Abstract:
A microphone comprising a diaphragm which has a front diaphragm surface on which sound waves impinge and a rear diaphragm surface which is at least partially acoustically separated from the front diaphragm surface, and a sound inlet, through which sound waves can go to the rear diaphragm surface. In order to improve a strongly frequency-dependent frequency response characteristic in respect of the directional effect of the microphone, the microphone includes at least one damping element and the slot-shaped sound inlet forms substantially an acoustic inductance so that at least a part of the sound waves to be picked up is passed with a delay to the rear diaphragm surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority of PCT Application Serial No. PCT/EP99/07869 filed Oct. 16, 1999 and German Application No. 198 50 298.2 filed Oct. 20, 1998, the complete disclosures of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   a) Field of the Invention 
   The invention concerns a microphone comprising a diaphragm which has a first diaphragm surface which is preferably towards a sound source and on which sound waves impinge and a second diaphragm surface which is at least partially acoustically separated from the first diaphragm surface, and which preferably faces away from the sound source, at least one, preferably slot-shaped, sound inlet, through which sound waves can go to the second diaphragm surface and which forms substantially an acoustic inductance so that at least a part of the sound waves to be picked up is passed with a delay to the second diaphragm surface, and at least one damping element. 
   b) Description of the Related Art 
   U.S. Pat. No. 3,585,317 A discloses such a microphone, in which an acoustic resistance is formed by the slot-shaped sound inlet, in order to damp the sound which passes through the slot-shaped sound inlet. The directional effect of the microphone can be influenced by the sound inlet between the air volume behind the diaphragm and the outside air. So that the slot-shaped sound inlet has the required acoustic impedance its width is small in relation to length. To achieve acoustic inductance, the slot-shaped sound inlet is provided with inserts of hardenable wax. 
   DE 22 17 051 also discloses a microphone which has a slot-shaped sound inlet to form an acoustic resistance. In that case, the sound inlet is in the form of a groove-shaped recess in the magnet system which is made from sintered material. In such a microphone the directional effect of the microphone is heavily frequency-dependent and can be used mostly only for low frequencies. Production of the corresponding slot-shaped sound inlets in the magnet system of sintered material requires special tools, and tuning of the directional effect by varying the slot-shaped sound inlet can be implemented only by replacing the entire magnet system. 
   OBJECT AND SUMMARY OF THE INVENTION 
   The object of the present invention is to develop a microphone of the kind set forth in the opening part of this specification, in such a way that it has a predetermined directional effect substantially over the entire frequency response curve or characteristic and in that respect its acoustic network is dependent substantially on the magnitude of the acoustic inductance and that of the damping element. 
   In a microphone of the kind set forth in the opening part of this specification, that object is attained in that the acoustic resistance which occurs in the sound inlet is less than the acoustic resistance of the damping element. 
   In such a microphone, the directional effect is achieved by a delay in the sound which passes in through the rear sound inlet. The delay in the sound is achieved by means of an acoustic network which has essentially an inductance formed by the slot-shaped sound inlet and a separate damping or attenuating element which forms an acoustic resistance. 
   The advantages of the invention are in particular that it implements a microphone with a directional effect which is constant substantially over the entire frequency range. In addition the acoustic network formed by the acoustic inductance and the damping element can be easily and precisely tuned so that the directional effect of the microphone can be predetermined within wide limits. 
   Admittedly, a parasitic acoustic resistance occurs in the sound inlet of the microphone according to the invention. In order however to make the acoustic network substantially dependent on the magnitude of the acoustic inductance and that of the separate damping element, the sound inlet in the microphone according to the invention is of such a configuration that the acoustic resistance which occurs in the sound passage is lower than the acoustic resistance of the damping element. 
   A preferred embodiment of microphone according to the invention provides that the damping element is formed by a sound passage which is provided with acoustic damping material and which connects a cavity to the volume delimited by the rear diaphragm surface. Tuning of the damping element is predetermined essentially by the size of the volume and the acoustic resistance of the sound passage which connects the cavity to the volume delimited by the rear diaphragm surface. 
   In a further preferred embodiment the sound inlet is of a substantially rectangular cross-section. That cross-sectional shape is easy to dimension in regard to the design of a microphone according to the invention and is easy to carry into effect in terms of manufacture. In that respect, the height of the sound inlet is particularly desirably smaller than its length, in which case the sound flow takes place along the longitudinal direction of the sound inlet and the length of the sound inlet is in turn smaller than the width thereof. The parasitic resistance of the sound inlet is held at a low level by virtue of the fact that the width of the sound inlet is large in relation to the length. In a desirable development the width of the sound inlet corresponds substantially to the periphery of the microphone. In that case, the sound inlet is interrupted only by support portions which are provided for the mechanical stability of the microphone and in particular the sound inlet. In this embodiment the sound inlet is thus formed not by narrow and long passages but by a substantially peripherally extending slot which has an only slight parasitic acoustic resistance and a predetermined acoustic inductance. 
