re: Alarm Glass Sensor

[Mike Schaublin <mschaublin@xxxxxxxxx>]

     <James wrote>
     
     Third question, does anyone know how to adjust the stock BMW 
     alarm's sensitivity?  I tried to hitting the winshield, dropping 
     pennies on the glass, shaking the car up and down, yelling inside 
     the cabin, even banging the glass sensor.  No luck, the alarm just 
     refuses to go off.  Only after I bang the glass sensor against the 
     dash a couple times, the alarm goes off.  I'm hoping to adjust the 
     sensitivity level up a bit.  The alarm was already installed when i 
     purchased the car used from someone, so I'm guessing it's installed 
     by the dealer. <end of snip>
     
     (The info below comes from my experiences with home alarms - I 
     believe the same principles apply to car alarms).  
     
     Glass breakage sensors won't go off when they are shaken, yelled 
     at, or banged.  They are specifically designed _not_ to go off in 
     these events to avoid false signals.  That is why most alarms will 
     have a separate glass breakage sensor and a motion (vibration) 
     sensor.  
     
     Glass sensors listen for the sound of the initial impact of an 
     object hitting the glass hard enough to break it.  This will 
     produce a high amplitude noise in a specific frequencey range - 
     probably in the high bass range between 100 and 1000 hz (just 
     guessing here); and I'm sure the frequency will be different in 
     each car, given body resonance, glass shape and thickness, etc.  To 
     get the glass sensor to trigger, you need to produce a sound in the 
     frequency range which the sensor is looking for.  The best, and 
     probably only way to do this (if the sensor was designed well) is 
     to break your windshield or side glass.  
     
     BTW, since the sensor is looking for a low frequency thud, not the 
     high frequency shattering sound, you cannot set of a glass breakage 
     sensor by breaking a beer bottle inside the car.  This is why glass 
     sensors on home alarms don't go off when you drop a plate or a 
     glass on the floor.  
     
     Hope this helps,
     
     Mike Schaublin
     '94 325i - still stock. Needs to be sharked & BL/SS/ERK'd.  
       

------------------------------