Abstract:
Disclosed are a sound generation system, an ultrasonic wave emitting device, and an ultrasonic wave emitting method which utilize the principle of parametric speakers (which uses the difference between frequencies of two ultrasonic waves) and which are capable of allowing target persons in a prescribed area to hear a sound only when a mobile body approaches said prescribed area. An alert system ( 10 ) has an ultrasonic wave emitting device ( 100 ) mounted on a mobile body ( 200 ), and an ultrasonic wave emitting device ( 300 ) permanently installed near an area (A 101 ). The ultrasonic wave emitting device ( 300 ) emits a first ultrasonic wave towards an area at least including the aforementioned area (A 101 ), and meanwhile, the ultrasonic wave emitting device ( 100 ) emits a second ultrasonic wave. The ultrasonic emitting device ( 100 ) forms the second ultrasonic wave by means of modulating the ultrasonic wave of frequency f 1  with an audible signal.

Description:
TECHNICAL FIELD 
     The claimed invention relates to a sound generating system, an ultrasound emitting apparatus, and a method of emitting an ultrasound wave. 
     BACKGROUND ART 
     A sound generating apparatus is used to allow a pedestrian to know the status inside an intersection by means of speech information, for example. 
     For example, a warning sound generating apparatus disclosed in Patent Literature 1 emits two types of ultrasound waves having different frequencies from different fixed parts, respectively, and allows a pedestrian present in an area in which the two types of ultrasound waves overlap with each other to hear a sound alarm. Specifically, the warning sound generating apparatus emits the first ultrasound wave (frequency fa) and the second ultrasound wave (frequency fb different from frequency fa) from two generators placed on street sides, respectively, and generates an audible sound having frequencies fa-fb in the area in which the first ultrasound wave and the second ultrasound wave overlap with each other. A principle of generating the audible sound is based especially on a “principle of a parametric speaker with reference to a frequency shift between two types of ultrasound waves.” Accordingly, allowing only the pedestrian present in a fixed area in which the first ultrasound wave and the second ultrasound wave overlap with each other to hear necessary information is possible. 
     In addition, an attention calling apparatus disclosed in Patent Literature 2 emits an ultrasound wave having a single frequency, and allows a pedestrian present in a specific area to hear a sound alarm. Specifically, the attention calling apparatus includes an ultrasonic sonar, and is mounted on a moving object (e.g., vehicle). The attention calling apparatus emits an ultrasound wave (i.e., frequency fd), which is generated by performing amplitude modulation on ultrasound waves (i.e., frequency fc) with audible signals (i.e., audio signals), at a sound pressure level of 110 dB or more to a predetermined region, and generates the audible sound within an area covered by the generated ultrasound wave. The principle of generating this audible sound is based especially on a “principle of a parametric speaker with reference to nonlinear characteristics when an ultrasound wave propagates through the air.” Accordingly, for example, it is possible to allow only a pedestrian who enters a dangerous area in front of a vehicle to hear warning information. 
     CITATION LIST 
     Patent Literature 
     PTL 1 
     
         
         Japanese Patent Application Laid-open No. 8-297798
 
PTL 2
 
         Japanese Patent Application Laid-Open No. 2005-306301 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, according to the above warning sound generating apparatus, an audible sound is always generated in a fixed area. Thus, for example, when an intersection is defined as the fixed area, a pedestrian always hear the audible sound even in the case of no vehicle. In other words, even when there is less need to call attention of the pedestrian, the pedestrian hears a warning sound. 
     In the meantime, the above attention calling apparatus can call attention of a target person who comes close to a moving object. However, there is a problem that the apparatus calls attention of even a person who is present in the same direction as the target person but whose attention need not be called. For example, although calling attention of a pedestrian on a sidewalk with a guardrail is not required, the apparatus calls attention of even a pedestrian on the sidewalk when a target person is present on a crosswalk in front of the guardrail. As an example, the apparatus calls attention of a person inside a building facing a street. 
     The claimed invention has been implemented taking into account the problems described above, and it is an object of the claimed invention to provide a sound generating system, an ultrasound emitting apparatus, and a method of emitting an ultrasound wave that can use a principle of a parametric speaker will reference to a frequency shift between two types of ultrasound waves and thus allow a target person present in a predetermined area to hear a sound alarm only when a moving object comes close to the predetermined area. 