   In a further preferred embodiment the diaphragm is connected to a diaphragm fixing portion. The diaphragm fixing portion serves to carry the diaphragm and to orient it by way of a suitable magnet system in such a way that an oscillating coil fixed to the diaphragm engages into an air gap provided in the magnet system. 
   In a further embodiment the microphone includes a closure element which is arranged in front of a mouth opening of the sound passage and which has an opening which substantially corresponds to the mouth opening of the sound passage and which is provided with the acoustic damping material. The closure element serves substantially to carry the acoustic damping material and to hold it in front of the mouth opening of the sound passage. The acoustic resistance of the damping element can be particularly advantageously changed by interchanging only the closure element and replacing it by another closure element with a different acoustic resistance. In that way microphone casings of the same structure can also be tuned differently by means of suitable closure elements. 
   In order to form the slot-shaped sound inlet through which sound waves can pass to the rear diaphragm surface, in a preferred embodiment of the microphone according to the invention the diaphragm fixing portion has an orifice which leads from the exterior to the rear diaphragm surface and which is substantially closed by a sealing element. In that arrangement the orifice is reduced by the sealing element to such an extent that the slot-shaped sound inlet is formed between the sealing element and the diaphragm fixing portion. 
   It is particularly advantageously possible in that way for the dimensions of the slot-shaped sound inlet to be predetermined by the sealing element which can be dimensioned and produced, independently of the diaphragm fixing portion. The microphone according to the invention can thus in turn be tuned in the desired manner by means of an inexpensive component which is simple to manufacture. There is therefore no longer any need for structural changes to the casing of the microphone or to other components which are to be manufactured with expensive tools. In that case, in a desirable development, the sealing element comprising a porous material and in particular a sintered material. Such material has a high level of internal damping which can improve the acoustic properties of the microphone and it can be easily put into a desired shape. 
   In a desirable development the cross-section of the slot-shaped sound inlet is essentially formed by a recess in the diaphragm fixing portion, the length of the sound inlet being essentially predetermined by the thickness of the sealing element. In that case, the sealing element is preferably of a substantially annular configuration and fits in an annular groove provided in the diaphragm fixing portion. The cross-section of the slot-shaped sound inlet can be predetermined in a simple fashion by the difference in size between the inside diameter of the diaphragm fixing portion and the outside diameter of the sealing element, in which case recesses do not have to be provided at the diaphragm fixing portion. That makes it possible to provide slots of different dimensions, with the same diaphragm fixing portion, insofar as only the annular sealing element is replaced by a different one involving a different outside diameter. 
   If however the slot-shaped sound inlet is completely closed by the sealing element, then only the damping element is effective and the directional characteristic of the microphone approaches a spherical shape. Therefore the microphone according to the invention also affords the possibility of embodying a microphone with a spherical directional characteristic, with the same design configuration and structure in respect of the microphone capsule. If a directional characteristic of that kind is to be implemented, then desirably the slot-shaped sound inlet can also be entirely omitted, in which case the other advantageous features described herein can nonetheless be provided in such an embodiment. 
   In a particularly desirable feature in that case the sealing element which substantially predetermines the size of the slot-shaped sound inlet is formed in one piece with the closure element which is arranged in front of the mouth opening of the sound passage and which carries the acoustic damping material. In that way, both the acoustic inductance by virtue of influencing the dimensions of the slot-shaped sound inlet and the acoustic resistance of the damping element by virtue of the choice of the acoustic damping material adopted can be particularly advantageously predetermined by means of a single component. 
   In an alternative embodiment the diaphragm fixing portion substantially surrounds the rear diaphragm surface and the sound inlet is formed between a holding portion provided on the diaphragm and the diaphragm fixing portion. The diaphragm fixing portion is essentially the portion of the diaphragm, with which the latter is connected to the diaphragm fixing portion. In a desirable development the holding portion is formed by a diaphragm ring connected to the diaphragm. Such a diaphragm ring advantageously enhances the stability of the diaphragm and can be easily manufactured. That is found to be advantageous in particular for the reason that, in a desirable development of the invention, provided in the diaphragm ring are recesses which substantially form the slot-shaped sound inlet. The diaphragm fixing portion can therefore have a substantially flat support contact surface for the diaphragm ring, wherein the dimensions of the slot-shaped sound inlet are predetermined by the recesses in the diaphragm ring. This embodiment also provides that the size of the slot-shaped sound inlet is formed by a component which is convenient in terms of manufacture so that, with identical diaphragm fixing portions, it is possible to provide for tuning of the microphone by altering the diaphragm ring. 
