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Jan. 7, 1947.
E. s. WINLUND
'
2,413,936
REVERBERATION METER
Original Filed June 30, 1942 '
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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.
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