Tuesday, November 15, 2011

Engineering Drums Part Five: The Kick Drum

Kick drums (bass drums) provide the low frequency, rhythmic foundation in pop music.  We want to capture this well as it is this LF content that provides the exciting 'thump' in the music that we feel in our chests.

Select a dynamic mic that has extended LF response (yes, occasionally you will find an engineer who uses a condenser mic on kick, but this is tremendously rare). Some common choices for kick mics include the AKG D112, Shure Beta 52, BeyerDynamic M88, Audio-Technica ATM25, Sennheiser MD421, and the Audix D4 or D6.

The D6 is an example of what I would call a 'tuned' kick drum mic, that is, its frequency response is not meant to approach linear, but rather has a purposefully manipulated curve designed to produce a pre-equalized, modern kick drum sound. Other tuned kick mic models include Sennheiser's E602 and Electro-Voice's N/D868. These tuned mics can be very cool, and helpful for getting sounds quickly, but be aware that the more tailored a mic's frequency response is, the less flexible it is as well (the more likely it will sound great on one drum, not so much on another). So if you choose a mic with a tailored frequency curve, make sure you really like what it does on your drum, or you may be swimming upstream later trying to undo its impact on the sound.

Kick drums sound quite different from one another depending on variables such as dimensions, shell material and thickness, bearing edges, types of heads used, tuning, and muffling, so unless you're tracking the same drum over and over, I recommend you select a mic with a little less personality, but rather one that yields a solid, complete tone and allows the engineer some freedom to tailor it as needed through the use of placement, EQ and dynamics processing. More often than not, for the kick I use an AKG D112 – it is a proven, well known performer that works well on a variety of kick drums, though there are many other mic options that also work quite well.

For pop/rock music kick mics are generally placed inside the drum. Access to the drum's interior is usually accomplished via a hole cut into the front head, allowing the boom arm of a mic stand to reach into the drum. You can also gain access to the interior of the drum by removing the front head altogether, but doing so will dramatically reduce the drum's ability to resonate compared to having a front head (even a head with a hole in it), and modern drum sounds often benefit from the additional resonance from having a front head.

Begin by placing the mic inside the drum about halfway between front and back heads, off center, pointed at the beater to capture the impact. When placing the mic I'm looking for a good balance of attack (from the beater) and tone (from the shell) – this placement is a usually a decent place to start. In this position it's close enough to the batter head to get some good definition, but the off center placement means that when the mic is pointed at the area of beater impact it is also pointed, somewhat, toward the shell as well, which increases the tonal component of the sound.

Listen critically to the sound with this initial mic placement, and decide what you'd like to change about it. No, don't reach for an EQ to try to make the sound better;  make it the best it can beget out there and move that mic around or tweak the drum to capture the sound you want.

The main components of the overall sound you're listening for are attack, tone (or body) and resonance.

Adjusting attack:

Placing the mic closer to the batter head increases the amount of attack in the sound. Pulling the mic back will reduce the attack.

Also, with directional mics high frequency sensitivity is greatest when the mic is pointed directly at the source. This HF response then rolls off as the sound becomes more off-axis. Therefore, if you like the amount of attack, but would like to lower the frequency of the attack, keep the mic a similar distance from the batter head, but turn it a little, so it's not pointed as much at the beater. Conversely, if you want the attack to consist of higher frequency content, point the mic directly at the beater.

If you find yourself wanting greater attack than mic placement alone can provide you may want to try fitting the batter head with a beater impact pad.  These vinyl/plastic pads are self-adhesive patches that you place at the point where the beater makes contact with the batter head.  These pads enhance attack (and also increase the durability of the head).  If you don't want to buy one of these pads, you can essentially make one by cutting a 3"x3" piece of old drum head and duct taping it in the same spot.  This will give more 'snap' to the attack.  

