US 3204031 A
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Description (OCR text may contain errors)
1965 R. GORIKE ETAL 3, 7
MQVING-COIL MICROPHONE ARRANGEMENT Filed Aug. 29, 1961 l MICROPHONE) MICROPHbfi Z I l x X I 6 T 14 Y i 1:
| FILTER Fi l-T ER 21 AMPLIFIEE RUDOLF G'C'DRIKEY KARL SCHUSTER INVENTORS.
AGENT United States Patent 3,204,031 MOVING-C011. MTCROPHUNE ARRANGEMENT Rudolf fiiirilre and Karl 'Schuster, both of Vienna, Austria, assignors to Akustische U. Kino-Gerate Geseilschaft m.b;H., Vienna, Austria, a firm Filed Aug. 29, 1961, Ser.=N0. 134,669 11 Claims. (Cl. 179-1) The necessity of causing sound waves to act on a diaphragm from both sides to achieve a directional pattern with moving-coil microphones involves difiiculties if a wide frequency range is to be utilized. This is due to the fact that the requirement for mass retardation of the vibrating system (diaphragm) necessitates a sufiiciently compliant mounting of the diaphragm (low restoring force). It has been found in practice that this results in an unsteady frequency response in the higher frequency range.
To provide dynamic microphone arrangements having a unidirectional pattern, two independent individual microphones, namely, a moving-coil pressure transmitter and a ribbon microphone, have been arranged one beside the other and electrically connected together. This resulted in a transformation of the omnidirectional and bidirectional patterns into a unidirectional pattern.
As contrasted therewith, the invention relates to a moving-coil microphone arrangement in which at least two mechanically independent individual microphones are arranged close to each other electrically connected together, and the individual microphones having substantially equaldirectional patterns and optimum frequency responses in different frequency ranges.
It is known to allocate different frequency ranges to two or more individual loudspeakers. This allocation has the purpose to obtain the necessary sound volume in the lower frequency range by the use of a sufiiciently large diaphragm area and to obtain a propagation of sound at high frequencies in a wide angle by the use of a stiff diaphragm which is as small as possible.
According to the invention, the diaphragms are of substantially identical surface area but differ substantially in their mounting and the acoustic elements coupled to the diaphragm.
The low-frequency diaphragm has suitably coupled to its rear side the acoustic analog of a resistive-reactive transit-time system, specifically an LR system, whereas the acoustic analog of another resistive-reactive transittime system, i.e., an RC system, is suitably coupled to the rear side of the high-frequency diaphragm. The analog of an LR system comprises an acoustic mass (L) in a duct which opens into a shallow air chamber behind the diaphragm, this air chamber having connected to it an acoustic frictional resistance (R) which opens into a sufficiently large chamber shut off from the external sound field. The analog of an RC system comprises an air chamber (C) and an acoustic frictional resistance (R), both of which are coupled to the rear side of the diaphragm, the resistance leading to the external sound field. Both systems are known to provide the required phase shift and are referred to as transit-time systems. The high-frequency diaphragm may also have the analog of an LR system coupled to it. Both RC and LR analog systems may be coupled to one and the same diaphragm.
The point of division between the frequency ranges is suitably so selected that the high-frequency microphone begins to predominate where the frequency curve of the low-frequency microphone begins to become unsteady. In practice, this point of division will lie between 300 and 1000 cycles, depending on the design of the individual microphones.
The diaphragm of the low-frequency microphone may be gripped under such stress as to resonate between 120 I ice and 400 cycles. Owing to the mass loading of the diaphragm by the acoustic mass L of the associated transit tirne system, the natural frequency of the overall vibratory system is shifted in known manner toward the lower end of the acoustic band (e.g., 40 cycles). [To increase the pressure gradient, one or more tubes 10-30 'centime ters long may be coupled to the diaphragm in known manner. In this case to constitute the analog of the' inductive branch (L) of the system in thefrequency re-' sponse are not to be expected up to 500 to 1000. cycles.
The high-frequency microphone may also be optimally designed and have a relatively stiffly gripped diaphragm so as to exhibit a relatively elevated inherent resonant frequency (400800 cycles). An acoustic transit-time system may be provided on the rear side of the diaphragm (RC or LRsystem) to ensure a very smooth frequency response in the higher frequency range and a constant twoway damping.
The two microphones are advantageously arranged so that the low-frequency microphone lies behind the highfrequency microphone. A disturbing influence, due to mutual interference, upon the frequency response and directional pattern in the overlap range is not to be expected because the sound wave length in that range is a multiple of the selected distance between the microphones which, therefore, receive the incident acoustic waves at these (and lower) frequencies nearly in phase.
The electrical circuitry connecting the microphones may include electrical filters. The microphone'outputs may be directly connected to a common amplifying channel, or individual amplifying channels may be provided which have outputs that are connected together. Such variations are basically lcnown per se.
it will be understood that any desired frequency range can be divided according to the invention into adjoining subranges or bands associated with more than two'microphones. Nor is the invention restricted to directional microphones; it is also applicable to pressure transmitters having an omnidirectional pattern.
An embodiment of the invention is diagrammatically illustrated in the accompanying drawing, in which:
FIG. 1 is a block diagram showing the microphone arrangement; Y
FIG. 2 is a diagrammatic representation of a high-frequency microphone provided with an analog RC system; and
FIG. 3 'is a diagrammatic representation of a low-frequency microphone provided with an analog LR system.
With reference to FIG. 1, a high-frequency microphone 1 and a low-frequency microphone 2 are secured in a common housing 3 and electrically connected together in series, and to an amplifier 4, via respective filters 21, 22 designed to suppress undesirable frequencies.
