JPH0474096A - Electroacoustic transducer - Google Patents

Abstract

PURPOSE:To prevent reduction in the directivity and to make the thickness of an acoustic filter thin by forming the acoustic filter with two layers comprising a 1st sound absorbing layer absorbing mainly a primary sound wave and a 2nd sound absorbing layer absorbing mainly a secondary sound wave. CONSTITUTION:A sound source 1 consists of a vibrator array in which lots of ultrasonic vibrators are arranged and has larger energy of primary sound waves. A cross section of an acoustic filter 4 is formed triangle and the filter 4 is arranged to a position corresponding to a reflecting face 5 and made of a synthetic resin foaming body or the like to suppress mainly primary sound wave and to transmit a secondary sound wave. Moreover, a sound absorbing member 6 to absorb primary and secondary sound waves to prevent irregular reflection in an inner side face of a cylinder 2 is arranged to an inner side face of a circumferential wall of a cylinder 2. Thus, the similar effect is obtained with the volume of nearly a half. Furthermore, the projection toward the side of the cylinder 2 is less by arranging the acoustic filter 4 in the inside of the cylinder 2 and the mount to other device is facilitated.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an electroacoustic transducer that generates an audible secondary sound wave by a nonlinear parametric effect of a primary sound wave.

[Conventional technology]

Conventionally, electrodynamic speakers have been widely used as electroacoustic transducers. By the way, electrodynamic speakers have relatively weak directivity, so if it is necessary to limit the sound field, it is necessary to strengthen the directivity. As a means of increasing the directivity of an electrodynamic speaker, it is known to increase the diameter of the speaker or to attach a horn to the speaker. However, the problem with either method is that the effect of improving directivity is small despite the increase in size. In particular, it is difficult to obtain directivity in the low audio frequency range. On the other hand, as an electroacoustic transducer with good directivity, a so-called parametric speaker is known that generates an audible secondary sound wave by a nonlinear parametric effect of a primary sound wave. Parametric speakers generate secondary sound waves that are more audible than primary sound waves based on the following principle. That is, as shown in Fig. 6, two different frequencies (F
1, F2) to drive a sound source 1 such as a speaker or an ultrasonic transducer, the primary sound wave (indicated by the solid line) output from the sound source 1 interferes with the air, and the difference component of the frequency of the primary sound wave ( F2-Fl) and the sum component (F2+F1) are generated as a secondary sound wave (indicated by a broken line). The sound pressure of the secondary sound waves generated by this phenomenon can be reduced by creating a very strong sound field using highly directional ultrasound waves as the primary sound waves.
to a level that can withstand practical use. Furthermore, as shown in Fig. 7, the frequency difference between the primary sound waves (Fl, F2) is the audible frequency (A
F), the difference component is 2 of the audible frequency.
A secondary sound wave (F2-Fl) will be generated. This phenomenon is called a nonlinear parametric effect, and the directionality of the generated secondary sound waves is weaker than that of the primary sound waves, but shows a pattern relatively close to that of the secondary sound waves (6th oval pattern in the figure). Therefore, if ultrasonic waves with strong directivity are used as the primary sound waves, the directivity can also be made strong in the audible frequencies generated as the secondary sound waves. The driving methods for driving the sound source 1 include a two-frequency driving method that drives at two frequencies as described above, and an amplitude modulated wave drive that drives using an amplitude-modulated signal as shown in FIGS. 8 and 9. The method is known. In the amplitude modulated wave driving method, the output of an audible signal generator 11 such as a microphone is passed to a modulation circuit 1 through an audio frequency circuit 12 including an amplifier circuit, etc.
4 and output from the carrier wave oscillation circuit 13. Amplifying circuit 1 transmits a signal obtained by amplitude modulating the carrier wave signal of
5 to the sound source 1. If the sound source 1 is driven by the amplitude-modulated signal in this way, a primary sound wave containing both-side wave components centered around the fundamental frequency F0 of the carrier wave signal will be output, as shown in FIG. Due to the interference, a secondary sound wave at an audio frequency (AF) is generated as a beat component corresponding to the frequency difference between the two side wave components. Parametric speakers that operate on the principle described above have the advantage of providing strong directivity down to the low audible frequency range, but they have the problem of low conversion efficiency from primary sound waves to secondary sound waves, making it difficult to produce a sound that is suitable for practical use. To obtain the pressure level, it is necessary to increase the energy of the primary sound wave. However, if the energy of the primary sound wave is increased, the ultrasonic wave, which is the primary sound wave, will have a negative effect on the human body.
A configuration is required that suppresses the primary sound waves and effectively extracts only the secondary sound waves. Acoustic filters are generally used to suppress primary sound waves. On the other hand, the space in which secondary sound waves are generated is called a parametric acoustic array, and the length of the parametric acoustic array from the sound source is called the effective array length. In order to effectively generate secondary sound waves from primary sound waves, it is desirable to prevent the primary sound waves from attenuating within the range of the effective array length. Therefore, as shown in Figure 10, a parametric acoustic - A configuration can be considered in which a cylindrical body 2 having a peripheral wall surrounding the array is provided, an opening 3 for transmitting secondary sound waves is formed at one end of the cylindrical body 2, and an acoustic filter 4 is disposed in the opening 3. ing. The acoustic filter 4 is formed using synthetic resin foam or the like so as to suppress primary sound waves and transmit secondary sound waves. In this way, the parametric acoustic array is
By surrounding it with a peripheral wall, the space through which the primary sound waves are transmitted is limited, and by allowing only the secondary sound waves to be taken out to the outside of the cylinder 2 by the acoustic filter 4, the influence of the primary sound waves on the human body is reduced. It suppresses Here, in order to miniaturize the parametric speaker, it is better for the cylinder 2 to be short, but if the cylinder 2 is short, the effective array length cannot be secured within the cylinder 2, and the primary sound wave is changed to the secondary sound wave. A problem arises in that the conversion efficiency of the antenna is reduced and the directivity is also reduced. In order to solve this problem, as shown in FIG. 11, it is conceivable to bend the cylindrical body 2 into an L shape or the like and form a reflective surface 5 at the bent part. In this configuration, since the primary sound waves are reflected by the reflecting surface 5, the length of the cylindrical body 2 (the horizontal length in the figure) can be shortened while ensuring the effective array length.

