Electronic musical instrument

Abstract

An electronic musical instrument (1) that generates a musical tone for a musical performance in accordance with a musical tone signal generated based on an actuated state of a control element (13), comprising: a soundboard (23) having a first end and a second end extending parallel to each other and attached to a predetermined part of the electronic musical instrument (1) by a plurality of first screws (TB) and a plurality of second screws (TB), respectively, and a vibration exciter (25) provided on the soundboard (23) and configured to cause the soundboard (23) to vibrate by being driven in accordance with the musical tone signal, thus causing the soundboard (23) to produce a musical tone, characterized in that the plurality of first screws (TB) and the plurality of second screws (TB) extend along the first and second ends, respectively.The second end is arranged in an offset manner such that a layer of each of the plurality of first screws (TB) and a layer of each of the plurality of second screws (TB) are not opposite each other in a symmetrical relationship in a direction orthogonal to the first and second ends, and a pad (33) for suppressing the resonance of the soundboard (23) is arranged between the vibration exciter (25) and the soundboard (23).

Description

BACKGROUND OF THE INVENTION Field of invention The present invention relates to an electronic musical instrument which generates a musical tone for a musical performance according to a musical tone signal generated based on an operated state of a control element. Description of related prior art One conventional electronic musical instrument of the type mentioned above is known, for example, as disclosed in Japanese patent application (Kokai) JP 2013-77000 A. This conventional electronic musical instrument is formed by adding the functions of an electronic piano to an acoustic, upright piano. The electronic musical instrument is equipped with a keyboard, a soundboard, a vibration exciter, and a control device, and has a plurality of operating modes, including a low-tone mode. In low-tone mode, key press information is detected by a sensor, and a detection signal indicating the key press information is input into the control device. The control device, which is implemented, for example, by a CPU, drives the vibration exciter according to the key press information detected by the sensor.This causes the vibration exciter to set the soundboard into vibration, thereby producing a musical tone. As shown in Fig. 8, an electronic piano constructed as described above generally has a soundboard SB, which is rectangular in shape, and the lower, upper, right, and left ends of the soundboard SB are attached to the front surface of an edge FR of the electronic piano by means of upper screws UB, lower screws DB, left screws LB, and right screws RB, respectively. Note that in Fig. 8, each of the upper to right screws UB to RB is shown at a slightly enlarged scale for convenience. The upper screws UB and the lower screws DB are arranged such that the position of each of the upper screws UB and the position of each of the lower screws DB are opposite each other in a symmetrical relationship in a vertical direction, and the left screws LB and the right screws RB are arranged such that the position of each of the left screws LB and the position of each of the right screws RB are opposite each other in a symmetrical relationship in a horizontal direction.For this reason, the conventional, upright electronic piano suffers from the following problem: The screws UB to RB for use in mounting the soundboard SB are arranged as above, and accordingly, the fixed ends of the upper, lower, right, and left ends of the soundboard are regularly positioned, and thus, when the soundboard SB is caused to vibrate by the vibration exciter according to the key press, the soundboard SB easily resonates, causing a large peak drop in the frequency characteristic of a musical tone, making it impossible to obtain an excellent musical sound. An electronic musical instrument according to the preamble of main claim 1 is disclosed in JP 2008-175866 A. Further examples of musical instruments are disclosed in JP H08-248946 A and DE 506 774 A. SUMMARY OF THE INVENTION It is an object of the present invention to provide an electronic musical instrument that is able to suppress the resonance of a soundboard and consequently reduce a peak drop in the frequency characteristic of a musical tone, in order to ensure excellent musical sound. To solve the above problem, the present invention provides for an electronic musical instrument with the features of main claim 1. In the construction of the electronic musical instrument according to the present invention, the vibration exciter sets the soundboard into vibration by being driven according to a musical tone signal, thus causing the soundboard to produce a musical tone. Furthermore, the first and second ends of the soundboard, which extend parallel to each other, are attached to the predetermined part of the electronic musical instrument by the first and second screws, respectively. The first and second screws are arranged along the first and second ends in an offset manner such that the position of each of the first screws and