US2496031A

US2496031A - Dual microphone sound detector system

Info

Publication number: US2496031A
Application number: US794740A
Authority: US
Inventors: Leslie J Anderson; Alfred H Kettler
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1947-12-30
Filing date: 1947-12-30
Publication date: 1950-01-31
Anticipated expiration: 1967-01-31

Description

Jan- 3l, 1950 -ANDERSON ErAl. 2,496,031

DUAL MICROPHONE SOUND DETECTOR SYSTEM Filed Dec. 50, 1947 WARM/v6 /26' 647-?6 /f 6 myZ/7l INVENToRs.

Patented Jan. 31,v 1956 DUAL MICROPHONE SOUND DETECTOR SYSTEM Leslie J. Anderson, Moorestown, `and Alfred H. Kettler, Collingswood, N. J., assignors to Radio Corporation of America, a corporation of Dela- Ware Application December ao, 194i, serial No. 794,740

7 Claims. l

It is a primary object of this invention, to provide an improved directional microphone system, having a pair of spaced microphone units, for producing an adjustable directional characteristic with minimum response to ambient sound, whereby the system may be utilized without prior limitations, for directional sound detection andthe like on moving vehicles, shipboard and the like.

It is also an object of this invention, to provide a directional microphone system as above referred to wherein each microphone unit has a unidirectional sound response characteristic superimposed upon the directional characteristic of the system.

It is a further object of this invention, to provide an improved directional microphone system of the type referred to for the detection of sound W-aves, having adjustable electrical means of the low pass lter type for producing a time delay which is substantially uniform over a relatively wide predetermined low frequency range with negligible amplitude effects, whereby a directional pattern, uniform with frequency, may be obtained from the system.

An improved directionalffmicrophone system in accordance with the invention may comprise a pair of acoustically spaced microphone units arranged in tandem on a common axis and facing in the same frontal direction,lan adjustable electrical network for controlling the directional pattern of the dual microphone arrangement, the electrical network comprising a. multiple unit low pass filter permitting relatively long electrical delay with corresponding improved low frequency response proportional to the spacing of the microphone elements, and an output low frequency band pass filter.

The dual microphone system of the present invention is particularly adapted for use in navigation to warn ships of the proximity of navigation hazards over short distances through fog and the like. and as a means for detecting the close approach of aircraft.

The invention is, however, not limltedto use in navigation on ships but may be used as a directional sound detector on stationary or movable equipment to warn of the approach or proximity to any sound emitting object such as an approaching vehicle or ship, or with respect to a fixed traflic point or beacon, for example.

In operation, a dual microphone comprising a pair of microphone units, each preferably beingA unidirectional in its acoustical response characteristic, is arranged or located in a regionA of low ambient sound such as at a point high up on a mast of a ship or frontally of a moving vehicle with the microphone units arranged on a common axis pointing upwardly or frontally as the case may be, in the samedirection on said axis.

The microphone units are suitably screened from wind noises and because of their unidirectional response characteristic are rendered substantially non-responsive to local sounds, as from the deck of the ship or from the moving vehicle as aforementioned, whereas the microphone units are relatively sensitive in the opposite direction, to sound waves applied thereto from sound sources to be detected.

Because of the nature of ambient sound on shipboard and on moving vehicles, the problem of spacing the microphone units to obtain a phase displacement in the response to sound Waves applied thereto involves a consideration of the overall space required for the dual microphone and for the electrical delay network required. This is because of the fact that the normal sound frequencies to be detected lie in a relatively low frequency band, for example between and 300 cycles. The electrical delay network elements required to introduce delay between the output of onemicrophone and the main system would assume relatively large proportions physically unless the microphone units are relatively Widely spaced and this has heretofore been one of the limiting factors in the production of a suitable dual microphone sound detector system of practical dimensions for use on shipboard, moving vehicles and the like.

It is therefore a still further object of this invention, to provide an improved dual microphone system having a controllable directional response characteristic, which may be made t0 operate with a relatively short acoustical spacing between the microphone units and with a `time delay and electrical control circuit connected therewith of compact design adapted to fit into a small space in the equipment.

Frequency discrimination between local or ambient sounds and sound waves to be detected is a serious problem by reason of the fact that the sound waves to be detected generally lie in -a low frequency range of approximately 100 to 300 cycles per second.

