Patent Publication Number: US-9407983-B2

Title: Microphone device

Description:
TECHNICAL FIELD 
     The present invention relates to a microphone device configured to reduce fluctuations in frequency characteristics due to diffracted sound and reflected sound. 
     BACKGROUND ART 
     A technique in which the acoustic characteristics of a speaker or a listening room are measured using a microphone and an audio signal is equalized on the basis of the results of the measurement has been put into practical use, and a technique for enhancing the accuracy of the measurement using the microphone has also been proposed (for example, refer to Patent Document 1).  FIG. 1  is an external view showing a microphone device  100  having been used conventionally for this measurement. This microphone device  100  has a housing being composed of a disc-shaped base section  101  and a neck  102  provided upright at the center of this base section  101 . At the top of the neck  102 , an opening section  102 A is provided, and inside the neck  102 , a microphone unit  103  is incorporated toward the opening section  102 A. In the above-mentioned measurement, the microphone device  100  is placed at a listening point, test sound is emitted from the speaker, and the test sound picked up by the microphone device  100  is analyzed to determine the acoustic characteristics of the speaker and the listening room. 
     Furthermore, as shown in  FIG. 2 , a technique wherein a microphone base  110  having three concave sections  111  is placed at a listening point, the microphone device  100  is mounted sequentially in the three concave sections  111 , and test sound is picked up sequentially to measure the acoustic characteristics of the listening room three-dimensionally has also been put into practical use. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: JP-A-2009-37143 
     SUMMARY OF THE INVENTION 
     Problem that the Invention is to Solve 
     The frequency characteristics of the microphone device for use in the above-mentioned measurement are desired to be flat. However, in the conventional microphone device  100  shown in  FIG. 1 , diffracted-reflected sound due to the edge of the base section  101  of the housing and reflected sound reflected by the microphone base  110  in the case that the microphone device  100  is placed on the microphone base  110  are picked up together with direct sound as shown in  FIG. 3 , and the frequency characteristics do not become flat because of interference due to the distance differences between the direct sound and the diffracted-reflected sound and between the direct sound and the reflected sound, thereby causing a problem that errors occur in the results of the measurement. The diffracted-reflected sound is herein the sound obtained in the case that the reflected sound from the surface of the microphone device  100  is diffracted (diffraction) and picked up by the microphone unit  103 , and the sound is hereafter referred to as “diffracted-reflected sound (due to the edge)” because the contribution of the diffraction from the edge is dominant to the fluctuations in the characteristics due to interference. 
     The present invention is intended to provide a microphone configured to suppress fluctuations in frequency characteristics due to diffraction and reflection as much as possible. 
     Means for Solving the Problem 
     The present invention is a microphone device comprising: a housing having an opening section in an upper face thereof; and a non-directional microphone unit incorporated in the housing and provided inside the opening section, wherein the upper face of the housing has a shape in which a distance from an edge defined as a boundary between the upper face and a side face or a bottom face to the opening section throughout a whole circumference of the upper face changes in ½ or more of the whole circumference of the edge and an average value of the distance from the edge to the opening section is shorter than ½ of the wavelength of a sound wave in a frequency range in which an auditory sensitivity of humans is low. 
     The frequency range in which the auditory sensitivity of humans is low may be 10 kHz. In addition, a ratio of a longest distance to a shortest distance, from the edge to the opening section, may be two or more. Furthermore, the opening section may be provided at a center of a circumscribed circle of a planar shape of the upper face. Moreover, a planar shape of the upper face may be a triangle. 