   In a further alternative embodiment the diaphragm according to the invention has a casing portion which is connected to the diaphragm fixing portion and which substantially encloses the rear diaphragm surface, in which case the sound inlet is formed between the diaphragm fixing portion and the casing portion. 
   Advantageous developments of the invention are characterized by the features of the appendant claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described by way of example hereinafter with reference to the drawings in which: 
       FIG. 1  shows a view in cross-section through a first embodiment of a microphone according to the invention; 
       FIG. 2  is a view on an enlarged scale of a part of  FIG. 1 ; 
       FIG. 3  shows a plan view of a sealing element which is fitted in the embodiment shown in  FIG. 1 ; 
       FIG. 4  shows a view in cross-section through the sealing element taken along line IV—IV in  FIG. 3 ; 
       FIG. 5  shows a second embodiment of the microphone according to the invention; 
       FIG. 6  is a view on an enlarged scale of a part of  FIG. 5 ; 
       FIG. 7  is a plan view of a diaphragm ring which is fitted in the embodiment shown in  FIG. 5 ; 
       FIG. 8  is a view in cross-section of the diaphragm ring taken along line VIII—VIII in  FIG. 7 ; 
       FIG. 9  shows a third embodiment of the microphone according to the invention; and 
       FIG. 10  shows a view on an enlarged scale of part of  FIG. 9 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a first embodiment of a microphone according to the invention in cross-section, comprising a diaphragm  3 , a diaphragm fixing portion  5 , a magnet system  7  and a microphone cover  9 . The diaphragm  3  is connected with its outer edge to the diaphragm fixing portion  5  and thereby centered over the magnet system  7 . An oscillating coil  11  secured to the diaphragm  3  extends substantially transversely with respect to the diaphragm  3  into an air gap  13  in the magnet system  7 . On its side towards the diaphragm  3  the microphone cover  9  is substantially matched to the contour of the diaphragm  3  and has a plurality of sound intake openings  15  through which sound to be picked up can impinge on the outside surface of the diaphragm  3 . The sound intake openings  15  are covered by a sound-transmitting material  17  in order to protect the diaphragm from fouling and contamination, in particular dust and moisture. 
   Also shown in  FIG. 1  and in the detail in  FIG. 2  is the diaphragm fixing portion  5  which has an orifice  19  leading from the exterior to the rear surface of the diaphragm  3 . Provided in the diaphragm fixing portion is an annular groove  21 , with the orifice  19  being provided in the annular groove  21  in the region of the edge between the bottom and the wall. Fitted in the annular groove  21  is a corresponding annular sealing element  23  which substantially closes the orifice  19  except for a slot-shaped sound inlet  25 . 
   The acoustic properties of the microphone can be predetermined to a wide extent by varying the geometrical dimensions of the slot-shaped sound inlet  25 . In the described embodiments, identified as the length  28  is that dimension of the sound inlet  25 , along which the sound flow essentially passes. The width is essentially determined along the periphery of the microphone, and the height  26  of the sound inlet is defined by the spacing of two complementary components ( 5 ,  23 ;  5 ,  37 ;  5 ,  51 ) which delimit the sound inlet  25 . Basically, in the illustrated embodiments of the invention, the height  26  of the sound inlet  25  is less than its length  28  and the length  28  of the sound inlet  25  is in turn less than the width thereof. 
   The height  26  (defined in the radial direction) of the slot-shaped sound inlet  25  as shown in  FIGS. 1 and 2  is in this case essentially predetermined by a recess  27  provided in the diaphragm fixing portion  5  and the length  28  is predetermined by the thickness of the annular sealing element  23 . 
   Provided on the annular sealing element  23  are ducts  29  which respectively form a sound passage, in a portion-wise manner. The ducts are provided with an acoustic damping material  31 . The ducts  29  in the sealing element  23  connect the volume  32  delimited by the rear surface of the diaphragm  3  to a cavity  33  which is closed in an outward direction (not shown). 
   The cavity  33 , together with the acoustic damping material  31  arranged in the ducts  29  in the sealing element  23 , forms a damping element, wherein the acoustic damping value on the one side depends on the size of the cavity  33  while on the other side it depends on the acoustic properties of the ducts  29  and the damping material  31 . The slot-shaped sound inlet  25  forms an acoustic inductance, the magnitude of which can be essentially predetermined by the geometrical dimensions involved. The acoustic inductance of the slot-shaped sound inlet  25 , together with the damping element, forms an acoustic network which passes a part of the sound waves to be picked up, with a delay to the rear diaphragm surface. 
