ZHOU SHAO KANG_1155021643_SENV7700

  • View
    9

  • Download
    0

Embed Size (px)

Text of ZHOU SHAO KANG_1155021643_SENV7700

  • Abstract

    This project is intended to design a concert hall for CUHK. The location of site is besides Sir Run Run Shaw Hall in

    central campus. Sir Run Run Shaw Hall is a multi-purpose cultural centre of The Chinese University of Hong Kong. It

    provides venue and services for large-scale functions such as congregations, examinations, assemblies, meetings

    and classes. However, acoustic property of concert hall is quite different from multi-purpose hall, sound intelligibility

    and reverberation time requirement is different. Thus, in this paper I want to design a concert hall for functional

    supplyment of Sir Run Run Shaw Hall in CUHK from acoustic point of study. I will further how research shape and

    material affects acoustic quality of concert hall in this paper.

    Keyword: Intelligibility, reverberation time, shape, material

    Fig. 1 Location of proposed concert hall in CUHK

    1. Review of basic acoustic definition in concert hall

    Before starting to design of concert hall, the preparation work is to review some basic acoustic definition of concert

    hall.

    Reflected Sound

    There are 3 categorizes of reflected sound: (1) early and middle-reflected sound; (2) reverberation (late-reflected

    sound); (3) standing waves. In concert design, the liveness of a concert hall is decided by early reflections, which

    contribute more to perception of reverberation. Early and middle reflection occurs within the 250msec after arrival of

    the direct sound. Early sound is considered to be 40msec after arrival of the direct sound for speech while for music

    80msec is more appropriate. The number and quantity of early and middle reflection delivered to any particular

    listening location depends largely on the rooms shape. For this reason, geometric analysis, which involves the study

    of reflected sound propagation paths modeled as rays radiating from the source of sound, is particularly useful for

    tracing echo paths and for studying the uniformity of early reflected sound in medium and large size spaces.

  • Echoes

    Echoes are reflections that can be heard distinctly and separately from the early reflected and reverberant sound.

    Design criteria for echo detection depend on the type of space being constructed. For most general purposes involving

    speech communication, echoes are normally heard due to intense reflection arriving 40 msec and later after the direct

    sound signal has reached the listener. In other words, the difference in path length between the direct sound and the

    reflected sound is at least 13.8m, corresponding to a propagation time of 40 msc or greater. In concert hall, echoes

    are most commonly detected in the front rows of an auditorium and on stage. This results from the front row being

    farthest from the rear wall, thus generating the largest path length difference between direct sound and the sound

    reflection from the rear wall or the combination of the ceiling and the rear wall.

    Reverberation

    Reverberation is directly proportional to room volume, inversely proportional to the surface area, and inversely

    proportional to the amount of sound-absorbing material. Excess reverberation results in blurring of sounds and can

    reduce speech intelligibility. It is possible to reduce reverberation by the following means: (1) adding sound-absorbing

    material; (2) reducing room volume; (3) increasing surface area.

    Reverberation time is measure used to quantify reverberation and is the time required for sound reflections to decay

    60 dB.

    Acoustic Shadow

    Due to some obstruction, some space cant get first reflected sound. These area

    calls acoustic shadow area (Refer to Fig.2). Acoustic shadow usually appears in

    space underneath of terraced seating which needs to be avoided in concert hall

    design.

    Fig. 2 Forming of acoustic shadow

    Focusing

    The cardinal rule in the design of rooms is to avoid sound reflections that focus in the plane of listening. A focusing

    surface concentrates sound energy, which may then be intense enough to be perceived as an echo. Surfaces such as

    domes, barrel-vaulted ceilings, and concave rear walls can cause sound focusing and are notorious for generating

    strong echoes

    2. Define volume and area of concert hall

    First I would need to define the volume and dimension of concert

    hall. Fig. 3 depicts typical volume requirement for different types of

    performance spaces. We can see from fig. 3 the volume of concert

    hall is 10-11.4 times required no. of seats.

    The proposed concert hall is supposed to accommodate 800 seats.

