Abstract:
An arrangement for optimizing the frequency response of an electro-acoustic transducer (10), comprising the electro-acoustic transducer ( 10 ) and a damping element ( 12 ) arranged behind the sound emanating or receiving side of a membrane ( 14 ) of the electro-acoustic transducer ( 10 ), wherein an air gap ( 16 ) is provided between the damping element ( 12, 13 ) and the membrane ( 14 ), which air gap ( 16 ) is sufficiently small for intimately acoustically coupling the membrane ( 14 ) with the damping element ( 12, 13 ), wherein the damping element ( 12, 13 ) is adapted to dampen an air flow created by the membrane ( 14 ) when moving.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to an arrangement for optimizing the frequency response of an electro-acoustic transducer. 
         [0002]    The invention further relates to a loudspeaker or microphone. 
         [0003]    The invention finally relates to a loudspeaker cabinet, especially the housing of small multi-media devices, e.g. mobile (i.e. cell) phones. 
       BACKGROUND OF THE INVENTION 
       [0004]    Membranes of electro-acoustic transducers such as loudspeakers or microphones have one or more characteristic inherent vibrations or eigenvibrations, also called “modes”, which influence the transducer frequency response. In particular, such modes cause undesired anomalies (peaks, dips and points of inversion) in the frequency response. However, a flat frequency response is desired in order to achieve an authentic sound reproduction. Especially, flat membranes, being formed from more or less rigid plates, are sensitive to modes, but also other membranes, such as cone-shaped membranes, as well as soft non-rigid membranes, are prone to the occurrence of modes. Thus, it is important to restrict the influence of modes on the frequency response in order to achieve an authentic sound reproduction. 
         [0005]    One known method of reducing the influence of modes is to dampen the membrane with absorbent material, for example glass wool. The absorbent material is brought into close contact with the membrane in order to obtain an optimal absorbing effect. U.S. Pat. No. 4,276,452 describes an electro-acoustic transducer in which inherent vibrations of the membrane are efficiently suppressed by absorbent material, which contacts the entire surface of the membrane uniformly. One disadvantage of the disclosed absorbing method is that it cannot be applied to existing electro-acoustic transducers since it requires a reconstruction of an electro-acoustic transducer, particularly of its membrane. Thus, this is an expensive method of reducing the influence of modes on the frequency response of an electro-acoustic transducer. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0006]    It is an object of the invention to provide an arrangement of the type defined in the opening paragraph, a loudspeaker and a microphone of the type defined in the second paragraph, and a loudspeaker cabinet of the type defined in the third paragraph, in which the disadvantage defined above is avoided. 
         [0007]    In order to achieve the object defined above, an arrangement for optimizing the frequency response of an electro-acoustic transducer is presented, comprising the electro-acoustic transducer and a damping element arranged behind the sound emanating or receiving side of a membrane of the electro-acoustic transducer, wherein an air gap is provided between the damping element and the membrane, which air gap is sufficiently small for rigidly acoustically coupling the membrane with the damping element, wherein the damping element is adapted to dampen an air flow created by the membrane when moving. 
         [0008]    In order to achieve the object defined above, furthermore a loudspeaker or a microphone comprises an arrangement for optimizing the frequency response of an electro-acoustic transducer according to the invention. 
         [0009]    In order to achieve the object defined above, finally a loudspeaker cabinet comprises speakers and damping elements as parts of an arrangement according to the present invention. The invention is also applicable to a housing of a mobile multimedia device, e.g. a mobile (i.e. cell) phone with one or more integrated loudspeakers and/or microphones. 
