Hello. I have a scene w/a fan whose blades slowly pick up speed and then slow back down again. I have a great sound effect of a giant fan cutting air, but I can't match the speed of the effect from my library to that of the picture. I've tried pitching it down and varispeed with Elastic audio in PT with little success. I was hoping to find a tutorial on a more calculated technique. Or, any tips from the pros on here? Thanks.
Have you tried slicing it? Slice it to a midi key board and follow the fan by playing (and readjusting afterwards). You get no pitching time stretching or other potential sound degradation, but still flexibility in time. For faster or slower turning you may want to adjust the start points of the slice.
I've just been reading up on HVAC equipment from ASHRAE chapters 8 of fundamentals and 47 of applications.
It gives a formula for finding the frequency emitted from fan blades depending on their size and speed.
If you can deal with the maths it may give you something to go on with EQing the sound effect.
This is something i just quickly found
Blade-Pass Frequency. The blade-pass frequency is represented by the number of times per second a fans impeller passes a stationary item: fbp = (rpm X number of impeller blades)/60. All fans generate a tone at this frequency and its multiples (harmonics). Whether this tone is objectionable or barely noticeable depends on the type and design of the fan and the point of operation
Rene's question makes a great point toward my thoughts on this subject, generally-speaking. If the fan is featured onscreen and then shifts to being offscreen (and onscreen in the distant background), I personally tend to maintain the rhythm of the action without doing a hard-and-fast sync match if it's going to distract attention and we're doing a lot of visual A/B cuts - in the same spirit that if I cut a fridge run in the background of scene where the fridge itself is not important to the scene and we don't go near it, I'll just park it in one spot as one continuous region with no POV cuts for onscreen/offscreen; help out the mixer at the and of the day.
This rhythm-over-sync method takes a variety of conditional forms. A direct example is a computer screen with a blinking beep - and we cut back to it many times and the speed slightly changes (e.g. 8 frames apart instead of 5). In these types of situations, I'll take a least common denominator approach to rhythm-over-sync. Whichever onscreen shot has the longest and most steady duration of the beep, I'll cut to that first, and then mirror that speed offscreen. And if we come back to other shots and the timing isn't dead on as-is, I may do gradual, 1-frame incrementally exponential shift the offscreen beeps in between so that by the time they leave the end of the first onscreen shot, the sync up to the returning onscreen shot with no audible giveaway. When doing this though, one should take into account the rhythm of the sound so as not to "stretch" this sneaky effect too far to the point that the rhythm breaks.
Maybe an approach like this will work for your fan depending on how's been framed (or not framed) in the scene. Possibly roll up your sleeves and hand-build 2 full cycles of this speed change (designing fan blades can look similar in layout to doing heartbeats, if that helps visualize). Then, just re-use that as a continuous loop, making small timing cheats to the offstage moments so that the onscreen moments are dead sync, or at least a tight but slightly loose sync (because sometimes there's a time you have to trade off dead sync for perfect rhythm). Offstage can be your friend :)
Again, these are only my personal ideas and experiences to try.
You don't provide enough info in your question to give a specific answer so I can only ask this: Whats the purpose of the fan?
If its purely functional, whats the problem? does its speed vary? over time? by how much? between picture cuts? you can crawl through the frames & work out how fast its spinning...
If its being used as a dramatic/emotional device then you'll have to explain the context more, because lets face it: the fan hanging over a suspected serial murderer is not going to sound the same as that in a mundane office scene....
As with most things, it will require layering: constant motor hum vs general air swishing vs blade thwok etc etc... each of these will need to be considered/recorded/recreated seperately... but without the context its hard to give advice....