timperrett Posted May 9, 2004 Share Posted May 9, 2004 I am aware this is a rather random question, but what is the reason behind Par cans buzzing/humming/whistling? Please answer asap as I need the answer for a presentation!Thanks Tim Link to comment Share on other sites More sharing options...
Pete McCrea Posted May 9, 2004 Share Posted May 9, 2004 Deadlines eh Tim? I'd have thought......actually I have no idea! Sorry! Pete Link to comment Share on other sites More sharing options...
robloxley Posted May 9, 2004 Share Posted May 9, 2004 It's due to physical vibration of the lamp's filament. Parcans are paticularly susceptible because of the filament shape.The buzz gets noisier at particular brightnesses as the timing of the dimmer triac turn-on and turn-off changes. The choke (inductor) in the dimmer should help reduce buzz as well as RF noise - how well it does this depends on how nice your dimmer is (i.e. how big the choke is). For a better understanding, look up how thyristor/triac dimmers work. Also note that IGBT and sinewave dimmers avoid much of this problem. Link to comment Share on other sites More sharing options...
paulears Posted May 9, 2004 Share Posted May 9, 2004 if you look at the way the fillaments are laid out on a duff lamp, they're quite long and resonate with the chopped waveform as the faders go up. most lamps do this, but they sit in a slightly decoupled socket, usually mounted on some sort of adjustable baseplate. these things prevent much of the vibration reaching the casing. in a PAR type lamp, the whole sealed beam assembly is rigid. if you tap it, it rings. the metalised reflector also behaves a little like a loudspeaker cone, focussing not just the light, but the noise too. stick it in a can with loose rear cover, loose retainer ring, loose frame holder runners - everything vibrates. that's not to mention the heat induced creaks and squeaks. a colleague is telling me sinewave dimmers will make them silent - I wait to be convinced on that one, although the theory says it should! Link to comment Share on other sites More sharing options...
DanielArkley Posted May 9, 2004 Share Posted May 9, 2004 Slighlt OT, but I find it quite useful that they ring. We just got 12 at school, and I can tell whether pre-heat is on or not by listening for the ringing - our venue isn't all that big, so I can hear them from the floor! Link to comment Share on other sites More sharing options...
Madmac Posted May 9, 2004 Share Posted May 9, 2004 There are several mechanism that cause the filament to emit sound or buzz. The primary one is down to the filament heating and cooling as the chopped mains waveform powers it. The buzz is greatest at 50% and normally not at all audible at 100%. At 50% the filament is being heated for 5 mSec and allowed to cool for 5mSec. As the temperature of the filament varies so does its size (thermal expansion) and this translates into mechanical stress that is coupled to the glass envelope. Generally lamps with long thin filaments make less noise that lamps with thick rigid filaments, the thick filament is able to transfer the movement with greater force. By increasing the time taken for the dimmer to go from off to on ( greater inductance in the case of chokes or slower switching in the case of IGBT's) the expansion occurs over a greater time and hence less mechanical disturbance. Sine wave dimmers recreate an AC voltage of the correct amplitude so no fast edges are present. The down side of sine wave dimmers is that they are heavier, more expensive and less efficient than phase angle dimmers. Madmac Link to comment Share on other sites More sharing options...
robloxley Posted May 9, 2004 Share Posted May 9, 2004 a colleague is telling me sinewave dimmers will make them silent - I wait to be convinced on that one, although the theory says it should! Well they seem to sound a lot quieter when dimmed through a variac Link to comment Share on other sites More sharing options...
Kevin Robertson Posted May 9, 2004 Share Posted May 9, 2004 Parcans don't buzz, they sing. :blink: :( Link to comment Share on other sites More sharing options...
timperrett Posted May 9, 2004 Author Share Posted May 9, 2004 Thank you guys - this is where blue room excels as a resource to me as an LD - invaluable Kind RegardsTim Link to comment Share on other sites More sharing options...
Martin Posted May 10, 2004 Share Posted May 10, 2004 Well they seem to sound a lot quieter when dimmed through a variac Indeed. We have a large variac for the orchestra RAT stands. Although this is to prevent interference with mic cables running alongside in the pit. Link to comment Share on other sites More sharing options...
Tomo Posted May 11, 2004 Share Posted May 11, 2004 We have a large variac for the orchestra RAT stands. Can I quote you on that? One [expletive] of a musician gets extremely arsey when I dim or gel the pit lights, and keeps insisting that "No professional theatres dim their pit lights" and offered to call a friend of his to confirm this - I told him not to bother as his friend wouldn't know anyway. He then proceeded to plug his ratstand into a spare hard power socket - after I'd already hardpowered the set as the dimmer hum had become an issue - and insisted that it made it brighter. (!?) I've been considering making up a set of 4-bars that put the ratstands in paired series.Although that may make them a little *too* dim, it would at least shut them up because they won't have any other option. So long as I hide the 'normal' 4-bars anyway... Incidentally, are there any 16th Ed guidelines on the labeling of series-splitter cables?Can't afford to buy a copy right now unfortunately. Link to comment Share on other sites More sharing options...
olistockman Posted May 11, 2004 Share Posted May 11, 2004 Assuming it's all safe, that's a perfect idea....one I may have to consider next time I make a journey into theatre! Link to comment Share on other sites More sharing options...
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