         [0010]    The characteristic features according to the invention provide the advantage that the influence of characteristic inherent vibrations or eigenvibrations, i.e. the modes of the membrane of the electro-acoustic transducer, on the frequency response may be efficiently reduced by the damping element or, more precisely said, by acoustically coupling the membrane and the damping element as rigidly as possible in order to reduce characteristic inherent vibrations or eigenvibrations. The word “rigid” and the term “rigid acoustic coupling” as well as the expression “intimate acoustic coupling” used in this patent, all mean that essentially the complete air masses moved by the membrane flow through the damping element and merely minor air streams bypass the damping element. It should be emphasized that “rigid acoustic coupling” does not mean a mechanical coupling of the damping element and the membrane, as known from U.S. Pat. No. 4,276,452. In particular, the frequency response of the electro-acoustic transducer can be improved in that it better approaches a flat frequency response with no significant anomalies through the measures according to the invention. This can make the sound reproduction of the transducer more authentic than without the measures of the invention. The damping element according to the invention causes damping of the air moved between the damping element and the membrane of the transducer by the membrane displacements. Thus, the damping element causes conversion of the kinetic energy of the air flow or the sound energy into thermal energy and, therefore, absorption of sound generated by the membrane of the electro-acoustic transducer. The air damping may decrease the Q (quality factor) of one or more anomalies in the frequency response. According to the invention, not only characteristic vibrations in the lower acoustic frequency range are reduced, but also characteristic vibrations in the higher acoustic frequency range may be efficiently reduced. The invention may be applied to any electro-acoustic transducer such as a speaker or a microphone, or to a loudspeaker cabinet, comprising one or more speakers. Furthermore, the invention may be applied to electro-acoustic transducers with plane-plate, cone- or dome-shaped membranes. In contrast to the transducer disclosed in U.S. Pat. No. 4,276,452, the invention is a simpler construction which requires no reconstruction of a transducer and may be applied to existing transducers. 
         [0011]    It should be observed that putting an absorbent material into the interior of a loudspeaker cabinet or its sound channels in order to dampen standing waves in the air cavity is known. These waves may occur at certain frequencies depending on the geometrical dimensions of the cabinet due to reflections of the sound waves at the hard inner walls of the cabinet (i.e. the modes of the cabinet caused by oscillations of the air). However, according to the prior art the absorbent material has not been used to optimize the frequency response of the electro-acoustic transducer, that is to say to damp the modes of the transducer membrane (i.e. waves in a solid body) through use of damped air flows (i.e. waves in air) rigidly coupled to the membrane. 
         [0012]    A further reduction in the influence of modes may be achieved if the damping element has dimensions such that its area facing the membrane is at least as large as the area of the membrane. This measure avoids air moved by the displacement of the membrane bypassing the damping element. 
         [0013]    In order to avoid the occurrence of eigenvibrations of the damping element, it may be rigidly mounted behind the sound emanating or receiving side of the membrane. 
         [0014]    In order to achieve a highly efficient reduction in membrane eigenvibrations, the air gap may be as small as possible since volume in the equivalent acoustic circuit means an acoustic compliance, which represents an imaginary part of the acoustic impedance of the electro-acoustic transducer and which should be as small as possible. A lower limit of the air gap size is determined by the maximum or largest possible displacement of the membrane. Thus, it is preferred to size the air gap slightly larger than the largest possible displacement of the membrane. Since this is a slit, the damping resistance R (see  FIG. 4 ) for sound is inversely proportional to the third power of d (d is the smallest significant dimension perpendicular to the air flow) of the gap between the membrane and an acoustically hard wall. Thus, the desired damping may be achieved by determining the air gap between the damping element and the membrane due to this relationship. 
         [0015]    According to one embodiment of the invention, the damping element may comprise a plurality of channels distributed over the element and with openings on the surface of the element in order to receive air moved by the membrane. The function of these acoustic channels is to receive sound energy. In the channels the received sound energy is converted into thermal energy mainly by viscosity friction between the air flowing in the channels and the walls of the channels. 
         [0016]    The channels may be slit-, hole- or amorphous-shaped. 
         [0017]    In order to achieve efficient conversion of the received sound energy into thermal energy, the channels may be formed such that they go through the damping element so that sound entering a channel on one side of the damping element may leave the damping element on another side. 
         [0018]    Alternatively, the channels may be closed. Especially if the channels are closed, their depth may be selected such that kinetic energy of the entering air flow generated by the membrane is almost completely converted into thermal energy within the damping element. 
         [0019]    In addition to holes and slits, the damping element may also comprise an amorphous porous material in order to absorb the sound from the electro-acoustic transducer. 
         [0020]    As a rule of thumb, the channels or pores should have a diameter less than about 0.1 millimeter since in this case the desired damping for efficient sound absorption has an effect in a frequency range up to 2.5 kHz. See  FIG. 4 . Most of the different types of membrane have their eigenvibrations in this frequency range. In general, the upper frequency up to which the damping has an effect increases as the diameter of the channels or pores decreases. Thus, the damping element or its effect on the eigenvibrations of a membrane may be adapted by selecting the diameter of the channels or pores to a certain electro-acoustic transducer and its membrane. By providing channels or pores of different diameter respectively different acoustic properties at the same time, different modes of the membrane can be damped. 
