This might help you: https://www.amazon.com/ask/questions/Tx10QT8C6RJEY9B?_encoding=UTF8&sort=helpful
Based on this, when someone did a frequency analysis, they found 17 us to over 17000 khz. (Edit: they can only hear up to 17000 hz, no one really has a definite answer.) They did say that they tested it by ear, but I am not totally sure how you can tell individual hz apart. (Even with perfect pitch, telling 17 hz apart from 18 hz is very unlikely...)
However, another source (http://www.gadgetexplained.com/2017/07/anker-soundcore-2-bluetooth-42-dual.html?m=1) says that the Soundcore 2, not the original, gets from 70 hz to about 20,000 khz, not saying where they got the source from.
I hope this helps!