each of the second screws are not symmetrically opposite each other in a direction orthogonal to the first and second ends.Unlike the conventional electronic musical instrument described above, according to the present invention the fixed ends of each of the first and second ends of the soundboard are arranged irregularly as described above, so that it is possible to suppress the resonance of the soundboard and thus reduce the peak drop of the frequency characteristic of a musical tone in order to obtain an excellent musical sound. According to the invention, a cushion for suppressing the resonance of the soundboard is arranged between the vibration exciter and the soundboard. In this setup, since the padding for suppressing the resonance of the soundboard is arranged between the vibration exciter and the soundboard, the resonance of the soundboard can be further suppressed, making it possible to obtain a more excellent musical sound. Preferably, the electronic musical instrument is an upright electronic piano in which the control element is a key, and the soundboard is rectangular in shape, with the first and second ends corresponding to an upper and lower end, respectively; the majority of the first screws and the majority of the second screws are arranged along the upper and lower ends, respectively, in an offset manner such that a layer of each of the first screws and a layer of each of the second screws are not symmetrically opposite each other in a vertical direction; a left end and a right end of the soundboard are fastened to the predetermined part by a majority of third screws and a majority of fourth screws, respectively; and the majority of the third screws and the majority of the fourth screws are arranged along the left and right ends, respectively.The right end is arranged in an offset manner so that a position of each of the third screws and a position of each of the fourth screws are not opposite each other in a symmetrical relationship in a left-right direction. In this preferred embodiment, the upper and lower ends of the soundboard are fastened to the predetermined part in a rectangular shape by the majority of first screws and the majority of second screws, respectively, and the left and right ends of the soundboard are fastened to the predetermined part by the majority of third screws and the majority of fourth screws, respectively. Furthermore, the majority of first screws and the majority of second screws are arranged in an offset manner along the upper and lower ends, respectively, such that the position of each of the first screws and each of the first of the second screws are not symmetrically opposite each other in the vertical direction; and the majority of third screws and the majority of fourth screws are arranged along the left and right ends, respectively.The right end is arranged in an offset manner such that the position of each of the third screws and each of the fourth screws are not symmetrically opposite each other in the left-right direction. Thus, the fixed ends of each of the upper, lower, right, and left ends are irregularly arranged, allowing the resonance of the soundboard of the upright electronic piano to be adequately suppressed, making it possible to obtain an excellent musical sound. The above and other problems, features and advantages of the present invention will become clearer from the following detailed description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of an upright electronic piano according to an embodiment of the present invention; Fig. 2 is a rear view of the electronic piano shown in Fig. 1; Fig. 3 is a cross-sectional view along line A-A from Fig. 2; Fig. 4 is an enlarged, partially cut-away cross-sectional view of a vibration exciter, and so on; Fig. 5 is a view useful in explaining the positions of the respective self-tapping screws for fastening a soundboard of the electronic piano in Fig. 1; Fig. 6 is a diagram showing the relationship between the frequency of a musical tone produced by the electronic piano shown in Fig. 1 and its sound pressure level, together with a comparative example; Fig. 7 is a schematic top view of a soundboard of an electronic grand piano to which the present invention is applied; and Fig.Figure 8 is a view that is useful in explaining the positions of the respective thread-cutting screws for fastening a soundboard of a conventional, upright electronic piano. DETAILED DESCRIPTION OF PREFERRED EXECUTION FORMS The present invention will now be described in detail with reference to the drawings, which show a preferred embodiment thereof. As shown in Figures 1, 2 to 3, an upright electronic piano 1 (electronic musical instrument) according to the present embodiment consists of a piano body 2 and a stand unit 3 for supporting the piano body 2. In the following description, a near side, a far side, a left side, and a right side of the electronic piano, as seen by the player, are referred to as the "front," "back," "left," and "right," respectively. End panels 4 and 4 are mounted on the left and right sides of the piano case 2 and the stand unit 3, respectively, in a manner that covers the piano case 2 and the stand unit 3. The piano case 2 has an exterior formed by left and right arms 5 and 5, a keyboard base 6 extending horizontally