We know someone with two large heat-pump-type A/C units that lie outside the
back of their house.

Does anyone know of any sound barriers that can be palced around the units
that can cut down on the noise but still let the air flow properly? I am
thinking like something like a playpen but only padded that can be placed
around the units.

Thanks

Someone wrote:
Does anyone know of any sound barriers that can be palced around the
units that can cut down on the noise but still let the air flow
properly?

If the noise is coming from the compressor then a compressor blanket
will help. However, if the noise is the result of air flow (fan noise)
then a blanket won't help. Is it practical to relocate the heat pumps
to a location where they won't be such an issue?