Loudspeakers

Transcript

May 25, 1965 H. F- OLSON ET AL v Filed Oct. 10, 1960 3,185,767 LOUDSPEAKERS 5 Sheets-Sheet l 12-1 \\ \ \ \\ \ \\\ \ INVENTORS BY JEAN Pris-rod 47771649’ May 25, 1965 3,185,767 H. F. OLSON ETAL LOUDSPEAKERS Filed Oct. 10, 1960 3 Sheets-Sheet 2 F57’.4. ll 6 103 Z 4. 6 104 May 25, 1965 3,185,767 H. F. OLSON ET'AL LOUDSPEAKERS Filed 061’,- 10, 1960 3 Sheets-Sheet 3 E77 73 7/ 7! By Jay/v PRESTON United States Patent ” ice ’ 3,185,767 Patented May 25, 1965 1 2 3,185,767 bodying the invention, with the helical spring shown in full view for clarity; FIGURE 2 is a sectional view of a loudspeaker in LOUDSPEAKERS Harry F. Olson, Princeton, and John Preston, Trenton, N.J., assignors to Radio Corporation of America, a corporation of Delaware Filed Oct. 10, 1960, Ser. No.61,537 9 Claims. (Cl. 179-1) ' corporating the preferred embodiment of the invention; FIGURE 3 is a view taken along the section lines 3-3 in FIGURE 2; p 7 FIGURE 4 is a graph diagrammatically illustrating the reverberation characteristics of the embodiment of the invention shown in FIGURE 2; The present invention relates to sound reproducing FIGURES 5 and 6 are partly broken sectional views devices such as loudspeakers and more particularly to 10 loudspeakers which synthesize sound reverberation en of other loudspeakers incorporating the invention; and, velopes. ‘I ‘ FIGURE 7 is a block diagram of a sound reproducing system including a loudspeaker reverberator constructed A listener in a concert hall hears a sound pattern in accordance with the invention. composed ‘of two separate acoustical effects, namely the Referring to the drawing, wherein like reference nu sounds which come directly from the performers and the 15 merals have been given to like components in the various multiple re?ections of these sounds from the surfaces of ?gures thereof, and particularly to FiGURE l, a loud the concert hall. The direct sound enables the listener speaker 10 constructed in accordance with the invention through his binaural sense to determine the auditory perspective, i.e. the spatial localization of the original includes an electro-mechanical transducer coupledto a sources of sound. The multiplicity‘of re?ected sounds 20 mechano-acoustical transducer by means of a mechanical transmission line. :from the surfaces of the hall provide the listener with More speci?cally, a permanent magnet 16 is mounted a reverberation envelope. These two acoustical effects on a metallic housing 18 which includes an inwardly constitute the artistic aspects of concert hall renditions. extending annular top pole plate 20. A voice coil 22 The phenomenon of reverberation is so common in everyday life and audiences have ‘been so long accus 25 is suspended in an air gap formed between the magnet 16 and the central aperture in the pole plate 20 by tomed to hearing performances in auditoriums with con means of a disc 24 and a pair of compliant annular siderable reverberation, that when reverberation is ab centering supports or spiders 26 and 28. The centering sent, the sound heard appears unnatural and lacking in supports 26 and 28 are cemented to both the annular expected depth. The lack of space in most recording studios makes it difficult to make recordings with proper 30 pole plate 20 and the disc 24 and serve to maintain the reverberation. Many systems heretofore proposed for synthetically adding reverberation to sound have been both exceed— ingly complex and expensive. Attempts to add reverbera tion to loudspeakers which also produce direct sound have resulted in insu?icient reverberation envelopes be ing developed and an absence of concert hall realism in the home. Accordingly, it is an object of this invention to pro vide a new and improved loudspeaker for the production of reverberant sound. It is another object of this invention to provide a new and improved loudspeaker reverberator which provides only reverberant sound. It is a further object of this invention to provide a loudspeaker which synthesizes reverberation by the use of simple, inexpensive mechanical components. voice coil 22 properly centered in the air gap with little or no transverse movement possible but capable of ex treme axial motion. The aforementioned components comprise the electromechanical transducer or dynamic driver ot‘ the loudspeaker 10. A spring 30, having a multiple number of coils ‘or turns wound in the form of a helix with the diameter of the coils being constant throughout the length ‘of the spring, is provided to comprise the mechanical trans mission line in accordance with the invention. The ends of the spring 30 are soldered to a pair of end plates 32 and 34 respectively. The end plate 32 is suitably ‘fastened ' to the disc 24 while the other end plate 34 is fastened to a hub 36. A conical truncated diaphragm 38 is provided to com prise the ‘mechano-acoustical transducer in accordance with the invention. The apex of the conical diaphragm cal transducer is coupled to a mechanoaacoustical trans ducer by a mechanical transmission line having both 38 is cemented to the hub 36 and the outer peripheral rim of the base of thediaphragrn 38 is clamped be~ tween and supported by a pair of annular rings 4% and In accordance with the invention, an