     Solution to Problem 
     According to an aspect of the claimed invention, a sound generating system including: a first ultrasound emitting apparatus that emits first ultrasound wave; and a second ultrasound emitting apparatus that emits second ultrasound wave, the second ultrasound emitting apparatus comprising: an ultrasound signal generating section that generates ultrasound signals that have the same frequency as the first ultrasound wave; an audible signal generating section that generates audible signals; and a modulation section that generates the second ultrasound wave through modulation of the ultrasound signals with the audible signals. 
     According to an aspect of the claimed invention, an ultrasound emitting apparatus that emits an ultrasound wave, the apparatus including: an ultrasound signal generating section that generates ultrasound signals; an audible signal generating section that generates audible signals; and a modulation section that generates the ultrasound wave through modulation of the ultrasound signals with the audible signals. 
     According to an aspect of the claimed invention, a method of emitting an ultrasound wave in an ultrasound emitting apparatus that is mounted on a moving object, the method including: generating ultrasound signals; generating audible signals; and generating an ultrasound wave through modulation of the ultrasound signals with the audible signals. 
     Advantageous Effects of Invention 
     The claimed invention can provide a sound generating system, an ultrasound emitting apparatus, and a method of emitting an ultrasound wave that can use principle of a parametric speaker with reference to a frequency shift between types of ultrasound waves and allow a target person present in a predetermined area to hear a sound alarm only when a moving object comes close to the predetermined area. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of an attention calling system according to Embodiment 1 of the claimed invention; 
         FIG. 2  is a block diagram showing a configuration of an ultrasound emitting apparatus according to Embodiment 1 of the claimed invention; 
         FIG. 3  is a block diagram showing a configuration of another ultrasound emitting apparatus according to Embodiment 1 of the claimed invention; 
         FIG. 4  explains the operation of the attention calling system; 
         FIG. 5  is a block diagram showing a configuration of an ultrasound emitting apparatus according to Embodiment 2 of the claimed invention; 
         FIG. 6  is a block diagram showing a configuration of a frequency controlling section; 
         FIG. 7  shows a correspondence of the speed of a moving object and a frequency deviation; 
         FIG. 8  is a block diagram showing a configuration of an attention calling system according to Embodiment 2 of the claimed invention; 
         FIG. 9  explains the Doppler effect; 
         FIG. 10  is a block diagram showing a configuration of an ultrasound emitting apparatus according to Embodiment 3 of the claimed invention; 
         FIG. 11  is a block diagram showing a configuration of an ultrasound emitting apparatus according to Embodiment 4 of the claimed invention; 
         FIG. 12  is a block diagram showing a configuration of another ultrasound emitting apparatus according to Embodiment 4 of the claimed invention; 
         FIG. 13  is a block diagram showing a configuration of an ultrasound emitting apparatus according to Embodiment 5 of the claimed invention; 
         FIG. 14  is a block diagram showing a configuration of another ultrasound emitting apparatus according to Embodiment 5 of the claimed invention; 
         FIG. 15  explains the operation of an attention calling system according to Embodiment 5 of the claimed invention; 
         FIG. 16  further explains the operation of the attention calling system according to Embodiment 5 of the claimed invention; 
         FIG. 17  is a block diagram showing a configuration of an ultrasound emitting apparatus according to Embodiment 6 of the claimed invention; 
         FIG. 18  is a block diagram showing a configuration of an ultrasound emitting apparatus according to Embodiment 7 of the claimed invention; and 
         FIG. 19  is a block diagram showing a configuration of an ultrasound emitting apparatus according to Embodiment 8 of the claimed invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the claimed invention will be described in detail with reference to the accompanying drawings. In the embodiments, the same components are denoted by the same reference numerals and their duplicated explanations are omitted. 
     [Embodiment 1] 
     [Summary of Attention Calling System  10 ] 
       FIG. 1  shows a configuration of attention calling system  10  according to Embodiment 1 of the claimed invention. In  FIG. 1 , attention calling system  10  includes ultrasound emitting apparatus  100  mounted on moving object  200  (e.g., a vehicle such as an automobile) and ultrasound emitting apparatus  300  fixedly placed close to area A 101 . Ultrasound emitting apparatus  300  is placed on a roadside structure such as a traffic light or a guardrail. 