Heretofore the design of such low frequency acoustical response systems of the dual microphone type has been limited for the reason that the sensitivity is largely determined by the spacing of the microphone units. To obtain a large electrical time delay with a single element filter as in a frequency range between 100 and 300 cycles per second, for example, would be impossible. Single filter elements render the system wholly limited to very small time delays.

Accordingly it is an object of this invention, to provide a dual-microphone sound-detector system with two acoustically spaced microphone units of the phase shifting type having a frontto-rear discrimination with respect to received sound waves and a directional characteristic which is frontally similar to the usual cardioid, and an adjustable electrical time delay and amplifying network provided with correlated low pass and band pass filters, whereby the directional characteristic of the microphone units may be superimposed on the directional characteristic due to the use of two acoustically spaced microphone units.

It is a further object of the invention also, to provide a directional microphone system of the character above referred to, in which the cornbined signal output from the dual microphone units with a controllable phase-shift time delay and band pass lter, is provided further with automatic volume control in advance of the band .g pass lter, whereby the system is adapted for operation in connection with sensitive tripping and warning devices.

The invention will further be understood from the following description, when considered in connection with the accompanying drawing illustrating a preferred embodiment of the invention, and its scope will be pointed out in the appended claims.

In the drawing;

Figure l is a schematic circuit diagram of a dual microphone system embodying the invention;

Figures 2 and 3 are vector diagrams illustrating certain operating characteristics of the system of Figure 1;

Figure 4 is a further diagram showing an acoustical response pattern for the system of Figure l; and

Figure 5 is a schematic circuit diagram of a portion oi' the system of Figure 1 further illustrating the embodiment of the invention shown therein.

Referring to Figure 1, a dual microphone assembly is provided by a frontal or upper microphone unit 6 and a rear or lower microphone unit 1, spaced on a common longitudinal axis 8 a predetermined acoustical distance (d) apart as indicated.

The microphone units are enclosed in a suitable wind screen or sound permeable housing 9 as indicated in dotted outline, which may be of the type as shown, described and claimed in our copending application Serial No. 5,436, filed January 30, 1948, for Microphone wind screens.

The microphone units 6 and 1 are preferably of the phase shifting type, having a front-torear discrimination for receiving sounds with a high degree of response or sensitivity from a frontal or vertical direction, as indicated by the arrow I, and having substantially negligible response to sounds or sound waves from the opposite longitudinal direction as indicated by the arrow II. However, satisfactory operation has been obtained with microphones having frontto-rear discrimination of approximately 20 db.

' The directional response characteristic is similar tothe cardioid pattern I0 drawn about the axis 8 with reference to the microphone units 8 and 1, as schematically shown in Figure 4, for example. It will be seen from the diagram that the microphone response characteristic has a high degree of rejection at angles between degrees and degrees. l

On shipboard, the microphone assembly and wind screen are mounted as high as possible above the deck and superstructure so that noises from the deck as well as noises produced by wind through the rigging may be attenuated as much as possible before reaching the microphones. The ambient or deck noises are further attenuated by the microphone characteristics as referred to in connection with Figure 4, orientated as shown, and in addition by utilizing two microphones spaced an acoustical distance (d) apart and associated with a suitable time delay network, thereby obtaining further improved ratio of signal to noise as will hereinafter appear. Y

A high degree of discrimination may therefore be obtained with respect to sounds coming from the direction I as compared with sounds coming from the direction II. The microphones 6 and 1 therefore, may be arranged to have unidirectional response or a high degree of sensitivity in the same direction, with regard to sounds coming from the direction I. The wind screen 9 must be effective to reduce relatively high wind velocities to substantially zero about the microphone units.

The frontal or upper microphone unit 6 is connected through a suitable electronic preamplifier I2 with a signal mixer device I3 in the form of a potentiometer resistor having an adjustable center tap output tap connection I4 and end terminals I5 and I6, the terminal I5 being connected with the output of the preamplifier I2 through a circuit connection indicated at I1.

In a similar manner, the rear or lower microphone unit 'I is connected through a second electronic preamplifier I8 with the mixer device I3 at the opposite terminal I6, with a phase-shifting time-delay network indicated at 20 and hereinafter described, interposed in circuit between the preamplier I8 and the terminal I6.

The signal output voltage from the frontal or upper microphone 6 and corresponding in amplitude to the amplitude of a received sound wave, is indicated at er, while the corresponding signal output voltage from the lower or rear microphone I is indicated at e2. In response to sound waves from below or the rear, the amplified output voltage e1' at the terminal I5 is opposed to the amplied and delayed voltage e2 which is indicated at the terminal I6 as e2'.