Problem M1 to be Solved by the Invention As mentioned above, the length of the cylinder 2 can be shortened by using the bent cylinder 2, but since the acoustic filter 4 is attached to the opening 3, the length of the cylinder 2 can be shortened. A problem arises in that the amount of lateral protrusion of the cylindrical body 2 increases by the thickness of the cylindrical body 2. Also,
If the reflective surface 5 exists, there is also the problem that the directivity is reduced due to diffuse reflection at the end of the reflective surface 5. Furthermore, it has been empirically found that the frequency of the primary sound wave with good generation efficiency of the secondary sound wave is about 10 times the frequency of the desired secondary sound wave. Therefore, if the frequency of voice is required as a secondary sound wave, the primary sound wave should be
Efficiency is good if the frequency is set to 30 kHz, however, 1
When the frequency of the next sound wave is low, it becomes difficult for the acoustic filter 4 to suppress the first order sound wave, and the problem arises that the acoustic filter 4 must be formed thickly, leading to an increase in size. The present invention aims to solve the above-mentioned problems, and by using a cylindrical body formed with a reflective surface, the length can be reduced, the amount of lateral protrusion of the cylindrical body can be reduced, and the directional An object of the present invention is to provide an electroacoustic transducer that can prevent a decrease in performance and reduce the thickness of an acoustic filter. [Means for Solving the Problems] In order to achieve the above object, in the structure of claim 1, electroacoustic conversion is performed to generate a secondary sound wave, which is an audible sound, by a nonlinear parametric action of a primary sound wave output from a sound source. In the device, a cylindrical body is provided, which has a peripheral wall surrounding a space in which secondary sound waves are generated, and openings for transmitting the generated secondary sound waves are formed at important points on the peripheral wall, and the internal space of the cylindrical body is bent. A reflective surface that changes the traveling direction of the sound wave so as to guide the secondary sound wave to the opening is formed at the bent portion, and an acoustic filter that suppresses at least the primary sound wave is disposed on the surface of the reflective surface. In the structure of claim 2, the acoustic filter is formed so that the amount of absorption increases at each part of the reflecting surface. In the configuration of claim 3, the acoustic filter is arranged such that the absorption amount is larger at a portion closer to the opening, and the absorption amount is larger at both ends than at the center on a line approximately equidistant from the opening on the reflective surface. It is formed in this way. In the structure of claim 4, the acoustic filter includes a first sound absorbing layer that mainly absorbs primary sound waves, and a second sound absorbing layer that is disposed between the reflective surface and the first sound absorbing layer and mainly absorbs secondary sound waves. It is formed by two layers.

