditoriums, theaters and large meeting rooms in libraries typically have flat floors and movable seats to allow flexibility of use. In children s theaters, where multiple types of uses often occur, this type of flexibility is an important feature. Occasionally, however, an auditorium may have fixed seating and a sloped floor for good viewing. In all cases, the location of projection screens will constrain the seating configuration and location. If a stage Page 20

21 is desired, particularly if musical performances will occur, then the configuration possibilities for the space are as shown in Figures Figure 19. End stage configuration. Figure 20. Corner stage configuration. Page 21

22 Figure 21. Thrust stage configuration. Note that in the thrust stage configuration, which creates a more intimate setting and works well for music performance, the audience members located at the side will not have good viewing of any projected images on the screen. A podium or lectern is often used in conjunction with these spaces. The podium should be located to the left or the right of the screen, and the stage dimensions should be large enough to accommodate it. Detailed design of podiums and operation of A/V equipment is treated below. For viewing typical front projection screens, the configuration of the seating and the screen should conform to the diagram of Figure 22 for video projection. As shown in this figure, the maximum depth of the seating should be eight times the height of the screen for video. Conversely, if the seating capacity is established and the depth is determined, the necessary height of the screen for good viewing from the back row will be one-eighth of the distance from the screen to that row. This will also indicate the height of the room needed to accommodate the screen. It is important to note that in training rooms and other rooms used for projection of datagrade images (numbers and text on a computer screen), the maximum depth of the seating should be only six or less times the height of the screen. Figure 22 also indicates an angle of 30 degrees from the centerline of the screen as the limit for seating in the front rows. Any seats located outside this angle will have relatively poor viewing of the screen from these front rows. Page 22

23 Figure 22. Seating area and projection screen configuration for good viewing. The width of the screen is determined by the aspect ratio desired (ratio of width to height). Normally, an aspect ratio of 1.3:1 is used, which is the same as current television and computer displays. Other design recommendations for the location and configuration of the projection screen are to allow six inches minimum below the ceiling to avoid ceiling reflections, and to set a minimum of four feet from the floor when a flat floor is used. Image projection is usually designed using a projector mounted above or behind the viewing audience and projecting the image on to a screen mounted on the viewing wall. This front projection screen requires a darkened room to some degree for high enough contrast to see the image clearly. The projector is usually mounted close to the ceiling, at a distance of approximately 1.5 times the width of the screen. If a projection booth is used, a larger and more intense type of projector is required. A rear projection screen can be used, where the projector is located in a separate room behind the viewing wall and the image is projected on to the back of a translucent window in the viewing wall. The rear projection screen provides clear images in a relatively lightened room, and the projector and any associated noise is not apparent to the viewing audience. Figure 23 illustrates these three location options of the projector and the dimensional Page 23

24 requirements for front and rear projection. Figure 23. Three typical video projector locations. The loudspeakers for the sound associated with the projected image should be located as close as possible to the projection screen. If the screen is a perforated type, the speakers can be located in the wall directly behind it. For a solid projection screen, the preferred location places the right and left channel speakers close to ear height. These two design approaches are shown in Figure 24. Page 24

25 Figure 24. Loudspeaker locations relative to the projection screen. A perforated screen (left) allows the speakers to be placed in the wall behind at ear level. A solid screen (right) should keep the left and right channel speakers at ear level. In large meeting rooms with a length of 40 feet or more, a speech reinforcement system is often necessary to amplify the speaker s voice so that the entire audience can hear. This implies that a second sound system is required in addition to the loudspeaker system associated with the video images. Microphones must be selected and located so that they adequately pick up the speech or music signal from the lecture or performer. Typically, microphones need to be located very close to the lecturer or performer and may not operate adequately if the microphone is located in the ceiling or on a table 15 feet away from where the speaker is located. Loudspeakers in speech reinforcement systems are typically arranged to provide the best coverage for room occupants. A central cluster groups all of the required loudspeakers at one location in the room as shown in Figure 25. This type of system provides the best match between the visual location of a performance and the audio spatial image. It also provides the most uniform timbre and loudness for the audience in different seating locations. The central cluster system, however, can have serious aesthetic impacts and can only be utilized in rooms with relatively high ceilings. In rooms with low ceilings, a distributed system works well because it uses many loudspeakers distributed throughout the room. Figure 26 shows a typical distributed loudspeaker system. Page 25