         [0021]    Furthermore, it is preferred that the damping element is shaped like a plate. A plate may be produced easily and can be applied to different membrane shapes. A damping plate may be most suitable for an electro-acoustic transducer with a plane membrane since the air gap between the damping plate and the membrane may be accurately adjusted. 
         [0022]    Preferably, the thickness of the plate is selected such that essentially no reflections of sound received by the damping element occur on its outer limiting surfaces. 
         [0023]    According to a further embodiment of the invention, the damping element may comprise several layers of damping material arranged one behind the other. Thus, the sound absorbing effects of different materials may be combined in order to achieve a highly efficient sound absorption. 
         [0024]    According to further embodiments, the transducers are selected from the piezoelectric type or the electromagnetic type or the iso-dynamic type or any other electro-dynamic type. The aspects defined above and further aspects of the invention are apparent from the exemplary embodiment to be described hereinafter and are explained with reference to this exemplary embodiment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The invention will be described in more detail hereinafter with reference to an exemplary embodiment. However, the invention is not limited to this exemplary embodiment. 
           [0026]      FIG. 1  shows a first embodiment of the arrangement according to the invention for optimizing the frequency response of an electro-acoustic transducer as an “open” arrangement. 
           [0027]      FIG. 2  shows a second embodiment of the arrangement according to the invention for optimizing the frequency response of an electro-acoustic transducer as a “closed” arrangement. 
           [0028]      FIG. 3  shows different embodiments of the openings of channels of the damping element according to the invention. 
           [0029]      FIG. 4  shows basic acoustic equations as well as acoustic elements and their electrical equivalent circuits. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0030]    Identical, similar, and functionally identical or similar elements can be denoted with the same reference numerals in the following description. 
         [0031]      FIG. 1  shows an electro-acoustic transducer configured as a loudspeaker  10  with a plane membrane  14 , for example a rigid plate, which is mounted in an opening of an acoustically “open” housing  28 , for example a wall with an opening for the loudspeaker  10 . The membrane  14  of the loudspeaker  10  is mounted at the housing  28  by a suspension  26 , which is flexible such that the membrane  14  may oscillate or vibrate unimpeded. 
         [0032]    A plate-shaped damping element  12  comprising a porous material is rigidly mounted immediately behind the sound emanating side of the membrane  14  such that only a small air gap  16  is provided between the membrane  14  and the damping element  12 . The air gap  16  is sized so small that the membrane  14  is acoustically coupled to the damping element  12  as rigidly as possible without touching it, in order to optimally reduce the influence of any eigenvibrations of the membrane  14  on the frequency response of the transducer  10 . 
         [0033]    Typically, the air gap  16  is sized a little larger than the largest possible displacement  30  of the membrane  14  in order to avoid the membrane  14  being able to bump against the damping element  12  which could result in distortions. The air gap  16  may not be sized so large that an essential part of the air flow  32  generated by vibrations of the membrane  14  can bypass the damping element  12  so that the effect of the damping element  12  is significantly reduced. 
         [0034]    The degree of sound absorption depends on the thickness of the damping element  12  according to the equations at the top of  FIG. 4 . As can be seen, the dimensions of the damping element  12  are selected such that they are larger than the dimensions of the membrane  14  in order to avoid an essential part of air flow generated by the membrane  14  being able to bypass the damping element  12 . It should be noted that the shape of the damping element is adapted to the shape of the membrane, i.e., if the membrane is cone- or dome-shaped the damping element  12  should also be cone- or dome-shaped in order to provide the small air gap  16  with an almost constant size over the entire area of membrane  14 . 
         [0035]    In  FIG. 1  moreover a small slit is provided between the damping element  12  and the housing  28 , which is also provided for damping. Such an arrangement may be useful if it is not possible to place a damping element  12  with the desired acoustic properties behind the membrane  14  (e.g. it is conceivable that due to limited space only a thin, rigid, airtight plate may be placed behind the membrane). In this case, said small slit provides the damping of the air flows as required for the inventive effect. In another case, a (homogeneous) damping element  12  as well as said slit are chosen in such a way that two different low-pass cut-off frequencies (see also  FIG. 4 , hole and slit) are defined and consequently higher order modes of the membrane  14  are damped with different degrees. Since the choices of cut-off frequencies may be made quite independently from another, a designer can more or less freely damp higher order modes. However, the slit shown in  FIG. 1  is not a mandatory feature of the invention. Instead the connection between damping element  12  and the housing  28  may be airtight too. 
