Patent Publication Number: US-5837915-A

Title: Electronic drum having flat sound producing characteristics

Description:
FIELD OF THE INVENTION 
     This invention relates to an electronic musical instrument and, more particularly, to an electronic drum beaten for generating an electronic drum sound. 
     DESCRIPTION OF THE RELATED ART 
     There are many kinds of electronic musical instrument. An electronic keyboard and a synthesizer are typical examples of the electronic musical instrument. Electronic drums have been developed, and are used by a drum player. An electronic drum has a drum pad struck with a stick, and sensors are attached to a back surface of the drum pad. The sensor is implemented by piezoelectric element, and converts an impact to an electric signal. The electric signal is representative of the strength of the impact, and an electronic drum sound is produced from the electric signal. 
     FIGS. 1 and 2 illustrates a prior art electronic drum. The prior art electronic drum 1 comprises a drum pad 1a and an electronic sound generating system (not shown) connected to the drum pad 1a. The drum pad 1a includes a supporting member 1b formed of iron and a pad member 1c formed of rubber. Both of the supporting member 1b and the pad member 1c are shaped into a disk configuration, and are equal in diameter to each other. The pad member 1c is laminated on the supporting member 1b, and is fixed thereto. 
     The prior art drum pad 1a further includes an absorbing member 1d attached to the back surface of the supporting member 1d and a piezoelectric element 1e fixed to the back surface of the absorbing member 1d. The absorbing member 1d is formed of sponge, and both surfaces of the sponge layer is coated with adhesive compound. The adhesive compound integrates absorbing member 1d, the piezoelectric element le and the supporting member 1d. The absorbing member 1d has a disk configuration much smaller than the supporting member 1b, and the piezoelectric element 1e also has a disk configuration slightly smaller than the absorbing member 1d. For this reason, only a central area of the supporting member 1b is covered with the absorbing member 1d, and most of the absorbing member 1d is covered with the piezoelectric element 1e. 
     When a drum player beats the top surface of the pad member 1c, vibrations take place, and the supporting member 1b and the absorbing member 1d propagate the vibrations to the piezoelectric element 1e. The piezoelectric element 1e converts the vibrations to an electric signal, and a lead wire 1f transfers the electric signal from the piezoelectric element 1e to the electronic sound generating system. The electronic sound generating system is responsive to the electric signal so as to generate an electronic drum sound. 
     The vibrations gradually decrease the amplitude thereof in proportional to the distance between a point beaten with the stick and the piezoelectric element 1e, and the amplitude of the electric signal is proportional to the amplitude of the vibrations. For this reason, when the drum player beats different points on the pad member 1c, the amplitude of the electric signal is decreased in inverse proportion to the distance between the point beaten with the stick and the piezoelectric element 1e as shown in FIG. 3. When the drum player beats the central area over the piezoelectric element 1e, the amplitude of the electric signal is maximized. On the other hand, a beat in the peripheral area results in the minimum amplitude of the electric signal. The electronic sound generating system determines the loudness of the electronic sound depending upon the amplitude of the electric signal. This means that the electronic drum sound is variable in loudness with the point beaten with the stick. However, such a variable drum sound is hardly controlled by the drum player, because he is expected to exactly control the stick in not only the strength of the stick but also the point beaten therewith. 
     In order to improve the sound generation characteristics of the prior art electronic drum, the piezoelectric element 1e is mounted on a sensor board 2a as shown in FIGS. 4 and 5. In detail, the drum pad 2 of the prior art electronic drum 1 includes the supporting member 1b, the pad member 1c and the piezoelectric element 1e as similar to the drum pad 1a, and the piezoelectric element 1e is attached to a central area of the rectangular sensor board 2a formed of synthetic resin. The absorbing member 1d is divided into a plurality of absorbing sub-members 2b, and the absorbing sub-members 2b attach the sensor board 2a to the supporting member 1b. 
     The sensor board 2a is wider than the piezoelectric element 1e, and changes the output characteristics as shown in FIG. 6. It is understood from FIG. 6 that the sensor board 2a makes the output characteristics mild. Even if a drum player beats the intermediate area between the central area and the peripheral area of the pad member 1c, the electronic drum sound is fairly equal in intensity to the sound at the impact in the central area. However, there are three peaks in the output characteristics, and the sensor board 2a steeply slopes the output characteristics from the intermediate area to the peripheral area. Thus, the prior art drum pad 2 still does not satisfy a drum player. The variation of the electric signal due to the different beaten points is hereinbelow referred to as &#34;local dependency&#34;. 