between the lower ends of the respective arms 5 and 5, an upper base 7 extending horizontally between the upper ends of the respective arms 5 and 5, and a back panel 8 for covering between the rear ends of the respective arms 5 and 5. Inside the piano case 2 are arranged a keyboard assembly 9, a control panel 10, midrange and tweeter 11, and tweeter 12. The keyboard assembly 9 consists of a keyboard 14, which has a plurality of keys 13 (controls) arranged side by side in a left-to-right direction on the keybed 6, a plurality of hammers (not shown) provided for the respective keys 13 and each configured to pivot in accordance with the key press of a corresponding key 13, and a plurality of key sensors (not shown) provided for the respective keys 13 and each configured to detect the key press information of a corresponding key 13. The key 13 consists mainly of a wood-based material and has essentially the same construction as a key on an acoustic upright piano. The key 13 is pivotally mounted at its center point. The key sensor is, for example,The circuit is formed by a rubber switch and detects the execution / non-execution (on / off) and the keystroke speed of pressing a corresponding key 13 via a hammer that pivots according to the pressing of the corresponding key 13. Note that in Fig. 1 some reference symbols for the keys 13 have been omitted for convenience. The control panel 10 is located above the keyboard 14 and is equipped with control knobs and levers for use in adjusting a tone color, a tone volume, an acoustic effect, etc. for the electronic piano 1 and with a display to show the settings. The midrange and treble loudspeakers 11 are formed by four midrange loudspeakers, each primarily reproducing a midrange and treble component of a musical tone. The loudspeakers 11 are arranged in the upper left and right rear ends of the piano case 2, with their sound-radiating surfaces facing upwards. The treble loudspeakers 12 are formed by two dome tweeters, each primarily reproducing a treble component of a musical tone. The loudspeakers 12 are arranged in the upper left and right rear ends of the piano case 2 facing an opening 17 (see Fig. 1) for sliding a key cover 16, with their sound-radiating surfaces facing forwards. The stand unit 3 is a box-shaped arrangement formed by a right and left foot block 18 and 18, a left and right side shelf 19 and 19, a left and right leg 20 and 20, a footboard 21, a lower panel 22 (counter shelf), a soundboard 23, and so on. A vibration exciter 25 is provided on the soundboard 23 to set the soundboard 23 into vibration, thus causing it to produce a musical tone. The soundboard 23 and the vibration exciter 25 form a soundboard loudspeaker. The footboard 21 is connected between the rear ends of the respective foot blocks 18 and 18 and extends from left to right, with three pedals 26 pivotally mounted in a central section thereof (see Fig. 1). Each pedal 26 is equipped with a pedal sensor (not shown) for detecting whether the pedal 26 is engaged or not. Note that the pedals 26 have been omitted from Fig. 3 for convenience. The lower panel 22 is made of a wood-based material and is rectangular in shape. The lower panel 22 is arranged to cover the front end of a space enclosed by the keybed 6, the footboard 21, and the left and right side panels 19 and 19 (hereafter referred to as the "stand space").Furthermore, the lower panel 22 has an upper section designed with a laterally elongated opening to direct a musical tone from the soundboard loudspeaker forward. This opening is covered by a wire mesh cover 22a. Similar to the soundboard of an acoustic upright piano, the soundboard 23 is formed in a laterally elongated, rectangular shape by assembling a plurality of solid wood boards, for example, made of spruce. A rim 30 (predetermined part) is mounted along the outer circumference of the rear end of the stand space, and the soundboard 23 is attached to the front surface of the rim 30 as follows: Each of the upper, lower, left, and right ends of the soundboard 23 has a plurality of pre-drilled holes (not shown) that are parallel to each other, and each pre-drilled hole extends through the soundboard 23 in the front-to-back direction.The rim 30 has a plurality of pre-drilled holes (not shown) formed in it in a manner associated with the respective pre-drilled holes of the soundboard 23, and each pre-drilled hole of the rim 30 extends in a front-to-back direction. The soundboard 23 is fastened to the front surface of the rim 30 by screwing a self-tapping screw TB (each of the first to fourth screws) into each of the pre-drilled holes of the soundboard 23 and an associated pre-drilled hole of the rim 30 from the front in the aforementioned sequence. The soundboard 23 is arranged parallel to the lower panel 22 in a manner opposite the lower panel 22 and covers the rear end of the stand space without a gap. Fig. 5 shows the positions of the self-tapping screws TB on the soundboard 23. As shown in Fig. 5, the self-tapping screws TB for fastening the upper end of the soundboard 23 and the self-tapping screws TB for fastening the lower end of the soundboard 23 are arranged in an offset manner along the upper and lower ends of the soundboard 23, respectively, such that the position of each self-tapping