     Advantage of the Invention 
     With the present invention, the influence of diffracted sound and reflected sound on the frequency characteristics of the sound picked up by the microphone unit can be suppressed to the minimum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an external view showing a conventional microphone device; 
         FIG. 2  is a perspective view showing a microphone base in which the microphone device is mounted; 
         FIG. 3  is a view illustrating diffracted sound and reflected sound entering the conventional microphone device; 
         FIG. 4  is an external view showing a microphone device according to an embodiment of the present invention; 
         FIGS. 5A to 5D  are graphs showing the frequency characteristics of the microphone device according to the above-mentioned embodiment and the frequency characteristics according to comparison examples; 
         FIGS. 6A to 6D  are graphs showing the frequency characteristics of the microphone device according to the above-mentioned embodiment and the frequency characteristics according to comparison examples; 
         FIGS. 7A to 7C  are views showing modification examples of microphone devices to which the present invention is applied; and 
         FIGS. 8A to 8D  are graphs in which the frequency characteristics of the microphone device shown in  FIG. 4  are compared with the frequency characteristics of the microphone devices shown in  FIG. 7A . 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
       FIG. 4  is an external view showing a microphone device  1  according to an embodiment of the present invention. This microphone device  1  is used as a measurement microphone for measuring the acoustic characteristics of an audio system and a listening room. The microphone device  1  has a housing  2  and a microphone unit  3  incorporated in the housing  2 . The planar shape of the housing  2  (the microphone device  1 ) is a nearly equilateral triangle, and its overall shape is such a shape as obtained by vertically cutting off a gently sloping cone so that the shape matches the planar shape of the above-mentioned nearly equilateral triangle. The upper face  10  of the housing  2  has an opening section  11  at the center and is inclined downward toward sides  13  serving as peripheral edges (edges) with the opening section  11  as an apex. With this configuration, the upper face  10  on the side  13  is a curved face being highest at an intermediate point  13 A nearest from the opening section  11  and lowest at apexes  13 B farthest from the opening section  11 . Hence, the side face  12  formed vertically downward from the side  13  of the upper face  10  is an arch-shaped plane being highest at the center portion, that is, the intermediate point  13 A, and lowest at both ends, that is, the apexes  13 B. 
     In addition, inside the opening section  11 , the non-directional microphone unit  3  is provided upward. 
     With this shape, the distance of the side (edge) of the upper face  10  from the opening section  11  (the microphone unit  3 ) is not constant. In other words, in the range from the intermediate point  13 A being nearest to the opening section  11  to the apex  13 B being farthest from the opening section  11 , the distance (to the opening section  11 ) changes gradually, and the ratio between the distance to the nearest point (the intermediate point  13 A) and the distance to the farthest points (the apex  13 B) is approximately 1:2.5. 
     Furthermore, as the planar dimensions of the microphone device  1 , the dimension from the center portion of the opening section  11  to the apex  13 B is approximately 2 cm, the dimension from the center portion of the opening section  11  to the intermediate point  13 A is approximately 1 cm, and the height of the microphone device  1  is approximately 1.5 cm. When it is assumed that the speed of sound is 340 m/s, 1 cm corresponds to ½ of the wavelength λ of a 17 kHz sound wave. 
     With this shape, frequency characteristics are improved because of the following reasons. 
     (1) Since the distance from each point of the side  13  of the upper face  10  to the opening section  11  (the microphone unit  3 ) changes gradually, the path length of the diffracted-reflected sound entering the microphone unit  3  from each point of the side (edge)  13  is different, whereby the influence on the direct sound entering the microphone unit  3  due to the interference is not concentrated on a specific frequency. 
     (2) Since the dimensions of the housing are short as described above, the path difference between the direct sound and the diffracted-reflected sound at the side  13  is small, and since the influence on the direct sound due to the diffracted-reflected sound appears in a high-frequency band (for example, an inaudible band), the influence on acoustic feeling is small. 
     Generally speaking, it is assumed that the audible range of humans is 20 Hz to 20 kHz. Within the range, the sensitivity of human ears is high for the sound in a frequency range of 2 kHz to 4 kHz, and the sound in this range is easy to hear. However, in frequencies higher than this range, the sensitivity lowers depending on the level of a signal, and humans gradually become unaware of sound; for example, it is difficult to hear the sound in a frequency range around 10 kHz and humans become unaware of the sound. For example, even if there is the influence of the diffracted-reflected sound, in the case that the frequency is, for example, approximately 10 kHz or more, it is assumed that the influence on acoustic feeling is negligible in practice. 
       FIG. 5D  and  FIG. 6D  are graphs showing the frequency characteristics of the microphone device  1  shown in  FIG. 4 .  FIGS. 5A to 5D  are graphs showing the frequency characteristics in the case that the microphone device  1  is placed in the air, and  FIGS. 6A to 6D  are graphs showing the frequency characteristics in the case that the microphone device  1  is mounted on a base (for example, such a base as shown in  FIG. 3 ). Both show the frequency characteristics of sounds arriving from a horizontal direction (θ=0°), a 20 degrees upward direction (θ=20°) and a 10 degrees downward direction (θ=−10°). These figures also show, as comparison examples, the characteristics ( FIGS. 5A and 6A ) of the microphone device having the conventional shape shown in  FIG. 1 , the characteristics ( FIGS. 5B and 6B ) of a microphone device having a shape with a neck longer than that of the conventional shape shown in  FIG. 1 , and the characteristics ( FIGS. 5C and 6C ) of a microphone device having a shape with a pedestal having a quadrangular planar shape and with a longer neck. 
