  We employ numerical simulations to study active transistor-like switches made
from two-dimensional (2D) granular crystals containing two types of grains with
the same size, but different masses. We tune the mass contrast and arrangement
of the grains to maximize the width of the frequency band gap in the device.
The input signal is applied to a single grain on one side of the device, and
the output signal is measured from another grain on the other side of the
device. Changing the size of one or many grains tunes the pressure, which
controls the vibrational response of the device. Switching between the on and
off states is achieved using two mechanisms: 1) pressure-induced switching
where the interparticle contact network is the same in the on and off states,
and 2) switching through contact breaking. In general, the performance of the
acoustic switch, as captured by the gain ratio and switching time between the
on and off states, is better for pressure-induced switching. We show that in
these acoustic switches the gain ratio between the on and off states can be
larger than $10^4$ and the switching time (multiplied by the driving frequency)
is comparable to that obtained recently for sonic crystals and less than that
for photonic transistor-like switches. Since the self-assembly of grains with
different masses into 2D granular crystals is challenging, we describe
simulations of circular grains with small circular knobs placed symmetrically
around the perimeter mixed with circular grains without knobs. Using umbrella
sampling techniques, we show that devices with grains with $3$ knobs most
efficiently form the hexagonal crystals that yield the largest band gap.
