Auditorium Acoustics


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Auditorium Acoustical design consideration

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Auditorium Acoustics

  1. 1. INTRODUCTION The auditorium, as a place for listening developed from the classical open-air theaters. An auditorium includes any room intended for :An auditorium includes any room intended for : - listening to music including theaters - churches - classrooms - meeting halls The design of various types of auditoriums has become a complex problem, because in addition to its various, sometimes conflicting, aesthetics, functional, technical, artistic and economical requirements, an auditorium often has to accommodate an unprecedentedly large audience.
  2. 2. INTRODUCTION …. • In some ways, even the largest hall is no different from the smaller rooms, the basic acoustic criteria are the same. - Must have a low ambient noise level from internal and external sources - Provide a reasonable level of acoustic gain - Provide appropriate reverberation time - Avoid artifacts such as echoes. • Hearing conditions in any auditorium are considerably affected by purely architectural considerations like: - Shape - Layout of boundary surfaces - Dimensions - Seating arrangements - Volume - Audience capacity Defects: 1. Echo 2. Delayed Reflection 3. Sound Shadow 4. Sound Concentration
  3. 3. • Reverberation is an important parameter that helps define the sound quality of an acoustic space. • This is especially true in large halls. • Reverberation time is closely linked to the intended purpose for any room, and to room volume. REVERBERATION AND ECHO • Halls designed for speech have shorter mean reverberation times than halls designed for music performance. • The recommended mean reverberation time increases as a function of room volume.
  4. 4. Large enclosed spaces are all potentially subject to the problem of discrete echoes. The long path lengths and multiplicity of seating positions near and far from the sound source can easily create echo problems. REVERBERATION AND ECHO ….
  5. 5. Sound amplification system are used for the following purpose: • To reinforce the sound level when the sound source is too weak to be heard. • To provide amplified sound for overflow audience. • To minimize sound reverberation. • To provide artificial reverberation in rooms which are too dead for satisfactory listening. REINFORCEMENT BY LOUDSPEAKER satisfactory listening. • To operate electronic organs, chimes etc. TYPES OF LOUDSPEAKER SYSTEM Three principal type of loudspeaker system are available: 1. The centrally located system with a single cluster of loudspeakers over a sound source. This system gives max. realism as the amplified sound comes from the same direction as original sound.
  6. 6. TYPES OF LOUDSPEAKER SYSTEM …. 2. The distributed system, using a number of overhead loudspeakers located throughout the auditorium. This system should be used when: • Auditorium height is too low to install central system. • When majority of listeners do not have an adequate sight line of central• When majority of listeners do not have an adequate sight line of central loudspeakers. • When sound has to be provided for overflow audience. • In large halls. 3. The stereophonic system, with two or more clusters of loudspeakers around the proscenium opening or the sound source. Stereophonic system preserves the illusion that, the sound is coming from the original , unamplified source.
  7. 7. PROBLEMS ASSOCIATED WITH LOUDSPEAKER SYSTEM 1. Audience will hear two sounds, arriving at two time. This difference should not be more than 1/30 sec. 2. When loudspeaker is placed halfway down2. When loudspeaker is placed halfway down the a large auditorium. Audience will hear loudspeaker first and direct sound as a weak echo. This problem can be overcome by introducing a delayed mechanism in loudspeaker system
  8. 8. • Speech Intelligibility = Power + Clarity • POWER is affected by : - Distance from speaker - Directional relationship to speaker - Audience absorption of direct sound - Reinforcement by reflectors - Reinforcement by loudspeakers - Sound shadows • CLARITY is affected by : - Delayed reflections : Echos, Near Echos, Reverberation - Duplication of sound source by loudspeakers - Ambient Noise - Intrusive Noise
  9. 9. VOLUME • For unamplified speech, it is often necessary to limit the overall room volume. This is because a large volume requires more speech power than a small room. • This volume minimization is contrary to rooms designed for music, where a relatively large volume is desirable. • In a face-to-face conversation, an unamplified talker may generate a SPL level of about 65 dB. This level decreases 6 dB for every doubling of distance. Sound is also attenuated as it travels through the hall because of air absorption. • To support audible levels, the audience area must be placed as close as possible to the speaker. This minimizes sound attenuation, provides a more direct sound path, and also improves visual recognition which improves intelligibility.
