  We discuss limits on the noise strength parameter in
mass-proportional-coupled wave function collapse models implied by bulk heating
effects, and examine the role of the noise power spectrum in comparing
experiments of different types. This comparison utilizes a calculation of the
rate of heating through phonon excitation implied by a general noise power
spectrum $\lambda(\omega)$. We find that in the standard heating formula, the
reduction rate $\lambda$ is replaced by $\lambda_{\rm eff}=\frac{2}{3
\pi^{3/2}} \int d^3w e^{-\vec w^2} \vec w^2 \lambda(\omega_L(\vec w/r_c))$,
with $\omega_L(\vec q)$ the longitudinal acoustic phonon frequency as a
function of wave number $\vec q$, and with $r_C$ the noise correlation length.
Hence if the noise power spectrum is cut off below $\omega_L(|\vec q| \sim
r_c^{-1})$, the bulk heating rate is sharply reduced, allowing compatibility of
current experimental results. We suggest possible new bulk heating experiments
that can be performed subject to limits placed by natural heating from
radioactivity and cosmic rays. The proposed experiments exploit the vanishing
of thermal transport in the low temperature limit.