     SUMMARY OF THE INVENTION 
     It is therefore an important object of the present invention to provide an electronic drum which generates drum sounds equal in loudness regardless of a point beaten by a player. 
     The present inventor contemplated the problem inherent in the prior art electronic drum, and noticed that the sensor board was formed of synthetic resin. The synthetic resin was small in internal loss, which was usually represented by &#34;tangent delta&#34; or &#34;loss tangent&#34;, and large in resonance sharpness. For this reason, when the present inventor beat the prior art drum pad 2, the nodes and the anti-nodes clearly took place in the sensor board 2a. The present inventor further observed that the waveform of the vibrations was varied depending upon the point beaten with a stick and the location of the absorbing sub-members 2b. This meant that the detected point on the waveform was variable with the point beaten with the stick. The present inventor concluded that an appropriate sensor board would improve the sound producing characteristics of the electronic drum. 
     To accomplish the object, the present invention proposes to form a sensor board of a material with a large internal loss. 
     In accordance with the present invention, there is provided an electronic drum comprising: a pad structure having a surface beaten by a player so as to generate vibrations therein; a sensor unit including a sensor board formed of a material having an internal loss equal to or greater than 0.02 and a vibration sensor attached to the sensor board so as to convert the vibrations to an electric signal; a vibration absorbing member provided between the pad structure and the sensor unit for propagating the vibrations to the sensor board; and an electric sound generating system connected to the vibration sensor for producing an electric drum sound on the basis of the electric signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features an advantages of the electronic drum according to the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a bottom view showing the prior art electronic drum; 
     FIG. 2 is a cross sectional view taken along line 2--2 of FIG. 1 and showing the structure of the prior art electronic drum; 
     FIG. 3 is a graph showing the amplitude of the electric signal in terms of the point beaten with the stick; 
     FIG. 4 is a bottom view showing another prior art electronic drum; 
     FIG. 5 is a cross sectional view taken along line 5--5 of FIG. 4 and showing the structure of the prior art electric drum; 
     FIG. 6 is a graph showing the amplitude of the electric signal generated by the prior art electronic drum shown in FIG. 4 in terms of the point beaten with the stick; 
     FIG. 7 is a bottom view showing an electronic drum according to the present invention; 
     FIG. 8 is a cross sectional view taken along line 8--8 of FIG. 7 and showing the structure of the electronic drum; 
     FIG. 9 is a graph showing a relation between an internal loss, a propagation velocity and the material of a medium; and 
     FIG. 10 is a graph showing the amplitude of the electric signal generated by the electronic drum shown in FIG. 7 in terms of the point beaten with the stick. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 7 and 8 of the drawings, an electronic drum embodying the present invention largely comprises a drum pad 3a and an electronic sound generating system 3b connected to the drum pad 3a. 
     The electronic sound generating system 3b is similar to that of the prior art electronic drum. An electric signal S1 is supplied from the drum pad 3a to the electronic sound generating system 3b, and the electronic sound generating system 3b generates an electronic drum sound S2 on the basis of the electric signal S1. 
     The drum pad 3a is broken down into a pad structure 3c beaten with sticks (not shown), a vibration sensor unit 3d for generating the electric signal S1 and a vibration absorbing member 3e provided between the pad structure 3c and the vibration sensor unit 3d. When a drum player beats the upper surface 3f of the pad structure 3c, vibrations take place in the pad structure 3c, and are propagated from the pad structure 3c through the vibration absorbing member 3e to the vibration sensor unit 3d. The vibration sensor unit 3d converts the vibrations to the electric signal S1, and the electric signal S1 is transferred from the vibration sensor unit 3d through a signal wire 3g to the electronic sound generating system 3b. 
