  In hole-doped cuprates there is now compelling evidence that inside the
pseudogap phase, charge order breaks translational symmetry leading to a
reconstruction of the Fermi surface. In $YBa_2Cu_3O_y$ charge order emerges in
two steps: a 2D order found at zero field and at high temperature inside the
pseudogap phase, and a 3D order that is superimposed below the superconducting
transition $T_c$ when superconductivity is weakened by a magnetic field.
Several issues still need to be addressed such as the effect of disorder, the
relationship between those charge orders and their respective impact on the
Fermi surface. Here, we report high magnetic field sound velocity measurements
of the 3D charge order in underdoped $YBa_2Cu_3O_y$ in a large doping range. We
found that the 3D charge order exists over the same doping range as its 2D
counterpart, indicating an intimate connection between the two distinct orders.
Moreover, we argue that the Fermi surface is reconstructed above the onset
temperature of 3D charge order.