     Ultrasound emitting apparatus  300  emits the first ultrasound wave (also referred to as frequency f 1 ) to an area including at least area A 101 , and ultrasound emitting apparatus  100  emits the second ultrasound wave (also referred to as frequency f 2 ). Accordingly, when moving object  200  comes close to an area covered by the ultrasound wave of ultrasound emitting apparatus  300 , area A 101  in which the first ultrasound wave and the second ultrasound wave overlap with each other is generated. In this area A 101 , the sound of frequency f 2 -f 1  is generated. Specifically, ultrasound emitting apparatus  100  generates the second ultrasound wave through modulation of ultrasound signals of frequency f 1  (that is, ultrasound signals having the same frequency as the first ultrasound wave) with audible signals. In other words, the ultrasound signals of frequency f 1  are used as carriers. Thus, in area A 101 , sounds corresponding to the audible signals superimposed on the carriers by ultrasound emitting apparatus  100  are generated. Those sounds can warn a target person present in area A 101 . 
     As described above, in attention calling system  10  using a principle of a parametric speaker using a frequency shift between two types of ultrasound waves, placing ultrasound emitting apparatus  100  on moving object  200  can generate an audible area (in which a target person can catch a sound alarm) only when moving object  200  comes close to an area covered by the ultrasound wave of ultrasound emitting apparatus  300 . 
     [Configuration of Ultrasound Emitting Apparatus  100 ]  FIG. 2  shows a configuration of ultrasound emitting apparatus  100  according to Embodiment 1 of the claimed invention. In  FIG. 2 , ultrasound emitting apparatus  100  includes ultrasound signal generating section  101 , audible signal generating section  102 , modulation section  103 , and emission section  104 . 
     Ultrasound signal generating section  101  generates and outputs ultrasound signals having a predetermined frequency to modulation section  103 . 
     Audible signal generating section  102  generates and outputs sound signals that are intended to be heard only by a target person (i.e., audible signals) to modulation section  103 . The contents of the audible signals may be a message, such as “dangerous” or “be careful,” melody or warning sounds. The bandwidth of audible signals is at least 20 Hz but not greater than 20 kHz. Since human ears are most sensitive to frequencies ranging from 1000 Hz to 3500 Hz, an audible region including this range is desirable. 
     Modulation section  103  forms modulation signals through modulation of the ultrasound signals generated at ultrasound signal generating section  101  with audible signals generated at audible signal generating section  102 . The modulation signals are emitted through emission section  104 . 
     [Configuration of Ultrasound Emitting Apparatus  300 ]  FIG. 3  shows a configuration of ultrasound emitting apparatus  300  according to Embodiment 1 of the claimed invention. In  FIG. 3 , ultrasound emitting apparatus  300  includes ultrasound signal generating section  301  and emission section  302 . 
     Ultrasound signal generating section  301  generates ultrasound signals having a predetermined frequency. This predetermined frequency is the same as that of the ultrasound signals generated at ultrasound signal generating section  101 . The generated ultrasound signals are emitted through emission section  302 . 
     [Operation of Attention Calling System  10 ]  FIG. 4  explains the operation of attention calling system  10 . In  FIG. 4 , ultrasound signal emitting apparatus  300  is fixedly placed close to a crosswalk and emits the first ultrasound wave (frequency f 1 ). This first ultrasound wave entirely covers area A 102 . Meanwhile, ultrasound signal emitting apparatus  100  mounted on moving object  200  emits the second ultrasound wave that includes superimposed audible signals. The second ultrasound wave entirely covers area A 103 . 
     Area A 102  and area A 103  overlap with each other at area A 101 , and audible signals superimposed on the second ultrasound wave are generated in this area A 101 . Consequently, it is possible to allow a target person present in area A 101  to hear the audible signals. Meanwhile, it is possible not to allow a non-target person (not shown in the drawing) present outside area A 101  to hear the audible signals. 
     According to the present embodiment, in attention calling system  10  using a principle of a parametric speaker using a frequency shift between two types of ultrasound waves, placing ultrasound emitting apparatus  100  on moving object  200  can generate an audible area only when moving object  200  comes close to an area covered by the ultrasound wave of ultrasound emitting apparatus  300 . 
     In the above explanation, the second ultrasound wave is generated through modulation of ultrasound signals of frequency f 1  with audible signals. Alternatively, a frequency other than frequency f 1  may be used to form the second ultrasound wave through modulation of the ultrasound signals having the frequency different from f 1  with the audible signals. The frequency other than frequency f 1  means a frequency that generates an audible sound with a difference in tone between f 1  and f 2 , 
     In the above description, ultrasound emitting apparatus  100  modulates ultrasound signals using audible signals and ultrasound emitting apparatus  300  does not modulate audible signals. Alternatively, ultrasound emitting apparatus  100  may not modulate audible signals and ultrasound emitting apparatus  300  may modulate the ultrasound signals using audible signals and emit the result. 