screw TB at the upper end and the position of each self-tapping screw TB at the lower end are not symmetrically opposite each other in the vertical direction. Furthermore, the self-tapping screws TB for fastening the left end of the soundboard 23 and the self-tapping screws TB for fastening the right end of the soundboard 23 are arranged along the left and lower ends of the soundboard 23, respectively.The right end of the soundboard 23 is arranged in an offset manner such that the position of each of the thread-cutting screws TB at the left end and each of the thread-cutting screws TB at the right end are not symmetrically opposite each other in the left-right direction. Note that in Fig. 5, each of the thread-cutting screws TB is shown on a slightly enlarged scale for convenience. Furthermore, the number of self-tapping screws TB for attaching the upper end of the soundboard 23 and the number of self-tapping screws TB for attaching the lower end of the same are set to four, and the number of self-tapping screws TB for attaching the left end of the soundboard 23 and the number of self-tapping screws TB for attaching the right end of the soundboard 23 are set to two and three, respectively. However, this is only an example, and the number of self-tapping screws TB can be adjusted as desired. Furthermore, a soundboard pad 31 is arranged between the soundboard 23 and the edge 30 to suppress the resonance of the soundboard 23. The soundboard pad 31 is made, for example, of PORON (model number HH-48), which is manufactured by Rogers Inoac Corporation. The soundboard pad 31 is formed in a plate shape, and its thickness is adjusted to a predetermined value according to the resonance characteristic (natural frequency) of the soundboard 23 by trials or the like. Furthermore, a plurality of soundbars 32 are mounted on the rear surface of the soundboard 23. The soundbars 32 serve to improve the transmission rate of the vibration on the soundboard 23 and extend parallel to each other. Furthermore, a plurality of mounting holes 23a are provided in the soundboard 23 at a predetermined location, which is somewhat closer to the side of the deep area relative to its center (see Fig.4) designed for use in mounting the vibration exciter 25, and each mounting hole 23a extends through the soundboard 23 in a front-to-back direction. The number of mounting holes 23a is set, for example, to four, and only three of them are shown in Fig. 4. Note that Fig. 4 shows a different cross-section than that shown in Fig. 3. The vibration exciter 25 is, for example, an electromagnetic vibration exciter with a vibration characteristic such that it oscillates in a predetermined frequency band (e.g., 30 Hz to 2 kHz) and comprises a body part 25a and an exciter part 25b to transmit a vibration to the soundboard 23, as shown in Figs. 2 and 4. The body part 25a has a flange 25c that projects outwards from the outer circumferential surface of a base (a rear end) thereof. The flange 25c has a flat surface that is orthogonal to the front-to-back direction, is rectangular in shape, and has four corners (see Fig. 2). Furthermore, the four corners of the flange 25c are each formed with insertion holes 25d (of which only three are shown in Fig. 4), and each insertion hole 25d extends through the flange 25c in the front to back direction. A vibration exciter pad 33 is arranged between the soundboard 23 and the vibration exciter 25 to suppress the resonance of the soundboard 23. The vibration exciter pad 33 is made, for example, of PORON (model number HH-48). The vibration exciter pad 33 is formed in a plate shape, and its thickness is adjusted, for example, to 6 mm according to the resonance characteristics of the soundboard 23, by trials or the like. Furthermore, the vibration exciter pad 33 is designed with a plurality of insertion holes 33a in a manner associated with the respective insertion holes 25d of the vibration exciter 25, and each insertion hole 33a extends through the vibration exciter pad 33 in the front-to-back direction. The number of insertion holes 33a is set to four, and only three of them are shown in Fig. 4.A countersunk screw B is inserted from the rear into each of the mounting holes 23a of the soundboard 23, the associated insertion holes 33a of the vibration exciter pad 33 and the associated insertion holes 25d of the vibration exciter 25 in the aforementioned order, and nuts N are attached to the respective countersunk screws B from the front, thereby fixing the vibration exciter 25 to the front surface of the soundboard 23. Furthermore, the electronic piano 1 is equipped with a tone generator implemented by an ECU comprising a CPU, RAM, and ROM (none of which are shown). Detection signals from the aforementioned key and pedal sensors are input to this tone generator. The tone generator produces a drive signal according to a program stored in the ROM in response to the input detection signals and then inputs the generated drive signal to the vibration exciter 25. This causes the vibration exciter 25 to be driven by the drive signal generated based on the depressed state of a key 13 and others, thereby setting the soundboard 23 into vibration and producing musical tones. Fig. 6 shows the relationship (indicated by a thick, solid line) between the