     FIG. 3  shows an annular sealing element which is used for example in the first embodiment of the microphone as shown in  FIGS. 1 and 2 .  FIG. 4  is a view of the annular sealing element in cross-section taken along line IV—IV in  FIG. 3 . The annular sealing element is of a substantially rectangular cross-section, with the sealing element  23  being provided with peripherally extending grooves  35  which are disposed in mutually opposite relationship on both sides. Provided in a portion-wise manner in the grooves  35  in the sealing element  23  are ducts  29  which are of the same width as the peripherally extending grooves  35  and which are substantially in the form of a slot. Arranged within the ducts  29  is an acoustic damping material  31  with which it is possible to predetermine the acoustic resistance of the ducts  29  in the sealing element  23 . 
     FIGS. 5 and 6  show a second embodiment of a microphone according to the invention,  FIG. 5  showing a view in cross-section and  FIG. 6  showing a view on an enlarged scale of part of  FIG. 5 . The microphone shown in  FIGS. 5 and 6 , like the microphone  1  in accordance with the first embodiment, also has a diaphragm  3 , a diaphragm fixing portion  5  which carries the diaphragm  3 , a magnet system  7 , a microphone cover  9 , an oscillating coil  11 , an air gap  13  which is provided in the magnet system  7  and into which the oscillating coil  11  secured to the diaphragm  3  at least partially engages, and a sound intake opening  15  which is covered by a sound-transmitting material  17 . 
   In contrast to the first embodiment, in the case of the microphone  1  in accordance with the second embodiment the rear surface of the diaphragm  3  is substantially enclosed by the diaphragm fixing portion  5 . The diaphragm fixing portion  5  has an annular groove  21  in a volume  32  which is delimited by the rear surface of the diaphragm  3 . An annular sealing element  23  is arranged in the annular groove  21 . The sealing element  23  is provided with ducts which are disposed in opposite relationship to adjoining sound passages which are formed in the diaphragm fixing portion  5  and which connect a cavity  33  (not shown completely) which is also enclosed by the diaphragm fixing portion  5 , to the volume delimited by the rear diaphragm surface. The annular sealing element carries acoustic damping material  31  with which the acoustic resistance of the damping element formed by the cavity  33  and the acoustic damping material  31  can be predetermined. 
   A diaphragm ring  37  of a substantially rectangular cross-section is secured to the outer, peripherally extending edge of the diaphragm  3 . Formed between the diaphragm ring  37  and the diaphragm fixing portion  5 , in a portion-wise manner, is a slot-shaped sound inlet  25  through which sound waves can pass to the rear diaphragm surface. The slot-shaped sound inlet  25  is formed by virtue of the diaphragm ring  37  having shallow recesses  39  at its surface which is towards the diaphragm fixing portion  5 . In this case, the length of the slot-shaped sound inlet  25  is determined by the part of the diaphragm fixing portion  5 , which is in opposite relationship to the shallow recess  39  of the diaphragm ring  37 . The height  26  of the slot-shaped sound inlet  25  can be predetermined by the size of the recess  39 . 
     FIGS. 7 and 8  show a view in detail of a diaphragm ring according to the invention, more specifically  FIG. 7  being a view from below and  FIG. 8  being a view in cross-section taken along line VIII—VIII. In the illustrated embodiment of the diaphragm ring  37 , eight recesses  39  are arranged uniformly around the periphery. The recesses  39  extend radially outwardly from the inner peripherally extending edge, on the underneath surface of the diaphragm ring, the outer downwardly disposed edge of the diaphragm ring  37  being maintained continuously. 
   The microphone in a third embodiment which is shown as a view in cross-section in  FIG. 9  and as a view on an enlarged scale in  FIG. 10  has a separate casing portion  51  which is connected to the diaphragm fixing portion  5  and which substantially encloses the rear diaphragm surface. Also provided in the housing portion  51  is the cavity  33  which is connected by a sound passage provided with acoustic damping material  31 , to the volume  32  connected through the rear diaphragm surface. In this case, the sound passage is formed by mutually oppositely disposed orifices  53  and  55  in the diaphragm fixing portion  5  and in the casing portion  51 , wherein the acoustic material  31  is arranged and held between the diaphragm fixing portion  5  and the casing portion  51 . 
   Formed between the diaphragm fixing portion  5  and the casing portion  51  is the slot-shaped sound inlet which represents the acoustic inductance. The height  26  (defined in the axial direction) and the length  28  of the slot-shaped sound inlet are predetermined by the diaphragm fixing portion  5  and/or the casing portion  51 . 
   While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention.