TCRM 10 focused on microphone techniques for recording guitars and bass as well as brass and woodwind instruments. This month the discussion will include some of the more difficult, complicated instruments to mike: mainly the piano and the drums. A special focus is given to stereo techniques, as they are the backbone of many successful recordings of either instrument. These methods are also invaluable for recording live music as well as in any circumstance where a more natural stereo image is desired.
Stereo Recording Techniques
Stereo recording involves using two (or more) microphone elements to capture a single performance to two tracks for the purpose of recreating a coherent, “realistic” stereo image.
The word “realistic” is in quotes here because, though the recording comes directly from the original source, the engineers recording technology, methodology, and mixing decisions invariably color the sound. The resultant recording does not sound the same as it did experiencing the performance directly. At the same time, stereo miking methods are likely to produce a more natural sense of stereo space than studio tricks using effects, EQ, delays, reverb and panning.
It is also important to note that simply using two mics and panning them hard left and right respectively around the stereo field is not necessarily a true stereo recording technique. Intention and image integrity are both key. For instance: using two microphones to record a guitar is not a true stereo recording technique if one is placed close and the other is a distant room mic. because there is no attempt being made to capture a natural sounding stereo image. It may, however, sound wicked cool.
Phase and Interference
As mentioned several times already in this series, time delays between two (or more) paths of the same sound can cause intense interference and comb filtering. When a pair of mics is used to capture the sound of an instrument, and difference in distance between these mics and the source can cause just such problems. These issues are exacerbated if the two channels are not panned hard to opposite sides of the stereo field. If they are panned to center or summed to mono for any reason, the resulting filtering can be extreme. For this reason the methods outlined below discuss these issues. Special care must be given when choosing which techniques to use and exactly where the mics should go.
Over the years, a number of stereo miking techniques have become quite popular, and even earned their own distinct names.
Coincident (also known as XY) is when two directional microphone diaphragms are placed as close together as possible and angled so that one picks up sound more from the left, the other from the right. The angle can vary from 60 to 120 degrees depending on the size of the instrument or ensemble, the extent of stereo spread intended, and the distance of the mics from the sound source(s). Since the mic diaphragms are very close together, there is little difference in the arrival times of sound from any angle. This makes coincident setups quite phase coherent and mono compatible.
When the diaphragms of two parallel microphones are separated by more than a couple inches, they are referred to as a spaced pair. The distance between the mics can be used to further accentuate the stereo image, encompass a larger sound source, or emulate the spacing of the ears. This feature also makes the spaced pair method less mono compatible.
Varying the distance between the microphones, or changing their position relative to the sound source will affect the exact frequencies filtered. Experimenting with these elements can help find the best microphone positions for a given studio situation.
When two mics are separated from each other (but kept within a couple feet) and angled like a coincident setup, they are said to be near coincident. This method adds to the flexibility of the spaced pair by allowing angling of the mics to help determine the stereo spread. Like the spaced pair, the near coincident method can exhibit mono compatibility issues. Since the mics are generally kept closer, however, the phase issues do not extend into the low frequencies. Again, only by experimentation and careful listening can the best angles and positions can be found.
The Blumlein technique is specific coincident method using two bi-directional mics. These two figure eights are placed close in a perpendicular fashion (90 degrees). While the front portion of the pickup patterns accentuate the stereo image of the performer, the rear portions add more of the acoustic context of the room. Since the diaphragms are as close as they can be, this technique tends to pass the mono compatibility test well.
The MS technique (short for Mid-Side) is another variety of the coincident setup. The cool aspect of MS is that it allows for electronic control of the width of the stereo image through cancellation. To accomplish this, a cardioid microphone is faced forward while a figure eight faces to the sides. When recorded directly to tape, these signals are said to be encoded and will not have any real stereo imaging if used “as is” without being decoded.
There are numerous devices, both hardware and software, which will automatically decode an MS signal. If one of these is not readily available, have no fear… almost every studio already has all the tools needed to do the job yourself. The cardioid signal should first be brought up into the stereo mix and panned to center (equally to left and right). The figure eight mic should be brought back into two separate channels and panned hard left and right respectively. Invert the phase of the left channel and group these two channels (so that the level of each is always equal). The more of these figure eight channels you bring into the mix, the wider the image will be.
