Ultrasonics are one of the most effective sensors for protecting the internal space in a vehicle and are much more reliable then a glass break sensor.
There are normally two sensors which are typically installed at the top of the “A” pillar (on the outside of the sun visors). One sensor transmits a high pitch sound wave (ultrasound) which is inaudible to the human ear, and the other sensor receives the signal.
The sound wave bounces off all the surfaces in the vehicle. If the wave pattern changes then the alarm system will trigger as shown in the video below.
Leaving a window open.
If you do wish to leave a window open, or a dog or person in the vehicle (not that I endorse doing this, so let’s not have any comments from any nutters out there!) when the alarm is armed the ultrasonics will need to be turned off to prevent a false alarm. Even a gust of wind through a slightly open window has the potential to trigger ultrasonics. The good news is that most decent alarms have the option to turn the ultrasonics off when arming. Most Cobra and Autowatch alarms allow you to turn the ultrasonics off by pressing the arm button on the remote a second time upon arming the system.
Ultrasonics come as standard on all Cobra alarms sold in New Zealand. The latest generation of sensors come in a very tidy grey capsule that suits the interior or most cars. Having said that not all cars have a grey interior and they can look silly if the car has black trim. The good news is that they can be painted to match and I try to carry some pre-sprayed black capsules in the wagon to cover this.
Note: Cobra ultrasonics plug directly into the Cobra alarm ECU so are not compatible with other brands of alarms.
Autowatch offer both a fixed eye ultrasonic sensor and the more common split eye version. The Fixed eye sensor comes as standard with the 458RL sold in NZ and has an LED built into the centre. It can be added on to any autowatch alarm system. Personally I would recommend having an alarm that gives the option to disable then upon arming which rules the Autowatch 279RL out.
The Split eye Autowatch sensors lack the original equipment appearance that the Cobra capsule style ultrasonics have, but are still tidy and come in black which looks acceptable regardless of the vehicle interior colour. They to can be added to most alarm systems, even if the alarm is not an Autowatch.
Not Suitable for all vehicles
Ultrasonics are not suitable for vehicles that are not sealed such as rag top convertibles.
If you have a cloth top car then a microwave sensor would be a better solution