To stop the whispers : silencing HMI noise

by Marc Galerne (K5600 Lighting)

par K5600 Lighting

[English] [français]

Following our discussions at the Satis and AFC Micro Salon shows, six members of the AFSI (French association of sound recordists) came to us, so that we could shed light on the issue of the noise made by HMI light sources.
Since we had to start somewhere, we proposed a meeting on our premises, so as to glean great ideas for future development. We therefore borrowed some used K5600 units from our rental house customers, as well as ballasts from different manufacturers, and brand new bulbs.

Present :
- For the AFSI : John Bourgon, Jean-Luc Raux-Cheynet, Denis Martin, Alexander Andrillon, Lawrence Blahay and Olivier Le Vacon.
- For Powergems Ltd. : Dr. Phil Ellam PhD, partner and head of the R & D department.
- For K5600 Lighting : Bruno Secorro, Marc Galerne.

We thank Pascal Lopinot of JC Lamps Prolux who couldn’t come, but who lent us a few bulbs to do our tests.

It should be understood that these tests are preliminary, with the sole purpose of providing a rough starting place. We had to start somewhere and, as much as possible, to try to create test procedures.
To move the project a little further would require a joint meeting between the AFC and AFSI. Ideally, these measurements should be made by an impartial body : the CST (French image and sound Technical Commission) is probably best placed to do this.

Here is a preview of some of our observations :
- The main problem comes from the bulbs.
This is made obvious by running a variety of new bulbs from different brands on the same unit. The 400 W bulb from Osram should be avoided. We have discussed this issue several times with the German company, with no response. The quietest bulbs are the GE / Koto anti-UVs. However they have difficulty with a hot re-strike.

- The sound of ignitor :
Between two generations of ignitor, we found a difference between those with a Tesla coil impregnated in silicone and the others. The presence of silicon reduces noise. On the Joker-Bugs 800, we have note that when the hood of the ignitor was not tightly fastened, the noise was very present. Simply tighten the screw to significantly reduce the noise.

- Aging is also a consideration
In particular there is a noticeable difference between the noise of ballast fans. There is a need for an industry standard for the maximum noise of ballast fans, as it is difficult for manufacturers of electronic ballasts to know what they can and cannot do.

- The higher the ballast frequency, the noisier the system
Phil Ellam brought Eproms configured with frequencies ranging from 50 Hz to 300 Hz. There is virtually no difference between 50 Hz and 100 Hz, which is the frequency most often used.

- The noisiest units to the ear are not necessarily the worst offenders
Some bulbs, particularly in 18Ks units, seemed noisier to us than others, but they were better for the sound department in the end, because they were easier to filter.
We would certainly improve the situation if we could transmit this information to all those involved (manufacturers of bulbs and ballasts, sound recordists, cinematographers, gaffers...)

PS : To put an end to a common myth : the distance between the head and the ballast is not limited to two extension lengths. It can go up to 100 meters on electronic ballasts, as they will compensate (to a certain limit) for losses due to distance. Although it should be noted that distances above 70 meters may cause a more or less significant loss in light intensity of the head, and may make the hot re-strike more difficult.