This technique uses the interference between the bi-directional and cardioid patterns to separate out the left and right information. Since the two figure eight channels are 180-degrees out of phase, each channel cancels out exactly the opposite information as the other.
A near coincident method adopted for use in French radio broadcasts, ORTF uses two cardioid microphones. They are spaced 17cm apart (to approximate the spacing between our ears) and are angled outwards at 110 degrees. The basic idea here is a rough emulation of the human method of stereo perception.
Another method of emulating human hearing, called baffled omni, takes into account some of the effects of the head itself. The human head can actually act as a barrier between the ear and sounds coming from the opposite side. Furthermore, since lower frequency sound can actually bend around the head to get to the ears (called diffraction) this effect varies by both angle and frequency (not to mention the size of your schnoz).
Dedicated Stereo Microphones
To facilitate making stereo recordings, many mic manufacturers offer dedicated stereo mics with two diaphragm elements built into a single unit. Some models include selectable patterns and angles, while others are fixed into a particular configuration. Most stereo microphones are variations on coincident designs including basic XY, Blumlein and even MS.
When using a pair of microphones to record in stereo, getting both of them (and their stands) into a proper and stable position can be challenging. Using a special device called a stereo bar allows a single stand to be used and makes placement both simpler and more secure. These bars come in two basic varieties: those with fixed wings and those with adjustable ones. While fixed bars offer the greatest ease of use and consistency, the moveable ones are much more versatile. Most of the major stereo techniques are possible using these bars including: coincident, near coincident, MS, Blumlein, ORTF, baffled omni, and closely spaced pairs.
As with all mono microphones and mic techniques, windscreens, pop blockers, and shock mounts are also handy accessories.
OK, now that stereo microphone techniques have been covered, the more complicated scenarios of miking drums and pianos can be addressed.
Pianos: The Grand
Grand pianos, whose strings are situated horizontally, seem to be deceptively simple instruments but are notoriously difficult to record convincingly. This is due to several acoustical factors: the size of the resonating area (20 to 50 or more square feet of resonating area), number of strings (usually over 230), expansive frequency range (ca. 28 to above 20,000 Hertz), and the tremendous sonic influence of sharp transient attacks (believe it or not the piano is classified as a percussion instrument).
The genre of the music being played on a piano also has a lot to do with the timbral and acoustical signatures desired. Classical music usually calls for a flat frequency response and a strong sense of acoustic space. Popular music, on the other hand, can utilize a wide range of sonic qualities depending upon artistic whims of the producer. Piano sounds here can range from aggressive, sharp and “in yer face”… to mellow… to awash in reverb. Microphone choice and placement techniques can help recording engineers to capture the full range of these sounds from a single instrument.
The sheer size of the piano and it’s resonating areas makes it very difficult to achieve balanced results by close positioning a single microphone. Generally, two or more microphones are best to capture a more balanced picture of the instruments tones and formants. One notable exception to this practice is the use of a single PZM boundary-style microphone attached to the bottom of the lid. The resonating lid acts as a sort of collector for the mic and, in a way, extends the pickup area of the mic element. Unlike many of the other close-miking techniques, in this case the lid can be either open or shut. Some people have even experimented with attaching the mic to the bottom of the soundboard… below the piano!
If only a single mic and/or track can be spared for the piano, a condenser could be placed just inside the piano, under the open lid towards the end of the strings around the A above middle C (440 Hz). This should be near the junction where the strings for the bass and middle notes intersect. Placing the mic about 12 to 16 inches up from the strings and angling it slightly towards the hammers will help balance out the sound. Of course, a coincident pair works even better in the same location.
An XY pair a foot or two outside the piano (lid open) located around the middle of the right side curve can achieve a more even tone. Placed around 16 inches to two feet above the rim of the piano, both direct sound and reflection from the lid are captured. This configuration is especially good for ballads and modern jazz styles. If the mics are moved further back from the instrument, an even smoother balance is achieved as well a greater sense of the acoustic space.
A close-miked popular music style sound can also be achieved by using a coincident pair above the hammers, centered with in the middle of the keyboard. A six to nine inch distance is standard. The mics should be pointed downward, but the angle between them can be used to adjust balance and pick up more or less of the pianos register extremes as needed. A spaced pair centered on either side of g4 (in the middle of the treble clef) will can produce an even more balanced tone. Unfortunately, it will likely have obvious phase issues, especially when summed to mono.