frequency (Hz) and the sound pressure level (dB) of a musical tone from the electronic piano 1, measured directly in front of the center of the soundboard 23, together with a comparison example (indicated by a thin double-dash line). The comparison example shows a case in which, unlike in the present embodiment, the screws for fastening the soundboard are arranged regularly, as shown in Fig. 8. In Fig. 6, the frequency is represented logarithmically. As shown in Fig. 6, it is possible according to the present embodiment to reduce the peak roll-off of the frequency characteristic of a musical tone mainly in a lower range compared to the case of the comparison example. As described above, in the present embodiment, each of the upper, lower, left, and right ends of the soundboard 23, which is rectangular in shape, is fastened to the edge 30 by a plurality of self-tapping screws TB. As described with reference to Fig. 5, the self-tapping screws TB for fastening the upper end of the soundboard 23 and the self-tapping screws TB for fastening the lower end of the soundboard 23 are arranged in an offset manner along the upper and lower ends of the soundboard 23, respectively, such that the position of each of the self-tapping screws at the upper end and the position of each of the self-tapping screws at the lower end are not symmetrically opposite each other in the vertical direction. Furthermore, the self-tapping screws TB for fastening the left end of the soundboard 23 and the self-tapping screws TB for fastening the right end of the soundboard 23 are arranged along the left and lower ends of the soundboard 23, respectively.The right end of the soundboard 23 is arranged in an offset manner such that the position of each of the thread-cutting screws at the left end and the position of each of the thread-cutting screws at the right end are not opposite each other in a symmetrical relationship in the left-right direction. Unlike the conventional electronic musical instrument described previously, as described above, the fixed ends of the upper, lower, left, and right ends of the soundboard 23 are irregularly arranged. This makes it possible to adequately suppress the resonance of the soundboard 23 and thus appropriately reduce the peak roll-off of the frequency characteristic of a musical tone, thereby enabling the production of an excellent musical sound. Since the vibration exciter pad 33 is positioned between the vibration exciter 25 and the soundboard 23 to suppress its resonance, the resonance of the soundboard 23 can be further suppressed, resulting in an even more excellent musical sound. It should be noted that the present invention is not limited to the embodiment described above, but can be implemented in various forms. Although in the embodiment described above the number of vibration exciters is set to one, it is possible, for example, to provide two or more vibration exciters. In this case, the vibration exciters, each exhibiting different vibration characteristics, can be provided for the high range, the medium range, and the low range, respectively. Although in the embodiment described above the soundboard pad 31 is made of PORON, any other material suitable for suppressing the resonance of the soundboard 23, such as urethane foam or rubber, can also be used. This variant of the soundboard pad 31 applies similarly to the vibration exciter pad 33. Although the soundboard pad 31 and the vibration exciter pad 33 are provided in the embodiment described above, at least the pad 31 can be omitted. While in the embodiment described above the self-tapping screws TB for fastening the upper, lower, left, and right ends of the soundboard 23 are arranged irregularly as described above, the self-tapping screws for fastening either the upper and lower ends or the left and right ends can also be arranged irregularly, and the self-tapping screws for fastening the other end of the upper and lower ends or the left and right ends can also be arranged regularly, as shown in Fig. 8. Although in the embodiment described above, self-tapping screws TB are used as screws for fastening the soundboard 23, general nuts and bolts can also be used. In this case, for example, each nut is firmly embedded in the rim, and then the bolts are inserted into the respective insertion holes of the soundboard from the front and secured in the respective nuts, thereby fastening the soundboard to the rim. Alternatively, each bolt is firmly embedded in the rim and inserted into an associated insertion hole of the soundboard from the rear, and then the nuts are fastened to the respective bolts from the front, thereby fastening the soundboard to the rim. In these cases, the bolts and nuts correspond to the first to fourth screws of the present invention. Although in the embodiment described above the present invention is applied to the upright electronic piano 1, it is understood that the invention can also be applied to an electronic grand piano, as well as to any other suitable electronic musical instrument, such as an electronic percussion instrument. Fig. 7 schematically shows a soundboard 41 of an electronic grand piano to which the present invention is applied. As shown in Fig. 7, the soundboard 41 has a perimeter which is fastened to an inner edge (not