  10. 10. ROOM SHAPE • The talker-to-audience distance can be minimized by carefully considering the room geometry. • A rectangular shoebox-type hall, with NORMAL SURROUND A rectangular shoebox-type hall, with the stage across one narrow end, may be excellent for music where an audience can be seated farther away and a greater ratio of reverberant sound is desirable. • However, a rectangular geometry is only suitable for a relatively small speech hall.
  11. 11. ROOM SHAPE • For greater seating capacity, the side walls should be splayed from the stage. • Splayed side walls allow greater seating area that is relatively close to the stage. • The splayed walls can usefully reflect sound energy to the rear of the hall. • A side-wall splay may range from 30° to 60°, the latter is considered a maximum angle, given the directionality of speech. Generally, fan-shaped halls are not used for musicgiven the directionality of speech. Generally, fan-shaped halls are not used for music performance.
  12. 12. ABSORPTION • In small speech halls, the majority of absorption is provided by the audience, therefore, the room surfaces can be relatively reflective. In larger halls, where there is greater room volume per seat, relatively greater room absorption is needed. • Beneficially, a reflective front stage area provides strong early reflections that are integrated with the direct sound and enhance it. On the contrary, strong late reflections and reverberation, such as from rear walls, would not be integrated and may produce echoes. • To accommodate this, the stage area and front of the hall are made reflective and absorption is placed in the seating area and rear of the hall.
  13. 13. CEILING • In many large halls, ceiling reflectors, sometimes called clouds, are used to direct sound energy from the stage to the seating area. • Both dimensions of a square reflecting panel should be at least five times the wavelength of the lowest frequency to be reflected. • When ceilings are high, care must be taken to ensure that path-length differences between direct and reflected sound are not too great, anddifferences between direct and reflected sound are not too great, and particularly should not exceed 20 msec. • In some cases, clouds are made absorptive, to avoid late reflections.
  14. 14. FLOORS • A sloping (raked) floor allows a more direct angle of incidence which in turn allows less absorption. Generally, the slope of an auditorium floor should not be less than 8°. • The floor of a lecture-demonstration hall might have a 15° angle of inclination. • Staggering of seats is also recommended. WALLSWALLS • Because of its potential to create undesirable late reflections, the rear wall of a large hall requires special attention. • Reflections from the rear wall would create a long path-length difference to a listener at the front of the hall. This can result in audible echoes, particularly because of the otherwise low reverberation level. • A reflective concave rear wall would also undesirably focus sound. • For these reasons, the rear wall of a large hall is usually absorptive. • In some cases, when added absorption is undesirable because of decreased reverberation time, reflective diffusers can be placed on the rear wall.
  15. 15. TYPES OF AUDITORIUM 1. FOR SPEECH - Conference Hall - Lecture Theatre - Law Court 2. FOR MUSIC - Concert Hall- Concert Hall - Music Practice Room 3. MULTI-PURPOSE - School Assembly Hall - Town Hall
  16. 16. DESIGN DATA
  17. 17. Developed Floor Slope For Unobstructed View
  18. 18. AUDITORIUM
  21. 21. ARCHITECTURAL DESIGN FOR CONCERT HALL • The first complexity rests with the music itself. • Different styles and culture of music have different acoustical requirements. • Perhaps the most difficult aspect of hall• Perhaps the most difficult aspect of hall design is the ambiguity of the goal itself. BALCONY • A balcony can be used to decrease the distance from the stage to some seating areas, and to provide good sight lines.
  22. 22. BALCONY …. • Generally, the balcony overhang depth should be less than twice the height of the balcony underside. • Ideally, the depth should not be more than the height.than the height. • Deep bal. can create acoustical shadows in the seats underneath bal. • In addition, reflecting surfaces on the ceiling and side walls, as well as the underside of the balcony, should be designed to add as much reflected sound as possible to the seating areas on the balcony and under it, to supplement the direct sound from the stage.
  23. 23. BALCONY • The front of a balcony parapet should be designed to avoid reflections that could affect sound quality in the seating areas in the front of the hall. This is particularly true when the plan view of the balcony has a concave shape. CEILING Ceiling height is usually determined by the overall room volume that is• Ceiling height is usually determined by the overall room volume that is required. • Ceiling height should be about one-third to two-thirds of the room width. The lower ratio is used for large rooms, and the higher ratio is used for small rooms. • A ceiling that is too high may result in a room volume that is too large, and may also create undesirable late reflections.