     A supporting member 3h and a pad member 3i form in combination the pad structure 3c. The supporting member 3h is formed of iron, and is shaped into a disk configuration. The pad member 3i is formed of natural rubber, synthetic rubber or a mixture thereof, and is also formed into a disk configuration. The pad member 3i is equal in diameter to the supporting member 3h, and is thicker than the supporting member 3h. The pad member 3i is laminated on the supporting member 3h, and is fixed thereto by means of adhesive compound. 
     The vibration sensor unit 3d includes a sensor board 3j and a piezoelectric element 3k connected to the signal wiring 3g. In this instance, the sensor board 3j is formed of cellular vinyl chloride. The sensor board 3j is shaped into a disk configuration, and is approximately equal in thickness to the supporting member 3h. Although the sensor board 3j is slightly smaller in diameter than the supporting member 3h, the sensor board 3j and the supporting member 3h are cocentric with each other, and the sensor board 3j is attached to the supporting member 3h by means of the vibration absorbing member 3e. 
     Other material is available for the sensor board 3j in so far as the internal loss, i.e., tangent delta is equal to or greater than 0.02 which is the internal loss of a typical synthetic resin already used for the prior art sensor board 2a. Although paper has the tangent delta greater than 0.02, the durability is poor. FIG. 9 illustrates the internal loss and the propagation velocity for vibrations. Regions A, B, C and D respectively stand for ceramics such as, for example, beryllia ceramics, silicones and alumina, light metal such as, for example, aluminum, titanium and magnesium, synthetic resin such as fiber-reinforced plastic resin, cellular vinyl chloride and paper including pulp. 
     The sensor board 3j formed of material with the internal loss equal to or greater than 0.02 make the vibrations uniform, and, accordingly, makes the amplitude of vibrations constant over the sensor board 3j. For this reason, even if the vibrations are propagated from any point on the top surface 3f to the sensor board 3j, the piezoelectric element 3k generates the electric signal S1 with a constant amplitude in so far as the drum player beats the top surface 3f at a constant impact. Thus, the sensor board 3j eliminates the local dependency of the drum sound, and, accordingly, allows a drum player to generate the electronic drum sound at a constant loudness regardless of the point beaten with the stick. 
     The uniform vibration property allows the manufacturer to enlarge the sensor board 3j, and the large sensor board 3j further eliminates the local dependency, because the vibrations immediately reach the sensor board 3j. 
     The piezoelectric element 3k detects vibrations propagated from the pad structure 3c through the vibration absorbing member 3e thereto. The vibrations give a strain to the piezoelectric element 3k, and the piezoelectric element 3k generates electric potential proportional to the strain. The variation of the electric potential is detected as the electric signal S1. 
     A plurality of vibration absorbing strips 3m form in combination the vibration absorbing member 3e, and are shaped into a rectangular configuration. In this instance, eight vibration absorbing strips 3m are radially arranged, and are equally spaced from one another along the outer periphery of the sensor board 3j. This means that the vibration absorbing strips 3m are angularly arranged at a constant pitch. The vibration absorbing strips 3m are formed of buthyle rubber, and have an appropriate thickness so as to achieve a good balance between the damping characteristics and the vibration propagation characteristics. The vibration absorbing strips 3m are coated with adhesive compound on both surfaces thereof, and adhere the sensor board 3j to the supporting member 3h. 
     The vibration absorbing strips 3m thus arranged cause the vibrations to enter into the periphery of the sensor board 3j, and smoothly transfer the vibrations from the supporting member 3h to the sensor board 3j. Even if a drum player beats a central area of the surface 3f, the vibrations radially spread over the pad structure 3c, and the vibration absorbing strips 3m transfer the vibrations from the periphery of the pad structure 3c to the periphery of the sensor board 3j. The sensor board 3j propagates the vibrations from the periphery thereof to the piezoelectric element 3k at the central area. The disk configuration of the sensor board 3j also promotes the elimination of the local dependency, because the vibrations travels over a constant distance between beaten points on the same circumference and the vibration absorbing strips 3m. Thus, the vibration absorbing strips 3m and the disk configurations equalize the distance between the beaten points and the piezoelectric element 3k, and effectively eliminate the local dependency. Especially, when a central area of the surface 3f is beaten, the vibration absorbing strips 3m improve a vibration transmission response. The improvement of the vibration transmission response means that the vibrations are propagated at high speed. 
