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Can a bank of ceiling fans being turned on or off mid-way through a venue cause a change in the way a room responds to or reflects/absorbs sound waves, thereby causing feedback problems?

Someone keeps blaming the fans for the sound troubles occasionally experienced at a church I attend. Is this a possibility? I want to give a "sound" and professional answer. I have a hard time believing the fans are truly the cause, but I've been wrong before.


mdb Wed, 01/11/2012 - 09:52

bouldersound, post: 382435 wrote: How high are the fans?

The fans are about 15-20 feet up and hang approximately 2 feet off the ceiling. There are 6 fans.
Not to say that I'm good at what I do, but I have never had the fans give me trouble (that I'm aware of) and have yet to produce feedback through the system. You watch... it will happen to me this week because I said that.

I just think it's manageable and that the fans couldn't possibly cause that much trouble. As I said, maybe I'm wrong and so I'm asking for some knowledgeable individuals to explain the theory as to why they could be an issue or to say, "no, that's not true."

dvdhawk Wed, 01/11/2012 - 10:46

Anything that disturbs the air, disturbs sound to some extent - but I would find it hard to believe switching the fans on or off would be a drastic change big enough to cause sudden feedback problems. The congregation would complain about the draft long before you got to gale-force winds. This time of year in a cold climate, such as yours, the fans should be turning slowly - in whichever direction pushes heat that has risen to the ceiling back down to circulate through the room. (CCW on my fan)

If there really is a correlation between turning the fans on/off and your audio problems - something I would have an easier time believing, (especially if it's an old building) there could be something sketchy going on with the electric. Some of the churches I get to work in were built decades before Edison had a working filament and decades more before he, Tesla, and Westinghouse were squabbling over the best way to generate and distribute electricity. Several generations of wiring standards and several well-meaning parishioners who volunteered their dubious electrical skills later....

mdb Wed, 01/11/2012 - 12:10

Thanks for all your responses.

From what you've all said, in summary we (the venue) have frequency-specific issues (which all places have to a varying degree). Standing waves or any other sound waves are not affected by the minimal airflow experienced within our space due to the fans being engaged. A variance in sound wave reflections due to the rotating fan blades (which is determined by how fast the fans are spinning) may cause a WoW (Wahwah) effect that can potentially be heard by some people depending on how severe it is, but feedback is not caused by or directly affected by a change in airflow or a fan being turned on or off (unless producing hurricane-like winds). The issue is most likely centered around poor gain-staging or incorrectly set EQ.

Cannonball Thu, 04/19/2012 - 10:07

This is a late addition to the conversation. It is possible that the Fans are causing the feedback but not for the reasons you think. Chances are with 6 fans going in such close proximity to the listening environment, their operation not only increases the noise floor of the venue but also probably generates frequency specific noise artifacts that the artist on stage and to a lesser degree the audience needs to overcome - for example a high pitched whine and low end rumble that acts as a masking tone. Simply put, the mains and monitors get louder to overcome the added noise and the operator is not fast enough or astute enough to compensate for it (or maybe your 13 band graph on your powered mixer just can't reach the freq without chewing out a ton of tone). But from your posts it seems that this is an occasional problem and one that can't be tied directly to the use of the fans.

Dollars to donut's its a problem more related to the performance expectations / gear limitations / operators ability and knowledge. That's not to discount acoustic anomalies, because they exist in every building, including anechoic chambers - its just to say that with the right combination of reasonable expectations, gear appropriate to the task and an operator that can mix with confidence in all situations even if it means making a tincan, piece of string, a magnet and a pie plate sound awesome then all is good.

But here are a few things to help you the next time feedback occurs. If the frequencies that tend to feedback during a performance are 800 hz and above chances are they are occurring from the monitors, below and its from the front of house. To determine if its a a time of flight issue (gain vs distance imbalance between speakers and microphones) or a room anomaly change the time of flight. If you induce a couple of milliseconds of delay in the system the time of flight of flight between source and destination changes and during a gain vs feedback event the feedback will begin to pitch up and down as you increase and decrease the delay. If its a room anomaly the feedback frequency will not change.

To determine if its a standing wave, more often than not the standing wave that is causing problems will attain a specific volume and hold that amplitude and you will begin to hear series of harmonic overtones that also hold at a specific amplitude - sometimes just moving the microphone will solve this or adjusting the gain. Also standing waves have a rise time that is very different from that of time of flight related feedback, once you know the difference between the two, you can quickly recognize which is which. Standing waves are quite common, occurring to some degree in almost every room constructed but tend to be most problematic in terms of mics / speaker interactions on small to midsize stages.

Occasionally, mostly in large venues with very symmetrical construction, you can experience phase induced feedback. The feedback usually only happens in the monitors (and sounds like crap in the FOH speakers). What happens is you can have two or more microphones that are spaced sometimes significantly far apart and instead of cancellations between sources you have summations. So lets say the singers mic appears to be feedbacking into thier own wedge but the source of the feedback is actually the violin mic across the stage (and the violin mic may or may not be feeding into the singers wedge), because of distance from source to destination and the coupling of indirect acoustic energy reflected by the rooms geometry coincidently creates a perfect set of conditions for a specific frequency to couple and feedback. This usually only occurs in really high frequencies, 5k and above and more often then not there is some comb filtering also occurring near the frequency that is or about to feed back, giving the mic a slightly distorted sound (Which leads to the all too often assumption that the mic's cap is blown or if the operator is smart enough to listen in the cans the mic sounds fine soloed in place but sounds like crap when everything is on so the boards channel must be hooped - yet everythign works hunky dory after the event - darn sound gremlins). Sometimes the simplest solution is to just move the microphones a few inches and problem solved. But this is not a super common problem, in the hundreds of venues I have worked, I have only experienced it in about a couple of dozen, mostly concert halls.

Incidentally church pulpits are notorious as focal points for all sorts of acoustically related problems because a lot of modern churches are designed to focus are the energy, visually, spiritually and so on to that position but unfortunately this also means acoustic energy as well, and its not always pretty.

I would suggest spending a few bucks and getting a real decent book on the basics of sound such as the Yamaha Book or if your feeling a little more advanced, Don Davis's book.

On a personal note I love having a couple of Feedback suppressors like the Shure DFR22 handy, the only thing to bear in mind is that they can be false triggered by things like rolled cymbals or clarinets. The beauty of devices such as these is with such super narrow que's the suppressor is like a scalpel for feedback, but it still takes a reasonably skilled operator to maximize a systems gain versus feedback in conjunction with the device to keep everything sounding natural. Where these things are super handy is in situations were you absolutely must use a lavalier mic, especially if the person wearing the darn thing has a habit of running all over the place and into the audience.

Hope this helps and best of luck.