    The volume of concert hall V=800*10=8000m

    Ceiling height also plays an important role in delivering early

    reflections in music halls. Typical ceiling heights yielding an adequate

    density of early sound are: (1) 500 seats: 9-11m; (2)1500

    seats:13.5-16m; (3)2500 seats:18-21m. (Source from: Architrectural

    acoustics principles and practice, 1999)

    Typically, the maximum ceiling height (h) can be determined from

    h= [3(V/T500)] *0,85

  • We can get expected best reverberation time of 8000m at 500 Hz is 1.6s (from Fig.4). h=[3(8000/1.6)] *0,85,

    h=14.1.

    So the maximum ceiling height of 800 seats is 14.1m

    The minimum area of proposed concert hall S=V/h, S=8000/14.1=567m

    So the minimum area of proposed concert hall for 800 seats is 567m.

    3. Geometric analysis and form test

    From the review part, we know that the liveness of a concert hall is decided by early

    reflections. Early reflections for concert hall are 80msec after arrival of direct sound. The

    shape and dimension decides the early reflections. So I want to have geometric test and

    analysis. The geometric test and analysis below are several conventional concert hall

    shapes.

    Option 1: 25m x 10m rectangular

    The computer simulation shows distribution of first reflected sound, the auditorium gets first reflected sound from the

    two side lateral wall. I also use hand sketch to show relationship between rectangular shape and first reflection basic

    on computer simulation result.

    Option 2: irregular polygon

    The computer simulation shows distribution of first reflected sound, the auditorium gets first

    reflected sound from the two side lateral wall. This shape can bring listeners closer to stage.

    Hand sketch is summary of computer simulation. It shows that first reflected sound at the

    back of auditorium is weaker than that at the front. It would be not easy for central part of

    concert hall to get first reflected sound from two side lateral wall.

    Option 3: sector

    The computer simulation shows distribution of first reflected sound. Hand sketch is

  • summary of computer simulation. We can see both from computer simulation and hand sketch that the center of

    auditorium is very difficult to get first reflected sound from lateral wall. With the angle of sector increases, it is more and

    more difficult to get first reflected sound from lateral wall.

    Option 4: irregular polygon

    We can see from the simulation that the central part of concert hall is

    also hard to get first reflected sound from lateral wall. Some

    amelioration is to provide corrugated shape for lateral wall. So central

    part can get first reflected sound from lateral wall because the

    orientation of reflector has been changed.

    Option 5: hexagon

    We can see from the simulation below that the central and front part

    of concert hall is also hard to get first reflected sound from lateral wall,

    in hexagon shape. But if we can see if we adopt corrugated

    arrangement of lateral wall, central and front part can get first

    reflected sound from lateral wall because the orientation of reflector

    has been changed.

    Compare 5 options, with the wider of concert hall, the auditorium will be closer to stage but the front part will be more

    difficult to receive early reflections. I need to make a balance between early reflections and distance. Option 1 the

    concert hall is too narrow which makes auditorium far away from stage. Option 2 is appropriate. Option 3 the angle of

    two lateral wall is too large which makes lateral wall difficult to provide early reflections. Option 4 the concert is too

    wide for central part to receive early reflections from lateral wall. Option 5 the width, angle of lateral wall, distance

    between stage and auditorium is appropriate.

    Here I would like to use option 5 for concert hall plan design.

    Option 5 has the problem of lack of first reflected sound for front parts. Ceiling design would be important for hexagon

    shape concert hall because ceiling can also reflect sound from sources and provides early reflections for the front and

  • middle part of concert hall.

    4. Geometric design of concert hall

    I would use ray tracing method to design the shape and dimension of ceiling, the sound source S is located 2.5m

    from the edge of stage, the height is 1.5m. The front part of concert hall is from A to B (A, B area both 1.1m from FFL).

    Fig. 5 Ceiling shape design by ray tracing method

    The stage height is 6m. I connect SA and AA and draw internal bisector AQ1, then I draw AA which is

    perpendicular to AQ1. Then I mirror S to S1 by AA. After I get S1, I connect S1 and B. Finally I get the position of A

    from the intersection of AA and S1B. And then I connect S and A, , I take B from the extensi