A typical HID bulb has a frequency between 10 KHz and 300 KHz. While the bulb is lit by an ultra frequency ballast, both ballast output frequency and bulb internal frequencies will gradually change. When those two frequency get close, resonance occurs. The HID bulbs, particularly CMH bulbs, will fail if resonance is not stopped quickly. However some ballasts may have a senor to avoid resonance. Check your ballast manufacture for detail information.