         [0036]      FIG. 2  shows a loudspeaker cabinet or box  36  comprising an acoustically closed housing  29  with an opening in which a loudspeaker  10  with a plane membrane  14 , for example a rigid plate, is mounted. As with the embodiment of  FIG. 1 , the membrane  14  of the loudspeaker  10  is mounted at the housing  29  by a suspension  26  which is flexible such that the membrane  14  may oscillate or vibrate unimpeded. 
         [0037]    In contrast to  FIG. 1 , the acoustically closed housing  29  is nearly completely filled with porous material as a damping element  13 . However, as with the embodiment of  FIG. 1  a small air gap  16  is formed behind the sound emanating side of the membrane  14  in order to acoustically couple the membrane  14  with the damping element  13  as rigidly as possible. The same rules as described in connection with  FIG. 1  apply for the size of the air gap  16 . However, it should be noted that the thickness of the layer of porous material should be so large that sound received from the membrane  14  is well absorbed within the porous material. Thus, it is important that only a minor portion of sound, or better none at all, running through the porous material is reflected within the housing  29 . As a rule of thumb, the thickness of the damping element  13  or the porous material is sufficient for the purpose of the invention if essentially no reflections occur on the outer limiting sides of the damping element  34  or the inner sides of the housing  29 . Then sound running through the damping element  13  is well converted into thermal energy and, therefore, absorbed. 
         [0038]      FIG. 3  shows the surface  24  of a damping element  15 . In particular, it shows in detail three different types of preferred material for a damping element  15  according to the invention. On the left, the surface  24  of a damping element  15  with a material comprising channels  19  with slit-shaped openings  18  is shown. The slit-shaped openings  18  are nearly equally distributed over the surface  24  in order to receive sound. In the middle, the surface  24  of a damping element  15  with a material comprising channels  19  with hole-shaped openings  20  is shown. The hole-shaped openings  20  have a diameter d. On the right, the surface  24  of a damping element  15  with a material comprising channels  19  with amorphous openings  22  is shown. 
         [0039]    Sound generated by an electro-acoustic transducer may enter the openings  18 ,  20 , or  22  and run into the channels  19 , in which they are converted into thermal energy due to the air damping. In detail, frictional forces occur between the air flow running into a channel  19  and the surface of the channel, which cause an acoustic impedance in the air flow direction. The real part of the complex acoustic impedance is inversely proportional to the third power of the channel diameter d. The plurality of openings on the surface  24  may be considered as a parallel arrangement of several acoustic impedances of the single channels  19 . Thus, a desired coefficient of friction may be achieved. 
         [0040]    The invention has the advantage that the frequency response of an electro-acoustic transducer  10  may be optimized by reducing the influence of modes on the frequency response. This is achieved without requiring a reconstruction of the electro-acoustic transducer  10  with a damping element  12 ,  13 ,  15  acoustically coupled with the membrane  14  of the electro-acoustic transducer  10  as rigidly as possible. Thus, the Q factor of modes may be reduced significantly and the frequency response of the transducer made flatter. The invention may be implemented at low cost with various materials as described above. It may also be applied to electro-acoustic transducers with different membrane shapes such as cone-, dome-shaped or plane membranes. It may be advantageously applied to loudspeakers and microphones, in particular improving the frequency response in the higher frequency range. 
         [0041]      FIG. 4  shows acoustic elements, their electrical equivalent circuits, and R, L, C values expressed in terms of mechanical dimensions: l, b, d [m], S[m 2 ], V[m 3 ] and frequency f [Hz]. Effective total length of a constriction in the flow direction is l. For a circular hole the total effective length l equals the geometrical length l 0  plus the end correction 0.85 d. The viscosity coefficient under normal conditions is η=1.86×10 −5  kg/(ms). It is proportional to pressure and roughly proportional to absolute temperature [K]. Air density is ρ=1.2 kg/m 3 . Speed of sound c 0 =340 m/s. Specific heat ratio γ=1.4 for air. P atmos =10 5  Pascal. The impedance analogy is used, where sound pressure [Pa] is equivalent to voltage [V], and volume velocity [m 3 /s] is equivalent to current [A]. The equivalences for L, R, and C are 1 H≡1 kg/m 4 , 1Ω≡1 kg/(m 4 s), 1 F≡1 m 4 s 2 /kg. 
         [0042]    Finally, it should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word “comprising” and “comprises”, and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.