Alternatively, you can change the beater to a harder type.  For instance, if the bass drum beater is made of felt, switch to one made of wood or hard plastic.  The harder material will create a stronger attack.

"Why not just use EQ?" you may be wondering.  Because even the best EQ will still affect everything the bass drum mic picks up, including the bleed from the bottom of the snare drum that is just on the other side of that batter head.  By altering the drum and/or beater, you affect the captured attack of the sound without affecting any of the leakage that may find its way into your kick drum microphone.  The result will be much more managable when mixing with less need for fiddling to get things to behave.  It requires a touch more discipline when tracking, but eliminates headaches later on and provides the best results in the final product.

Adjusting tone:

To increase the drum's tone simply back the mic away from the batter head, closer to the front head. With this greater distance more of the drum's depth will be captured, creating a fuller, deeper tone. To further emphasize the tonal component, turn the mic away from the beater, and more toward the shell instead. This rounds off the attack content and further features the shell's contribution to the sound.

Sometimes the tuning of the drum may seem right, but it's tone is too bright and can actually approach an almost basketball-like 'ping' in the midrange. This happens when the interior shell of an unmuffled bass drum is hard enough that the sound actually bounces around inside the drum rather than being evenly diffused or absorbed. This is particularly true of acrylic and fiberglass drums, as well as those from a couple of manufacturers who like to seal the interior of their wood drums with a thick coat of hard polyurethane. To solve this annoying resonance place a dish towel, washcloth, hand towel, or similar sized soft cloth along the inside bottom of the shell, not touching either head. Since it doesn't touch the heads it doesn't dampen the resonance of the drum, yet the soft texture of the towel gives the soundwaves a place to go. This works surprisingly well.

For greater absorption use either a thicker towel or a larger one folder into a smaller footprint (still not touching the heads).

If you want to further increase the fundamental low-frequency content of the tone there is an old trick that can help – you must dampen the shell's resonance. To understand why this is so I'll make a slight detour, but one that I promise you will find useful, not only for understanding kick drums, but ALL drums.

For the past 25 years or so it seems that drum manufacturers have been striving to create drums that resonate well. Sounds like a great thing.... who doesn't love resonance? They make thinner shells that resonate easier and design mounting hardware that allows the drum to resonate fully, all in an attempt to create more 'tone.' The funny thing is that this 'tone' everyone is striving for is actually comprised of less fundamental tone, and more overtones or harmonics.

When someone strikes a drumhead the sound goes down the inside column of the drum shell and gets the bottom head moving too. The tuning of these heads, along with the dimensions of the drum, determines the drum's fundamental pitch. When the vibration of the heads is unhindered, the fundamental pitch is strong. But thin shells with mounting hardware designed to let them resonate fully respond easily to the vibration of the heads by doing a little vibrating of their own. The shell's involvement absorbs some of the heads' vibrations which actually dampens the fundamental. What, then, is the product? More harmonic content. This dampened fundamental/increased harmonic results in a brighter overall tone that is common to modern drums. This is why thinner shelled drums have a brighter tone.

Conversely, a thick shell isn't as easily set in motion by the vibration of drum heads, so the fundamental pitch is stronger. This is why thick drums are louder and darker with more fundamental pitch than their thinner-shelled cousins. This is true of other acoustic instruments as well.

For example, Martin's D-35 acoustic guitar is known for its darker tone, compared to, say, their D-28. Both are dreadnoughts of the same size and materials with same level of craftsmanship, but the D-35 has a three-piece back rather than the two-piece back of the D-28. Three pieces of wood, along with the greater amount of glue needed to combine them, create a guitar back that is slower to respond to vibration than a two piece design that uses less glue. Since it can't vibrate so easily, it doesn't hamper the fundamental content as much, and the guitar's tone becomes darker and louder.

Make sense?

So back to kick drums. Since hindering the drum shell's ability to resonate increases the fundamental pitch, then bass drums that have a couple of toms mounted to its top will have a stronger fundamental than that same drum with no toms mounted to it (since the weight of the toms will hamper the shell's ability to resonate freely). 

