--There are a few different aspects where frequency will apply for these HID ballasts. First, would be EMI / RF from the ballast itself or output cord; along with EMI being sent back through the line. This point is fairly well known and why some choose magnetic ballasts over electronic. I suppose it's also, in part, why some companies (ex.
gavita) mount the ballast to the socket directly. The patents provided above are most certainly focused on this and it would seem that the main aspect (for design) of the BLI ballast is to reduce this component.
--The second item, which is mentioned in their brochure, is related to "acoustic resonance" or "acoustic streaming". So, this is a "possible" issue with all HID's; though applies most largely to HID's with relatively symmetrical and spherical packets.
**What this means / is: Basically, the plasma that is emitting the light in operation is "oscillating" from various factors (temp, pressure, input frequency, packet dimensions etc). In certain cases where the input frequency to the bulb is close to the "acoustic resonance" of that bulb (at that point in time), or a harmonic thereof; a standing pressure wave can develop in the packet. Where the acoustic resonance frequency is; 1/2 wavelength equals the diameter of the packet (based on pressure velocity at that time).
-- If these standing waves are created then they will result in either A. Bulb flicker. Or B. Arc Extinguishing. Note: this flicker is most often of a low frequency and is visible, however this can also be tested with a photodiode and an oscilloscope to see flickers at higher frequencies.
--The acoustic resonant frequency of a bulb is defined first by it's shape and structure for the packet itself. The power delivered to the bulb, in general, determine the pressure and temperatures; which correspond to the pressure velocity. Because of this, the resonant frequency is constantly changing as the bulb warms up until full operation (i.e. there is not 1 set resonance with any single bulb).
--For frequencies then. Frequencies, less than 2,000 hz will not cause acoustic resonance in any known HID. Frequencies most commonly associated with this phenomena are in between 35,000 to 300,000 Hz (depending on bulb size / wattage). From an electrical design standpoint; higher frequency is more efficient and easier to produce than low frequency, so high frequency is always preferred ** from a electrical design standpoint.
--There are 3 main ways that this issue has been corrected with ballast design (over the last 40 years). 1. Low frequency square wave ballasts and these are the "most common" method. 2. Frequency modulation ballast which vary the frequency so resonance cannot be achieved (osram). 3. Ultra High frequencies over 500,000 hz also prevent acoustic resonance as the energy contained in any pressure waves is small in comparison to the plasma.
