Yes, it is possible to understand the situation by measuring the reflected sound waves from the ground or concrete. This technique is commonly known as sonar or ultrasound imaging. By emitting sound waves and analyzing the reflected waves, information about the surrounding environment can be obtained.
The frequency range used for such measurements depends on the specific application and the equipment being used. In general, ultrasound frequencies ranging from a few kilohertz (kHz) to several megahertz (MHz) are commonly used for non-destructive testing and imaging purposes.
Lower frequencies, such as a few kilohertz, can penetrate deeper into materials, but they provide lower resolution. Higher frequencies, such as a few megahertz, offer better resolution but have a shallower penetration depth. The choice of frequency depends on factors such as the desired resolution, the thickness and composition of the material being inspected, and the specific requirements of the application.
To summarize, by measuring the reflected sound waves in the appropriate frequency range, it is possible to gather information about the presence of a gap or another wall behind the transducer.