The potentiometer I3 and variable contact I4 is provided for the purpose of making the magnitude of the voltage e1 equal to the magnitude of the voltage e2 at the output connection I4, in case the voltages e1 and e2 are not exactly equal in magnitude or the gain of the two preampliers are not identical. This is effected by shifting the tap I4 to the exact zero potential point on the resistor I3 in each case, thereby reducing substantially to zero, the voltage output in response to sound waves from the rear or below.

In response to signals or sound waves to be detected, received from the front or from above. the voltages e1 and e2 and e1 and e2' to a greater extent, are out of phase and vectorially aiding to provide a vector voltage at eo which may be of appreciable magnitude. For' the usual range of sounds to be detected, the output voltage eo is at low audio frequency and in the relatively low frequency range of from 100 to 300 cycles for example. This operating or output voltage to be detected, is further amplified in an audio frequency amplifier 2| connected with the output contact |4 whereby it is raised to a useable magnitude for operation of a detecting device or other utilization means. In the present example, the amplifier 2| is provided with an output connection indicated at 22, through an automatic volume control circuit indicated at 23, and a band pass lter indicated-at 24, with utilization or detectingmeans comprising an electronic tripyping or detecting device responsive to an ampliiied voltage eo of predetermined magnitude, a suitable warning device 26 connected with and controlled by the tripping device 25.

The automatic volume control system 23 controls the gain of the amplifier 2| through an automatic volume control connection indicated at 21. The band pass filter 24 is designed to pass a predetermined band of audio frequency signal which is from 100 to 300 cycles, beingthe correspond to the time interval between sound arriving at the two microphones when coming in at some angle from below or from the rear, rather than from directly below (direction II), then the response will be a minimum at that angle. Thus it is possible to reduce the effect of sounds which may not be directly below or in rear of the microphone assembly. Since the individual microphone units are also preferably directional, their characteristic is superimposed on the directional characteristic due to the use of two microphones with a time delay. This arrangement gives very high rejection to sounds from below or from the rear.

In detecting the approach of aircraft, for example, from above it has been noted that the frequency spectrum for aircraft in flight extends from the fundamental, usually between 40 and 100 cycles per second. At the low frequency end, the second and third harmonics are usually prominent though somewhat below the fundamental. Harmonics above the fourth decrease rapidly to about 1000 cycles.

At the low end of the spectrum the harmonics appear as peaks and provide a, fairly high background because of the engine exhaust and other plane noises. With regard to ambient noises on shipboard, for example, excluding wind noise, the chief source of noises are engine exhaust, blowers and machinery.

The combined wind noise and ship noise as experienced at various points on the ship has been found to present a fairly continuous time spectrum which attenuates rapidly above 130 cycles. Furthermore, the noise produced by specific pieces of machinery shows a spectrum with distinct peaks, the largest of which occur at or below 130 cycles. Accordingly. in the present example the low frequency cutoff of the lter 24 is made of the order of 150 cycles.

The band pass filter 24, therefore, effectively reduces the amount of undesired noise pickup by the microphone. The pass band is made as narrow as possible without interfering with the desired sound pickup, which in the present example may be considered to be of the order of from to 300 cycles per second. The pass band -is so located that a favorable ratio of sound level to ship and wind noise is provided. However, it is possible that' for some particular installations a narrow, lower or higher band may be desired.

The automatic volume control device operating on the overall output signal before it reaches r the band pass filter, serves to improve the operation in reducing the amount of attention required. The ratio of sound inside the filter band to that outside the filter band is generally fairly constant for a given installation and the operating conditions involved, and the automatic volrume control device is adjusted to operate in response to a change in the prevailing condition.

The time constant of the automatic volume control circuit may be made such that for small changes in noise level the automatic volume control output will return to-near its former value in a few seconds, in this case in slightly over two seconds. When a plane approaches, the overall noise level will not rise sufficiently to operate the volume control before the sound level in the filter band has increased sumciently to operate the tripping device 25 and the warning device 26 controlled thereby. The control by the automatic volume control system may be reduced under very low noise conditions if desired.

