Room Acoustics Lecture_Lec 1Room Acoustics Lecture_Lec 1Room Acoustics Lecture_Lec 1.pptx

THIS IS A PAPER FOR ACOUSTICS

1. ROOM ACOUSTICS

2. Room Acoustics Objectives and Considerations Objectives: Creating suitable interior acoustic environment Providing the best condition for both production & reception of desirable sound Creating good condition for hearing of speech / music Considerations: Reverberant field / Enclosure Satisfactory production of sound Satisfactory distribution of sound Exclusion of unwanted sound /Noise/

3. Pattern of Distribution of Sound in an Enclosure The behavior of sound paths inside an enclosed space can be affected by the mechanisms of reflection, absorption, transmission and diffraction. Reflection and absorption play the largest roles in room acoustics. The distribution and decay of sound energy in an enclosure depends mainly on the nature of interior materials – roughness/smoothness/porosity/hardness and the angle of the surfaces .

4. Reflection Reflection is the return of a sound wave from a surface; the angle of incidence will equal the angle of reflection. When an array of suspended panels is used to direct reflected sound energy toward the audience, the individual panels should be of varying sizes to prevent creating a "rasping" sound.

5. Diffusion Diffusion is the scattering or random redistribution of a sound wave from a surface. The direction of the incident sound wave is changed as it strikes a sound-diffusing material. Diffusion is an extremely important characteristic of rooms used for musical performances. When satisfactory diffusion has been achieved, listeners will have the sensation of sound, coming from all directions at equal levels.

6. Diffraction Diffraction is the bending or "flowing" of a sound wave around an object or through an opening . It helps the sound to "bend around" the obstacles. 

7. Absorption is a reduction in the sound energy reflected from a surface. It is a major factor in producing good room acoustics, especially when controlling reverberation. The effective absorption of a particular surface depends on both the absorption coefficient of the surface material and the area of that particular surface exposed to the sound. Absorption of surface = area of surface x absorption coefficient of that surface. Unit: m2 Sabin’s or ‘absorption units’ Total absorption = Σ ( area x absorption coefficient) Absorption

8. Types of absorbers Panel / membrane are absorbers for lower frequencies- 40-400Hz. Cavity absorbers are volume resonators for specific lower frequencies. Practical absorber - combination of several methods. E.g. acoustic tiles- the basic material of the tile, such as fiberboard, is porous and acts as an absorbent for higher frequencies. The tile material may also be drilled with holes which then act as cavity absorbers. Absorption

9. Absorptive surfaces are primarily used for the following applications: - Reverberation Control: reduction of reverberant sound energy to improve speech intelligibility and source localization. - Sound Level Control: reduction of sound or noise buildup in a room to maintain appropriate listening levels and improve sound isolation to nearby spaces. - Echo and Reflection Control: elimination of perceived single echoes, multiple flutter echoes, or unwanted sound reflections from room surfaces. - Diffusion Enhancement: mixing of sound in a room by alternating sound absorptive and sound reflective materials.

10. Absorptive surfaces be any of three basic types of materials: Porous materials include fibrous materials, foam, carpet, acoustic ceiling tile, and draperies that convert sound energy into heat by friction. Example: fabric-covered 1 in. (2.5 cm) thick fiberglass insulation panels mounted on a wall or ceiling . - Vibrating panels thin sound-reflective materials rigidly or resiliently mounted over an airspace that dissipates sound energy by converting it first to vibrational energy. Example: a 1/4 in. (6 mm) plywood sheet over an airspace (with or without fibrous materials in the airspace). - Volume resonators - materials containing openings leading to a hollow cavity in which sound energy is dissipated. Example: slotted concrete blocks (with or without fibrous materials in the cores).

11. Distribution and Decay of Sound Energy Nature of interior surfaces characterized by porosity, smoothness, being reflective or absorptive and Angle of the interior surfaces like walls and ceilings are responsible for the distribution and decay of sound energy in the interior. Reflectors near the source properly distribute the sound energy while absorbers significantly reduce or kill the sound energy. Reflecting Surfaces and Pattern of Reflected Sound Sound Reflectors An effective sound reflector has a hard surface, such as thick plaster, double-layered gypsum board, sealed wood, or acrylic plastic, and is significantly larger than the wavelength of sound it is designed to reflect. In the church example shown below, the organ and console are located within the sanctuary, not in a gallery or other deep recess.

