Jan. 7, 1947. E. s. WINLUND ' 2,413,936 REVERBERATION METER Original Filed June 30, 1942 ' 3 / ~ /7, Z 19MB 04/ FFCWFE)? 42 IIIIIIIIII L H w2 ' INVENZ‘OR. ltlrllondci?rl?lwd BY cg 197' Tam/£2! Patented Jan. 7, 1947 2,413,936 1 UNITED SKATES PATENT OFFICE 2,413,936 REVERBERATION METER Edmond S. Winlund, Moorestown, N. J., as signor to Radio Corporation of America, a cor poration of Delaware Original application June 30, 1942, Serial No. 449,178. Divided and this application January 24, 1945, Serial No. 574,289 3 Claims. 1 (Cl. 181-05) This application is a division of my copending application, Serial No. 449,178, ?led June 30, 1942, upon which U. S. Patent 2,384,868 was granted September 18, 1945. This invention relates generally to reverbera tion indicators and particularly to a new and im proved method of and means for providing a di rect indication of the sound decay characteristics of an acoustic chamber. 2 for measuring the applied control electrode po tential is connected between the control electrode and the cathode. Among the objects of the invention are to pro vide an improved method of and means for indi cating directly the sound decay characteristics of a studio. Another object of the invention is to provide an improved method of and means for indicating directly the sound decay characteris Heretofore, various methods have been used for 10 tics of the studio, as a function of voltages de measuring the sound decay characteristics in rived from a sound translating device, combined sound studios. However, the systems known in with a bias voltage, and applied to the control the prior art either require comparatively com electrode of a thermionic vacuum tube whose an plicated apparatus, or provide an indication ode circuit time constant adjusts itself to equal which requires computation to ascertain the the time constant of decay in the studio. Another sound decay characteristic. object is to provide a direct reading reverberation The instant invention contemplates the use of simple and inexpensive apparatus for indicating directly the sound decay characteristics of a studio as a function of the voltage applied to the control electrode of a thermionic tube including an anode circuit which has the same time con-, stant as the time constant of sound decay in the studio. By suitably choosing the thermionic tube, and the associated circuit values, the voltage ap- ~ plied to the tube may provide a direct indication meter circuit having suf?cient range for the meas urement over a sound range of the order of 60 db. of the sound decay characteristics of a studio. The invention will be described in detail by ref - erence to the accompanying drawing of which Figure l is a schematic diagram of one embodi ment of the invention, and Figure 2 is a graph illustrating the operation of the invention. Referring to the drawing, a studio I is provided with a sound source, which may be a reproducer of the tube anode resistance, thereby permitting 2 designed to establish constant amplitude sounds. the calibration of the indicating meter in sec The reproducer may be energized by an audio onds of sound decay time constant, which is pro oscillator 3, the output of which may be inter~ portional to the instantaneous anode resistance of 30 rupted by a key 4. A sound translating device 5, the thermionic tube. which may be a microphone of any well known By providing su?icient gain in the sound trans type, is connected, through a pie-amplifier i‘! and lating circuits, the reverberation may be meas a gain control to the input of an audio ampli?er ured over a minimum period of approximately two l having desired gain and power characteristics. seconds, during which time the actual sound de- .‘ This apparatus may comprise customary studio cay in a relatively “dead” studio would be in ex sound pickup equipment. The output of the cess of 60 db. Since some indicating systems used audio ampli?er l is applied to the input of a con heretofore have lacked volume range, the actual ventional recti?er 8 comprising, for example, a time during which observations could be made diode or a diode-connected triode. The output of has in many cases been limited to a small irac~~ . i: the rectifier, comprising pulsating, preferably ?l~ tion of a second, thereby reducing the accuracy of such indications. Brie?y, the system to be described includes a means for producing and interrupting a continu ous audible sound, of constant amplitude, within ~ the studio. The reproduced sound, and the re verberation produced thereby, is detected by a standard microphone, ampli?ed and recti?ed and combined, at a predetermined level, with a con stant D.-C. standard voltage. The combined voltages are applied to the control electrode of a ther mionic vacuum tube having at least a cathode, a control electrode and an anode. The anode is grounded, and is connected through a capacitor of predetermined value to the cathode. A meter tered, direct current having a voltage preferably in excess of 200 volts, is applied across a second potentiometer 9. The positive terminal of the second potentiome ter 9 is grounded. The sliding contact of the second potentiometer 9 is connected to the cathode of a diode recti?er iii, and to the control electrode of a triode thermionic tube 5 l. The negative ter~ minal of a source of constant D.-C. potential £2, for example, a battery, is connected to the anode of the diode recti?er iii. The positive terminal of the constant D.