     [Embodiment 2] 
     In Embodiment 2, a frequency of ultrasound signals is controlled in consideration of the Doppler effect. 
     [Configuration of Ultrasound Emitting Apparatus  400 ] 
       FIG. 5  shows a configuration of ultrasound emitting apparatus  400  according to Embodiment 2 of the claimed invention. Ultrasound emitting apparatus  400  is mounted on moving object  200  as ultrasound emitting apparatus  100 . 
     In  FIG. 5  ultrasound emitting apparatus  400  includes speed information acquiring section  401 , frequency controlling section  402 , and ultrasound signal generating section  403 . 
     Speed information acquiring section  401  acquires speed information on the moving speed of moving object  200 . The speed information is, for example, speed v 1  acquired by a speed sensor provided in moving object  200 . Instead, the number of turbine rotations of a torque converter in a transmission, or the number of drive shaft rotations in the transmission or the like may be used. 
     Frequency controlling section  402  calculates a target frequency based on the speed of moving object  200 . In other words, based on the speed of moving object  200 , frequency controlling section  402  firstly calculates a finite difference between a Doppler frequency and reference frequency f 1  (i.e., frequency deviation Δf) when the ultrasound wave of frequency f 1  is emitted. Then, frequency controlling section  402  calculates the target frequency through addition of reference frequency f 1  and frequency deviation Δf. The frequency of ultrasound signals generated at ultrasound signal generating section  403  is adjusted based on this target frequency. 
     To he more specific, frequency controlling section  402  includes storage section  411  and calculation section  412  as shown in  FIG. 6 . 
     Storage section  411  stores the correspondence between the speed of moving object  200  and frequency deviation Δf.  FIG. 7  shows the correspondence in the case where reference frequency f 1  is 40 Hz. 
     In the correspondence stored in storage section  411 , calculation section  412  acquires the value of frequency deviation Δf associated with the speed of moving object  200  acquired from speed information acquiring section  401 , and calculates the target frequency based on the acquired value of frequency deviation Δf and reference frequency f 1 . 
     Ultrasound signal generating section  403  generates ultrasound signals having the target frequency calculated at frequency controlling section  402  and outputs the result to modulation section  103 . 
     [Operation of Attention Calling System  20 ]  FIG. 8  explains the operation of attention calling system  20  according to Embodiment 2 of the claimed invention. In  FIG. 8 , ultrasound emitting apparatus  300  is fixedly placed close to a crosswalk and emits the first ultrasound wave (i.e., frequency f 1 ). 
     Meanwhile, ultrasound emitting apparatus  400  emits audible signals by using ultrasound signals having a frequency corresponding to the speed of moving object  200  as carriers. 
     When moving object  200  moves, the ultrasound emitted from ultrasound emitting apparatus  400  is influenced by the Doppler effect. 
       FIG. 9  explains the Doppler effect.  FIG. 9A  shows an ultrasound wave when moving object  200  does not move,  FIG. 9B  shows an ultrasound wave when moving object  200  comes close to a target person, and  FIG. 9C  shows an ultrasound wave when moving object  200  moves away from the target person. 
     When the ultrasound emitted from ultrasound emitting apparatus  400  is influenced by such Doppler effect, a shift between the frequency of the carriers used in ultrasound emitting apparatus  400  and the frequency of the carrier used in ultrasound emitting apparatus  300  is caused. Consequently, the quality of audible signals to the target person present in area A 101  may decrease. In other words, when moving object  200  is not moving, the target person can hear the sound corresponding to the audible signals. However, when moving object  200  moves toward the target person, the Doppler effect occurs in accordance with the speed of the moving object, and thus the frequency increases. For this reason, the generated sound becomes higher in pitch than the intended sound and may be inaudible by one who has a narrow audible frequency range. 
     In contrast, since ultrasound emitting apparatus  400  adjusts the carriers to a frequency corresponding to the speed of moving object  200 , the carrier frequency emitted from ultrasound emitting apparatus  400  in audible area A 101  is equal to the carrier frequency emitted from ultrasound emitting apparatus  300  in audible area A 101 . Accordingly, the quality of the audible signals can be maintained even when moving object  200  moves at a fast speed. 
