I, too, am not sure if ultrasonic repellers really work, but Spencer
Chase mentioned trying some industrial ultrasonic units.  These
generally operate at 20 kHz or 40 kHz, so will not be audible to most
people.  Might drive the dog mad, though!  The noise you hear when
ultrasonic units are operating is from other mechanical resonances
within the housing etc.

The piezo type are usually not adjustable for frequency and are very
easy to damage if dismantled.  The transformer type are often
adjustable but are not as common due to cost and size.

The other point to keep in mind is that they are designed to work into
a load, e.g., a tank of detergent etc, so if operated for long periods
in air they may self-destruct.

I have an ongoing battle here with bush rats as big as small cats
invading the house, and I find the best means of defense is to prevent
them from getting into the house and into the musical instruments, by
fitting wire mesh screens etc.  I've tried baiting them (they love the
bait and squeal with delight whilst fighting over it) but then they
expire within the wall cavity and stink for a week or so.

In summer a friendly goanna, about 1.5 meters long, keeps the rats at
bay and we only hear it occasionally as it slithers under the roof to
keep warm.

Colin MacKinnon