Complex sentence: The first detection, made while the TPL was descending, lasted over two hours before it was lost; however, it was made at at 33kHz, while the ULBs on Flight 370's flight recorders emit a pulse at 37.5 ± 1kHz.
Split into simpler sentences: The first detection, made while the TPL was descending, lasted over two hours before it was lost. This detection was made at at 33 kHz, while the ULBs on Flight 370's flight recorders emit a pulse at 37.5 ± 1kHz.