"I downloaded an application on the internet that allows me to detect ultrasound and infrasound, but it seems that it is not that sensitive?"
Applications depend on the sensors your phone has. So, depending on the type, you cannot expect much from what the cell phone will do. Microphones, for example, used in applications that detect sound, work very well in the audible range, but they were not manufactured to detect ultrasound and infrasound. In some more advanced cell phones that have a barometric pressure sensor, these sensors can be used to detect infrasound, based on the variations in sound pressure that ultrasound produces. However, you need to check if your phone has this feature.