Operation of the dual microphone system is I as follows:

Assume a sound wave to reach the microphones from a direction II, that is from the rear or below, the voltage e2 will be generated in the lower microphone 1 corresponding to the instantaneous sound pressure. As the sound wave reaches the upper microphone 6, a corresponding voltage e1 will also be generated there. However, there will exist a time interval between action at microphone 1 and microphone 6 corresponding to the time required for the sound wave to travel from one microphone to the other since they are acoustically spaced. If now the electrical time delay is so set by the delay network 2|) that the delay which it applies to the voltage of the microphone 1 is the same as that occurring to the voltage from microphone 6 resulting from the travel time of the sound wave from microphone 1 to microphone 6 the two voltages e1 and ez will cancel when added vectorially at phase by the mixer device |3-l4.

Likewise, if the sound approaches from an angle, such as indicated by direction lII, of Figure l, there will again be a delay in the voltages generated by the two microphones 6 and 1 because of a difference in the acoustic paths to said microphones. The amount of this delay is determined by the particular direction, from which the sound approaches, always being less 76 tical time delay plus an electrical time delay for the voltage from the lower or rear microphone 1. When added in opposition vectorially in the mixing circuit, there will be a phase diiierence corresponding to the double time delay, and the output voltage eo will be a resultant of appreciable magnitude suiiicient to operate the detecting means if within the predetermined selected limits of the band pass filter 24 in frequency.

Specifically, the relations may be illustrated in the vector diagrams of Figure 2 and Figure 3. Referring to Figure 3, when sound approaches from below or the rear (direction II) the voltage output ez corresponding to the lower microphone output is opposed to the voltage output el' corresponding to the voltage of the upper microphone channel, which has been acoustically delayed by the phase angle due to the path difference which may be 1/4 wavelength.

The voltage e2 is reversed in phase effectively by the circuit connections to the mixer device l3-l4, with respect to ei and is shown as -ez in the vector diagram. This voltage -ez is delayed by the electrical network by a corresponding phase angle 0. The electrical network is adjusted to provide a condition where 0' equals 0, making e2 the vector which will be added to e1 and with the contact I4 adjusted to the neutral point the vector sum at eo will be zero.

For the representation of sound approaching from above or frontally, Figure 2 shows ei' as the vector corresponding to the forward or upper microphone voltage, While e2 is the vector corresponding to the voltage from the rearward or lower microphone which has been acoustically delayed by the angle 0. This is reversed in phase, becoming -e2. It is then delayed by the electrical network 20 by the angle 0 corresponding to its adjusted delay characteristic and becomes e2'. The vector sum of e2' an'd er' is eo which is the output voltage of the mixer network iii-I4, and may thus be of such value that it operates the utilization means, which in this case is the tripping device and the warning device 26 connected therewith.

An electrical time delay may be obtained from zal.:

fiavel of the incident sound wave to be detected.

The time delay of a low pass filter is virtually constant for frequencies up to .4 of the cut-oi! frequency. The phase delay in degrees in this region is very nearly fre cutoii' freq.

and the time delay is then very nearly l 3 (cutoi freq.)

The delay in the present example may be considered as 1 millisecond and the corresponding microphone spacing to be 13.4 inches, for the Vfrequency range presently considered. Figure 5 shows the electrical system used in the network Assuming sounds coming from a direction generally indicated at I in Figure 1 the phase angle between the signal input p1 and p2 for the microphones E and 1 respectively is equal to where w=21rf d=distance between two pickup points 6 and 'I C=velocity of propagation of sound in air This equation represents two vectors,

d b 1 cos 21:(- cos dri-) and sin 21C:- cos ivi-itl) at right angles to each other. They are added vectorially by taking the square root of the sum of their squares.

filter circuit one or more small incremental elements or sections, thereby to change the directional characteristic of the microphone system by If the individual voltages e1 and ez are cardioid in characteristic, the above Equation III may be used as an operator to secure variations of the cardioid characteristic by adjusting as indicated in Figure 4. In Figure 4 the main cardioid peak I0 results when d or b are wavelength and the double lobed response indicated by the dotted figures 28 and 29 results when d or b are equal to 1/2 wavelength of the sound at the frequencies being received.

With the microphones arranged in tandem relation, facing in the same direction on a common axis in accordance with the invention, the optimum directional characteristic may be obtained when the microphone spacing (d) corresponds to 1A wavelength at approximately the highest frequency at which directional properties are desired, and the phase shift in the time delay circuit is of corresponding value.

Further in accordance with the invention,

III

therefore, the time delay is obtained through the use of a multi-section low pass lter. As pointed out, such a filter structure shows approximately constant time delayup to .4 of the cutoff frequency and numerically the time delay is approximately equal to ,/axcut-oif frequency. In the present case, where it is desired to introduce a xed and a variable increment in time delay below 400 cycles per second, a cut off frequency of 1000 cycles per second may be used.