     According to the present embodiment, in ultrasound emitting apparatus  400 , frequency controlling section  402  adjusts the frequency of the ultrasound signals generated at ultrasound signal generating section  403 , based on the speed of moving object  200 . 
     Consequently, since the frequency of the ultrasound signals generated at ultrasound signal generating section  403  can be adjusted to the frequency in consideration of the Doppler effect, the carrier frequency emitted from ultrasound emitting apparatus  400  in audible, area A 101  is equal to the carrier frequency emitted from ultrasound emitting apparatus  300  in audible area A 101 . Accordingly, the quality of the audible signals can be maintained even when moving object  200  moves at a fast speed. 
     In the above description, ultrasound emitting apparatus  100  modulates ultrasound signals using audible signals and ultrasound emitting apparatus  300  does not modulate audible signals. Alternatively, ultrasound emitting apparatus  100  may not modulate audible signals and ultrasound emitting apparatus  300  may modulate the ultrasound signals using audible signals and emit the result. 
     [Embodiment 3] 
     In Embodiment 3, the contents of audible signals are varied in accordance with the speed of a moving object. 
       FIG. 10  shows a configuration of ultrasound emitting apparatus  500  according to Embodiment 3 of the claimed invention. Ultrasound emitting apparatus  500  is mounted on moving object  200 . In  FIG. 10 , ultrasound emitting apparatus  500  includes audible signal controlling section  501  and audible signal generating section  502 . 
     Audible signal controlling section  501  acquires speed information on moving object  200  from speed information acquiring section  401 , and varies the contents of audible signals generated at audible signal generating section  502 , based on the speed of moving object  200 . To be more specific, audible signal controlling section  501  outputs identification information on the audible signals in accordance with the speed of moving object  200 , to audible signal generating section  502 . For example, the contents of the audible signals are defined as “dangerous” when the speed of the moving object is fast, and “a vehicle is coming close” when the speed of the moving object is slow. By this means, an easily-understandable warning can be given to pedestrians. 
     Audible signal generating section  502  generates audible signals corresponding to the identification information on the audible signals received from audible signal controlling section  501  and outputs the result to modulation section  103 . 
     According to the present embodiment, audible signal controlling section  501  varies the contents of the audible signals generated at audible signal generating section  502  based on the speed of moving object  200 . 
     By this means, for example, the easily-understandable warning can be given to the pedestrians. 
     Although the carrier frequency and the contents of the audible signals are varied based on the speed of moving object  200  in the above description, only the contents of the audible signals may be varied without changing the carrier frequency. 
     [Embodiment 4] 
     Embodiment 4 indicates an embodiment applicable even in a case where a plurality of fixedly placed ultrasound emitting apparatuses, which has different types of carrier frequencies, is present together in an attention calling system. 
       FIG. 11  shows a configuration of ultrasound emitting apparatus  600  according to Embodiment 4 of the claimed invention. In  FIG. 11 , ultrasound emitting apparatus  600  includes radio signal transmitting/receiving section  601 . 
     Radio signal transmitting/receiving section  601  transmits reference frequency information on the frequency of the ultrasound signals generated at ultrasound signal generating section  301 . Accordingly, ultrasound emitting apparatus  700 , which is described, hereinafter and present around ultrasound emitting apparatus  600 , can acquire the reference frequency information. A frequency common to a plurality of ultrasound emitting apparatuses  700  is transmitted as the reference frequency information. 
       FIG. 12  shows a configuration of ultrasound emitting apparatus  700  according to Embodiment 4 of the claimed invention. Ultrasound emitting apparatus  700  is mounted on moving object  200 . In  FIG. 12 , ultrasound emitting apparatus  700  includes radio signal transmitting/receiving section  701 , frequency controlling section  702 , and ultrasound signal generating section  703 . 
     Radio signal transmitting/receiving section  701  receives the reference frequency information transmitted from ultrasound emitting apparatus  600  and outputs the information to frequency controlling section  702 . 
     Frequency controlling section  702  receives the reference frequency information from radio signal transmitting/receiving section  701  and sets a target frequency based on the reference frequency information. Frequency controlling section  702  sets the target frequency to the frequency indicated by the reference frequency information. 
