GB2157128A - Microphone apparatus - Google Patents

Abstract

A microphone apparatus consists of a plate (1) and a microphone (2) located off centre and spaced from the plate surface by no more than 4.25 mm. Two microphones may be located at symmetrical positions. Over the audible frequency range (20 Hz to 20 KHz) sound reaching the microphone(s) directly is in phase with sound reaching the microphone(s) after reflection from the plate. <IMAGE>

Description

SPECIFICATION Microphone apparatus The present invention relates generally to a microphone apparatus, and is directed more particularly to a microphone apparatus suitable for use upon collecting sound by utilizing a sound field near the surface of a rigid body plain plate and so on.

Recently, such a sound collecting method for utilizing the sound field near the surface of a rigid body plain plate becomes a topic in the art. In case of employing such sound collecting method, it is necessary to clearly grasp the relation among the setting state, frequency characteristic, directivity at a sound receiving point and so on. As to the sound field near the surface of a rigid body plain plate analysis and experiments have been carried out in various view points by many researchers from the end of the 19th century. In order to perform severe analysis of such the sound field, it is necessary to consider the diffraction of sound through one side of the surface of a rigid body plain plate to its back or rear side. However, when such severe analysis is performed, complicated calculations must be achieved.Therefore, in the prior art satisfactory results are not always obtained and hence the prior art sound collecting method utilizing the sound field near the surface of the rigid body plain plate is lack in practice and difficult to provide a desired microphone apparatus for practising such sound collecting method.

According to the present invention there is provided a microphone apparatus comprising; a rigid plate having a plain surface with a predetermined area; and a microphone element attached near the surface of said plate, said microphone element being attached at a peripheral position offset from a center of said plate within such distance from the surface that the signal by the sound pressure directly reaching the sound collecting point thereof from the sound source is substantially the same in phase as the signal by the sound pressure caused by the primary reflection on the surface over all audible frequency.

Preferably the microphone element is spaced by a distance of 4.25 mm or less from the surface of the plate.

The microphone element can be above, i.e. spaced outwardly from, the plate or below, i.e. buried in it.

The above features and advantages of the present invention will become apparent from the following description take in conjunction with the accompanying drawings through which the like references designate the same elements and parts.

Figures 1 and 2 are respectively schematic views used to explain the fundamental theory of the present invention; Figure 3 is a perspective view showing an embodiment of the microphone apparatus according to the invention; Figure 4 is a model for explaining the operation of the embodiment shown in Figure 3; Figures 5to 7 are respectively characteristic graphs used to explain the operation of the embodiment shown in Figure 3; Figure 8 is a perspective view showing another embodiment of the present invention; Figures 9 to 11 are respectively diagrams used for the explanation of the operation of the embodiment shown in Figure 8; Figure 12 is a side view showing a further embodiment of the invention; and Figure 13 is a characteristic graph used to explain the operation of the embodiment shown in Figure 12.

The present invention will be hereinafter described with reference to the attached drawings.

At first, the fundamental theory of the present invention will be now described with reference to Figures 1 and 2.

In Figure 1, reference letters W1, W2, W3 and W4 designate four walls, respectively, which form a sound field surrounded thereby, So a sound source and Ma sound collecting point which are both located within the sound field surrounded by the four walls W1 toW4. In this case, it is assumed that the sound pressure caused by the sound which propagates along the direct path from the sound source So to the sound collecting point Mistaken as PO, the sound pressure caused by a primary mirror image sound source S1 generated by the wall W1 as P1, and the sound pressures similarly caused by primary mirror image sound sources S2, S3 and S4 generated by the walls W2, W3 and W4 as P2, P3 and P4, respectively.Further, it is assumed that the sound pressures caused by the secondary mirror image sound sources, which are generated such that the sounds from the sound source So are reflected on two walls, are respectively taken as P12, P13, P14, P21, P23, P24, P31, P32, P34, P41, P42, and P43. Similarly, it is assumed that the sound pressures caused by the mirror image sound sources, which are generated such that the sounds from the sound source So are reflected on three walls and more, as Pijk (where i i j k k...). Under such assumption, the ratio S/N at the sound collecting point M is expressed by the following equation (1)

where i jok2F.... j * Now, the ratio S/N is considered under the above condition when the sound collecting point M is located very close to or near the wall W2.

