Ultrasonic sensor

An ultrasonic sensor comprising:

• an element storage case including at least one case-side diaphragm that is a vibration plate having a thickness direction along a directional axis;
• and
• at least one ultrasonic element accommodated in the element storage case and spaced apart from the case-side diaphragm, wherein:
• the ultrasonic element includes an element-side diaphragm that is a vibration film having the thickness direction along the directional axis and provided by a thin part of a semiconductor substrate having the thickness direction along the directional axis;
• the semiconductor substrate is arranged to provide at least one closed space as a resonant space between the case-side diaphragm and the element-side diaphragm;
• and
• the semiconductor substrate is fixed and supported by the element-storage case, wherein:
• a resonant frequency of the ultrasonic element is defined as a first resonant frequency;
• a resonant frequency of the closed space is defined as a second resonant frequency;
• a resonant frequency of the case-side diaphragm is defined as a third resonant frequency;
• the first to third resonant frequencies are substantially equal to each other;
• wherein:
• a maximum difference among a difference between the first resonant frequency and the second resonant frequency, a difference between the second resonant frequency and the third resonant frequency and a difference between the first resonant frequency and the third resonant frequency is defined as Δfr;
• a bandwidth of a widest resonance band among a resonance band of the ultrasonic element, a resonance band of the closed space and a resonance band of the case-side diaphragm is defined as BW;
• and
• an equation of Δfr≤BW is satisfied.