     Ultrasound signal generating section  703  generates ultrasound signals having the target frequency set at frequency controlling section  702  and outputs the result to modulation section  103 . 
     According to the present embodiment, ultrasound emitting apparatus  700  can use the ultrasound signals of the frequency corresponding to the frequency information transmitted from ultrasound emitting apparatus  600  as carriers. Consequently, even when a plurality of ultrasound emitting apparatuses  600  having different types of the carrier frequency is present together in the attention calling system, ultrasound emitting apparatus  700  can use the carriers equal to the frequency of ultrasound emitting apparatuses  600  present around ultrasound emitting apparatus  700 . 
     In the above description, ultrasound emitting apparatus  700  has a configuration formed by adding radio signal transmitting/receiving section  701  to the configuration of ultrasound emitting apparatus  100 . However, the claimed invention is not limited to such an embodiment, and ultrasound emitting apparatus  700  may have a configuration formed by adding radio signal transmitting/receiving section  701  to the configuration of ultrasound emitting apparatus  400  of Embodiment 2. In this case, frequency controlling section  702  calculates the target frequency base on the speed of moving object  200  received from speed information acquiring section  401  and the reference frequency information received from radio signal transmitting/receiving section  701 . Specifically, frequency controlling section  702  calculates frequency deviation Δf based on the speed of moving object  200 . Then, frequency controlling section  702  calculates the target frequency through addition of the reference frequency indicated by the reference frequency information and frequency deviation Δf. 
     [Embodiment 5] 
     In Embodiment 5, a fixedly placed ultrasound emitting apparatus switches a frequency of ultrasound signals based on traffic light information indicating the status of a traffic light. This ultrasound emitting apparatus transmits signal status information and frequency information on a frequency associated with a traffic lane. Additionally, the ultrasound emitting apparatus mounted on a moving object generates carriers using the frequency corresponding to the traffic lane on which a moving object runs. 
       FIG. 13  shows a configuration of ultrasound emitting apparatus  800  according to Embodiment 5 of the claimed invention. Ultrasound emitting apparatus  800  is fixedly placed close to an intersection. In  FIG. 13 , ultrasound emitting apparatus  800  includes traffic light information acquiring section  801 , ultrasound signal generating section  802 , and radio signal transmitting/receiving section  803 . 
     Traffic light information acquiring section  801  acquires the traffic light status information representing the status of a traffic light that is connected without or with a cable to ultrasound emitting apparatus  800  and is placed on an intersection. The traffic light status information includes information on the traffic light status (e.g., red or green) corresponding to each traffic lane. 
     Ultrasound signal generating section  802  switches a frequency of ultrasound signals based on the traffic light status information acquired at traffic light information acquiring section  801 . Accordingly, ultrasound signal generating section  802  generates ultrasound signals having the frequency corresponding to the traffic light status. 
     Radio signal transmitting/receiving section  803  transmits frequency information on correspondence between each traffic lane and the frequency. 
       FIG. 14  shows a configuration of ultrasound emitting apparatus  900  according to Embodiment 5 of the claimed invention. Ultrasound emitting apparatus  900  is mounted on moving object  200 . Ultrasound emitting apparatus  900  includes radio signal transmitting/receiving section  901 , frequency controlling section  902 , and ultrasound signal generating section  903 . 
     Radio signal transmitting/receiving section  901  receives the frequency information transmitted from ultrasound emitting apparatus  800  and outputs the result to frequency controlling section  902 . 
     Frequency controlling section  902  acquires traffic lane information on the traffic lane on which moving object  200  runs. The traffic lane information is acquired from a vehicle navigation system, for example. Frequency controlling section  902  outputs the information on a frequency associated with the traffic lane indicated by the traffic lane information in the frequency information to ultrasound signal generating section  903 . Accordingly, ultrasound signal generating section  903  generates ultrasound signals having a frequency corresponding to the traffic lane. 
       FIGS. 15 and 16  explain the operation of the attention calling system according to Embodiment 5 of the claimed invention. 
     in  FIG. 15 , a traffic light in the vertical direction is green and a traffic light in the horizontal direction is red. Meanwhile, in  FIG. 16 , a traffic light in the vertical direction is red and a traffic light in the horizontal direction is green. 
     As described above, ultrasound emitting apparatus  800  switches a frequency of ultrasound signals based on the traffic light status information acquired at traffic light information acquiring section  801 . In other words, ultrasound emitting apparatus  800  uses carriers of frequency f 3  in the case of the traffic light status as in  FIG. 15 , and uses carriers of frequency f 4  different from those of frequency f 3  in the case of the traffic light status as in  FIG. 16 . 