     It is desirable for the sensor board 3j to have the following damping characteristics so as to cause the piezoelectric element 3k to generate the electric signal S1 with a constant amplitude. The desirable damping characteristics cause the first wave of the vibrations to have a large amplitude in the dead range or the maximum detectable range and the second wave to rapidly enter through the dead range into the non-detectable range, or damp the first wave to the nth wave in the dead range and the (n+1)th wave, the (n+2)th wave, . . . in the non-detectable range. Such a damping characteristics is achieved by selecting the material and the configuration of the sensor board 3j, the material and the configuration of the vibration absorbing strips 3m and the relative relation of dimensions between the sensor board 3j and the vibration absorbing strips 3m. The cellular vinyl chloride, the buthyle rubber and the disk configuration are the most appropriate materials for the sensor board 3j and the vibration absorbing strips 3m. 
     The term &#34;non-detectable range&#34; is defined as a time period when a tone generator supplies an audio signal to a speaker system in response to a potential signal higher than the threshold (or the lower limit of the potential level for the tone generator). Even if the next wave higher in potential level than the threshold reaches the tone generator in the non-detectable range, the tone generator can not respond to the next wave. 
     The most appropriate arrangement of the vibration absorbing strips 3m is shown in FIG. 7. Namely, the vibration absorbing strips 3m are spaced from the center of the sensor board 3j as far as possible, and are arranged in such a manner as to form a circle cocentric with respect to the disk-shaped sensor board 3j. The vibration absorbing strips 3m are spaced from each other at a constant pitch. 
     The relative relation between the vibration absorbing strips 3m and the sensor board 3j falls within the following range: 
     i) the diameter of the sensor board 3j ranges from 50% to 100% of the diameter of the circle of the vibration absorbing strips 3m; 
     ii) the thickness of the supporting member 3h ranges from 10% to 50% of the thickness of the pad member 3i; 
     iii) the thickness of the vibration absorbing strips 3m ranges from 10% to 50% of the thickness of the pad member 3i; 
     iv) the thickness of the sensor board 3j ranges from 10% to 50% of the thickness of the pad member 3i; and 
     v) the internal loss or tangent-delta is equal to or greater than 0.02. 
     In the above described embodiment, the pad member 3i has the diameter of 179.6 millimeters, and is 6 millimeters in thickness; the supporting member 3h has the diameter of 152.8 millimeters, and is 1.6 millimeters in thickness; the vibration absorbing strips 3m are 20 millimeters in length, 5 millimeters in width and 1 millimeter in thickness; the sensor board 3j has the diameter of 110 millimeters, and is 1 millimeter in thickness; and the internal loss or the tangent-delta is 0.04. 
     The present inventor evaluated the drum pad 3a in comparison with a comparative example which had a sensor board with the internal loss less than 0.02. The present inventor beat the surface 3f with a stick at a constant impact, and changed the point beaten with the stick over the surface 3f so as to plot the amplitude of the electric signal S1 in terms of the distance from the center of the pad member 3i. The amplitude of the electric signal S1 was substantially constant as shown in FIG. 10; however, the constant amplitude was never observed in the comparative example. A flat output characteristics of the piezoelectric element 3k were observed in so far as the relative dimensions between the sensor board 3j and the vibration absorbing strips 3m fell within the above described ranges. 
     As will be appreciated from the foregoing description, the sensor board 3j with the internal loss not less than 0.02 equalizes the amplitude of the vibrations propagated therethrough, and effectively eliminates the local dependency. As a result, the electronic drum according to the present invention generates the electronic drum sound at a constant loudness in so far as a drum player beats the drum pad 3a at a constant impact. 
     Although particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. 
     For example, an electronic drum according to the present invention may be beaten with hands or mallets. 
     The sensor board 3j may be shaped into a polygonal configuration closed to a circle. 
     The vibration absorbing member 3e may have a ring configuration. 
     The pad structure may be constituted by a supporting member sandwiched between pad members or a pad member formed of hard rubber. 
     The electronic sound generating system may impart a timbre different from a drum sound to the electronic sound or simply amplitude the electric signal. 
     The sensor board 3j may be formed of a material having the internal loss not less than 0.02 and a vibration propagation velocity larger than the cellular vinyl chloride. The material improve the vibration transmittability of the sensor board.