Getting the enveloppe of a signal requires more a simple band-pass filter.
However, you could rectify the signal and bandpass-filter the rectified signal to get the enveloppe. That's how I used to do it with ultrasound signals.
https://en.wikipedia.org/wiki/Envelope_detector
You could also use more elaborate techniques, like the Hilbert transform (or Hilbert filter) to get a complex signal that would allow use to use the module of the complex signal to get the enveloppe.