From the horse's mouth :
Virtually no sensor dead zone, objects closer than 6 inches range as 6 inches
FWIW, a lot of these off-the-shelf rangers send streams of eight cycles of 40kHz.
The wavelength in air is around 8.5mm, so eight cycles is nearly 7cm.
Given that the transmitter usually rings for another five or six cycles after the eight that are driven, this limits your minimum range.