the present invention will be described in detail with reference to the drawings showing a preferred embodiment thereof . fig3 shows the construction of a reverberation system according an embodiment of the present invention . in fig3 a storage unit 1 stores data used for producing reverberant sound , namely , coefficient parameters ( impulse responses ) for use in digital filters that will be described later . a cpu 2 reads out the coefficient parameters stored in the storage unit 1 , and supplies the parameters to digital filters ( cnv1 - cnv8 ) 5 - 12 that perform convolution operations for respective loudspeakers . the cpu 2 also executes control programs for adjusting delay time and volume as described later . a microphone 3 is positioned at a real sound source location on a stage , and an a / d converter 4 converts an analog voice signal received through the microphone 3 , into a corresponding digital signal . the signal supplied from the a / d converter 4 , namely , digital signal ( source signal ) of sound generated from the real sound source ( microphone 3 ), is supplied to the digital filters 5 - 12 , and processed according to a convolution algorithm using the coefficient parameters supplied from the cpu 2 , so that virtual reverberation signals are produced . the outputs ( virtual reverberation signals ) of the digital filters 5 - 12 are then supplied to respective signal delay circuits 13 - 20 . the signal delay circuits 13 - 20 serve to delay the virtual reverberation signals , respectively , by finite lengths of time that are set as desired by a delay time setting circuit 61 . to provide the signal delay circuits 13 - 20 , tone control devices , which are generally called &# 34 ; effect devices &# 34 ;, may be used . while the tone control device incorporates lpf function , hpf function , bpf function , and others , the present invention focuses attention on the signal delay function , out of these functions , and utilizes this function for adjusting the initial delay time of reverberant sound as described later . the delayed virtual reverberation signals output from the signal delay circuits 13 - 20 are respectively supplied to volumes 21 - 28 , where the levels of the signals are adjusted under control of the cpu 2 . the virtual reverberation signals whose levels have thus been adjusted are then fed to d / a converters 29 - 36 , where the digital signals are converted into analog signals , and the analog signals thus obtained are amplified by respective amplifiers 37 - 44 , and generated through sub - loudspeakers 45 - 52 for producing reverberation . thus , the storage unit 1 , cpu 2 , a / d converter 4 , digital filters 5 - 12 , signal delay circuits 13 - 20 , volumes 21 - 28 , d / a converters 29 - 36 , amplifiers 37 - 44 , and the delay time setting circuit 61 , described above , constitute a virtual reverberant sound synthesizing device of the present invention . in the meantime , the analog signal received from microphone 3 is amplified by an amplifier 62 , and generated from a source reproduction main loudspeaker 63 that is placed in the vicinity of the real sound source location . if necessary , a signal delay circuit 64 for delaying the signal , as described later , may be inserted between the microphone 3 and the amplifier 62 . as shown in fig5 the sub - loudspeakers 45 - 52 for producing reverberation are installed at respective positions 45b - 52b that are identical with or different from actual observation positions . these positions 45b - 52b will be called &# 34 ; simulation positions &# 34 ;. by generating reverberant sounds from the simulation positions , a virtual hall 73 in which seats 72 for audience are surrounded by virtual walls 74 is simulated in front of the real sound source location ( where the microphone 3 is placed on the stage 71 ). fig4 is a view useful in explaining the case where reflected sound is measured in an actual hall ( model acoustic room ). in this case , a plurality of reflected sound measurement points 45a - 52a are set or established along walls 74a that physically exist and define an actual hall 73a that surrounds seats 72a in front of a stage 71a on which the real sound source location is set . the measurement of reflected sound in the actual hall , and production of reverberant sound for a virtual hall ( virtual acoustic room ) based on the measurement result will be described referring to fig6 a - 6c . fig6 a illustrates the manner in which direct sound 150 from a real sound source ( or sound already reflected by a wall ) is reflected by an actual wall 81 . the reflected sound 160 from the wall 81 is measured by a sound receiver ( microphone ) 82 that is installed at a position spaced apart from the wall 81 , as shown in fig6 b . subsequently , a loudspeaker 83 is placed at the same position as the microphone 82 , as shown in fig6 c , such that the loudspeaker 83 is oriented in the same direction as the reflected sound . even in the absence of the actual wall 81 , the loudspeaker 82 located in the above manner is able to create reverberant sound as if the wall 81 existed at the position shown in fig6 c . while the above description concerns only one reflected sound for the sake of brevity , a plurality of reflected sounds are serially generated on the wall 81 in real situations . the manner of generation of reflected sounds is different from one wall to another . for the purpose of measuring reflected sound , the direct sound may be directly measured by a sound receiver ( boundary microphone ) 82 that is installed on the wall 81 , as shown in fig7 a , or may be directly measured by a sound receiver 82 that is located at a position spaced apart from the wall 81 , as shown in fig7 b . with these arrangements for measurement , an impulse response at each measurement point can be observed . if the sub - loudspeakers 45 - 52 for producing reverberant sound are located at exactly the same positions as the respective measurement points 45a - 52a in a hall that is different from the actual hall of fig4 in which reflected sounds were measured , or in an outdoor hall , and the signal delay circuits 13 - 20 function to delay input signals with substantially the same delay times as actually measured , a virtual hall that is similar to the actual hall in which the reflected sounds were measured can be reproduced . suppose it is desired to create a virtual hall having relatively large width and small depth as shown in fig5 which is different from the actual hall of fig4 having relatively small width and large depth , with the sub - loudspeakers 45 - 52 located at exactly the same positions as described above . to create such a virtual hall , reverberant sounds from the sub - loudspeakers 45 , 25 46 , 47 and 50 , 51 , 52 located at the left - side and right - side portions of the virtual hall 73 of fig5 respectively , are generated with delays that are larger than actual measurement values , and reverberant sounds from the loudspeakers 48 , 49 located at the rear end of the virtual hall 73 are generated with delays that are smaller than the measurement values . in this manner , the left and right virtual walls 74 of the virtual hall 73 move backward , and the rear virtual wall 74 moves forward , so that the virtual hall 73 is designed with a larger width and a smaller depth than the hall 73a in which the reflected sounds were measured . thus , reverberation can be produced in any type of virtual hall 73 , only by adjusting the delay time . where it is desired to reproduce the hall 73a of fig4 based on the impulse responses observed in the hall 73a of fig4 with the sub - loudspeakers 45 - 52 located at the positions 45b - 52b in fig5 the delay time setting circuit 61 is set to operate the signal delay circuits 13 - 20 , so that the delay times of reverberant sounds from the left and right sub - loudspeakers 45 - 47 and 50 - 52 are made smaller than delay amounts obtained by actual measurements , and the delay times of reverberant sounds from the rear sub - loudspeakers 48 , 49 are made larger than the measured delay amounts . although the virtual acoustic room is considered as a plane in the above description , for the same of brevity , the actual acoustic room is defined by a three - dimensional structure including a ceiling and a floor in addition to side walls , as reflecting surfaces , and the reflection in this room takes place three - dimensionally in a more complicated manner . in any event , the present invention aims at producing reverberant sound that matches any type of virtual halls ( acoustical rooms ) having a wide variety of shapes , based on a single set of impulse response data associated with reflected sounds measured in one model room as shown in fig4 . a method of determining how much delay should be added to direct sound in accordance with a desired virtual hall will be now described . fig8 a illustrates an actual hall in which the impulse responses for producing reverberant sounds are measured , and fig8 b illustrates a virtual hall that is simulated based on the impulse responses measured in the actual hall , while fig9 a shows the actual hall 73a of fig8 a and the virtual hall 73 of fig8 b in a manner being superposed on each other . in fig9 a , points p1 , p2 , . . . , p5 indicated along the wall 74a of the actual hall 73a represent a plurality of measurement points . as shown in fig9 b showing an impulse response that is measured at point p5 as one of the measurement points , initial reflected sound 160 is generated upon a