In addition, some aftermarket alarms have provisions for optional sensors (these must be purchased separately). The tilt sensor can sense the vehicle being tilted (alerting to towing). Tilt sensors come in digital or mercury. A digital sensor is more accurate since it sets itself, allowing for the vehicle to be placed on a hill and not cause false triggers. A sound discriminator or glass breakage sensor senses only the sound of glass breaking. Typically, a sound discriminator sensor can be eliminated using a shock sensor. Proximity, infrared, or motion sensors sense motion inside or outside the vehicle; these are typically installed on convertible or T-top vehicles. These sensors are usually adjustable in order to avoid false alarms. For example, a shock sensor will sometimes vibrate due to loud noises in the area such as heavy trucks, loud vehicle exhaust, or an accidental bump to the car from a passerby. Proximity sensors often cause false alarms in parking lots when a passerby is entering or exiting a vehicle parked next to the armed car. These often cause the alarm to falsely sense an attempted break-in.