When it comes to recording live musicians, few instruments are as misunderstood or mysterious as the drums.
The drum kit presents a complexity that just doesn’t exist when recording other instruments, simply because of the sheer number of components that make up even the most basic drum setup, as well as its wide frequency response and abundance of transients.
Even when you’re using a single microphone to record a full drum kit, there are still several drums and cymbals (not to mention the room) that need consideration and attention.
To try and maintain control over the balance of the drum kit, we recording engineers will often use multiple mics to allow us to manipulate the level and tone of each component while, hopefully, maintaining some sense of realism or believability.
But as you add more microphones, you also increase the likelihood of destructive interactions between the microphones, putting your whole sound at risk. These destructive interactions between mics are caused by phase cancellation.
The best way to combat phase cancellation is by placing your mics carefully to begin with. However, if you’re left with significant phase issues when it’s time to mix, a little bit of time-alignment can dramatically improve the clarity, coherence and impact of your drum sound. As you become more and more adept at this technique, you may even find that well-recorded drums can benefit from a little time spent experimenting with alignment.
To hear the sound of drums before and after time-alignment, listen below:
In just a few moments, we’ll look at a clear-step-by-step process for time aligning drums that works every time. But first, a little bit of background:
Phase Cancellation: The Arch Enemy of the Perfect Drum Sound
When you combine two signals of equal amplitude and frequency that are “in–phase” with one another, they will reinforce each other, and your signal will get louder as a result. This is known as “constructive interference“.
On the other hand, if you combine those same signals but they are 180˚ out of phase with each other they will cancel each other out completely. This is destructive interference or phase cancellation.
This basic concept is easy to understand by combining two sine waves of the same frequency and timing, or using two identical audio tracks. Simply open your DAW, take any audio file, and duplicate it so you have two copies. Then, invert the polarity on one track and bring up both faders to the same level. The combined output will be -∞, or rather, nothing.
This same idea is still at play when you are using multiple microphones on a full-frequency source like a drum kit, though becomes slightly more complicated. Not only are the signals more complex, but also there are multiple arrival times to deal with.
Each mic on the kit will “hear” each drum and cymbal at a different level, at a different time, from a different angle, with a different frequency response. This creates multiple instances of constructive and destructive interference simultaneously.
The results can be unpredictable: Some frequencies will cancel completely while others will be slightly reduced; still others will be reinforced, and so on.
To make things even more confusing, the signals are not simply “in” or “out” of phase with each other: They can be out of phase by anywhere from 1˚-359˚, and by different amounts at different frequencies!
This is not a problem that can be easily fixed with just a polarity switch, either. Without the right techniques to deal with it, it can be a chaotic mess.
This kind of chaos is more common than you might expect, and less experienced recording engineers will often make matters worse by working on the sound of each mic individually without regard to how it will interact with the other mics as they are added to the drum mix.
For example, a new mixer might bring up the kick drum, make it sound good by itself, then solo the snare drum, make it sound good by itself, and so on and so on, down the line until each mic is “perfect”.
However, once you bring every fader back up and listen to the drum mix, there is shock and awe. (And not the good kind). This approach usually results in tracks that don’t fit together well, and when combined, the sound is nothing like the sum of its parts.
Drum tracks like this often suffer from uneven bass response, peaky midrange, a lack of high frequency detail, a hollow or non-existent center image in stereo, and a general absence of size and impact. These are not the adjectives you want to use when describing your work to prospective clients!
More experienced engineers will usually take a more comprehensive approach to working on a drum sound. They will pay attention to the overall balance, and show awareness of how the addition of each mic affects the balance and tone of the drum mix. They may still work on the sound of each mic, but only after the cumulative effects of all of the mics have been accounted for and dealt with through proper mic placement.
Better Phase Coherence = Less Work!
The most obvious benefit to ensuring that you have good phase coherence at the tracking stage is that your drum tracks end up needing less EQ and other processing to have impact and size, because everything is operating as a whole. It’s one drum sound, not just a bunch of drums combined (like you may get when using samples). The sound is more coherent, clear and focused, and yes, bigger.
Once the inter-mic relationships have been accounted for, it becomes far easier to manipulate the internal balance. For instance, a change in level of one mic won’t have a significant negative impact on the rest of the kit, and will instead just change the balance between elements. When everything is moving in the same direction, then each mic becomes mostly constructive rather than mostly destructive.
Tracks that are properly aligned will accept compression and EQ more seamlessly and with fewer surprises. But if your tracks aren’t well aligned, even careful use of compression and EQ can just make matters even worse.
Because compression affects the decay and sustain of a drum mic, the whole balance or tone of the kit can shift radically as the compressor changes the level of the sustain. The same goes with EQ: High frequency boosts cant reveal non-linear peaks and dips in the midrange or high frequencies as destructive phase interactions at these frequencies are increased or minimized.
While compression, EQ, reverb and distortion can all be effective at helping to maximize the size, impact and character of a drum sound, nothing will have a greater positive effect than having drum tracks that have good phase coherence. (Well, maybe having a good drummer and a good kit in a good room, but that’s another story.)
Just remember that “good phase coherence” doesn’t mean that everything is 100% “in phase”, because that’s essentially impossible with multi-mic techniques on a full drum kit. There are just too many varied arrival times and frequency response differences to bring everything completely in phase. These are complex, full-frequency signals at close proximity to each other, not simple sine waves. The best we can do is assure that destructive phase relationships are minimal, and that each new mic we fold in adds far more to the tone than it takes away.