26 Figure 25. A central cluster loudspeaker system. The speaker coverage or throw is indicated by the dashed lines. Figure 26. A distributed loudspeaker system. The speaker coverage is directed downward toward the audience and overlaps slightly. Page 26

27 7.2 Teleconference Rooms Teleconferencing systems can be just audio, or both audio and video. For video conferencing, cameras and video monitors or projection facilities are required at each location, and the video images are typically transmitted over high-speed ISDN telephone lines to provide smooth image reception. When a room is designed for video conferencing, it usually has one of two floor plans for effective front camera imaging. One plan has a V-shaped table opened toward the camera to allow more conferees to be viewable and also to allow attendees to see a central screen or monitor. A second type of plan features a smaller curved table where all participants are equidistant from the camera, and therefore all appear to be the same size when the video signal is transmitted. This room configuration is most appropriate for a small group. For both types of floor plans, microphones should be located along the length of the table. Ceiling-mounted microphones should not be used since they pick up room reverberation and background noise. Advances in the technology of large flat wall video screens will ultimately allow attendees at the distant location to become effectively in the room as their images can be projected at life size. 7.3 Electronic Classrooms and Training Rooms Many of the design considerations concerning seating and projection screen configurations that were treated in the previous section apply as well to these types of library spaces. Distance Learning Classrooms are a hybrid between a presentation environment and a videoconferencing environment. An instructor at the front of the room presents material to, and interacts with, both local and distant participants. Both audiences can ask questions and see the response images. Video monitors or projection screens are located at the front of each classroom to display instruction material, and the same type of equipment is located at the rear of the room to allow the instructor to see the distant audience at the same time as the local participants. Usually, a technician is required to operate the camera in order to allow the instructor to move naturally at the front of the room. Training Rooms often require two screens, one that shows the computer screen display and the other that is used for other instructional images. Also, with the emergence of integrated control systems, the instructor can control a wide variety of equipment from an A/V lectern that is typically included in the training environment. This lectern is an island of electronic Page 27

28 playback devices with local controls for remotely located equipment. Only a VCR is included in the lectern to allow local media insertion. Some lecterns have a side-mounted graphics table for presenting flat images; a camera is mounted at the ceiling, directly above the graphics table. Figure 27 illustrates a typical A/V lectern. Figure 27. A/V lectern. 7.4 Lighting in A/V Spaces The general lighting of A/V spaces must be dimmable to allow some form of note-taking illumination. This lighting does not have to be bright, but it should provide fairly even coverage of the entire seating area, with no spill light on the projection screen. Lighting for the speaker or instructor must be carefully designed to avoid interference with the projected image and to allow proper viewing if the image is captured in video format. The presenter must be visible to people in the room and also to viewers of video. Spotlights with tungsten-halogen lamps and louvers, internal shutters or barn doors can be aimed so they light the presenter at a podium while avoiding spill light on to the screen. Borrowing from the lighting techniques used in broadcast TV studios, the presenter should be lit with key light, fill light, and back light. Key and fill lights are positioned 45 degrees to the left and right of the presenter, and 40 degrees above horizontal as measured from the presenter s face. The key light is brighter than the fill light. The backlight is positioned above and slightly behind to light the top of the presenter s shoulders and head. All three lights are needed to make the presenter appear natural on video and to separate the presenter s image Page 28

