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Phase and Time Alignment
Getting the timing right in your drum recordings...
By Eric Ferguson

The use of multiple microphones is standard in modern drum recording. While old-school purists often lecture on the merits of less-is-more miking, the reality of modern production requires the flexibility of a multi-mic setup. While more mics does mean more flexibility in the final mix, the caveat is that each additional microphone further complicates the phase relationships of the various sonic elements. If these relationships are not dealt with properly, a single microphone can actually subtract tone and clarity from the overall drum mix. This is why the heralded old-school approach to drum recording often sounds better.

In this column, I will briefly describe the concept of phase and give several real-world examples of how to track and mix phase-phat drums.

Background

As you probably know, sound takes time to travel. This means that two mics placed at different distances from a drum will capture the same drum hit at slightly different times. If the distance between two mics is less than approximately 15 feet, our ears continue to perceive the drum hit as a single event and do not notice the distance as a flam or delay. When mixed together, though, the frequency content of these two mics is added and subtracted in relationship to the distance. This is commonly known as phase cancellation. When two mics are mixed together, desired frequencies are often canceled out. This can create a mix thinner than what each single mic presents on its own.

To further understand phase cancellation, a simple physics lesson is required. As sound travels through air, rapid oscillations in pressure create the audible frequencies you and I perceive as sound. Since all frequencies travel at the same speed, their oscillations, measured in Hertz (cycles per second), create waveforms of different lengths. These various waveform lengths mean that at different distances from a sound source, instances of positive and negative air pressure are present. If two mics are placed at distances in which a particular frequency has opposite air pressures, cancellation of this frequency occurs. This thins out that particular frequency from the mix.

Fix it in the mics

In the real world, the multiple mics we use on a drum kit all have phase relationships that cancel or combine to change the overall mix. The most obvious way to minimize these interrelationships is to limit the number of microphones used. Does the hi-hat mic actually help? Do you really need to use the overheads when the room mics do the job alone? Tom gating also helps in this way. While individual mics are necessary for huge-sounding toms, tom tracks almost always interfere with overheads and snare mics.

Using a noise gate or software automation to mute the toms except when played is standard in minimizing leakage. I leave all muting and gating to the mixing stage, choosing to track as many mics as possible and make major decisions later.

Mic choice and position also affects phase relationships. Dynamic mics are more resistant to leakage than condensors, and can therefore help greatly. Hypercardoid mics, if available, can further limit leakage. (I often use a Sennheiser MD 441 on the snare to give more isolation and attack.) Mic positioning should also be experimented with. Try angling the hat mic so it doesn’t also point at the snare. Try moving the room mics around to find the sweet spots. You may also want to try building a tent of packing blankets around the outside of the kick drum—this can better isolate a mic not placed fully inside the drum.

Fix it in the mixer

The phase/polarity switch on your console or mic pre is the next line of defense. Phase switches invert the signal polarity of an individual microphone, so its phase is exactly opposite. Professional engineers often take time while getting drum sounds to try multiple combinations of these switches, to see what sounds best.

Sometimes reversing the phase of the overheads helps the low end of the kick drum. Sometimes flipping the phase of the snare makes the drum have more attack. I recommend trying as many combinations of polarity as possible—you never know which microphone is seriously affecting the mix. You may even discover a mic cable that is wired backwards and save many future headaches!

Fix it in the DAW

Often, inexpensive consoles and preamps lack polarity switches. This means you need to experiment with phase combinations inside your digital audio workstation. Many standard plug-ins, such as eq and compressors, feature polarity switches that should be experimented with when mixing. If your DAW does not have capacity for flipping polarity in a plug-in, then you can do it destructively to the audio file itself. This function is commonly called “Invert.” (Check out Figure 1.)

An important side note to flipping the polarity in regards to snare miking: It is common practice to mike a snare from both above and below the drum. The snare bottom mic is great for adding the sizzle of the snares without direct hi-hat leakage. The problem with double-miking the snare is that the two mics are almost exactly out of phase with each other. This is clearly shown in Figure 1A. For this reason, it is standard practice to flip the polarity (Figure 1B) of the snare bottom mic to minimize this serious phase cancellation.

A more advanced method of dealing with phase requires editing in a digital audio workstation. In Figure 2 we have an example of a double-miked kick drum where one mic was placed inside the drum, the other on the outside. By zooming in on the two kick drum tracks in Figure 2A we can show the timing differences caused by the difference in distance of the two mics from the source. By selecting and sliding the initially delayed “KickOut” track to match the earlier “KickIn” track, you can compensate for this timing difference, as shown in Figure 2B. The two mics are now time-aligned and the kick sounds huge!

In a similar manner, I often move the room mic tracks earlier or later. This can both drastically transform the perceived size of the room and totally change which frequencies are affected by phase cancellation.

Although often overlooked, it is the knowledge of phase that can transform an average drum sound into a phat, punchy, and professional recording. Experimentation is the key, so remember, there are no rules!

Eric Ferguson (talkback@recordingmag.com) is based in Los Angeles, where he gets to participate in a wide variety of projects when he’s not working on his upcoming CD. Check out his website at www.bluecupmusic.com.

 

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