Terminology, Definitions, and Conversion Factors
Total Page：16
File Type：pdf, Size：1020Kb
APPENDIX 1 Terminology, Definitions, and Conversion Factors @IMENSIONAL QUANTITIES IN CHAPTER 3 AND APPENDIX 2 In Chapter 3 and the tables of Appendix 2 various terms are used for the architectural and acoustical parameters of concert halls and opera houses. The meanings of those terms as used in that chapter and elsewhere in the text are as follows: N = number of seats in the hall (usually wheelchair space is not counted). V = Volume of the hall in cubic feet (cubic meters). In concert halls, V includes the volume of air in the main hall and in the orchestra enclosure. If there is a stagehouse, the volume V does not include that volume of the stagehouse that lies outside the orchestra enclosure unless the con struction is lil.>e that of Hall No. 10 in Chapter 3. V is measured as though there were no seats in the hall. The volume occupied by the solid balcony structures is not included. In opera houses, V includes the volume of air contained in the house forward of the main curtain. It does not include the volume of air in the stagehouse or the volume occupied by the solid balcony structures. Sa = area of floor space over which the audience chairs are located. The seating areas given in this book are projected areas. That is to say, in sloped (raked) floors, such as in balconies, the slant area is not measured. Instead, the area is that shown on the drawing. SA = acoustical audience area. It includes the sum of (a) the area Sa (see above) and (b) the areas of strips 20 in. (0.5 m) wide around the separated blocl~s of the seating area, except that such strips are neither included at the front edge of a balcony where the audience is seated against a balcony rail nor where the seats abut a wall. In this book the range of SA/Sa for the 100 halls is l.1 to l.5. The average for European and American halls built since 1975 is 1.25. The largest ratios are often found in Japapese halls, where the buJding codes require more aisles. In boxes, the same rule applies, unless the seats are far apart or the box is much larger than the seating area Sa' In that case, use SA (per chair) = 7.5 ft2 576' ApPENDIX 1 (0.7 m 2). However, the number of seats in the box times 7.5 (or 0.7) must not exceed the area of the floor of the box. Sc = area occupied by the chorus, or by the audience if seated in the chorus area. It is not considered as part of audience area if closed off or unused during non-choral performances. ' So = area of stage. When the stage area exceeds 1,940 ft2 (180 m 2), So is limited to that value. This numerical area is deemed the acoustical area of a 100- piece orchestra. No side strips are added. Spit = area of the open surface of the pit. Sp = area of the proscenium curtain. It is assumed that the fire curtain is pulled up during the acoustical measurements and that the proscenium curtain is present and sound absorbent. The reason for needing this area is for calculating the reverberation of the audience space without including the volume and absorption of the fly (scenery) tower. ST = SA + So (+Sc) (for concert halls). The acoustical absorption area ST is used in formulas for calculating reverberation times. It includes the surfaces that are highly sound absorbent. The other areas-namely, ceilings, sidewalls, and those floor areas that are not included in ST-usually contribute between 15 and 25 percent of the total sound absorption. Sc is included when it is normally used for audience seating. ST = SA + Spit + Sp (for opera houses). The proscenium area is included when the main performance curtain is lowered. If the measurements are made without lowering the curtain, the fly (scenery) tower will affect the measured rever beration times in unknown ways. H = average room height, measured from main floor to ceiling in that part of the main-floor audience area not covered by balconies. This height is needed to determine tbe time delay of the first ceiling reflection, which may be long relative to the arrival of the direct sound when the reflection goes to a main floor seat and much shorter when it goes to a balcony seat. W = average width, measured between sidewalls in the audience area on the main floor, disregarding any balcony overhang. This width is an indication of the "intimacy" of the hall. The first reflection after the direct sound that reaches a listener on the main floor may come from a balcony face. However, most side balconies are only a few rows deep, so that the average width between sidewalls is a more general indication of the "intimacy" factor. L = average room length, measured from the stage front to the average of the back wall positions at all levels. This length is a general indication of the mag nitude of the fall-off in lo~dness with distance from the stage. The maximum fall off is determined by D. Terminology, Dej;nitions, and Conversion Factors 577 It is not intended that H X W X L should be the exact cubic volume of the auditorium. D = distance from the front of the stage to the most remote listener, measured on the centerline, unless the rear wall is not flat or curved outward, in which case a more suitable location on the rear wall must be chosen. SD = average stage depth. SW = average stage width SH = mean ceiling height above the stage area, measured relative to the front of the stage. ~COUSTICAL PARAMETERS IN APPENDIX 2 Appendix 2 contains a detailed listing of the data obtained from acoustical measurements in concert halls and opera houses as outlined in Chapter 2. The terminology and definitions of the parameters are covered here. RT = reverberation time in sec. It is defined as the time, multiplied by a factor of 2, that it takes for the sound in a hall to decay from - 5 to - 35 dB below its steady-state value. The factor of 2 is necessary because RT must conform to the original definition of sound decay that was from 0 to - 60 dB. RT is usually measured in octave or one-third octave bands and the sound source is usually a pink (random) noise or a sound impulse. Originally, RT was determined from a plot of sound pressure level vs. time as recorded on the moving paper of a graphic level recorder. Today it is determined by the Schroeder (1965) method which involves computer integration of the equivalent of a bacl~ward-played tape-recording of the decaying signal. The mid-frequency RT is the average of the RT's at 500 and 1,000 Hz. The measurement is generally made in both occupied and unoccupied halls, the former being more important because it is the sound field in which a listener actually participates. When time permits, measurements are made for several sound source positions on the stage (and the pit in an opera house) and at 16 to 24 positions in a hall. When a hall is symmetrical the number of positions may be halved and the measurements made in only half of the hall. The data at each frequency for the various positions are generally averaged to obtain an average value of the RT for the hall. EDT = early-decay-time in sec. EDT is measured in the same fashion as RT except that it is the time iHal~es for a signal to decay from 0 to - 10 dB relative to its steady state value. A multiplying factor of 6 is necessary to mal~e EDT time 578 ApPENDIX 1 comparable to RT. There are a limited number of halls in Appendix 2 for which EDT was measured with full occupancy. IACCA = measure of the difference in the sounds arriving at the two ears of a listener facing the performing entity in a hall. It is called the "interaural cross correlation coefficient." For the data reported in this book the source of sound is an omnidirectional (dodecahedron) loudspeaker fed by an impulse sound. To mea sure IACC a digital recorder is connected to the outputs of two tiny microphones located at the entrances to the ear canals of a person or a dummy head, and quan tifying the two ear differences with a computer program that performs the operations of Eqs. (A3.1) and (A3.2) of Appendix 3. IACCA is determined with a frequency bandwidth of about 100 to 8,000 Hz and for a time period of 0 to about 1 sec. No frequency weighting is used. There are a limited number of halls in Appendix 2 for which the IAce s were measured with full occupancy. IACC (bands) = the same as above except determined in each of the six frequency bands with mid-frequencies of 125, 250, 500, 1,000, 2,000, and 4,000 Hz. The time periods and band-combinations are selected as follows: IACCE3 = the interaural cross-correlation coefficient determined for a time period of 0 to 80 msec, where 0 msec is the time at which the direct impulse sound from the omnidirectional source reaches the tiny microphones. It is the average of the values measured in the three octave bands with mid-frequencies of 500, 1,000, and 2,000 Hz. It is demonstrated in Chapter 4 to be fairly highly correlated with the subjective ratings of acoustical quality as expressed by qualified judges seated in the audience.