     As described above, since  FIGS. 5A to 5D  show the frequency characteristics in the case that the microphone device  1  is placed in the air, it is assumed that only the diffracted-reflected sound due to the housing  2 , more particularly, the side (edge)  13 , affects the frequency characteristics of the sound signal picked up by the microphone unit  3 . 
     In both the comparison examples shown in  FIGS. 5A and 5B , the characteristics of sounds arriving from any directions are changed at 2 kHz or more, and the changes in the characteristics are not the same depending on the arrival angle. Furthermore, in the sound arriving from the 20 degrees upward direction, a dip (minimum value) occurs in an audible range of 10 kHz or less. Moreover, in the comparison example shown in  FIG. 5C , although small changes occur in the characteristics at 2 kHz or more, the characteristics are flat as a whole. However, since the changes in the characteristics vary depending on the arrival angle, it is difficult to make correction. On the other hand, in the microphone device  1  shown in  FIG. 5D  according to the embodiment of the present application, the characteristics of the sounds arriving from any angles do not fluctuate up and down extremely, and the characteristics of the sounds arriving from any angles are similar to one another, that is, slightly rise at approximately 3 kHz or more; hence, correction can be made in post-stage circuits, and accurate measurement can be made. 
     Next, since  FIGS. 6A to 6D  show the frequency characteristics in the case that the microphone device  1  is mounted on the base as described above, it is assumed that the diffracted-reflected sound due to the sides (edges)  13  of the housing  2  and the reflected sound reflected by the surface of the base affect the frequency characteristics. 
     In all the examples shown in  FIGS. 6A to 6D , as the frequency becomes high, the gain (characteristic) of the sound arriving from the 20 degrees upward direction rises due to the influence of the reflection by the base, and as the frequency becomes high, the gain of the sound arriving from the 10 degrees downward direction lowers due to the influence of the shielding by the base. Furthermore, in the case of the sound arriving from the 20 degrees upward direction, since the reflected sound reflected by the base is picked up by the microphone unit  3 , a dip (minimum value: path difference ½λ) and a peak (maximum value: path difference λ) occur in the frequency characteristics depending on the path difference between the direct sound and the reflected sound. As the distance between the face of the base and the microphone unit is longer, that is, as the neck is longer, the path difference becomes larger, and the frequencies at the peak and the dip are shifted to lower frequency bands. In the comparison example in  FIG. 6A , a dip occurs at around 6500 Hz, in the comparison example in  FIG. 6B , a dip occurs at around 5000 Hz, and in the comparison example in  FIG. 6C , a dip occurs at a lower frequency, that is, around 2500 Hz. On the other hand, in the microphone device  1  according to the embodiment of the present application shown in  FIG. 6D , a dip occurs at a frequency higher than 10000 Hz in which the influence on acoustic feeling is small, whereby the influence of the dip on the adjustment of hi-fi audio is small. As described above, in the case of the shape of the microphone device  1  shown in  FIG. 4 , the frequency characteristics are less susceptible to the influence of the diffracted-reflected sound due to the sides (edges)  13  of the housing  2  and the reflected sound due to the face of the base than the frequency characteristics in the other comparison examples; even if the frequency characteristics are affected, correction can be made easily. 
     Furthermore, the shape of the microphone device  1  according to the present invention is not limited to that shown in  FIG. 4 . The shape may merely be such that the distance from the side  13  of the upper face  10  of the housing to the opening section  11  is not constant or such that the dimensions of the housing are short (the path difference between the diffracted-reflected sound and the direct sound is smaller than ½λ of an audible frequency), and various shapes, such as those shown in  FIGS. 7A to 7C , can be conceived. The planar shape of the housing shown in  FIG. 7A  is a quadrangle (square). The housing having this shape is easy to produce and stable when mounted on the base. The planar shape of the housing shown in  FIG. 7B  is a polygon with re-entrant angles (starfish shape). In the housing having this shape, since the distance difference between the nearest point and the farthest point thereof is large, the influence of the diffracted-reflected sound due to the edges can be reduced further. Moreover, the housing shown in  FIG. 7C  is made small to the extent that it can accommodate the microphone unit, and is provided with three legs so as to be fitted in the concave sections  111  of the microphone base  110  shown in  FIG. 2 . With this shape, the influence of the diffracted-reflected sound due to the housing is almost negligible. 