The structure of such a filter 'network 20 is shown more in detail in Figure 5 to which attention is now directed. In Figure 5, the lter comprises an input terminal 30 and an output terminal 3| together with a grounded throughconnection provided `between the terminals 33 and 34, with a series of low-pass audio frequency lter inductances 35 and 36 connected serially between the terminals 30 and 3| through a fixed inductance 31"'at the terminal 3| and a connection lead 38. The series inductance elements 35 together with shunt connected filter` capacitors provide a fixed, multiple section, low-pass filter terminating in a switch 4| having a connection point 42 with the rst of the series of inductance coils 36 and a second connection point 43 with the lead 38.

The first lter inductance 36 has an output connection with a second switch arm 44 and a shunt filter capacitor 45, and the switch arm in turn has a connection point 46 with a succeeding filter inductance 36 in the series before mentioned, and an alternative switching connection 41 with the lead 38. The successive coils following in order are similarly connected through switches indicated at 50, 5|, 52 and 53 for connecting successive series filter inductances 36 and shunt lter capacitors in circuit with the terminal 3| through the common inductance 31 and the lead 38 to add or subtract from the overall adjusting the time delay in the filter network. This provides for the discrimination against a particular source of noise or sound from below or to the rear of the microphone array as previously described, and a time delay which is very uniform over a considerable range of frequency with negligible amplitude effects, thus'giving a directional pattern uniform with frequency. The number of fixed units is such that with the switch 4| on -point 43, the electrical time delay is minimum, and with the last of the switches 53 closed to the lead 38 the electrical time delay is maximum, the other switches being closed to complete the series connection through all of the inductance elements.

By providing simple switching for a series of ladder type low pass filter elements as shown in Figure 5, for use in the time delay network and adjustment may be provided from any remote point, thereby to adjust the directional pattern of the microphone system.

The use of a multiple-section electrical time delay rather than a single reactive element permits long electrical delays relatively, with consequently greater low frequency response, which for a given microphone element, will be proportional to the spacing of the two elements.

It may be pointed out that the sensitive tripping device shown in Figure 1 is capable of operating on changes of input signal level from several db. to a fraction of a db. depending upon the adjustment of the circuit and the sensitivity of the tripping device which may be of any suitable electronic type.

From the foregoing description it will be seen that in a low frequency sounddetection system, a low pass filter providing approximately constant time delay up to four tenths of its cut-ofi frequency may be used as an effective phaseshift time-delay network in which the phase shift characteristic is used to obtain a time delay effect. In this manner limitation on the amount of the time delay is removed as a problem of design and any desired spacing of the microphone elements may be obtained.

Furthermore with such a delay network, for the detection of sounds in the low frequency range of from 150 to 300 cycles, the microphone units may be spaced only slightly over one foot apart, and to reduce Wind noises to a minimum, the screen 9 may be less than one and one-half feet in diameter, so that the overall dimensions of the sound pickup device or microphone arrangement may be relatively small and easily placed on any vehicle or ship and even more readily at a fixed location.

Likewise the phase-shift delay-network may be provided within reasonably small dimensions,

and the remainder of the circuit is easily designedfor compact mouning in small space, s0 that a sound system embodying the invention may be designed to the practical limitations of relatively small physical size while retaining and providing desirable electrical directional characteristics which may be adjusted as desired and adapted for remote control.

We claim as our invention: 1. In a dual microphone sound detector sys- 20, the electrical system is readily adjustable,A

tem, the combination of a pair of microphones having directional characteristics, said microphones being mounted in predetermined spaced relation on a common axis for preponderant sound response in one direction, a circuit connected with said microphones for mixingr the signal voltage output therefrom, a multiple section W pass filter network connected in circuit with one of said microphones, said network being adjustable in incremental steps to provide a predetermined electrical time delay in circuit with said microphones corresponding to a predetermined acoustical time delay established by the spaced relation thereof. an output circuit connected jointly with said microphone units through said signal mixing circuit for receiving output voltagestherefrom in aiding phase relation vectorially to provide a resultant operating voltage, a band pass filter, and a utilization device responsive to said voperating voltage connected with the output circuit through said band pass filter. l' i 2. In a dual microphone sound detector ystem, the combination of a pair of microphones having substantially directional response characterisics. said.microphonesv being: Inountedlinipr determined spaced 'relationgon"` a common siffnal mixing device connected between lsaid f microphones for jointly' receiving signal output voltages therefrom. a low pass lternetworkiin circuit between said mixing device and yone of said microphones for adjusting the vectorial yre,- lation of said voltages to a predetermined lvalue. a circuit forderiving the vector sum of sairi output voltages from said mixing device, va band i pass filter in said circuit, and utilization means responsive to a predetermined output voltage connected with said last named circuit through :iii-

