Do as many impacts as possible per spot.

Try to be careful while testing: cleaning the surface is really important, coupling the signal is a key...

Do as many impacts as possible per spot.

Try to be careful while testing: cleaning the surface is really important, coupling the signal is a key aspect, applying constant pressure on the sensor, applying constant force on the hammer, vertically aligning the sensor on the concrete surface, etc.

Then try to calibrate the wave velocity and revise if you are getting good signal. If that is not the case, find another alternative spot in the same pile / wall and repeat the testing until you receive good signal.

If you have a clear signal, data processing will be really easy as you can always use the high and low pass filters to eliminate the signal noise, and also the time gain compensation to increase the signal amplitude.