  24. 24. CEILING • To avoid potential flutter echo, a smooth ceiling should not be parallel to the floor. • In many halls, the ceiling geometry itself is designed to direct sound to the rear of the hall, or to diffuse it throughout the hall • Concave surfaces such as domes, barreled ceilings, and cylindrical arches should be avoided because of the undesirable sound foci they create.
  25. 25. WALL • The rear wall must avoid any large, unbroken concave geometry. • Side walls must avoid parallelism. This can be avoided by tilting or splaying wall surfaces. • These angles can also be advantageously used to direct reflected sound to the audience seating area, and to provide diffusion.the audience seating area, and to provide diffusion. • Any surface that unavoidably introduces concave geometry or an undesirable angle should be covered with absorptive material.
  26. 26. FLOOR • In halls designed for either music or speech, a sloping (raked) floor is desirable especially for large halls. • In halls designed for either music or speech, a sloping (raked) floor is desirable especially for large halls. • A sloping floor improves sight lines, and also improves fidelity in the seating area. • When sitting on a sloping floor, the listener receives more direct sound than would be available on a flat floor.
  30. 30. RECORDING STUDIO • Good recording studio? • Only one ultimate criterion—the acceptability of the sound recorded in it for its intended audience. AMBIENT NOISE • Studio must have low ambient noise levels.• Studio must have low ambient noise levels. • Many noise and vibration problems can be avoided entirely by choosing a site in a quiet location. • The maximum external noise spectrum must be reduced to the background noise criteria goal within the space by the transmission loss of the walls, windows, and doors. • A reinforced concrete building, while providing high transmission-loss partitions, also efficiently conducts noises throughout via structural paths. • The HVAC (heating, ventilating, and air-conditioning) system must be designed to provide the required noise criteria goals.
  31. 31. DIRECT AND INDIRECT SOUND • Sound picked up by a microphone in a studio consists of both direct and indirect sound. • Direct sound is observable a short distance away from the source, but farther away, the indirect sound dominates. • Sound picked up by a microphone would, for• Sound picked up by a microphone would, for the first few milliseconds, be dominated by the direct component, after which the indirect sound arrives at the microphone as reflections from room surfaces. • Another component of indirect sound results from room resonances. • Resonances dominate the low frequency region in which wavelengths of the sound are comparable to room dimensions. • Indirect sound also depends on the materials of construction, such as doors, windows, walls, and floors. These too are set into vibration by sound from the source
  32. 32. OPTIMAL REVERBERATION TIME • If the reverberation time is too long, speech syllables and music phrases are masked and a deterioration of speech intelligibility and music quality results. If reverberation time is too short, music and speech lose character and suffer in quality, with music suffering more. • There is no specific optimum reverberation time, because many other factors are involved. e.g. is it a male or female voice, slow or fast talker, English or German language, vocal or instrumental, solo flute or string ensemble, hard rock or a waltz? DIFFUSION • Diffusion contributes a feeling of spaciousness through the spatial multiplicity of room reflections and the control of resonances. • Distributing the absorbing material is a useful means of not only achieving some diffusion, but increasing the absorbing efficiency as well.
  33. 33. ROOM SIZE & SHAPE The size and shape of a room determines its natural resonances - often called room modes. Every rectangular room has three sets of primary modes, with one each for the length, width, and height. Generally speaking, larger rooms are better acoustically than smaller rooms. Another important factor in the design of studios is the ratio between the length, width and height.length, width and height. The worst shape is a cube, because all three dimensions resonate at the same frequencies. HEIGHT WIDTH LENGTH 1.00 1.14 1.39 1.00 1.28 1.54 1.00 1.60 2.33 Ideal Ratios RATIO SMALL STUDIO MEDIUM STUDIO LARGE STUDIO HEIGHT (ft.) 1.00 8.00 12.00 16.00 WIDTH (ft.) 1.28 10.24 15.36 20.48 LENGTH (ft.) 1.54 12.32 18.48 24.64 VOLUME (cu.ft.) 1000 3400 8000 Sizes
  34. 34. Thanks….