This little miracle device is highly recommended. The baby's heartbeat was found quickly and reliably in our case, and confusion with the maternal pulse is absolutely excluded (71 vs. 158 !!!). The volume on the head listening is super, super well, if you connect an external speaker instead of headphones. Only the recording, which is connected via a SoundCloud available for download, is very low, but can be edited easily with various sound editing programs ... With Sony Xperia phones it seems obviously to be incompatible, but it eg works with Samsung Garäten properly. Connect, turn on, search, record and be amazed: o) Since the unscrupulous use of ultrasound in the SS is not without controversy, I would still not too often use, because the fact is: the little one while and then very restless! But against an occasional preview is certainly nothing wrong, especially for anxious mothers-that is a great purchase! : O)