29 from the background. Room lighting should be controlled by a dimming system with preset scenes for bright and dim settings of the general lighting and independent controls for the front-of-room and podium lights. The preset dimming system allows easy selection of predetermined looks, and it also allows the lighting to be controlled by a touch-screen or other A/V equipment via an A/V lighting interface. Adequate emergency lighting must be provided to allow safe exiting. However, circuiting some of the general lights as night lights that remain on all the time should be avoided because it often will be desired to use the room with all lights off. Separate, normally turned off, emergency lights with battery packs or an automatic emergency transfer switch to bypass the dimming system are two other options. Glossary of Acoustical Terminology Absorption Coefficient Ambient Noise Aspect Ratio A-weighting Breakout Noise Broad-band Noise dba Decibel (db) Frequency The fraction of the incident sound power that is absorbed by a material on a scale from 0 to 1. The background noise, including sounds from many sources near and far, associated with a given environment. The ratio of dimensions usually normalized to the smaller dimension (width to height for screens). A standard frequency weighting that de-emphasizes low-frequency sound similar to average human hearing response and approximates loudness and annoyance of noise. A-weighted sound levels are frequently reported as dba. Noise associated with fan or airflow noise that radiates through the walls of a duct into the surrounding area. Noise comprised of a wide frequency range and not characterized by any tonal component. See A-weighting The measurement unit used in acoustics for expressing the logarithmic ratio of two sound pressures or powers. Typically used to describe the magnitude of sound with respect to a reference level equal to the threshold of human hearing. A descriptor for a periodic phenomenon. The frequency is equal to the number of times that the pressure wave repeats in a specified period of time. Page 29

30 In the case of sound, frequency is measured in units of Hertz (Hz), which correspond to one cycle per second. Hertz Noise Criteria (NC) Curves Noise Reduction Coefficient (NRC) Reverberation Sound Sound Insulation Sound Transmission Class (STC) Sound Transmission Loss (TL) Speech Reinforcement System Vibration Isolation Videoconference Room See Frequency. A set of spectral curves used to obtain a single number rating describing the noisiness of environments for a variety of uses. NC is typically used to rate the relative loudness of ventilation systems. A single number rating of the sound-absorption of a material equal to the arithmetic mean of the sound-absorption coefficients in the 250, 500, 1,000 and 2,000 Hz octave frequency bands rounded to the nearest multiple of Collection of time-delayed sounds following a direct sound that result from reflections indoors. (1) An oscillation in pressure, resulting from molecular motion, in a viscous or elastic medium such as air, water, wood, steel, etc. (2) Sound is an auditory sensation evoked by air molecules vibrating in a frequency range between 20 Hz and 20,000 Hz. (1) The capacity of a structure to prevent sound from being transmitted from one space to another. (2) Insulation used in a wall, floor or ceiling cavity to add damping and decrease transmitted sound. (See Sound Transmission Loss.) A single-number rating derived from laboratory measurement of sound transmission loss. STC is calculated in accordance with ASTM E413, Classification for Rating Sound Insulation. The STC describes the soundinsulating properties in the 100-4,000 Hz frequency range, primarily for assessing speech transmissions through a structure, such as a partition. A laboratory measurement of sound insulation indicative of the sound intensity flow transmitted through a partition without regard to the partition size, usually measured in one-third octave bands. An electronically amplified audio system designed to reproduce speech at a sufficient level for intelligibility to overcome distance and other acoustic limitation. The methods used to reduce vibration in a structure caused by vibrating equipment, including the use of springs and elastomeric materials. A room designed for simultaneous audio and video communication between two groups at different locations. Page 30

31 Reference Sources Charles M. Salter Associates. Acoustics: Architecture, Engineering, The Environment. San Francisco: William Stout Publishers, c1998. The Author Charles M. Salter Associates is an acoustical consulting firm with over 22 years of experience in acoustical and audio/visual design. Charles Salter, the firm s President has taught acoustics for twenty-five years and is an adjunct professor of acoustics at the University of California, Berkeley. For more information, please visit Charles M. Salter Associates on the Internet at Charles M. Salter, P.E. Charles M. Salter Associates, Inc. 130 Sutter Street, 5 th Floor San Francisco, CA Illustrators Michael Flynn Michael Flynn Illustration Edward M. Dean, AIA Page 31