Mic Question for Engineers

M

MrLeadFoot

Silver Member
I'd like some opinions in regards to positioning two overheads relative to phase cancellation.

I hear a lot of people saying that if you don't have overheads positioned equidistant from the snare, you get phase cancellation that cuts out low tones. My overheads are not the same distance from my snare, and I don't think I've ever encountered this, or maybe I have but just don't know it.

Nevertheless, the theory doesn't make sense to me. For example, if low tones are what's affected by the phase cancellation, then why is it so important to have the overheads equidistant from the snare? What about all the toms and kick? You can't position two overheads equidistant from everything.

I always close-mic my 6 drums in addition to using two overheads. Does that change things? One of my overheads is 41" from the snare, and the other is 45". Would that discrepancy cause phase issues? Even if I were to have both overheads, say 43" from the snare, my snare mic is only 1.5" from the snare, so wouldn't that defeat the whole purpose of trying to position overheads the same distance from the snare?.

I have also heard that if you have close-mic'd drums you should use the overheads to pick up cymbals only, and engage the low-cut switch on the overheads. But, if I do that, wouldn't they still pick up the drums, but only pick up the highs from the drums, and subsequently emphasize more of the highs of the drums in the mix?

FWIW, I've tried engaging the low-cut switch, but to be honest, while listening carefully with in-ears when striking cymbals I can detect a bit of difference, once I start playing the full kit, I don't really notice any difference, let alone when the whole band plays.

Can someone with experience please share some light on these issues?
 
Hi,

okay first of all I'm not an engineer but I have talked with many professional engineers about sound.

Nevertheless, the theory doesn't make sense to me. For example, if low tones are what's affected by the phase cancellation, then why is it so important to have the overheads equidistant from the snare? What about all the toms and kick? You can't position two overheads equidistant from everything.
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I only know that phase cancellation appears when mics point against each other. So for example your overheads and snare-bottom mic.
One mic points from top to down and the snare bottom mic points from bottom to top.
That is where phase cancellation will appear.
Regarding the low tones.
You always have a low-Cut on the overhead EQ when you use close micings. (as you said) When I watched a lot of mixing tutorials (which I still do) I always see an EQ where everything under 2khz (or higher) is lowered down by -20db or something. So you only get the cymbals. In my overheads I actually hear nearly to nothing from bassdrum, snares and toms.

I know that will sound a bit dumb now, but there's a lot of unneccessary math going on when it comes to mic a drumset.
I once was in a studio where the guys setted up 18 drum mics in 2 1/2 hours. The calculated everything and the result sounded like crap.
What I wanna say is: Check all the mics for phase cancellation, like here:
http://www.youtube.com/watch?v=dSpHKVdIlgM&feature=related

I was told by many soundengineers "if it sounds good to you, it will sound good in the result - you can do a lot by ear".
Just play around with the mic positions and check what sounds good :)
 
One of the difficulties of audio engineering is that sound is inherently dynamic and variable. You may not experience any issues with a particular mic setup but somebody else using the same setup in a slightly different room might be experiencing serious issues.

Just to clarify. Phase cancellation occurs at any frequency. Low frequencies are more obviously affected because there are fewer cycles which makes any cancellation appear more severe. Any cancellation at low frequencies will be much more obvious than cancellation at higher frequencies.

To clarify why the snare drum is usually the 'problem', it's simply because the snare drum is usually the loudest drum on the kit. The snare drum typically covers a huge range of frequencies too - right across the audible spectrum - which means that any phase issues can become painfully obvious because of both the amplitude and the frequency content. Having two mics different distances from the snare can be an issue because the wave front entering each microphone should be as in-phase as possible. If one microphone is placed physically further away - even by an inch or so - the change in distance means that the wave entering that microphone will most likely be at a slightly different phase. This isn't necessarily a problem because phase isn't just a bipolar phenomenon, it is measured in degrees. Something that is 180 degrees out of phase will cancel totally, something that is 10 degrees out of phase will make next to no audible difference.

Also remember that different frequencies will have different phases, so if your microphones are out of phase at a high frequency, then it will be much less noticeable than being out of phase with a low frequency. Low frequencies also have a longer wavelength, which makes the 'out of phase' alignment cover a bigger physical distance.

Part of the issue you describe with the snare drum mic is alleviated because you are not picking up the same sound source with the snare mic as you are with the overheads. For phase to happen, you have to be picking up an identical - or very similar - sound source and combining them together. Your snare mic's signal is radically different from the signal that your overheads are picking up. There is the potential for phase but it's much, much less likely.

As for engaging the low-cut when you're close-micing? Ask a hundred engineers the same question and you'll probably end up with different approaches and different rationale each time. Live, I might do that but in the studio, I wouldn't.

What you also have to remember is that theory is all fine but it ultimately comes down to what you're hearing. If what you're hearing is good then you are doing it right. Sometimes you can know all the theory in the World but you still end up with bad results because of variables you are unaware of and a radically different approach works. There are literally millions of ways of micing a drum kit so it's always down to your ears in the end.
 
Kalma said:
(...) I only know that phase cancellation appears when mics point against each other. (...)
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Not only. E.g. when recoding/miking up guitar cabinets with 2 microphones (pointing in the same direction, or more or less similar direction if you apply some angle) phase cancellation can/will occur also. It's related to the sound "arriving" at the microphones after different traveling time (= various distance between the source and mics). And those timing discrepancies can be anything between 0 and 180 degrees. So switching on the "phase reverse" button won't help in most cases. But inserting cotinuously variable delay lines will, or some little helpers like e.g. the Radial Engineering "Phazer" (which I'm using for guitar cab recording). That device just "slows down" the "faster" signal (=from the mic with less distance to the source) and brings the phase in line. In fact, the phase alignment can be done gradually so there is a whole range where the combined sound (both microphones) will vary from "crappy sound" (=noticeably out of phase) to "it's getting better now" to "sounds right/time aligned".

