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The formula for calculating how far the object is, is really simple, basically you just measure the time it takes for the pulse to travel, multiply it with the speed of sound which is 341m/s in air, and divide the whole thing with 2, since the pulse travels from the sensor to the object and back.
Figure 3One of the drawbacks is that the ultrasonic sensor is that there is no way of telling the difference between small and large objects because the pulse that's emitted is cone shaped, because of the shape, an echo will be returned by all objects the pulse comes into contact with. One way to work around this is to use rotating sensor or multiple sensors to find out the shape and objects size.
Figure 34 (Ultrasonic pulse from the transducer sensor)
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