To choose an ultrasound sensor , first of all, you’d better read the sensor specifications carefully or have a good knowledge of what it is used for. the output frequency or signal of different sensors are different ! Currently the output for industrial use is a bunch of pulse signal which contains information of modulating and ?, In addition to being used to cut waves, ? can also be used to encrypt ,isolate same signal type or against interference. Modulated signal contains working information of date bits etc. Generally speaking, you can not judge a sensor by no or
discontinuous waves. You can connect a headset with the information output terminal after the frequency counter. If you can hear the signal, it means the ultrasound transmitting works.