12. Section View of Church Reflectors in order of increasing effectiveness for distributing sound are concave, flat, and convex sound-reflecting surfaces.

13. Flat Reflectors as building elements Flat, hard-surfaced building elements, if large enough and oriented properly, can effectively distribute reflected sound. The reflector shown below is tilted slightly to project sound energy toward the rear of an auditorium.

14. These figures make a direct comparison between the reflections from flat, convex and concave surfaces. The wave front from the convex surface is considerably bigger than that from the flat surface, and is diminishing. It follows then that sound waves reflected from convex surfaces are more attenuated, and therefore weaker, than sound waves reflected from a flat surface. Similarly , sound waves reflected from a concave surface are more considered and therefore of greater intensity. It also be noted that sound waves reflected from a concave surface, unlike those reflected from a flat of convex surface, may actually increase in intensity the further they travel. In the example illustrated they will pass through a region of focus in which the sound heard may be as loud as that heard near the source.

15. Ceiling Shape The preferred ceiling shape and height depend on the intended use of the room . For example, ray-diagram analysis indicates that the hard, sound reflecting flat ceiling provides useful sound reflections which cover the entire seating area in a lecture room . Useful sound reflections for speech are those which come from the same direction as the source and are delayed by less than 30 ms. However, by carefully reorienting the ceiling the extent of useful ceiling reflections can be increased so that, the middle-rear seats actually receive reflections from both ceiling planes.

16. Sloped Ceiling For concert halls, where long reverberation is a design goal, high ceilings are preferred and all walls should be sound-reflecting. In addition, ceilings that are diffusing can improve audibility of lateral sound by diminishing the strength of ceiling reflections. Flat Ceiling

17. Convex reflector : Widely spread reflected sound Enhances diffusion most effective Concave Reflector : Focused reflection Causes hot spots & echoes Poor distributors

18. Concave and Convex Reflector as building elements Convex, hard-surfaced building elements, if large enough, can be most effective as sound-distributing forms. The reflected sound energy from convex surfaces diverges, enhancing diffusion, which is highly desirable for music listening. In addition, reflected sound from convex surfaces is more evenly distributed across a wide range of frequencies. Reflecting surfaces in a room are used to help the even distribution of sound and to increase the overall sound levels by reinforcement of sound waves. There can also be unwanted reflections such as echoes. Concave sound-reflecting surfaces (such as barrel-vaulted ceilings in churches and curved rear walls in auditoriums) can focus sound, causing hot spots and echoes in the audience seating area. Because concave surfaces focus sound, they also are poor distributors of sound energy and therefore should be avoided where sound-reflecting surfaces are desired (e.g., near stage, lectern, or other source locations in rooms).

19. Reflection of limited size For effective sound reflection to occur a reflector must be large in relation to the wavelength of the sound and, in all cases, the power of the reflected sound will be affected by diffraction at the edges of the reflector. Figure below shows sound waves reflected from reflectors of different widths together with the diffracted waves (or wave fringes) which develop from the edges of the reflectors. The frequency of the sound is the same in both cases, as indicated by wavelength, and therefore the degree of diffraction is also similar. However, in both cases, the energy in the diffracted sound waves is being extracted from the main reflected waves and, because the latter are smaller in the second example, the effect of this loss will be greater. For a given frequency therefore small reflectors are less efficient than large ones. When reflectors are used in auditoria to reinforce sound they must therefore be of adequate size. Nevertheless, since the intelligibility of speech is much more dependent on hearing the middle and high frequencies than on the reception of low frequencies, reflectors of about five times the wavelength of middle frequencies (about 3m) are very effective in reinforcing speech sounds.

