Scene But Not Heard

I am forced to admit that we in the lighting profession can be a source of unwanted noises for our audio brethren.
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I recently had a lively discussion with an audio person over just how much noise comes from a lighting system. (I guess it was really an argument, but we all know what audio people are like.) On consideration of the matter, I am forced to admit that we in the lighting profession can be a source of unwanted noises for our audio brethren. Having made that admission, however, I am anxious to point out that it is usually with no consciously malicious intent, and frequently something beyond our immediate control.

When I was in high school, lighting plays and musicals with few luminaires and even fewer slider resistance dimmers, I lived under the illusion that the only sounds in a production came from performers' voices and the orchestra. I was utterly devastated when I worked with a contemporary dance company at college and heard the sounds of the dancers' feet pounding on the stage floor, and their panting like long-distance runners at the end of a piece. This was further compounded, when later I heard the horrendous wooden hammering sound that is a classical corps de ballet, floating gracefully en pointe. Since then I have gradually learned to accept the growing cacophony of noises that are part of our craft.

TICK... TICK... TICK

One source of lighting noise that has been with us since long before television is that ticking sound that accompanies temperature changes in our luminaires. The sound of 350 kW of fixtures fading up at the end of a break is quite impressive, although very rarely heard on-air. The more gentle ticking that emanates from a running rig has occasionally gotten me into trouble with sound recordists when shooting drama. It can be disconcerting when a big softlight creaks during quiet, intense dialogue. The best way to minimize this variety of noise is to let the rig stabilize by keeping the luminaires and the studio air conditioning at their working levels, for as long as practicable before transmission or recording.

Since the advent of phase-controlled dimming, the "singing lamp" has been another common source of luminaire-related noise. In this type of dimmer, the incoming voltage waveform has variable-sized chunks chopped out of it by an electronic switch such as a Thyratron valve, SCR, Triac, IGBT or MOSFET. The abruptly chopped power can cause lamp filaments to vibrate at twice the line frequency (120 Hz) and its harmonics (240 Hz, 360 Hz, etc.). The effect is worst when a lamp's filament has gone a little saggy from age, and the dimmer is in the middle of its range (40-70 percent). The noise can be quite noticeable in small studios, especially in the quiet moments during interviews or acoustic music. The quick fix is to take the offending dimmer(s) either up to full, or down to zero. Generally neither of these options is very acceptable, which is exactly why the dimmer was at an intermediate level in the first place.

Luminaires with discharge lightsources, such as HMI, HID, MSR, MSD and fluorescent tubes, don't have filaments to sing but they all have ballasts and igniters that add to the lighting hubbub. The inductive type of ballast regulates the current flow to the lamp by electromagnetic means. It is basically a big coil of wire, wound on an iron former. The ballast coil will tend to hum noticeably at mains-derived frequencies (60 Hz, 120 Hz, 180 Hz, 240 Hz), unless the designer has gone to substantial lengths to damp the vibrations. I have noticed that if a ballast has been in heavy field use for a long time, it may start to develop additional rattles and hums, as bolts and mountings work loose. These symptoms can generally be alleviated by a visit to the maintenance shop for some TLC.

Electronic ballasts regulate the current to the lamp by electronically switching the power supply on and off at very high frequencies. The absence of large inductive coils reduces both their weight and the hum produced. These ballasts have the advantages of increased portability, substantially reduced frame-related light flicker and for some light sources such as fluorescent tubes, increased efficiency. Unfortunately the switch-mode regulation process introduces harmonic distortion into the mains supply - something that can upset other local electronic devices such as cameras and audio equipment and eventually even your utility transformer. There is some acoustic noise from these ballasts, usually at harmonics of their switching frequency (which varies from 400 Hz to more than 10 kHz) but rarely enough of it to cause problems for audio. As a result of working on rock concerts in my teens without wearing ear protection, I usually fail to notice these high-frequency noises. The crew, however, assure me that they are audible.

Dimmers themselves have long been a source of noises. Reactors and autotransformers hum, while the inductive chokes on early thyristor dimmers rattled for the same reason that the lamps sing. Although silent in the dimming process, the only resistance dimmers I have worked with in television were driven by anything-but-quiet electric motors and magnetic clutches. Even the most modern electronic dimmers invariably have cooling fans. In most permanent studio setups we can banish the dimmers to an equipment room where a few fans or hums, more or less, will hardly be noticed. Temporary rigs on outside broadcast remotes rarely have the luxury of locating the dimmers at any real distance from the working stage and may require the installation of baffling around the dimmers to keep the audio people happy. I have, on various occasions, used everything from a pile of black woolen stage drapes to a truckload of sand to restore the peace.

NOISE NUISANCES

The most recent additions to our collection of lighting noise sources are the motorized devices that have come in to wide use over the last decade. Although color scrollers (especially on a 5 kW fresnel or a large molefay) can be quite noisy, that noise only occurs during a color change, something that doesn't often occur during very quiet periods in a production. If the major prize has just gone off in your game show, there will be so much canned applause and music playing that the noise from a few scrollers will probably pass unnoticed.

Scrollers generally have a couple of motors and possibly a cooling fan, but this is nothing compared to robotic fixtures. The Martin 2000 Performance is a typical modern robotic-moving head spot. It has a 1,200 W metal halide lamp and features that include CMY color mixing, indexing rotating gobos, animation effects, color temperature correction, zoom optics, four independent framing shutters and optical dimming. Its complement of potential noise sources includes four cooling fans in the luminaire and another one in the base for the electronic ballast. But that's only the cooling. The robotic functions use two motors each for pan and tilt, three for the CMY color mixing, nine for the framing shutters, three for the rotating gobos, two for the fader and several more, giving a grand total of 28 stepper motors. It's a tribute to the design engineers that these luminaires don't sound more like a race-car derby. Special quiet(er) versions of most top-of-the-range robotic spots are available for use in studios, although there is, of course, a price premium for quiet.

Finally, there is the hum and rattle in the supply and load wiring caused by large, severely distorted currents. Our phase control dimmers and electronic ballasts can produce extraordinarily nonsinusoidal currents in the cabling; especially the supply neutrals. The electromagnetic fields resulting from these currents can cause noticeable vibrations in switch cabinets, cable ducts, troughs and raceways. I have noticed this most particularly in older studios where oversized neutrals have not been installed. As the distorted currents can overload some older wiring, it is important to talk with an electrical engineer about the problem, before getting out a pack of cable ties to anchor the offending cables to reduce the noise.

I had always thought of lighting as an artform that was seen, but not heard. After my conversation, though, I realized that unwanted noise from lighting gear was almost as much of a problem as unwanted boom mics in frame. I guess we could just consider it long-overdue payback.