Quick and Inexpensivew way to block outside noise

[Allan Black]

Originally Posted by Jacques E. Bouchard

Thanks Allan, very good points. We're using a muslin screen, which as a fabric won't bounce sound.

Tell me more about OOPhase. I find it VERY interesting. Is there a tutorial on line that I can read to teach me more about this technique?

Thanks everyone, as always you've given me a lot more detailed info than I had anticipated, which is why I love this forum. :-)

OOP (out of phase) works well when lay you 2 identical sound tracks in sync in post, and you put one of them out of phase with the other. Most NLEs have that feature, Vegas does and the theory goes, ALL audio is cancelled out, zero zilch.

In this case, because the lavs on the talent would 'hear' different sounds to the cardioid mic, it might only work with louder intrusive truck sounds, so you'd be adjusting the cardioid track in the mix on auto. It would affect the voice frequencies if you overdo it.

In fact it might work better if you boomed the talent with a hypercardioid, with the live side facing the windows after you blocked them as much as possible .. you'd have to experiment.

Cheers.

[Greg Miller]

Richard,

I concur with your comments about out-of-phase cancellation. The only way the noise signal from the "noise" mic will match the noise signal from the "talent" mic is if the two mics are identical in type, response, polar pattern, and location. If you use a different kind of mic for the "noise" mic, and it's several feet from the "talent" mic, then the two noise signals will be quite different and 180º cancellation will probably not work.

Glad you concur with my "window blocking" scheme. It would be a viable solution, but not practical. It would not be easy or convenient to insert and remove them very frequently. If I were renting office space for a year, and wanted to turn it into a studio space, that might be a reasonable solution, but not on a daily or even weekly basis... having to caulk them in, and then remove the caulk, would be a bit pain.

Your other comments about OOPhase, Adobe Audition's dynamic noise filter, etc., are all spot on, IMHO.

[Greg Miller]

Originally Posted by Josh Bass

Greg Miller, YOU have just BLOWN MY mind.

Josh,

Thank you for your kind comments. That makes it all worthwhile. I was lucky enough to encounter a few good mentors along the way (although most "experts" hoarded their knowledge and tried to make everything seem much more difficult than it really was). I believe in the "pass it forward" philosophy, so if I can help someone with an answer or explanation now and then, I find that very rewarding.

Now if you'll just add up what you spent on all those books, and send me half that amount... ;-)

Seriously, I'm glad my remarks were helpful to you. Please let me know if anything was unclear or incomplete.

Happy Trails!

[Predrag Vasic]

Inverting phase of a signal from another microphone will likely never produce a solution. This method is perfect for those active noise suppression headphones, where the microphone that picks up outside noise sits just a few millimetres away from the listener's ear. One must consider the actual wavelength of the frequencies one wants to suppress by inverting phase. For lower frequencies, this wavelength is fairly long (almost 2m for 180Hz, or 1m for 350Hz). For higher frequencies, though, it gets quite short (5cm for 6kHz). The sound of such high frequency will be in opposite phase at just 2.5cm (1 inch) of distance.

Putting two microphones at two different spots in hopes of getting rid of unwanted noise by inverting phase will not get any meaningful results because of the wavelength problems. If those two mics are 1m apart, frequencies around 180Hz will be arriving at opposite phase to these two mics, while frequencies around 350Hz would be arriving at the same phase. Obviously, since noise isn't locked to any single individual frequency, when mics are any meaningful distance apart, the difference in wavelength of different frequencies will cause certain specific frequencies (whose wavelengths are half the distance between mics, or multiple thereof) to be cancelled out (due to inversion of phase), while others would be boosted (due to synching of phase).

The only way phase inversion works well to suppress noise is when two sources to be mixed together with phase inverted on one of them are physically extremely close, so that all of the unwanted frequencies have wavelengths that are significantly longer than the distance between the sources. In active headphones, the microphone and the listener's ear are close enough to effectively suppress frequencies below 4kHz (about 9cm wavelength). This is also how balanced cabling works: two wires that are extremely close to one another carry the signal from the same audio source, where one of them has inverted phase. Along the run of the cable, these two wires can pick up RF or EM interference (buzz from power lines, etc). At their end, the phase of one of them is inverted again. Any interference that was received along the lines by both of these wires will be cancelled out by the inversion of phases, while the signals, now in proper phase, would be added.

[Jacques E. Bouchard]

Thanks to everyone for offering invaluable advice. I'll have to weigh the pros and cons of each solution and pick what is best for our particular situation.