[Effect]

According to the structure of claim 1, since the acoustic filter that suppresses at least the primary sound wave is disposed on the surface of the reflecting surface, the acoustic filter can be placed inside the cylinder, and as a result, the acoustic filter can be disposed to the side of the cylinder. It is possible to eliminate the protruding part of the
This makes it easier to incorporate it into other devices. In addition, since the sound waves pass through the acoustic filter twice, once when entering the reflecting surface and when exiting, the amount of absorption per volume can be increased compared to the conventional configuration in which the opening is covered with an acoustic filter. This allows the acoustic filter to be made smaller and more compact. Furthermore, since the acoustic filter is placed on the surface of the reflecting surface, the side lobes caused by diffused reflection that occur near the ends of the reflecting surface can be suppressed by the acoustic filter, and the overall length of the cylindrical body is shortened. Even though it forms a surface, the directivity of secondary sound waves can be strengthened. In the structure of claim 2, the acoustic filter is formed so that the amount of absorption differs at each part of the reflecting surface, so that depending on the directivity pattern of the primary sound wave and the directivity pattern of the secondary sound wave,
This can prevent unnecessary sound waves from leaking outside. In the configuration of claim 3, the acoustic filter is arranged such that the absorption amount is larger at a portion closer to the opening, and the absorption amount is larger at both ends than at the center on a line approximately equidistant from the opening on the reflective surface. , it is possible to reduce side lobes due to diffused reflection near the ends of the reflecting surface and strengthen the directivity of the secondary sound waves. In the structure of claim 4, the acoustic filter includes a first sound absorbing layer that mainly absorbs primary sound waves, and a second sound absorbing layer that is disposed between the reflective surface and the first sound absorbing layer and mainly absorbs secondary sound waves. Since it is formed of two layers, the effect of reducing side lobes is further enhanced.

[Example 1] The basic configuration of this example is as shown in Fig. 1,
The configuration is similar to that shown in FIG. 10 except for the position and shape of the acoustic filter 4. The sound source 1 is constituted by a transducer array in which a large number of ultrasonic transducers are arranged, and is designed to increase the energy of the primary sound wave. The driving method of the sound source 1 is the amplitude modulated wave driving method shown in FIGS. 8 and 9. The acoustic filter 4 has a triangular cross section and is disposed at a location corresponding to the reflective surface 5. This acoustic filter 4 mainly suppresses primary sound waves, and
This is a transmission type filter that allows secondary sound waves to pass through, and is made of synthetic resin foam or the like. Furthermore, a sound absorbing material 6 is disposed on the inner surface of the peripheral wall of the cylinder 2 to absorb primary and secondary sound waves and prevent diffuse reflection on the inner surface of the cylinder 2. Here, regarding the path of the sound wave, if the distance from the incident surface to the acoustic filter 4 to the reflecting surface 5 is 11, and the distance from the reflecting surface 5 to the output surface of the acoustic filter 4 is 12, then the sound wave will travel along the acoustic filter 4. The acoustic filter passes through the inside for a distance of 1++12, and compared to the case where the acoustic filter 4 is provided in the opening 3 as in the past, the same effect can be achieved with about half the volume. By arranging the acoustic filter 4 inside the cylinder body 2, the amount of lateral protrusion of the cylinder body 2 is reduced, making it easy to incorporate into other devices. If we were to create an alarm clock that uses this strength to allow only certain people to hear the alarm, it would be easy to incorporate it into the alarm clock case or bed board. Furthermore, by arranging the acoustic filter 4 on the surface of the reflective surface 5, it is possible to suppress the occurrence of diffused reflection near the end of the reflective surface 5, and prevent a reduction in directivity.