electro-mecha-ni time delay vibration-transmitting and vibration-re?ecting 42. characteristics. More speci?cally a dynamic driver such as that of a loudspeaker is coupled to a sound radiating a frame member 44 and a plurality of threaded rods cone or diaphragm by means of one or more coiled 46. The frame member 44 is fastened to the pole plate springs. Electrical impulses transduced into mechanical 20. The rods 46 are inserted through a ‘plurality of apertures on the ‘frame member 44 and the annular rings 40 and 42 and then fastened to a suitable cabinet vibrations by the dynamic driver are transmitted by the ~ springs to the diaphragm, the piston-like movement of A supporting structure for the loudspeaker 10 includes 48 which holds and encloses the loudspeaker 10. A The en plurality of nuts 49 are provided to secure the rods 46 ergy not absorbed by the diaphragm is re?ected back through the springs to the dynamic driver. The energy 60 to the cabinet 48, and the frame member 44 and the annular rings 40 and 42 to the rods 46. is again re?ected by the dynamic driver and then re transmitted by the springs. Thesuccessive transmis When electrical impulses are applied to the voice coil sions and re?ections of the energy synthetically produce 22, the voice coil 22 is sent into vibratory motion. This a reverberation envelope. is due to the force exerted thereon by an electrical The novel ‘features which are considered to be char 65 current ?owing, in the magnetic ?eld created between permanent magnet 16 and pole plate 20. The oscillatory acteristic of this invention are set forth with particu larity in the appended claims. The invention itself, both motor vibration of the voice coil 22 is applied to the as to organization and'method of operation, as Well as spring 3%) and mechanical vibrations are transmitted down which causes sound to be radiated therefrom. additional objects and, advantages thereof, will best be understood by referring to the accompanying drawing and the following description in which: FIGURE 1 is a sectional view of a loudspeaker em the length of the spring 30 by the axial displacement of successive coils. After a predetermined time delay, the mechanical vibratory energy arrives at the diaphragm 38. A part of the energy in the spring 30 is transmitted answer Q! where p=density of the material of the spring in grams per cubic to the diaphragm 38 causing piston-like vibrations to be set up therein which radiates the energy in the form of sound Waves. centimeter Substantially all the mechanical vibratory energy that Eszshear modulus in dynes per cubic centimeter is not absorbed by the diaphragm 38 is re?ected back Rzradius of the coils or turns in centimeters through the spring 30. The re?ected wave travels back rzradius of the wire in centimeters to the voice coil where it is substantially completely re n=total number of turns ?ected again. The Wave then is again transmitted toward l=length of spring in centimeter the diaphragm 3S and the process repeats itself. For steel wire, the shear modulus and density are Successive transmissions, re?ections and retransmissions 10 8X1011 dynes per cubic centimeter and 8 grams per cubic of energy occur with decreasing amplitude in each cycle centimeter respectively and Equation 4 for steel reduces until the acoustical energy radiated is reduced to the point to of inaudibility. Thus the loudspeaker 10 synthesizes the multiple re?ections of sound in a reverberant auditorium V Where acoustical energy impinging on one surface is 15 partially absorbed and partially re?ected. The re?ected sound again impinges on a second surface, etc., the proc ess repeating until inaudibility is reached. No sound is directly radiated from the dynamic driver of loudspeaker 10. X3.5><104 Steel springs designed to provide transit times of from 20 to 30 milliseconds have provided proper time delays. The characteristic mechanical impedance of a helical spring may be determined from the equation All energy radiated in the form of sound waves occurs after the introduction of a time (6) delay by the mechanical transmission line or spring 30. Energy received by the voice coil 22 in the form of elec trical impulses is stored in the spring 30 during the time it takes to be transmitted along the spring to the dia phragm 38. The diaphragm 33 radiates sound waves only after the spring 30 transmits energy thereto. Therefore, no sound emanates from the loudspeaker 10 during the where the terms in the equation have been de?ned above. As long as the spring transmission line is not terminated in a resistance having a magnitude equal to the char acteristic impedance, re?ection of vibratory wave energy Will occur. However it is important that there be a large mismatch between the characteristic mechanical im storage time period. pedance of the spring 30 and the terminating resistance, Any form of transmission line that introduces a time delay in the transmission of wave energy and also causes so that a major portion of the vibratory wave energy successive and diminishing re?ections of the Wave energy may be used in the invention. Ti However a mechanical transmission line in the form of a spring is very practical. The velocity of longitudinal wave propagation in a spring 35 