The signal by the sound pressure P0 directly reaching the sound collecting point Mfrom the sound source So is same in phase as the signal by the sound pressure P2 caused by the primary reflection on the wall W2 over all frequencies (the term "all frequencies" is explained below), so that the above equation (1 ) becomes as follows:

where i + j + k + ....

At this time, since P0 -. P2 is satisfied, the numerator of the equation (2) becomes 2P2,. Further, since in general the denominators of the equations (1 ) and (2) are approximately equal to each other, it is understood that the ratio S/N is improved by about 3 dB.

Next, such a case will be now considered where a sound source S is positioned in a free space, a disc D which will become an obstacleforthe sound emitted from the sound source S is presented and a sound receiving point R is located above the surface of the disc D by a height Z as shown in Figure 2.

In case of Figure 2, a direct sound qimpthrough a direct path L from the sound source Sto the sound receiving point R is expressed as follows:

A particle velocity U on a surface dS by the direct sound 4)p is expressed as follows:

and a reflected sound d5 on the surface dS becomes as follows:

Therefore, a sum cPs of the reflected sounds is expressed as follows:

Thus, if a sound pressure P at the sound receiving point R is Expressed by the ratio for a sound-pressure Pp of the direct sound, its approximate equation becomes as follows::

where AZ (0 = {A (A (8))2 + Z The characteristics on the axis of a plane wave upon its coming ( t= 0 and L ^ ) - L) or the characteristics on the center of the disc D with the radius a when the plane wave is directly incident on the disc Dare expressed from the equation (7) as follows:

where

As a result, as expressed by the equation (8), the frequency characteristics at the center of the disc D include the ripple components of about 10 dB. The reason of this is by the fact that since the same boundary conditions are superimposed on with one another, the interference by the diffraction becomes large.In order to reduce the ripple components, it is necessary to locate the sound receiving point R eccentric or apart from the center of the disc D. By this it is possible to smooth the frequency characteristic, but in accompany therewith the directional characteristic becomes out from the symmetry and the directional characteristic appears in the direction opposite to that into which the sound receiving point is displaced.The reason of this is that the mirror image effect (reflection effect) is reduced in the direction near the edge of the dise D from the sound receiving pointMas explained in connection with Figure 1,the level of the directional characteristic becomes low but in the opposite direction the reflection surface which will cause the mirror effect exists large and the level of the directional characteristic increases.

The present invention is effected based on the fact that the directional characteristic appears in the opposite direction into which the sound receiving point is displaced.

Figure 3 shows an example of the microphone apparatus according to the present invention. In this example, a plain plate 1 with a predetermined shape and a constant area, for example, a disc with a radius a is located as a plain surface of a rigid body and a microphone element 2 is located on the disc 1 at its peripheral position different from a center c of the disc 1, for example, at the position apart from the center c by 3/4a. In place of the disc, a plain plate such as a square shape plain plate, a rectangular shape plain plate or other shape plain plate can be used as the plain plate 1. A sound source 3 is located above the microphone element 2 on the plain plate 1 apart therefrom by a predetermined distance.

Figure 4A is a schematic side view of Figure 3 and Figure 4B is a schematic plan view of Figure 3, respectively. In Figure 4A, reference letter designates the incident angle of the sound from the sound source 3 (shown in Figure 3) on the microphone element 2. When the incident angle (I) is changed, the change in the sound pressure at the microphone element 2 by the sound source 3 reveals the directional characteristics indicated by the black points in the graph of Figure 5 (practically measured values). The condition in this practical measurement is, for example, such that a = 85 mm, 314a . 65 mm, and the distance between the sound source 3 and the plain plate 1 is about 2.5 - 3 m.In the graph of Figure 5, the solid line curve shows the calculated value by an approximate analysis under which the diffracted sound through the side of the plain surface of the rigid body is neglected in view of practical point. It is understood from the graph of Figure 5 that the measured values are substantially coincident with the calculated values.

Further, from the graph of Figure 5 it is understood that the collected sound pressure becomes high for the sound in a constant direction (from the position of the center direction) and minimum at the position of the plane flush with the plane of the plain plate 1. In this case, the sound from the sound source 3 is not a so-called burst-shape interrupted wave but a continuous wave with a constant frequency and a constant sound pressure.