lapse of an initial delay time ta after occurrence of direct sound 150 , followed by attenuation of the initial reflected sound 160 , or reverberation 180 . a change in the initial delay time ta through signal delay processing is considered to be equivalent to a change in the distance between the real sound source location and the relevant wall . if the amplitude level of reverberant sound is adjusted by means of a corresponding one of the volumes 21 - 28 at the same time that the initial delay time is changed , so that the amplitude level is reduced with an increase in the delay , or increased with a decrease in the delay , the reproduction accuracy is further improved , namely , the desired virtual hall can be created with improved accuracy . fig9 c shows an impulse response that is to be produced at point p5 &# 39 ; ( fig9 a ) on the virtual wall 74 of the virtual hall 73 , which corresponds to the measurement point p5 on the wall 74a of the actual hall 73a . it will be understood from fig9 c that the initial delay time ta &# 39 ; is increased by an amount proportional to an increase in the distance from the real sound source location to the point p5 &# 39 ;, as compared with the distance to the point p5 , and the amplitude of the reflected sound is accordingly reduced . while the impulse response of fig9 c is obtained by shifting the whole reverberation to a later period on the time axis , the increase in the initial delay time ta &# 39 ; would be predominantly perceived as if the wall itself was retracted rearwards with an increased distance from the real sound source location . according to the present invention , such changes in the initial delay time ta &# 39 ; can be easily accomplished by means of the time delay circuits 13 - 20 , rather than the digital filters of fig3 . fig1 and fig1 illustrate examples in which the shape of the virtual hall is changed as desired , into more complicated shapes . in the example of fig1 , a multiplicity of sub - loudspeakers sp for producing reverberation are positioned in the same manner as described above , along virtual walls 74 - 1 of an actual or virtual model hall 72 having a modified hexagonal shape . to create a circular virtual hall 73 - 2 as shown in fig1 , with the sub - loudspeakers sp located in the same positions , a positive (+) or negative (-) amount of delay is added to the initial delay time of reverberant sound from each sub - loudspeaker sp , so as to correct the delay time in view of a difference in the shape between the virtual halls 73 - 1 and 73 - 2 . in fig1 , the delay amount α a for each loudspeaker sp is represented by the length of a corresponding arrow . the positive (+) and negative (-) signs of the delay amounts indicate a lag and a lead in time , respectively , which may be considered as relative relationships among the delay amounts . for example , the maximum value of the negative (-) delay may be set to zero delay , and the other delay amounts may be increased by a difference between the maximum delay value and zero . this also makes it possible to create an atmosphere in which the shape of the acoustic room has been changed enough . in this case , however , the acoustic room tends to expand to greater dimensions than desired values . to improve this point , a main reproduction system ( left and right main pa loudspeakers 63r , 63l ) for reproducing the source signal may be controlled by the time delay circuit 64 of fig3 so that sound represented by the source signal is generated with the maximum value of negative (-) delay , to thereby provide an acoustic room having desired dimensions . in the system of the present embodiment , the shape of the virtual hall is changed by adjusting the initial delay time of virtual reflected sound by means of the time delay circuits 13 - 20 of fig3 . although the same function may be accomplished by means of the digital filters 5 - 12 , the use of the digital filters 5 - 12 for this purpose will require multiple sets of impulse response data to be prepared for all different types or shapes of halls . in this respect , the present reverberation system only requires one set of impulse response data to be prepared , which leads to simplified construction of the system . also , the control scheme of the present system is simple and self - explanatory , and thus can be easily designed . furthermore , the system of the present invention produces virtual reverberant sound , such that reflected sound from a single imaginary sound source is reproduced by a plurality of sub - loudspeakers , with certain time differences and level differences , and such that the reproduction is carried out with respect to a plurality of imaginary sound sources , thus assuring a sufficiently large sound receiving area .