s relatively small, making the transit time of the Wave is re?ected. A simple introduction of a single time delay by the spring 3% would not produce reverberation but only a single echo. The terminating impedance in the loudspeaker 10 is substantially equal to the mechanical impedance of the diaphragm 38 and the mechanical radiation resistance of the air load on the diaphragm 38. The mechanical radiation resistance is mainly determined by the dimen namic driver and the radiation of sound from the loud speaker is attained. Furthermore the dissipation of 40 sions of the diaphragm 38. A diaphragm 38 is chosen so that the radiation resistance is very small compared to energy in a spring is relatively small providing a substan the characteristic impedance of the spring 30. For ex~ tially lossless transmission line. A spring transmission ample, at 500 cycles the mechanical radiation resistance line is also easily terminated in an impedance much smaller energy relatively long. Thus the desired comparatively long time delay between the initial vibration of the dy is 1/10 that of the characteristic impedance of the spring. than its characteristic impedance to cause the desired re ?ection of wave energy. Therefore a spring is well suited as As a consequence, the sound waves in the spring must make a large number of excursions in order to transfer the major or signi?cant amount of energy stored in the spring to radiated ‘sound energy. Thus, it will be seen that the radiated sound from the loudspeaker 10 simulates the re?ected sound in an auditorium, namely, the ?rst element of radiated sound is delayed with respect to the for providing the delay and re?ection necessary for syn thesizing reverberation in a sound reproducing device. The velocity of Wave propagation in a helical spring is given by the equation, (1) V= electrical impulses in the dynamic driver and the elements ii H of sound which are radiated persist for a time after the where electrical impulses have ceased. In other words, the out put of the loudspeaker 10 possesses all the characteristics of reverberation. V=velocity in centimeters per second m=mass of the spring per unit length Reverberation time, which is a measurement of acousti C=comp1iance of the spring per unit length. In terms of the constants of the spring the mass and compliance per unit length are: cal quality, is arbitrarily de?ned as the time required for sound intensity to decrease to one-millionth of the origi nal intensity. Any desired reverberation time may he 0 obtained in a loudspeaker constructed in accordance with the invention. Assuming a sine wave electrical signal input to the dynamic driver of the loudspeaker 10, the sound pressure amplitude at any time after the input signal has been removed is given by the equation Rmt Thus the velocity of Wave propagation, and therefore (7) P=Poe 2M sin 21rft the transit time, in terms of the constants of the spring where may be determined from the equation: 70 P=sound pressure in dynes per square centimeter at any time t (4) t=time in seconds R1 Rm=effect1ve mechanical resistance of the mechanical 75 system III mechanical ohms 8,185,767 5 6 M=effective mass of the vibrating system, in grams Po=souud pressure at t=i0 in dynes per square centimeter loudspeaker St} in order to simulate exactly the acoustics j=frequency in cycles per second. loudspeaker was about the optimum in the mid-frequency ' The term range, centered about 1000 cycles. In a concert hall, the reverberation time is of the order of 1.5 seconds with use 6 of a concert hall, the reverberation time measured for this 2M determines the exponential decay of the sound emanating the optimum time depending on the volume and vgeomet rical con?guration of the hall. Referring to FIGURES 5 and 6, loudspeakers embody ing the invention are shown which use only a single spring from the loudspeaker 10 and therefore the reverberation 10 rather than ‘a multiple number of springs. Byvvarying the time. coil or turn diameter along the length of a spring, con The time required for the radiated sound intensity to tinuous Wave energy re?ection characteristics will be decay to one millionth after the electrical impulses have created in the spring itself by the mismatch of mechanical ceased is the reverberation time. Since sound intensity impedance between successive coils. Consequently a is propoitional to the square of the sound pressure, the reverberation time then is given by the expression ‘ , <8) 1 __ mar _Rmt 2M large number of reflections of wave energy with varying ‘ time delays occur because of the introduction of a suc cession of impedance discontinuities along the mechanical transmission line or spring. In FIGURE 5, a spring 60 wound in the form of a helix From the equation the following expression may be obtained for the reverberation time, M having an hourglass shape is utilized, while in FIGURE 6 a spring 62 wound in the form of a t'runcated'helical cone is utilized to accomplish the continuous re?ections of vibratory wave energy. ‘ As shown in FIGURE 7, a system for simulating con In one embodiment of the loudspeaker of FIGURE 1, 25 cert hall realism in the home includes a stereophonic sound reproducer with the addition of a loudspeaker the cone diameter was 