Some recommend to adjust the mic distance until you get no phase problems/cancellations but this approach also means you might sacrifice the sweet spot or a spot at which the mics sound really good. Inserting some time alignment devices has the benefit of not needing to care about mic distances and adjust the phase before hitting the REC button (if you use hardware) - or even afterwards (you could micro delay some tracks against others to clear up phase problems).

I understand that usually there's much more mikes involved in drum recording vs. guitar recording.

I might add that phasing is a natural occurrence to some degree (depending on the instrument - e.g. drums). But in those situations we are so used to the sound as to perceive it as "normal".
 
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Milt Hathaway said:
For live reinforcement, or recording? Each of these situations requires different techniques.
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While I intitally thought that I'm concerned with a live application, since we use IEMs which would be like listening to a recording, I would have to say both, simultaneously. When I think about it, I'm not so sure if one should even try to address phase cancellation when live, since there's still so much ambient sound and bleed from the drums into vocal mics, right?
 
mediocrefunkybeat said:
As for engaging the low-cut when you're close-micing? Ask a hundred engineers the same question and you'll probably end up with different approaches and different rationale each time. Live, I might do that but in the studio, I wouldn't.
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Thanks for all the details. Would engaging the low-cut on the overheads help ensure that there's no phase cancellation?
 
OK, I'll engage the low-cut engaged tomorrow night's gig, and see what the recording comes out like. Thanks again.
 
mediocrefunkybeat said:
As for engaging the low-cut when you're close-micing? Ask a hundred engineers the same question and you'll probably end up with different approaches and different rationale each time. Live, I might do that but in the studio, I wouldn't.
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After re-reading your post, and reading INTO it a bit, maybe I really am experiencing phase issues. You see, we always record our performances out of the board, and I have noticed that although our mix is not very different at outdoor gigs compared to indoors, I have noticed that outdoor recordings ALWAYS sound SO much better than indoor recordings, which tend to sound a bit hollow. While I've always thought that that's because the sound bounces off walls and ceilings indoors and is more apt bleed into vocal mics. While that undoubtedly happens, your comment about maybe engaging low-cut live but not in the studio made got me to thinking that it stands to reason that by virtue of being indoors with sound reflection that being in ANY room probably contributes significantly to potential phase issues, right? Or, is the fact that outdoor recordings are better simply due to sound having no chance to reflect back into the other mics?

Also, do you think that since I close-mic all the drums that there really is no reason NOT to engage low-cut in the overheads?
 
One of the basic rules of audio engineering is that mic placement and the room have to be working well with each other. If your room has a lot of reflections, you could be introducing all kinds of problems - especially if the room has large, parallel walls. I would advise you to look into sound treating the room as well as mic placement - quite simply that is what 90% of recording is about.

As for not engaging the low-cut in the studio? I wouldn't do it because I want as much information going into the recording system as possible. It's the same reason I wouldn't EQ when tracking but only during the editing process. If you have the low-end from the microphones, you could just choose to remove it later. You might find that it sounds good to have it but if you engage the low-cut, you just don't have that option. I always record flat and EQ later and with modern systems there is very little reason why you would do otherwise.

Live sound and studio sound are different animals. The priorities of live sound are different from the priorities of the studio. With live sound, you're going for the best sound you can straight away, taking into account the other instruments, the stage, etc. In the studio you're going under the microscope and it's usually much more precise.
 
mediocrefunkybeat said:
One of the basic rules of audio engineering is that mic placement and the room have to be working well with each other. If your room has a lot of reflections, you could be introducing all kinds of problems - especially if the room has large, parallel walls. I would advise you to look into sound treating the room as well as mic placement - quite simply that is what 90% of recording is about.
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We have absolutely no control of any of that when playing indoors at commerical venues, for wedding receptions, special events, corporate functions and the like. Our recordings of outdoors gigs sound so much fuller with more lower tones than indoor gigs. You would think it would be the other way around, which is why I was asking if reflection can contribute to phase issues.

The more I think about it, the more I am convinced that it's a moot point to try and address phase issues via mic placement in indoor venues because the sound is going to bounce all over the place anyway, so even if both overheads are equidistant from the snare, that sound is going bounce all over the walls and ceiling and hit the mics again at different intervals anyway.

So far, I have always had the overheads wide open in both situations. Maybe I should try engaging low-cut for indoor gigs, and turning it off for outdoor gigs.
 
Sound screens placed strategically could certainly help if you think reflections are the issue. That and slightly closer mic placement should make a difference.
 
In my experience, some of the most problematic overhead miking comes from the overheads being too far away from the kit or too close to the ceiling above the kit. I still like using a spaced stereo pair for a greater stereo image, especially with the cymbals when all the drums are close miked. I have the snare and bass drum in the center with the hats and toms panned either as I see them or a mirror image, it all depends if an audience can see the kit or it is being recorded. The overheads are panned extreme right and left.

My suggestion would be to just experiment with the microphone placements, even to the extent that you may have it in your gut that there's no way in hell it will work. This is where pleasant surprises sometimes pops up.

Dennis
 
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