In FIG. 2, which illustrates a representative embodiment of high-frequency microphone 1, a shallow air chamber 5 is defined by a diaphragm 6 and a rear wall 12 comprising three acoustic frictional resistances (R) designated 7, 8 and 9. The resistances 7 and 9 lead to the outside air. The resistance 8 leads to a closed chamber 10 confined by a pot-shaped housing member 11 and constituting an acoustic capacitance (C).
In FIG. 3, showing a representative embodiment of low-frequency microphone 2, a shallow air chamber 13 is defined by a diaphragm 14 and a rear wall 15 comprising an acoustic frictional resistance (R), designated 16, leading to a closed chamber 17 confined by a pot-shaped housing member 18. Chamber 17 contains a mass of air, larger than that present within chamber 10, which adjoins the rear side of the frictional resistance 16 and is otherwise separated from the outside air. Radially outwardly of the chamber 17, tubes 19, which contain air columns acting as inertances or acoustic inductance equiv- 3 alents (L), open into the air chamber 13 at the rear wall 15. At their other end, these tubes 19 open into the atmosphere. The air cushions 5 and 13 can, of course, also be considered as acoustic capacitances (C).
As will be seen from FIGS. 1-3, microphones 1 and 2 have their diaphragms 6, 14 facing in the same direction and centered on a common axis, these diaphragms spanning the open front ends of the microphone housings (Whose rear ends are partly open into the atmosphere at 7 and 19). The diaphragm 14 of the low-frequency microphone 2 faces the rear of the high-frequency microphone 1, the two microphones being separated by a distance which, as previously pointed out, should be a fraction of the acoustic-Wave length in the overlapping region of their respective frequency bands. Since the axial length of microphone 1 is less than that of microphone 2, as seen in FIGS. 2 and 3, this arrangement affords maximum flexibility in. selecting the spacing of the two microphones from each other.
The diaphragms 6 and 14 are of the moving-coil type, yet their conventional coils and associated circuitry have not been illustrated.
What is claimed is:
1. A microphone arrangement comprising at least two mechanically independent directional moving-coil microphones separated from each other by a fraction of a wavelength at a predetermined acoustic frequency, each of said microphones having a housing with an open front end and a partly open rear end, each of said microphones further having a vibratory system in said housing including an exposed diaphragm spanning said front end and an air mass adjacent the rear surface of said diaphragm, said diaphragms facing in the same direction, the vibratory system of one of said microphones having a natural frequency above said predetermined frequency, the vibratory system of the other of said microphones having a natural frequency below said predetermined frequency, and a common output circuit for said microphones.
2. A microphone arrangement as defined in claim 1 wherein the housing of said one microphone has at least one rear aperture relatively close to its front end, the housing of said other microphone having at least one rear aperture relatively remote from its front end.
3. A microphone arrangement;'as- -defined in claim 2 wherein said rear aperture of said one microphone is provided with frictional acoustic resistance means, the housing of said other microphone forming at least one rearwardly directed elongated tube terminating at said rear aperture thereof.
4. A microphone arrangement as defined in claim 3 wherein the diaphragms of said microphones are centered on a common axis, the diaphragm of said other microphone facing the rear of the housing of said one microphone.
5. A microphone arrangement as defined in claim 1 wherein said common output circuit includes filters individual to said microphones.
6. A microphone arrangement as defined in claim 1 wherein said predetermined acoustic frequency lies in the range of 300 to 1000 cycles per second.
7. A microphone arrangement comprising a high-frequency and a low-frequency directional microphone of the moving-coil type facing in the same direction while being separated from each other by a fraction of a wavelength at a predetermined acoustic frequency, said highfrequency microphone including a first housing With an open front end spanned by a first diaphragm and a first transmit-time system constituted by a rear portion of said first housing behind said first diaphragm, said rear portion having at least one restricted rear aperture relatively close to said first diaphragm, the combination of said first transit-time system and said first diaphragm having a natural frequency above said predetermined acoustic frequency, said low-frequency microphone including a second housing with an open front end spanned by a second diaphragm and a second transit-time system constituted by a rear portion of said second housing behind said second diaphragm, each of said second systems being the acoustic analog of a predominantly resistive-reactive electric circuit, the last-mentioned rear portion having at least one restricted rear aperture relatively remote from said second diaphragm, the combination of said second transit-time system and said second diaphragm having a natural frequency below said predetermined acoustic frequency; and a common output circuit for said microphones.
8. A microphone arrangement as defined in claim 7 wherein said first diaphragm is stressed to an inherent natural frequency in a range of 400 to 800 cycles per second, said second diaphragm being stressed to an inherent natural frequency lying in a range of 120 to 400 cycles per second.
9. A microphone arrangement as defined in claim 8 wherein the natural frequency of the combination of said second diaphragm and said second transit-time system is near the lower end of the acoustic band.
10. A microphone arrangement as defined in claim 7 wherein said first transit-time system is the acoustic analog of a predominantly resistive-capacitive electric circuit.
11. A microphone arrangement as defined in claim 7 wherein said second transit-time system is the acoustic analog of a predominantly resistive-inductive electric circuit.
References Cited by the Examiner UNITED STATES PATENTS 2,262,146 11/41 Massa 179-1 2,305,599 12/42 Bauer 179-1 2,702,318 2/55 Dvorsky 179111 FOREIGN PATENTS 884,516 7/53 Germany.
OTHER REFERENCES Friedman et al.: (A New Cardioid) Tele-Tech and Electronic Industries, October 1955; pages -72 and 129-433.
ROBERT H. ROSE, Primary Examiner.