2 questions

[cellardweller]

What affect would insufficient phantom power have? Is there just no way of knowing without a/b comparison?

Where can I find info on recognizing phase issues. I understand the obvious drop in volume when out of phase, but how do you recognize when you are "a little" out of phase(without being able to look at the waveform on a DAW)?

I did search The Mother Of All Threads, any probably overlooked it...
I read what SAE had on it, but the "a little" out of phase thing is still unanswered... I think ...

 

[MadAudio]

I've read a couple of things about insufficient phantom power, but can't remember it all. There are some partial answers here though: https://homerecording.com/bbs/showthread.php?t=148430

 

[Bassman Brad]

If two mics are partially out of phase, it causes a "comb filtering" effect which simply means that certain frequencies will get cut, while other frequencies don't. Obviously, this changes the tonal character of the instrument. Since this is usually more likely to happen in the high frequencies, than the low frequencies, you usually hear it as a softening or blurring of the high frequencies - sort of like a phase shifter that doesn't swoosh, but is stuck at one position in the cycle.

The best way to check for phase problems, if you don't have an oscilloscope or any way to visually check the waveforms, is to get a single speaker and wire it up so that you can combine both left and right channels together through that one speaker. This gives you an audible check of "mono compatibility" and will make any phase problems much more apparent, because it will be easier to hear things that drop out when you switch over to mono.

Brad

 

[mshilarious]

It can affect low frequencies too. One priest at my parish has a habit of leaving his lapel mic on when he preaches at the lectern--the phase cancellation makes him sound very tinny.

I think that's more associated with larger phase differences though. I agree with Brad that small phase differences are more likely to be apparent in high frequencies.

 

[Bassman Brad]

By the way, I probably should have mentioned that you shouldn't get too wrapped up in getting things in "perfect phase," because there simply is no such thing. Two mics in two different positions will NOT be in perfect phase with each other on all frequencies. It just ain't gonna happen. When I was an engineer in a commercial studio, we had an O-scope permanently patched into a monitor bus, so that we could use it to check phase problems on any two mics at any time. But, what you need to understand is that, whenever you sent two signals to the 'scope, it didn't stay in one place, it tended to move around a little bit, especially whenever the guitar or piano (or whatever) moved into a different register. So, the bottom line is this: As long as phase issues don't cause you any audible problems, such as dropouts when you go to mono, or weird frequency distortion effects, then the mics are "close enough" to being in phase. By the way, some recordists have even used out of phase microphone effects intentionally for artistic purposes. I remember reading about a technique for using two mics - that are purposely set up out of phase - to make a 6 string guitar sound like a 12 string guitar. Pretty cool, huh?

Hope this helps.

Brad

 

[cellardweller]

Thanks, good info here.

The best way to check for phase problems, if you don't have an oscilloscope or any way to visually check the waveforms, is to get a single speaker and wire it up so that you can combine both left and right channels together through that one speaker. This gives you an audible check of "mono compatibility" and will make any phase problems much more apparent, because it will be easier to hear things that drop out when you switch over to mono.
Brad

If I were to pan, say both guitars, center, and mute one, listening for freq./volume changes would this be a slightly less effective way of recognizing a prob w/phasing?

 

[Bassman Brad]

If I were to pan, say both guitars, center, and mute one, listening for freq./volume changes would this be a slightly less effective way of recognizing a prob w/phasing?

Yeah, that should work. It's still a good idea to set yourself up with an option to monitor the whole mix in mono, for a whole lot of reasons. This is, I suppose, the reason that I thought of that approach, first. But for just checking for phase problems on a couple of mics, yeah, that should do it.

Brad

 

[Bassman Brad]

It can affect low frequencies too.

Oh, absolutely! I didn't mean to suggest that bass frequencies can't be out of phase or that they can't cause any problems. It's just that, in recording situations, it is usually the high frequencies that you have to worry about being out of phase. Remember that high freqs. have wavelengths measured in inches or fractions of inches, low bass waveforms are measured in FEET.

Here's an example of where bass frequencies would be out of phase: Many of the smaller bass amps can be tilted back so that they can be used as a personal monitor. Well, I've seen bass players who used a full size bass amp at the back of the stage AND also put one of those little bass amps up by their mic stand to serve as a monitor. The problem with this is that the two bass cabinets might be, let's say, eight feet apart and are facing in opposite directions (i.e. are 180 degrees out of phase)! What this means is that, whatever frequency corresponds to a wavelength of eight feet (I'm too lazy to look it up) would be notched out of the frequency response of the bass guitar signal when you listen out in the audience. Obviously, this isn't going to make the bass sound very good! And yet, I've seen more than one bass player do this. And believe me, it seriously damages the tone of the bass.

But, in the recording studio, when you're using multiple mics, they are usually not far enough apart to cause problems in the bass frequencies.

Brad

 

[noisedude]

In terms of mic'ing, is it all about the 3:1 rule, or is it more complex than that?

 

[cellardweller]

Thanks, that should help immensely.

whatever frequency corresponds to a wavelength of eight feet (I'm too lazy to look it up)

Where do you find the above info? I'm not sure how I'd even go about searching for this, but I'll start @SAE... I think I remember seeing this there...

Finally, are we talking about cancellation and boosting of certain high freq? Do I remember correctly that the two usually occur together?

 

[darnold]

Dont be too concerned about phase unless its very severe and causing problems. The fact is, like mentioned above, their are phase problems in everything. Infact phase has alot to do with sound in general. Phase cancellation can also be created from things being out of tune. Its a different sound generally but it is phasing that makes us tell if things are out of tune or not. And when things are in tune, it makes the instrument bigger. Think of an orchestra, when you have 20 violins or more playing at one time, the same thing, theres no way their frequencies are going to be in phase. Thats what creates the orchestration effect. Then you have everything in its own register, and distant creating its own phase sound.

