Acura 2011 RL Owner's Manual page 385

The system may not sense thin or
low objects, or sonic-absorptive
materials such as snow, cotton, or
sponge.
The system cannot sense objects
directly under the bumper.
If a beeper sounds several times or
does not sound when you put the
transmission into reverse, there is a
problem with the backup sensor
system. You should have the system
checked by your dealer.
Canadian Owners:
This device complies with Industry
Canada Standard RSS-210. Operation is
subject to the following two conditions:
(1) this device may not cause
interference, and (2) this device must
accept any interference that may cause
undesired operation of the device.
This ISM device complies with Canadian
ICES-001.
Backup Sensor System
379