6 inches. The mechanical radia reverberator constructed in accordance with the invention. tion resistance, Rm, for a cone diameter of 6 inches at A stereophonic phonograph 70 includes a motor 71, a 500 cycles per second is 1400 mechanical ohms. This record turntable 72 suitably mounted on the motor 71 is the eifective mechanical resistance because the other mechanical resistances in the system are negligible. The 30 to be driven thereby, and a sound signal pick-up device or stylus 73 mounted to track the grooves in a stereo ‘effective mass of the vibrating system is the mass of cone phonic disc record 74. The pick-up device 73 is con 38, the end plates 24 and 36, the voice coil 22, and the nected to left 75 and right 76 variable audio frequency effective mass of the spring 30. The effective mass of 7 ampli?ers, the ampli?ed outputs of which are fed to left a spring in which a wave system exists is 1/2 of the 77 and right 78 loudspeakers respectively. The loud mass of the spring. For one embodiment of FIGURE speakers 77 and 78 are known direct-radiator conical 1 in which the cone diameter was 6 inches, the total diaphragm type loudspeaker units and are preferably effective mass of the vibrating system as outlined above spaced laterally apart from each other. i was 420 grams. For the above constants of effective The ampli?ed sound signals from the ampli?ers 75 mechanical resistance and effective mass of the vibrating and 76 are also mixed or combined in a mixer and system, the reverberation time as computed from the ampli?er stage 79 before application to a loudspeaker above equation is four seconds at 500 cycles per second. reverberator 80 which is constructed in accordance with . Reverberation times of from 0.5 to v5 seconds provide excellent simulation of concert hall acoustical effects and _ the invention. The loudspeaker reverberator 80 is pref are attained in a loudspeaker constructed in accordance erably located to the rear of a listener. with the invention. Referring to FiGURE 2, a preferred embodiment of the invention includes a plurality of springs 52, 54, 56 and 58 connected in a loudspeaker 50 between the disc 24 and the hub 36. FIGURE 3 shows the arrangement of the springs with respect to each other when mounted and the difference in coil or turn diameter in each spring. The inclusion of a multiple number of springs with different diameters introduces different wave propagation velocities into the wave energy transmitted by the springs. The more springs that are included in the loudspeaker 50, the more closely would the sound emanating from the loudspeaker 50, simulate the multiple reverberations in a concert hall, where the tremendously large number of re ?ected sound waves arriving at a listener over different ‘ ‘In listening to sound being reproduced by the (above system, a greatly enhanced concert hall realism may be attained using ordinary stereophonic disc records. The direct sound left and right loudspeakers 77 and 78 pro vide the auditory perspective ‘and the time delayed and reflected sound from the loudspeaker reverberator 80 provides the reverberation envelope. ,No direct sound emanates ‘from the loudspeaker reverberator 80. The only direct sound that is heard comes from the loud speakers 77 and 78, with the reverberator loudspeaker 80 supplying the reverberation after a predetermined time delay, which is the normal sequence in auditorium acoustical effects. The ratio of direct sound to reverberant sound is also controlled in this system by varying the volume controls paths having different transit times creates an envelope 60 on the ampli?ers 7'5, 76 and 79. Thus, controlled rever of reverberant sound. berant sound may ‘be added to a st-ereophonic sound re The loudspeaker 50 shown in FIGURE 2 was con producing system in the home ‘by adding a loud-speaker structed using steel wire as the material for the springs. constructed in accordance with the invention, thereby The diameter of the wire was selected to be 0.1 inch achieving substantially all the acoustical effects ‘experi and the coil or turn radii in each of the four springs 65 enced in a concert hall. What is claimed is: 52, 54, 56 and 58 were % inch, % inch, 1 inch and 1% inches respectively. The total number of coils or 1. A loudspeaker reverberator comprising in combina turns in each spring was 60. The dynamic driver and tion an electro-mechanical transducer; a spring wound in diaphragm of a standard 8 inch loudspeaker were also the form of a helix and having a given characteristic used. The loudspeaker 50 was mounted in a cabinet of 70 mechanical impedance, coupled at one end :to said electro one cubic foot volume. The reverberation time versus frequency characteristic of the loudspeaker 50 is graphically illustrated in FIG URE 4. While presumably a very large number of springs would have to be included in the construction of the mechanical transducer; said spring having :a time delay vibration-transmitting characteristic; and a mechano acoustical transducer coupled to the other end of said spring, said mechano-acoustical transducer having a lesser mechanical resistance than the characteristic impedance 3,185,767 8 of said spring to receive mechanical vibrations therefrom after a predetermined time delay measured ‘from the initial vibration of said dynamic driver, said mechano— acoustical transducer translating a portion of said of said spring whereby vibration re?ection occurs at said mechano-acoustical transducer. 