1. Technical Field
The present disclosure relates to an ultrasonic sensor including a piezoelectric element for detecting ultrasonic wave and an acoustic matching member for conducting the ultrasonic wave received with an oscillation surface to the piezoelectric element.
2. Description of Related Art
Patent Document 1: JP-2009-20086A corresponding to US/20080072675A
Patent Document 2: JP-2009-49776A corresponding to US/20090054784A
Some ultrasonic sensors include a piezoelectric element and an acoustic matching member (see Patent Documents 1 and 2 for example). When the ultrasonic sensor is downsized, an oscillation surface (also called “a reception surface”) for receiving the ultrasonic wave and its sound pressure may become smaller. When the oscillation surface is made smaller, a receivable sound energy becomes smaller. Thus, it is difficult to maintain ultrasonic wave detection performance while the ultrasonic sensor is being downsized. A conceivable way for addressing the above difficulty may be the following. The acoustic matching member is formed so that an area of the oscillation surface is larger than an area of a region (also called “a contact surface”) contacting the piezoelectric element (see Patent Document 1 for example). In this configuration, the energy received with the oscillation surface is conducted to the contact surface, which is smaller in area than the oscillation surface. In this case, sound pressure concentration may occur and detention performance may improve. Thus, when the area of the contact surface is decreased so as to satisfy the above-described relationship between the oscillation surface and the contact surface, it is possible to maintain the ultrasonic wave detection performance while downsizing the ultrasonic sensor.
In the above related art, when the area of the oscillation surface is assumed to be constant, the ultrasonic wave detection performance improves with decreasing area of the contact surface. However, the piezoelectric element has a size limit from viewpoint of manufacturability. Thus, the above related art has a limit on the improvement of the detection performance. In addition, a decrease in the contact surface between the acoustic matching member and the piezoelectric element leads to a decrease in joint strength between the acoustic matching member and the piezoelectric element. With this regard, the above related art has a difficulty.