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Novice Question: On Production - Phase-Cancellation

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Nate Hurwitz
Novice Question: On Production - Phase-Cancellation
on Dec 16, 2008 at 1:35:16 pm

Hey this is a real noob question, but I am trying to understand more about the production aspect of audio recording.

Fact: Phase-Cancellation can occur while recording as stereo.

If this is true, is the majority of audio for film/video recorded as mono (dialogue) during production? I don't have much experience with this in the field and I am wondering what some best practices are used for avoiding problems like phase cancellation on set nowadays. Would appreciate any input from those who know, thanks!!


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Ty Ford
Re: Novice Question: On Production - Phase-Cancellation
on Dec 16, 2008 at 2:19:39 pm

Hello Nate and welcome to the Cow Audio Forum.

Phase cancellation is a naturally occurring phenomenon. It's all around us every day. Most of the time we don't notice it. People who do sound for a living may notice it a bit more because they are always listening on some level.

Yes, phase cancellation occurs during stereo and mono recording. It is frequently positional, that is, dependent upon the position of the mic(s). It happens in the air before the sound hits the mic. Stick a mic in a null and you will record the results of cancellation. Move x number of inches or feet and you may walk into a peak where the opposite of cancellation occur. At these positions the sound accumulates to be louder than in the average sound field. It is not unusual for this to occur for a small spectrum of sound instead of the entire audio spectrum.

Film dialog is almost always recorded in mono. With multiple voices, it's not unusual to record each voice to a separate track. In post, the tracks are brought up in the editing program. There, typically, when actor A is speaking, the gain from tracks of actor's B, C and D are reduced to keep actor A's voice (which may be heard in the other actors' mics) from being heard.

If this isn't done, there is a hollowness to actor A's voice. That's phase cancellation! Actor A's voice gets to the other actors mics slightly after it gets to his. Putting on my aluminum foil Physics Hat, that's about 1 millisecond per foot, because that's how fast sound travels. So if actor A's voice is getting into three other mics, all at different times, you get a phase wash that's very nasty. This is made worse by the fact that in those other mics, with actor A so far away, you are also hearing a lot of the room acoustics of actor A's voice banging around in the room.

In my seminars, I try to get people to imagine a pool table. The bumpers are walls. The ball is the sound. Roll the ball and it bounces until it runs out of energy. Sound works the same way. Now imagine a three dimensional pool table; ceiling, walls, floor. The ball still rolls until it runs out of energy.

Any three dimensional space has, depending on its geometry, places where certain frequencies might add or subtract. That's the wonder of acoustics. That's why recording room tone in every setup is important, even if the setups are in the same room. They MAY sound the same, but they probably won't.

If you only use a mono mic for dialog -- like a boom mic -- for interiors, you'll want a hyper cardioid or supercardioid pattern, and NOT a shotgun mic. Shotgun mics are only directional at high frequencies. In a room they pick up mid and low bounce which, because it gets to the mic later than the direct sound you're aiming at because it has bounced off the wall, ceiling, floor, blackboard, conference table, window, etc., before getting to the side of the mic.

There are and endless number of individual situations in which moving the boom mic a little here or there make a profound difference in the amount of phase cancellation heard. That's one of the things a good boom op and mixer know and know how to deal with. Ears are important. Knowing how to process what you hear to make the sound better is the job. Using different tools and techniques to get the job done are the art and craft of the sound person.

Regards,

Ty Ford




Want better production audio?: Ty Ford's Audio Bootcamp Field Guide






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Nate Hurwitz
Re: Novice Question: On Production - Phase-Cancellation
on Dec 16, 2008 at 3:35:38 pm

Ty thanks a lot!

A couple questions:
1.) What is a null (In audio terms)?

2.) Is there anywhere online that's resourceful with examples of bad production audio (many different situations)? I may just try recording the bad sound myself even.

I've read about this mic, it's a shotgun, but it's hyper-cardioid and gets good reviews - http://www.trewaudio.com/sankencs3.htm - It's a tad expensive, but it looks like it might do the job for any indoor shooting.

Nate Hurwitz
Digital Media Arts/Music Major
Do you love it?


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Ty Ford
Re: Novice Question: On Production - Phase-Cancellation
on Dec 16, 2008 at 4:15:01 pm

A couple questions:
1.) What is a null (In audio terms)?

> Where stuff cancels out. In a room, the sides of a figure of eight mic pattern,

2.) Is there anywhere online that's resourceful with examples of bad production audio (many different situations)? I may just try recording the bad sound myself even.

> Youtube. :) Well the desired effect is to start with bad and move toward good. Good headphones help. I use Sony MDR7506 and Audio Technica ATH-M50.

I've read about this mic, it's a shotgun, but it's hyper-cardioid and gets good reviews - http://www.trewaudio.com/sankencs3.htm - It's a tad expensive, but it looks like it might do the job for any indoor shooting.

The CS-3e is a good choice. I have it on the "Good Gear" list in my little book.

Regards,

Ty Ford

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide






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Nate Hurwitz
Re: Novice Question: On Production - Phase-Cancellation
on Dec 20, 2008 at 2:24:15 am

Ah yes youtube is one heck of a mud hole for bad sound! Thanks again!

Nate Hurwitz
Digital Media Arts/Music Major
Do you love it?


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