     Meanwhile, ultrasound emitting apparatus  900  mounted on moving object  200  A has acquired the traffic lane information and sets the carrier frequency to the frequency associated with the traffic lane indicated by the traffic lane information in the frequency information transmitted from ultrasound emitting apparatus  800 . In other words, since the traffic lane of moving object  200  A corresponds to the traffic lanes in the horizontal direction in  FIGS. 15 and 16 , ultrasound emitting apparatus  900  mounted on moving object  200  A uses the carriers of frequency f 4 . In contrast, ultrasound emitting apparatus  900  mounted on moving object  200  B uses the carriers of frequency f 3 . 
     Accordingly, in the traffic light status in  FIG. 15 , while audible signals are generated in the area in which target person T 102  is present, the audible signals are not generated in the area in which target person T 101  is present. On the contrary, in the traffic light status in  FIG. 16 , while the audible signals are generated in the area in which target person T 101  is present, the audible signals are not generated in the area in which target person T 102  is present. 
     According to the present embodiment, in ultrasound emitting apparatus  900 , radio signal transmitting/receiving section  901  receives the frequency information transmitted from ultrasound emitting apparatus  800 , and frequency controlling section  902  adjusts a frequency of the ultrasound signals generated at ultrasound signal generating section  903  to the frequency associated with the traffic lane of moving object  200  in the frequency information. 
     Consequently, when ultrasound emitting apparatus  800  emits the ultrasound wave of the frequency associated with the traffic lane in which the traffic light status is green, the frequency is equal only to the carrier frequency emitted from ultrasound emitting apparatus  900  mounted on moving object  200  entering an intersection. For example, since a vehicle in the traffic lane in which the traffic light is red stops, calling attention of pedestrians on a crosswalk to this vehicle is not required. However, calling attention of the pedestrians to a vehicle that makes a right or left turn from the traffic lane in which the traffic light is green is necessary. Thus, warning can be given only to threatened target persons. 
     [Embodiment 6] 
     In Embodiment 6, the direction of output ultrasound wave is controlled to be the direction in which a moving object makes a turn. 
       FIG. 17  shows a configuration of ultrasound emitting apparatus  1000  according to Embodiment 6 of the claimed invention. In  FIG. 17 , ultrasound emitting apparatus  1000  includes direction sensor  1001  and emission section  1002 . 
     Direction sensor  1001  acquires information on the direction in which moving object  200  provided with ultrasound emitting apparatus  100  makes a turn, and outputs the result to emission section  1002 . 
     Emission section  1002  controls the direction of output ultrasound wave so that the direction of the output ultrasound wave is equal to the direction in which moving object  200  makes a turn. In other words, emission section  1002  controls the directivity of the ultrasound wave in accordance with the direction in which moving object  200  makes a turn. 
     Consequently, when moving object  200  makes a turn, an area that the moving object is to arrive can be defined as an audible area before the anteroposterior direction of moving object  200  is equal to the direction toward the area that the moving object is to arrive. For this reason, it is possible to call attention of target persons present in the area that the moving object is to reach, in the early phase. 
     [Embodiment 7] 
     In Embodiment 7, when the direction of the emitted ultrasound wave and the direction of a moving object (that is, the anteroposterior direction of the moving object) form an angle (i.e., angle not equal to 0°), the frequency of ultrasound signals is varied based on components of the speed of the moving object corresponding to the direction of the emitted ultrasound signals. 
       FIG. 18  shows a configuration of ultrasound emitting apparatus  1100  according to Embodiment 7 of the claimed invention. Ultrasound emitting apparatus  1100  is mounted on moving object  200 . In  FIG. 18 , ultrasound emitting apparatus  1100  includes frequency controlling section  1101 . 
     Frequency controlling section  1101  acquires the direction of the emitted ultrasound wave, from emission section  104 . Frequency controlling section  1101  calculates the speed components corresponding to the direction of the emitted ultrasound wave based on the moving speed of moving object  200  and the direction of the emitted ultrasound wave, and calculates the target frequency based on the calculated moving speed. 
     Consequently, a carrier frequency in consideration of the Doppler effect can be provided, even when the direction of the emitted ultrasound wave and the direction of moving object  200  (that is, the anteroposterior direction of the moving object) form an angle (i.e., angle not equal to 0°). 