Without phase there would be very little stereo imaging in general. Our ears localize things by difference in time that it hits your ears. Thats what stereo imaging is.

I dont think its more important that the high frequencies are inphase than the lows. Both can give the same amount of problems. But the thing is, its alot harder to hear the problems in the highs when they are out of phase than the lows. Its obvious when bass frequency cancels out, because its a complete dropout. Thats what causes the loss of volume. then you still hear the highs but they sound combfiltered instead of just gone.

This is also why it is recommended to do X/Y Stereo micing instead of very spread A/B in alot of situations. Think of a drumset. You have them spaced 3 or 4 feet apart, then the phase is completely different from the left mic to the right mic. This is what is giving it a very wide spread. But what if the snare mic is 180 degrees out of phase with the left mic, but completely in phase with the right mic. You notice this during tracking and do a polarity reverse on the snare mic, now its just reversed but you still have the same problem but its in the right mic now. So based on this theory, theirs no way to get the snare mic completely in phase with the overheads, because not even the overheads are in phase with each other.

This can work as a good effect if you spend the time to get it right. But you will lose alot of the tightness in the whole drum mix. But with X/Y where there are very minimal phase problems because the stereo is based on amplitude levels. Then you can much easier get the snare mic perfectly in phase with the over head mics, which will focus the sound and make it larger. But you wont have a wide stereo feild.

This is why a 3 mic technique with kick and 2 overheads is a good idea for those who cant recognize and fix phase problems. Its a really big deal in these kinds of applications, and most likely you will get a better sound by using just 3 mics.

Danny

 

[Bassman Brad]

In terms of mic'ing, is it all about the 3:1 rule, or is it more complex than that?

Well, it's a little bit more complicated than that. Keep in mind that the 3:1 rule will keep you out of trouble in MOST situations, but isn't really a hard and fast rule, just a general guideline. But you could have a situation where two mics follow the 3:1 rule, but still cause phase problems. For example, if you have two mics, one of which is 8 feet from the source and the other is 16 feet from the source and pointed in the opposite direction then you have the very same problem with phase cancellation that we discussed in the bass guitar example, and this is equally true even if the mics are 24 feet apart from each other (thus honoring the 3:1 rule). Remember that the 3:1 rule doesn't eliminate phase problems, it just minimizes the likelihood that they will cause you grief in the real world.

 

[mshilarious]

Oh, absolutely! I didn't mean to suggest that bass frequencies can't be out of phase or that they can't cause any problems. It's just that, in recording situations, it is usually the high frequencies that you have to worry about being out of phase. Remember that high freqs. have wavelengths measured in inches or fractions of inches, low bass waveforms are measured in FEET.

That's right. In my example the two mics would be about two to three feet apart, with about a foot difference to the source.

 

[Bassman Brad]

Where do you find the above info? I'm not sure how I'd even go about searching for this, but I'll start @SAE... I think I remember seeing this there...

The wavelength of a particular frequency is based upon a mathematical formula, which in turn utilizes the speed of sound as one of the factors. Unfortunately, I don't know the formula OR the speed of sound off of the top of my head. I just look up the values in a table whenever I need them. I think that the Yamaha Sound Reinforcement Handbook has one of these tables in it, but I'm sure you can find them in many places. A simple google search would probably find it for you.

And, as Danny and I have both said, the whole phase thing needs to be kept in proper perspective. If you have a single audio signal from a single source and you split it in half and send it two different places under exactly identical conditions (equal lengths and capacitance of cable, etc...), then these two signals will be perfectly in phase. If you combine those signals back together, they will sound exactly the same as they did separately. But, make no mistake, NOTHING ELSE is going to be perfectly in phase. Basically, phase is not a problem unless it sounds bad. Otherwise, it's all good.

 

[mshilarious]

The wavelength of a particular frequency is based upon a mathematical formula, which in turn utilizes the speed of sound as one of the factors. Unfortunately, I don't know the formula OR the speed of sound off of the top of my head.

It's pretty simple: speed of sound at sea level = about 1100 ft/sec

wavelength = 1100/frequency

frequency = 1100/wavelength

So an 8 ft wave is 137.5 Hz.

 

[Bassman Brad]

It's pretty simple: speed of sound at sea level = about 1100 ft/sec

wavelength = 1100/frequency

frequency = 1100/wavelength

So an 8 ft wave is 137.5 Hz.

Thanks, mshilarious. I really should know that.

(Please excuse me while I give myself twenty lashes.)
Brad

 

[mshilarious]

If you have a single audio signal from a single source and you split it in half and send it two different places under exactly identical conditions (equal lengths and capacitance of cable, etc...), then these two signals will be perfectly in phase.

Let's look at that for a minute. Cable capacitance is really small, so it would have to be a massive difference in length to be audible. Here's a good explanation:

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/skincoax/page8.html

As for cable length causing a delay, recall that signals travel at roughly the speed of light, or 300 million m/sec. So if you used a 1 km cable for one channel and a 1 m cable for the other, the time difference would be about 3 microseconds - that's 1/300 of a millisecond, which corresponds with an audio frequency of 333kHz (edit: sorry!).

I may have dropped a digit or two there, but the principle is sound: cable length can't create an audible phase difference.

Oh and since I've gone metric:

speed of sound = about 340 m/sec

 

[Bassman Brad]

That may very well be true. The point I was trying to make, though, was that if you have two signals that are perfectly in phase with each other in all frequencies, they are not merely similar, they are the same exact signal. Obviously, when we are dealing with multiple signals in the recording environment, they usually are NOT exactly the same, and so they would not be perfectly in phase.

Brad