20. Reflections form re-entrant angles Sound entering a right-angled corner of a room will be reflected back towards the source if the adjacent surfaces are of a reflective material. This is shown in the figure below. Such reflections are often the case of Disturbing echoes. As in case of all reflections, the phenomenon is frequency dependent, that is, related to wavelength and the dimensions of the reflecting surfaces. Quite small areas of reflecting material in the corner of a room, for example between ceiling and wall, can, however, result in high-frequency echoes. To prevent this return of sound towards the source the corner can, however, be modified in any of the three ways as shown in the figure above a) It may be other than a right angle b) One surface may be made absorbent, or c) One Surface may be made Dispersive. Absorbent or dispersive treatment, if employed for this purpose, must, however, be taken right into the corner, as shown.

21. Useful Reflections and Sound Reinforcement Useful reflections Useful reflections are found near the source ;1 st order reflection carry information about actual location of walls around receiver , 1 st and 2 nd order reflection are early reflections . Useful sound reflections come from the same direction as the one coming from the source have generally a delay of less than 30m.s. (milliseconds). Lateral reflections are early sound reflections from side walls can add strength to the direct sound. Designing for strong early reflections can increase clarity, sound strength and spaciousness.

22. In most rooms accommodating more than say 25 listeners, it is of relevance to consider how early reflections can help distribute the sound evenly from the source(s) to the audience. This will increase the early reflection energy and so improve clarity/intelligibility and perhaps even reduce the reverberation time by directing more sound energy towards the absorbing audience. Unwanted reflections come at a distance from the source. Sound reinforcement can be understood as adding strength to direct sound, either naturally, physically or through electronic equipment. Outdoors, with absence of reflecting surfaces sound energy falls with a distance. But in an enclosure reflection reinforces the direct sound

23. Pattern of Distribution of Sound in an Enclosure Sound path: can be generally defined as the distance and direction sound travels during propagation . Sound paths can be represented by ray diagrams . Ray diagrams are used to study the distribution of sound and to identify surfaces which may produce direct and reflected sounds and echoes.

24. Reflected Path Lengths Reflected sounds have a significant impact on the intelligibility of the speaker. After hearing the direct sound, an audience hears a series of reflections or echoes. When the reflections arrive late, they are perceived as a discrete echo and obscure the intended message. Therefore, the difference between reflected and direct sound paths should be less than 34-feet /10.3m/for adequate speech intelligibility. Table 3 provides a design guide for evaluating the listening conditions by understanding sound paths and their corresponding time delays. Table 3 Optimal Path Lengths & Sound Delays

25. Path difference = Reflected path – direct path Front location #1: Path difference = (11 + 18) – 12 = 17 ft. →Excellent speech and music because path difference is less than 23 ft .

26. Example Shown here is an auditorium section with sound path differences calculated to front and middle- rear audience locations from a typical source location. Path difference = Reflected path – direct path Front location #1: Path difference = (11 + 18) – 12 = 17 ft. →Excellent speech and music because path difference is less than 23 ft . Middle location #2: Path difference = (16 + 26) – 33= 9 ft. → Excellent speech and music because path difference is less than 23 ft.

27. Sound Paths from Stage in Auditorium The listener in the auditorium will hear the direct sound first and then, after the initial-time-delay gap, reflections from the walls (path 1 on the drawing), ceiling (path 2), stage enclosure (path 3), and so on. These arrival times and sound levels are indicated by the bars on the sound level vs. Time graph is shown below. Sound Level vs. Time Graph for Auditorium

28. Sound path in auditoriums The initial-time-delay gap is the time interval between the arrival of the direct sound and the first reflected sound of sufficient loudness. It should be less than about 30 ms (path difference < 34 ft /10m) for good listening conditions because sounds within this time interval can coalesce as one impression in a listener's brain. Early-arriving reflected sound energy is important for clarity and definition of music. "Early" sound is usually defined as the direct and reflected sound arriving within the first 80 m s. Clarity can be defined as the ratio of early sound energy to late or reverberant sound energy. Auditoriums with narrow shapes support direct and early-reflected sound because the initial-time-delay gaps will be short. In the design of auditoriums, ray diagrams can be used to determine initial-time-delay gaps. The initial-time-delay gap also strongly influences a listener's perception of the size of an auditorium (called intimacy).

29. 0.34 is sound traveled by 1mili second Direct Sound path=D Sound path for reflection=(R1+R2) Time delay in milliseconds= (R1+R2)-D 0.34