"substantially uniform with frequency over the 5. In a sound detector system, the combination of a pair of acoustically spaced microphone units arranged in tandem relation on a common axis and facing in the same frontal direction, each of said microphones having a cardioid response characteristic and adjustable electrical network for controlling the directional pattern of said dual microphone arrangement. said network comprising a multiple section low pass filter providing relatively long electrical delay in circuit with a rearward one of said microphone units, a mixing circuit for vectorially adding the output voltages of said microphone units through said electrical network, and a band pass filter connected with said mixing circuit for deriving therefrom the vector sum of said output voltages within a. predetermined frequency range.

6. In a dual microphone sound detector system, the combination of a pair of microphones mounted in predetermined spaced relation on a common axis in a forward'and rearward relation to each other for preponderant sound response and detection in the same forward direction, said microphones being of the phase shifting type having a front-to-rear discrimination with respect to received sound waves, an

'adjustable electrical time delay network in circuit with `the rearward onev of said microphones comprising `a series of low pass filter sections provi'ding a relatively long electrical delay which is l` `operating range of said system, a signal output said band pass filter for indicatingl the presence of sound waves applied to said microphones within the passband ofsaid lter.

3. In a dual microphone sound detector systern, the combination defined in claim 2, wherein the low pass filter network comprises a series of low pass lter sections and switching means for changing the number of sections included serially in said network, therebyprovidingr an.

adjustable and substantially uniform time delay over a relatively wide predetermined frequency range with negligible amplitude effects, and a directional sound response pattern uniform with frequency.

4. In a dual microphone sound detector system, the combination of a pair of `microphones having substantially unidirectional response characteristics, said microphones being mounted in predetermined spaced relation on a common axis for preponderant sound response and detection in the same direction, a signal mixing device connected between said microphones for jointly receiving signal out-put voltages therefrom in opposed relation` a low pass filter network in circuit between said mixing device and one of said microphones for adjusting the vectorial relation of said voltages to a predetermined valuesubstantially uniform with frequency over the response range of said system, a circuit for deriving the vector sim of said output voltages from said mixing device, and a band pass lter in said circuit having a high frequency cutoff below substantially four tenths of the high frequency cutoff of said low pass-filter network.

voltage mixing circuit for said microphones, an output signal amplifier connected with said mixing circuit, the gain of said amplifier being responsive to variations in the amplitude of sig- Inalsapplied thereto, utilization means responsive to a predetermined signal output voltage connected' with said amplifier, and a band pass lter Aconnected in circuit between said amplifier and said utilization means.

'7. In a dual microphone sound detector system, thehcombination as defined in claim 6 further characterized by the-fact that the low pass lter sections serially included in the electrical rtime delay network are adjustable in number, Awith a high frequency cutoff Acharacteristic such that the highest received operating frequency for the system is of the order of four tenths ol said cutoff frequency, and that the pass band of the band pass filter is limited to a desired range of operating frequencies for which the system is responsive.

LESLIE J. ANDERSON.

ALFRED H. KETTLER.

REFERENCES CITED The. following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,166,991 Guanella- July 25, 1939 u, 2,251,708 Hefele Aug. 5, 1941 2,349,370 orner May 23, 1944 2,441,554 Barker May 18, 1948 FOREIGN PATENTS Number Country Date 493,340 Great Britain Oct. 6, 1938 495,515 Great Britain Nov. 15, 1938 Certificate of Correction Patent No. 2,496,031 January 31, 1950 LESLIE J. ANDERSON ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

AColumns 9 and l0, lines 4 and 5, right-hanlportion of the equation, for

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oce.

Signed and sealed this 26th day of February, A. D. 1952.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,496,031 January 31, 1950 LESLIE J. ANDERSON ET AL.

It is hereby certied that error appears in the printed specification of the above numbered patent requiring correction as follows:

lColumns 9 and l0, lines 4 and 5, rght-hanportmn of the equation, for

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oice.

Signed and sealed this 26th day of February, A. D. 1952.

THOMAS F. MURPHY,

Assistant Uommz'ssz'oner of Patents.