[Embodiment 2] In this embodiment, as shown in FIG. 2, the acoustic filter 4 is disposed corresponding to a part of the reflecting surface 5, and the end of the reflecting surface 5 near the opening 3 is provided. Acoustic filter 4 only in the section
has been set up. The acoustic filter 4 has two layers, as shown in FIG. It is set to mainly absorb secondary sound waves. In this way, by forming the acoustic filter 4 into two layers and providing the second sound absorbing layer 4b at the end of the reflecting surface 5, the side lobes of the secondary sound waves can be reduced and the directivity can be increased. . In addition, in order to further reduce side lobes, the opening 3
In addition, as shown in FIG.
In contrast, the amount of absorption at both left and right ends of the reflective surface 5 is larger than that at the center (thick wall). The other configurations are the same as those in the first embodiment, so their explanation will be omitted. In addition to changing the thickness of the acoustic filter 4, the material of the acoustic filter 4 may be changed to control the amount of absorption.

[Embodiment 3] In this embodiment, as shown in FIG. 3, the ultrasonic transducers 1a constituting the sound source 1 are arranged so that the emission surface forms a concave surface.
This is so that the primary sound waves from each ultrasonic transducer 1a are converged to approximately one point. With such a configuration, the sound pressure level near the convergence point of the primary sound wave can be increased, and the effective L and A lengths can be shortened. That is, the length of the cylindrical body 2 can be shortened and the size of the cylindrical body 2 can be reduced. However, since the wavefront of the sound wave is no longer in the form of a plane wave, diffused reflection on the reflecting surface 5 is likely to occur, and many side lobes will occur. Therefore, as in the second embodiment, the absorption amount of the acoustic filter 4 is made larger at both the left and right ends of the reflecting surface 5 than at the center, thereby suppressing the generation of side lobes and strengthening the directivity. Here, it is time-consuming to form the acoustic filter 4 into a smoothly curved shape as in the second embodiment, so in this embodiment, the acoustic filter 4 is formed by combining three partial filters 4 c and 4 d. It is oval. The other configurations are the same as in the first embodiment.