Some people have 'virgin' kick drum shells that are drilled for nothing other than lugs and spurs. Such free-standing drums, particularly those with thinner shells, allow maximum vibration which dampens the fundamental yielding a more harmonic tone. So... to increase the fundamental pitch you must dampen the shell's tendency to resonate. To do this place a a weighted object(s) inside the drum, resting on the bottom of the shell. I have a few pieces of heavy 4” iron pipe that I place on a towel inside kick drums when I want a stronger fundamental pitch. I know of folks who use a canvas bag with around 25 lbs. of bb's in it. Any weight you add to the inside of the shell will dampen it, bringing out the fundamental pitch.

Be aware , however, that leaving such a weight inside the drum continuously will eventually warp the shell. I have seen kick drums from studio 'house kits' that have become very warped from years of having a heavy weight laying inside. So use the weight as needed, but please use caution, so you don't damage the drum itself.

Adjusting resonance:

If the drum is too resonant, or 'boomy,' apply (or increase) padding, such as a folded towel or blanket or even a small pillow against the batter head. This will tame the duration of the head's vibration, reducing the drum's resonance. The more padding you use, the shorter the resonance will be.

You can also pad the front head some, if needed, though I often find that muffling the batter head of a well-tuned bass drum is enough.  Trust your ears.

Supplemental mics:

It is common practice these days to use a second mic to capture another perspective of the kick sound to be combined with the internal mic for a more complete presentation. The ability to balance these mics in post gives the engineer additional flexibility for shaping the kick drum sound. Usually these mics are placed outside the drum, a few inches off the front head.

Two types of 'mics' are most commonly used for this (one of which isn't really a microphone at all): 1) a large-diaphragm condenser mic and 2) a speaker wired in reverse.

For a more natural sound I use a large diaphragm, cardioid condenser to capture the 'out front' perspective of the bass drum. I place it in close, so the proximity effect of the cardioid pattern works to capture the low frequency energy of the drum. This mic usually sounds a little incomplete by itself, but coupled with the internal mic yields a defined kick with a solid LF component and natural feel. Because of the sensitivity of condensers mic it can be helpful to angle the mic down a bit to help minimize bleed from the rest of the drum kit. If necessary you can use a heavy packing blanket to cover the front of the kick to further isolate this outside mic. In practice this isn't usually required, but you may find it helpful.

For a more exaggerated LF punch wire a 6”-8” speaker to an XLR connector (speaker leads to pins 2 and 3 and speaker metalwork to pin 1) and place it a few inches off the front head, pointed right at it. Yamaha actually makes a subkick mic that is essentially such a device. It's handy to use, but you don't have to spend $300 on it if you have a speaker or two laying around already. A speaker is essentially a very large dynamic microphone in reverse anyway, so wiring it for use as a microphone actually makes more sense than one might think at first.

Trivia time: did you know that the bass guitar sound for the Beatles' “Paperback Writer” was achieved using a reverse-wired Tannoy speaker placed directly in front of Paul McCartney's bass cabinet? (I'm a huge Beatles fan and just had to share).

Back to the speaker wired as a mic – when you audition this 'mic' alone it isn't gonna sound very good. It'll be foggy and muddy with almost no definition, but coupled with the clarity and punch of the internal mic, you will have a HUGE bass drum sound. Use it sparingly as the effect is easy to overdo.

These mics offer lots of options on selection and placement. Just trust your ears and play around with them, and you'll get some excellent results.

And remember that anytime you use two mics on one source always be sure to check your polarity settings. If they are in opposite polarity much the wonderful low end that you're working so hard for will be cancelled, leaving a very nasty, filtered, unsatisfying result.

Next comes the snare drum. Thanks for reading!


1 comment:

  1. Joel this is awesome stuff man. Thanks for posting