The gain of the collected sound pressure relative to the frequency is shown in the graph of figure 6 in which the solid line curve represents the calculated value while the black points denote measured values.

From the graph of Figure 6, it is understood that the gain of the collected sound pressure for the frequency is such that the ratio between its increase and decrease becomes large as the frequency becomes high.

Figure 7 is a graph showing the frequency characteristics or the relation of the directional characteristics to the frequency characteristics when as shown in Figure 4 the incident angle cm) of the plane wave is set at + 45", 0 and -45 under the same condition. In the graph of Figure 7, the slid line curves represent the calculated values and the other marks represent the measured values. In this case, the mark X is the case that the incident angle cp is selected as +453, the mark A the case that the incident angle 4) as 0t and the mark 0 the case of the incident angle 9 as - 45q respectively.From the graph of Figure 7 it will be clear that the relation between the directional characteristic of the collected sound and the frequency is such that the frequency characteristic of the sound appears more remarkable as the sound becomes near the radius direction of the plain plate 1 and the isolation between the left and the right is established over 800 Hz to 6 kHz which is important for the auditory sense.

As described above, according to the above example of the invention, by locating the microphone element 2 at the psition apart from the center c of the plain plate 1 with a predetermined distance i.e. 3/4a, the gain of the collected sound pressure becomes high as the sound comes nearer from the center c of the plain plate 1, the frequency characteristcs there of becomes remarkable and the various characteristics such as sensitivity, clarity and so on thereof are improved.

As regards the term "all frequencies" as used above in relation to equation 1, this term refers to the frequency range from 20 Hz to 20 kHz because the audible frequency range of a human body is generally taken to be this range. In order that the signal by the sound pressure P0 directly reaching the sound collecting point M from the sound source So is same in phase as the signal by the sound pressure P2 caused by the primary reflection on the wall WO throughout the frequency range from 20 Hz to 20 kHz, if the above two signals are in phase at the frequency 20 kHz, the above condition is satisfied.

In orderthatthe signals are in phase at the frequency kHz, since 1 period is 50 us, the time difference therebetween should be at most 25 us.

If this is expressed as a distance z from the rigid plain surface, from the conditions 2z Us 25 us x 340 m/s (sec.) (340mls= sound velocity) the following condition is derived z ç 4.25 mm Accordingly, if the distance z of the microphone from the rigid plain surface is selected as equal to or smaller than 4.25 mm (z S 4.25 mm), the above mentioned condition is satisfied.

Whether the microphone is above or buried in the rigid plain surface, if the distance z satisfies the condition z S 4.25 mm, a similar phenomenon (and theory) is established.

Figure 8 shows another example of the invention in which microphone elements 4 and 5 are respectively located at positions each apart from the center c of the plain plate 1 by 3/4a and symmetrical with respect to the center c. When the measuring condition of the microphone elements 4 and 5 are selected same as that of the first example, this example represents the same characteristics.

Under the above arranging condition, now such the case is considered that, as shown in Figure 9, the radius a of the plain plate 1 is selected as 85 mm, the distances of the left (L) and right (R) microphone elements 4 and 5 from the center c of the plain plate 1 are each selected as 65 mm and the sound source 3 is positioned in the direction at the intersecting angle of about 45O to the right microphone element 4 and apart therefrom about 2.5 - 3 m.When the sound from the sound source 3 is a continuous wave with a constant frequency and a constant sound pressure, as described above the collected sound pressure at the right microphone element 4 is higher than that at the left microphone elementS. Thus, if the sounds from the respective microphone elements are recorded or heard as the left sound comes from the left side and the right sound comes from the right side, the sound is different from the location of Figure 9 and the localization of the sound image is shifted to the right direction.Accordingly, when a continuous sound with a constant frequency and constant sound pressure is recorded by a recording apparatus such as a tape recorder and so on under the above stereo microphone system as mentioned above, it is necessary that the output from the left microphone element is supplied to the right input of the recording apparatus and the output from the right microphone element is supplied to the left input of the recording apparatus. In other words, in this case since the directivity is opposite to the setting position for the sound collection different from the prior art sound recording and reproducing, upon the recording and reproducing the localization is set opposite in the left and right positions.