[Embodiment 4] In each of the above embodiments, an example was shown in which a synthetic resin foam was used as the acoustic filter 4, but in this embodiment, a so-called perforated sound absorbing plate as shown in FIG. 4 is used. . This acoustic filter 4 includes a front plate 8 in which a large number of sound absorption holes 7 are perforated, and a back plate 9 arranged at a distance behind the front plate 8. The front plate 8 and the rear plate 9 are made of aluminum plate material, etc., and as shown in FIG. 4(a), the front plate 8
In some cases, an independent sound absorption chamber 10 corresponding to each sound absorption hole 7 is formed between the front panel 8 and the rear panel 9, or in a case where a plurality of sound absorbing chambers 10 are formed between the front panel 8 and the rear panel 9, as shown in FIG. Some acoustic filters 4 have one sound-absorbing chamber 10 spanning the holes 7. In such an acoustic filter 4, the aperture ratio, which is the total area of the sound-absorbing holes 7 relative to the total area of the surface plate 8, and the The resonant frequency is determined by the distance from the back plate 9, etc., and as shown in Fig. 5, it has a characteristic that the sound absorption coefficient of reflected waves increases at frequencies higher than the resonant frequency fr. In other words, it functions as a low-pass filter for reflected waves.Therefore, by placing this acoustic filter 4 on the reflecting surface 5, it is possible to reflect the secondary sound waves and attenuate the primary sound waves. The configuration is the same as that of Example 1, so a description thereof will be omitted. [Effects of the Invention] As described above, in the configuration of claim 1, an acoustic filter for suppressing at least the primary sound wave is disposed on the surface of the reflecting surface. , so the acoustic filter can be placed inside the cylinder.
As a result, it is possible to eliminate the lateral protrusion of the cylindrical body, and there is an advantage that it can be easily incorporated into other devices. In addition, since the sound waves pass through the acoustic filter twice, once when entering the reflecting surface and when exiting, the amount of absorption per volume can be increased compared to the conventional configuration in which the opening is covered with an acoustic filter. This has the advantage that the acoustic filter can be made smaller and more compact. Furthermore, since the acoustic filter is placed on the surface of the reflecting surface, the side lobes caused by diffused reflection that occur near the ends of the reflecting surface can be suppressed by the acoustic filter, and the overall length of the cylindrical body is shortened. Despite forming a surface, the effect is that the directivity of secondary sound waves can be strengthened. In the structure of claim 2, the acoustic filter is formed so that the amount of absorption differs at each part of the reflecting surface, so that depending on the directivity pattern of the primary sound wave and the directivity pattern of the secondary sound wave,
This has the advantage that unnecessary sound waves can be prevented from leaking to the outside. In the configuration of claim 3, the acoustic filter is arranged such that the absorption amount is larger at a portion closer to the opening, and the absorption amount is larger at both ends than at the center on a line approximately equidistant from the opening on the reflective surface. This has the advantage that side lobes due to diffuse reflection near the end of the reflecting surface can be reduced and the directivity of secondary sound waves can be improved. In the structure of claim 4, the acoustic filter includes a first sound absorbing layer that mainly absorbs primary sound waves, and a second sound absorbing layer that is disposed between the reflective surface and the first sound absorbing layer and mainly absorbs secondary sound waves. Since it is formed of two layers, the effect of reducing side lobes is further enhanced.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing Embodiment 1 of the present invention, and FIG.
1) (b) are respectively a front view and a sectional view showing an acoustic filter used in Example 2 of the present invention, FIG. 3 is a perspective view showing Example 3 of the present invention, and FIGS. 4(a) and (b) are 5 is a cross-sectional view showing an example of the acoustic filter used in Example 4 of the present invention, FIG. 5 is an operation explanatory diagram showing the characteristics of the acoustic filter used in the same example, and FIG. 6 is an explanation of the principle of the electroacoustic transducer according to the present invention. 7 is an explanatory diagram of the same operation, FIG. 8 is a block diagram showing the drive circuit of the same as the above, the ninth factor is an explanatory diagram of the same as the above, and the first
0 and 11 are schematic configuration diagrams showing conventional examples. 1...Sound source, 2...Cylinder, 3...Opening, 4...
- Acoustic filter, 5... reflective surface. Agent Patent Attorney Ishi 1) Chief 7 Figure 3 @ Figure 5

Claims (4)

[Claims] (1) In an electroacoustic transducer that generates an audible secondary sound wave by a nonlinear parametric action of a primary sound wave output from a sound source, the second sound wave generated by the electroacoustic transducer has a peripheral wall surrounding a space in which the secondary sound wave is generated. A cylindrical body with openings for transmitting secondary sound waves formed at important points on the peripheral wall is provided, and the internal space of the cylindrical body is bent to change the traveling direction of the sound waves so as to guide the secondary sound waves to the openings at the bent parts. An electroacoustic transducer comprising a reflective surface and an acoustic filter for suppressing at least primary sound waves disposed on the surface of the reflective surface. (2) Claim 1, wherein the acoustic filter is formed so that the amount of absorption differs at each part of the reflecting surface.
The electroacoustic transducer described. (3) The above-mentioned acoustic filter is formed so that the portion closer to the opening has a larger absorption amount, and the absorption amount is larger at both ends than at the center on a line approximately equidistant from the opening on the reflective surface. The electroacoustic transducer according to claim 2, characterized in that it consists of: (4) The acoustic filter has two layers: a first sound-absorbing layer that mainly absorbs primary sound waves; and a second sound-absorbing layer that is disposed between the reflective surface and the first sound-absorbing layer and mainly absorbs secondary sound waves. The electroacoustic transducer according to any one of claims 1 to 3, characterized in that it is formed by.