The NoBark SBC-18 uses a patented Perfect Bark sensor system to provide the most reliable bark detection available. The unit delivers a static stimulation via the contact points only if it detects both vibration from the vocal cords and the sound of the bark. External noise or another dog's bark won't trigger a correction. The NoBark SBC-18 provides six levels of correction in each of the low, medium and high stim ranges, for a total of 18 levels. In each stim range, the correction starts at the lowest level and progresses to the next level (to a maximum of 6 levels) each time your dog barks within 30 s of the previous bark. If your dog doesn't bark again within 30 s, the correction will be reset to the lowest level in that mode. As a built-in safety feature, if your dog barks 15 or more times in 50 s, the unit will automatically shut off for 3 mins.