However, if the sound source 3 is made to generate an interupted wave of a burst shape variable in frequency and different in sound pressure as shown in Figure 10, the sound arriving at the right microphone element 4 is delayed by the distance amount of 1301A/2 mm from that arriving at the left microphone element 5 in time as shown in Figure 9. In other words, the arriving time of the interrupted sound wave to the microphone element 4 is delayed by 0.26 ms from that to the microphone element 5 as shown in Figure 11.

Therefore, when the sound is heard by head phones or the like whose directivity is substantially determined by the phase difference of the arriving sounds, it is preferred that the ouput from the left microphone element is supplied to the left input and the output from the right microphone element is supplied to the right input. That is, when the interrupted sound wave is heard through the head phones and the like whose directivity is determined by the phase difference of the sounds, the localization (directional sense) by the auditory sense is sensed to the left side more. This is based on a so-called law of the first wavefront (Has's effect) that when the above time difference is less than about 5 ms, the localization moves to the side of the large level.

Accordingly, in case of using the head phones and so on set forth above, it is desired that similar to the normal recording mode, the output from the left microphone element is fed to the left input and the output from the right microphone element is fed to the right input, respectively. However, when a reproduced sound is heard through a speaker, a preceding sound becomes dull and the sense for the distance becomes opposite, so that similar to the stationary state of the sound with the constant frequency and the constant sound pressure, the left microphone element is connected to the right input and the right microphone element is connected to the left input.

As mentioned above, according to the second example of the present invention, the same operation and effect as those of the first example are achieved and further the stereophonic sound collection becomes possible by effectively utilizing the above sound field phenomenon.

Figure 12 shows a further example for the present invention in which a cloth 7 with a constant thickness and sound absorbing characteristics is bonded to the surface of the plain plate 1 under the state similar to that shown in Figure 3 while the sound absorbing surface of the microphone element 2 is exposed. The cloth 7 may be made of, for example, wool, glass wool, felt and soon.

Figure 13 is a graph showing the frequency characteristics of the third example shown in Figure 12. In the graph of Figure 13, the broken line curve represents the frequency characteristics of the case where the cloth 7 is not provided and the solid line curve represents those with the cloth 7. From the graph of Figure 13, it will be understood that the high frequency region higher than, for example, Hz of the frequency characteristics can be suppressed by the provision of the cloth 7.

Accordingly, if the third example or microphone apparatus of the invention shown in Figure 12 is employed to record the sound in a conference or the like, sound components of relatively high frequencies generated from such as a shelf, desk, turning over the leaves and so on can be removed from being collected or unnecessary sounds other than voices and so on are not collected so that the conference can be proceeded effectively. Further, the third example of the invention may be used under the stereophonic sound collection mode as shown in Figure 8.

As described above, according to the present invention, since the microphone element is located on the plain plate with a constant area at its peripheral position at least different from its center, the sound collecting system which effectively utilizes the sound field near the plain surface of the rigid body can be presented.

Further, according to the present invention, the various characteristicssuch as sensitivity, clarity and so on can be improved as compared with the prior art microphone apparatus.

In addition, the high frequency region higher than about 1 kHz is raised by the invention so that the sense for the distance is substantially compressed to make the sound collection area wide and hence the microphone apparatus is very effective for use as a sound collection system to collect the sound in the conference and so on.

The above description is given on the single preferred embodiments of the invention, but it will be apparent that many modifications and variations could be effected by one skilled in the art without departing from the spirits or scope of the novel concepts of the invention, so that the scope of the invention should be determined by the appended claims only.

Claims (2)

1. A microphone apparatus comprising; a rigid plate having a plain surface with a predetermined area; and a microphone element attached near the surface of said plate, said microphone element being attached at a peripheral position offset from a center of said plate within such distance from the surface that the signal by the sound pressure directly reaching the sound collecting point thereof from the sound source is substantially the same in phase as the signal by the sound pressure caused by the primary reflection on the surface over all audible frequency.

2. Apparatus according to claim 1 wherein the microphone element is spaced by a distance of 4.25 mm or less from the surface of the plate.