     [Embodiment 8] 
     Embodiment 8 is a combination of Embodiment 6 and Embodiment 7. 
       FIG. 19  shows a configuration of ultrasound emitting apparatus  1200  according to Embodiment 8 of the claimed invention. In  FIG. 19 , ultrasound emitting apparatus  1200  includes frequency controlling section  1201 . 
     Frequency controlling section  1201  acquires the direction of output ultrasound wave, from emission section  1002 . Frequency controlling section  1201  calculates the speed components corresponding to the direction of the emitted ultrasound wave based on the moving speed of moving object  200  and the direction of the output ultrasound wave and calculates the target frequency based on the calculated moving speed. 
     By this means, Embodiment 8 can acquire the effect of Embodiment 6 and Embodiment 7. 
     [Other Embodiments] 
     (1) Embodiment 1 assumes that ultrasound emitting apparatus  100  and ultrasound emitting apparatus  300  use ultrasound signals having the same and single frequency, and the audible signals transmitted from ultrasound emitting apparatus  100  are also one kind. However, the claimed invention is not limited to such an embodiment, and ultrasound emitting apparatus  100  and ultrasound emitting apparatus  300  may emit a plurality of ultrasound waves, each having a frequency common to the two apparatuses. In this case, ultrasound emitting apparatus  100  superimposes audible signals having different contents on a plurality of ultrasound signals having different frequencies. 
     (2) Although the above embodiments have described an example where the claimed invention is implemented with hardware, the claimed invention can be implemented with software. 
     Each function block employed in the explanation of each of the aforementioned embodiments may typically be implemented as an LSI constituted by an integrated circuit. These may be individual chips or partially or totally contained on a single chip. “LSI” is adopted here but this may also be referred to as “IC,” “system LSI,” “super LSI,” or “ultra LSI” depending on differing extents of integration. 
     Furthermore, the method of circuit integration is not limited to LSI&#39;s, and implementation using dedicated circuitry general purpose processors is also possible. After LSI manufacture, utilization of a programmable field programmable gate array (FPGA) or a reconfiguraable processor where connections and settings of circuit cells within an LSI can be reconfigured is also possible. 
     Furthermore, if integrated circuit technology comes out to replace LSI&#39;s as a result of the advancement of semiconductor technology or a derivative other technology, it is naturally also possible to carry out function block integration using this technology. Application of biotechnology is also possible. 
     The disclosure of Japanese Patent Application No. 2010-152046, filed on oar Jul. 2, 2010, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
     Industrial Applicability 
     A sound generating system, an ultrasound emitting apparatus, and a method of emitting an ultrasound wave of the claimed invention is useful since they use a principle of a parametric speaker with reference to a frequency shift between two types of ultrasound waves and allows a target person present in a predetermined area to hear a sound alarm only when a moving object moves close to the predetermined area. 
     Moreover, the claimed invention is advantageous because the invention is, for example:
     usable regardless of weather conditions such as rain (the same effects are brought about on the road side and the moving object side);   usable in a place other than an intersection;   usable in an object other than a vehicle and also as a train alarm;   usable as a means for announcing advertisement or the like to a pedestrian;   configured to allow a pedestrian to know that a vehicle comes close, by being attached to the vehicle, such as a hybrid vehicle and an electric vehicle having small running sound;   configured to allow other people to know the presence of a wheel chair that is in a blind corner in a hospital, for example; and   applicable to a visually handicapped person in the same way described above.   

     REFERENCE SIGNS LIST 
     
         
           10  Attention calling system 
           100 ,  300 ,  400 ,  500 ,  600 ,  700 ,  800 ,  900 ,  1000 ,  1100 ,  1200  Ultrasound emitting apparatus 
           101 ,  301 ,  403 ,  703 ,  802 ,  903  Ultrasound signal generating section 
           102 ,  502  Audible signal generating section 
           103  Modulation section 
           104 ,  302 ,  1002  Emission section 
           200  Moving object 
           401  Speed information acquiring section 
           402 ,  702 ,  902 ,  1101 ,  1201  Frequency controlling section 
           411  Storage section 
           412  Calculation section 
           501  Audible signal controlling section 
           601 ,  701 ,  803 ,  901  Radio signal transmitting/receiving section 
           801  Traffic light information acquiring section 
           1001  Direction sensor