shown) by a plurality of self-tapping screws tb (first and second screws). A vibration exciter (not shown) is mounted on the lower surface of the soundboard 41 to set the soundboard 41 into vibration, and between the soundboard 41 and the inner edge and between the soundboard 41 and the vibration exciter, a soundboard pad and a vibration exciter pad (neither of which are shown) are arranged, respectively, to suppress the resonance of the soundboard 41.The soundboard 41 has a left and a right end 41a and 41b (a first and second end) which extend parallel to each other in the front-to-back direction, and the thread-cutting screws tb for fastening a front section of the left end 41a and the thread-cutting screws tb for fastening the right end 41b are arranged along the left and right ends 41a and 41b of the soundboard 41 in an offset manner such that the position of each of the thread-cutting screws at the left end 41a and that of each of the thread-cutting screws at the right end 41b are not opposite each other in a symmetrical relationship in the left-right direction. Thus, in the electronic grand piano, the fixed ends of each of the left and right ends 41a and 41b are also irregularly positioned as described above, making it possible to suppress the resonance of the soundboard 41 and thereby reduce the peak roll-off of the frequency characteristic of a musical tone, thus enabling an excellent musical sound. Since the vibration exciter pad is arranged between the vibration exciter and the soundboard 41, the resonance of the soundboard 41 can be further suppressed, making it possible to obtain an even better musical sound. Furthermore, it is understood that in the electronic grand piano to which the present invention is applied, at least one of the soundboard pad and the vibration exciter pad can be omitted. Moreover, general nuts and bolts can be used instead of the self-tapping screws tb. In this case, for example, each nut is firmly embedded in the inner rim, and then the bolts are inserted from above into the respective insertion holes of the soundboard and secured in the respective nuts, thereby fixing the soundboard to the inner rim. Alternatively, each bolt is firmly embedded in the inner rim and inserted from below into an associated insertion hole of the soundboard, and then the nuts are fastened to the respective bolts from above, thereby fixing the soundboard to the inner rim. In these cases, the bolts and nuts correspond to the first and second screws of the present invention. It is furthermore clear to those skilled in the art that the foregoing are preferred embodiments of the invention, and that various changes and modifications can be made without departing from its spirit and scope.

Claims

An electronic musical instrument (1) that generates a musical tone for a musical performance in accordance with a musical tone signal generated based on an actuated state of a control element (13), comprising: a soundboard (23) having a first end and a second end extending parallel to each other and attached to a predetermined part of the electronic musical instrument (1) by a plurality of first screws (TB) and a plurality of second screws (TB), respectively, and a vibration exciter (25) provided on the soundboard (23) and configured to cause the soundboard (23) to vibrate by being driven in accordance with the musical tone signal, thus causing the soundboard (23) to produce a musical tone, characterized in that the plurality of first screws (TB) and the plurality of second screws (TB) extend along the first and second ends, respectively.The second end is arranged in an offset manner such that a layer of each of the plurality of first screws (TB) and a layer of each of the plurality of second screws (TB) are not opposite each other in a symmetrical relationship in a direction orthogonal to the first and second ends, and a pad (33) for suppressing the resonance of the soundboard (23) is arranged between the vibration exciter (25) and the soundboard (23). Electronic musical instrument (1) according to claim 1, wherein the electronic musical instrument (1) is an upright electronic piano (1) in which the control element (13) is a key (13), wherein the soundboard (23) is formed in a rectangular shape, and the first and second ends correspond to an upper end and a lower end of the soundboard (23), wherein the plurality of first screws (TB) and the plurality of second screws (TB) are arranged along the upper and lower ends in an offset manner such that a position of each of the first screws (TB) and a position of each of the second screws (TB) are not opposite each other in a symmetrical relationship in a vertical direction, wherein a left end and a right end of the soundboard (23) are fastened at the predetermined part by a plurality of third screws (TB) and a third screw (TB), respectively.a plurality of fourth screws (TB) are fastened, wherein the plurality of third screws (TB) and the plurality of fourth screws (TB) are arranged in an offset manner along the left and right ends respectively, such that a position of each of the third screws (TB) and a position of each of the fourth screws (TB) are not opposite each other in a symmetrical relationship in a left-right direction.

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