-C. potential source i2 is con nected to the cathode of the triode thermionic tube H. A capacitor it, of predetermined value, is con 2,413,936 3 4 nected between the cathode of the triode ther mionic tube H and ground. The anode of the triode thermionic tube 1 l is grounded. A suitable known circuit modi?cations will permit the use meter l4 and a second capacitor l5 are connected between the control electrode and the cathode of of a logarithmic meter scale covering a wide range. The capacitor l3 may be also varied to provide an instrument having several useful ranges. Since the indication provided by the meter M will be signi?cant only during the actual the triode thermionic tube H. The anode-to sound decay interval in the studio, the meter must cathode resistance of the triode thermionic tube be read during this interval. As explained here I l is represented by the resistor is shown in dash tofore, the circuit described may be readily de lines. In operation, a steady audio signal is intro 10 signed to cover a range of the order of 60 db., thereby providing a signi?cant and steady meter duced into the studio by the reproducer 2, and at reading for approximately 2 seconds after the a desired instant, is interrupted by opening the key A is opened. The values mentioned hereto key 4 in the reproducer circuit. Before the key fore are for a very “dead” studio, and in most is opened, the gain control potentiometers 6 and 9 are adjusted to provide a D.-C. voltage at the 15 instances the signi?cant or steady meter reading may be observed for substantially longer inter point V of the order of 200 volts. The negative potential derived from the sliding contact of the vals. Fig. 2 is a graph indicating the grid potentials second potentiometer 9 will, by means of the diode (as indicated on the meter It) with respect to recti?er l0, drive the cathode of the triode H to time. As explained heretofore, the meter may almost 200 volts below ground, if the battery be calibrated in seconds of time constant of sound voltage is chosen so that the control electrode decay. Curve A is an indication for one prede will be su?iciently more negative to just block off termined reverberation while curve B is a similar the anode current of the triode II. indication for a second predetermined reverbera If it is assumed that the sound decay charac teristic in the studio is logarithmic 25 tion. The substantially ?at portion, of the curves A and B, between the dash lines, is the time in terval during which signi?cant indications may be observed. where E is the sound density at time t after decay In Fig. 1, an anode battery 18 may be inserted, has commenced, E0 is the original‘ sound density, 30 by means of the switch l9, between the anode of e is the natural logarithmic base, it is the time of the triode l i and ground. By proper selection of sound decay, and the triode H, with this arrangement, the signi? cant portions of the graphs A and B may be made 4V extremely ?at, since during the useful observa-. 35 tion interval, the tube transconductance will be where V is the volume of the studio,‘ 0 is the substantially constant, With the switch in either velocity of sound and A is the total number of position, the anode resistance of the tube H is Sabine absorption units exposed to the sound. substantially constant during the interval of sig Then the voltage V Will decay likewise. As the ni?cant or steady meter'reading. voltage at the negative terminal of the recti?er 40 I claim as my invention: falls, i. e., approaches ground potential, the re 1. The method of measuring reverberation of “at sultant voltage is applied to the control electrode ' sound waves in a closed chamber by means in of the triode ll. As the grid Voltage approaches ‘cluding a thermionic tube, comprising translate ground, the capacitor 13 will tend to maintain its ing said sound waves into potentials proportional charge and will tend to maintain the cathode of to the amplitude of said waves, applying said po the triode H at its previous potential. Since the tentials to said tube to vary the anode resistance grid voltage is approaching ground the triode ll thereof, and indicating the time constant of said becomes conducting, thereby tending to bring the tube anode circuit in terms of sound decay time. cathode potential up toward the grid potential. 2. The method of measuring the reverberation Should the triode ll draw too much anode cur time of sound waves by means including a ther 50 rent so that the cathode potential tends to rise mionic tube having a reactive anode circuit, com above the control electrode potential at a given prising translating said sound waves into a1ter~ instant, the tube will tend to cut off until the con nating potentials characteristic of the amplitudes trol electrode potential again rises to the point of said waves, rectifying said potentials, applying of equilibrium. The e?ect of these potential said recti?ed potentials to said tube to vary the variations will be an automatic compensation of anode resistance thereof, and indicating the time the anode resistance of the triode H to a value constant of said tube anode circuit in terms of such that the time constant (RC) of the anode sound decay time. circuit is equivalent to the time constant of the 3. The method of measuring the reverberation sound decay in the studio. As this automatic compensation is obtained, the cathode and con- - trol electrode potentials will automatically tend to equalize. If the tube and associated circuit values are properly chosen, the grid bias, as indicated by the meter ill, will be a direct indication of the time constant of the anode circuit. This charac teristic follows because the time constant is pro portional to the resistance of the anode circuit (which is varying) and to the capacity (which is constant). The meter It may be calibrated in “seconds” of time constant. If a tube having a remote cut on" potential is used, suitable well time of sound waves by means including a there mionic tube having a reactive anode circuit, com prising translating said sound waves into alter nating potentials characteristic of the amplitudes of said waves, rectifying said potentials, normally ' biasing said tube to anode current cut-off condi tion, applying said recti?ed potentials to said tube in opposite polarity to said bias to vary the anode resistance of said tube, and indicating the time constant of said tube anode circuit in terms of said reverberation time. EDMOND S. WINLUND.