     As described above, the microphone device  1  is desired to satisfy the following conditions. The shape of the upper face is desired to have lengthily protruding portions and deeply recessed portions, that is, the distance difference (distance ratio) between the nearest point and the farthest point is larger the better, so that the distance between the side (edge) of the upper face of the housing and the microphone unit does not become constant throughout the whole circumference. However, the overall size is smaller the better, and in the case that the distance to the farthest point is smaller than ½λ of an audible frequency, the band where interference occurs is in an inaudible region, whereby it is not necessary to consider the shape. In the case that the distance from the side  13  of the upper face of the housing to the opening section  11  changes as in the present invention, it has been confirmed from experiments that there is no problem in acoustic feeling, provided that the average value of the distances is approximately ½ of the wavelength in the frequency of approximately 10 kHz. As the microphone device is smaller, the characteristics becomes better as described above; however, the microphone device is required to have a certain amount of weight because the microphone cable thereof is drawn therefrom; otherwise, the microphone device is unstable. 
     In addition, in the case that measurement is made in the state that the microphone device  1  is mounted in the circular concave section  111  of the microphone base  110  shown in  FIG. 2 , it is preferable that the planar shape (the shape of the bottom face) of the microphone device is formed such that the circumscribed circle thereof has the same size as that of the circular concave section and such that the microphone unit  3  (the opening section  11 ) is placed at the center of the circumscribed circle so that the position of the microphone unit  3  does not change depending on the mounting direction of the microphone device  1 . 
       FIGS. 8A to 8D  are graphs in which the frequency characteristics in the case that the housing has the quadrangular planar shape shown in  FIG. 7A  selected from among the various shapes shown in  FIGS. 7A to 7C  are compared with the frequency characteristics in the case that the housing has the triangular planar shape shown in  FIG. 4 . Although both the apparatuses have the housings having shapes satisfying the above-mentioned conditions and both the apparatuses exhibit excellent characteristics in comparison with the conventional apparatus, it is found that the apparatus having the housing having the triangular shape, in which the number of corners is small and the distance difference (distance ratio) between the nearest point and the farthest point is large, exhibits better characteristics. It is assumed further preferable that the distance ratio between the nearest point and the farthest point is two (in the shape of the equilateral triangle). 
     In the embodiments shown in  FIG. 4  and  FIGS. 7A to 7C , the boundary between the upper face and the side face of the microphone device is formed by a single line (side (edge)  13 ); however, round-chamfering may be performed from the upper face to the side face, whereby a ridge line having a gently changing shape and having a certain width may be used as the boundary. Furthermore, chamfering may be performed at a plurality of corners, whereby the boundary may be formed of a plurality of belt-like portions. In these cases, the range having this round-chamfered ridge line shape or this belt-like shape, having the width, may be assumed to be the edge. Moreover, in the case of a shape in which its upper face is inclined gradually to its bottom face (having no side faces), the contour line (planar shape) of the upper face (bottom face) may be assumed to be the edge. 
     In the embodiments shown in  FIG. 4  and  FIGS. 7A to 7C , the distance (that is, the contour shape of the upper face) from each point of the side (edge) of the upper face to the opening section (the microphone unit) changes gradually and continuously; however, portions having the same distance may exist in a short range. In the case that the range having the same distance is ½ or less of the whole circumference of the edge, it is assumed that the advantage of the present invention can be obtained. 
     In the embodiments shown in  FIG. 4  and  FIGS. 7A to 7C , the shape of the upper face has been described as a shape in which the distance from the side (edge) of the upper face to the opening section (the microphone unit) changes, and the base point for the distance at the opening section has been described as the center of the opening section for convenience sake. However, in the embodiment according to the present invention, the base point for the distance at the opening section is not limited to the center of the opening section. Any point on the edge portion of the opening section may be used as the base point, or an intermediate point between any point on the edge portion and the center portion may be used as the base point. The whole of the opening section may also be used as the base point. 
     Although the measurement microphone device  1  for measuring the acoustic characteristics of an audio system or a listening room has been described in this embodiment, the present invention is not limited to measurement microphone devices but may be applied to recording microphones. 
     Although the present invention has been described in detail referring to the specific embodiment, it is obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. 
     This application is based on Japanese Patent Application (2012-034892) filed on Feb. 21, 2012 and Japanese Patent Application (2012-269546) filed on Dec. 10, 2012, the contents of which are hereby incorporated by reference. 
     DESCRIPTION OF REFERENCE NUMERALS AND SIGNS 
     
         
           1  microphone device 
           2  housing 
           3  microphone unit 
           11  opening section 
           13  side (edge) 
           110  microphone base 
           111  concave section