2. A loudspeaker reverberator comprising in combina tion a dynamic driver for converting electrical impulses into mechanical vibrations, a helical spring having a given characteristic impedance and a transit time for mechan mechanical vibrations into radiated sound Waves and re ?ecting the remaining portion. 6. A loudspeaker as de?ned in claim 5 wherein said spring is wound in the form of a helix having an hour ical vibrations of from 20 to 30 milliseconds coupled at one end to said dynamic driver, and a diaphragm hav ing an air load thereon exhibiting a mechanical radiation glass shape. 7. A loudspeaker as de?ned in claim 5 wherein said resistance less than the characteristic impedance of said 10 spring is wound in the form vof a helix having a truncated spring coupled to the other end of said spring, said dia cone shape. phragm re?ecting a major portion of the mechanical 8. A loudspeaker reverberator comprising in combina vibrations transmitted thereto and radiating the rem-ain tion a voice coil having a given mass suspended in a ing portion in the form of sound waves. magnetic ?eld ‘for translating electrical impulses into 3. A loudspeaker reverberator comprising in combina mechanical vibrations, a helical spring having a given mass for transmitting the mechanical vibrations, a dia phragm having a given diameter to obtain a predeter mined mechanical radiation resistance and a given mass, tion an elect-ro-mechanical transducer to translate re ceived electrical impulses into mechanical vibrations, a helical spring having a given characteristic impedance and a given time delay vibration transmitting character said diaphragm translating the mechanical vibrations into acoustical vibrations, and ?rst and second end plates each istic coupled at one end to said electromechanical trans ducer to transmit said vibrations, and a mechano acoustical transducer coupled to the other end ‘of said spring to receive the mechanical vibrations, said mechano-acoustical transducer having a mechanical resist having a given mass for coupling said voice coil to said spring and said spring to said diaphragm respectively, said mechanical radiation resistance bearing a predeter mined relationship to the effective masses of said spring, voice coil and end plates so that said loudspeaker pro vides a predetermined reverberation time. ance less than the characteristic impedance of said spring in order to re?ect a major portion of the mechanical vibrations and translate the remaining portion into sound waves, said loudspeaker providing a reverberation time of 9. A sound reproducing system for simulating concert hall acoustical etlt‘ects comprising in combination a pair of direct sound radiating loudspeakers; means for apply from 0.5 to 5 seconds. 4. A loudspeaker having reverberation synthesizing ing to said loudspeakers ampli?ed stereophonically characteristics comprising in combination a dynamic driver for converting electrical impulses into mechanical vibrations, a plurality of reverberator springs Wound in the form of helixes, each of said springs having a dif ferent diameter, means coupling said dynamic driver to one end of each spring for imparting to said springs 'mechanical vibrations, each of said springs having a related sound signals; a reverberator loudspeaker com prising an electiro-mechanical transducer, a mechano acoustical transducer and a mechanical transmission line coupling together said transducers, said mechanical trans mission line having a time delay vibration-transmitting and vibration-re?ecting characteristic, and means :for applying to said reverberator loudspeaker a mixed com different time delay vibration-transmitting characteristic; posite sound signal derived from said stereophonically related signals. a diaphragm for translating mechanical vibrations into sound waves; means coupling said diaphragm to the other ends of each of said springs to receive mechanical vibra Reterences Cited by the Examiner UNITED STATES PATENTS tions having different time delays, said diaphragm re?ect ing a portion of the mechanical vibrations back to said springs. 5. A loudspeaker having reverberation synthesizing 45 characteristics comprising in combination a dynamic driver for converting electrical impulses into mechanical vibrations, a spring having a given characteristic mechan ical impedance coupled at one end to said dynamic driver 50 for receiving and transmitting the mechanical vibrations, said spring comprised of a plurality of helical coils of different diameters for continuously re?ecting a portion of the mechanical vibrations at each successive coil, a mechano-acoustical transducer coupled to the other end 1,932,461 2,230,836 2,375,004 2,548,235 2,768,235 2,853,145 6/32 2/4l 5/43 4/51 10/56 9/58 Van Deventer _______ __ 179—l09 Hammond ____________ __ 179--l Knowles _____________ __ 35—-25 Olson ________________ __ 1'79—1 Knoblaugh ___________ ._ 1'79—1 Martin _______________ __ 179-1 ROBERT H. ROSE, Primary Examiner. L. MILLER ANDRUS, WILLIAM C. COOPER, Examiners.