A spaced-pair technique, which can work well, places the two mics around 9 inches above the treble and bass strings respectively. The treble mic is situated over the a5 strings (first ledger line above the treble clef) about halfway down the strings. The bass mic is located above the center of the overlapping bass and middle strings section, much further away from the keys than the treble mic.
When the piano must be recorded at the same time as other loud instruments, and leakage between those instruments mics becomes a problem, there are a few things you can try. First, the closed lid PZM method above offers a very good solution. Placing heavy blankets over the open lid and forming an isolation tent around mics placed within the piano also works pretty well. Finally, don’t forget that microphone directionality, room placement, gobos, or the magic of overdubs can also aid you with this tricky problem. (see TCRM #9)
Pianos: The Upright
Because there are somewhat more limited options as to how to get at the strings and soundboard, the upright piano is a bit easier to deal with than the grand.
With the lid open, an XY pair can be placed centered over the open back just a few inches above the rim. A spaced pair can also work well when placed in a similar fashion. One mic should be used above the bass strings and the other above the treble. In both cases, removing the front faceplate will help reduce reflections if desired.
With the faceplate off, an XY or spaced pair can be positioned in front of the piano. In these positions, care should be taken so that they do not get in the way of the performer or get bumped.
Finally, a PZM can be attached to the back of the soundboard near the center. This acts in a similar fashion to the grand piano method described above. A spaced pair can also be used to capture treble and bass off of the back of the soundboard. In both cases it is especially important to avoid reflections off from nearby walls that can easily build up between them and the large parallel surface of the piano back. This can be accomplished by angling the piano, facing the back towards the center of the room, or using blankets and/or gobos to absorb those pesky reflections. As with the grand, blankets can also be used to avoid leakage by building a small iso-tent over the mics and top of the piano.
The Drum Kit
Before the discussion of how and where to place which mics around or above the kit begins I must take a moment to underline a few key points. First of all, it is hard to get a good sound from a mistuned or crappy drum kit (never mind the skill of the drummer). Tuning is an all-to-often ignored part of the session. Time should be taken to tune all drums before mics are set up and recording begins. If you are not already familiar with these techniques, take some lessons or get pointers from a drummer you know who always seems to have a great tone. Even if you have to shell out a few bucks… the time and money to gain drum tuning skills will be well worth it.
Another important factor to consider is what perspective you want of the kit. There’s the sense of the kit from the drummers perspective (from behind) or from the normal listeners position right out front. The left to right orientation of the kit is exactly opposite depending on which of these is chosen. Once the decision is made, great care should be taken to keep track of which microphones are left and which are right. It will be hard to get a coherent, clear sense of the overall set if these get confused. Mixing down will be both frustrating and tiring.
Those Ubiquitous Overheads
While many people are going crazy these days putting one or more close-positioned microphones on each and every piece in a drum kit, the art of the stereo pair (or even single miking for mono) is increasingly being overlooked. Less is sometimes better: especially when it comes to leakage and interference issues on drum tracks.
Many great sounding stereo drums tracks have been recorded using only a stereo pair, or a pair plus one or two close-miked key instruments (kick, snare, possibly hi-hat) for good measure. In quite a few cases, mixes have also been saved by muting half of the recorded drum tracks, and working back from the overhead pair. For these reasons, and because the overhead pair is relied on so heavily to pick up the cymbals, great care should be taken to choosing and placing these microphones. Listen to the results a particular placement is achieving and move them or reconsider the methods used until you get the basic tone you’re after.
Hung over the center of the kit, a coincident pair can be used in a basic XY setup or in either Blumlein or MS configurations. In most cases, these microphones are small diaphragm condensers. Large diaphragms can also be used when a more “classic”, less bright or “sharp” quality is desired. It should also be noted that some of the most recent large diaphragm designs use materials and dimensions which make their transient response close to that of some smaller diaphragm designs. Sonically, this places them somewhere in between the traditional sounds.
Hoisting the mics higher above the kit can give a better balance overall, but the image will narrow and the immediacy of the sound may be lost. Also consider adjusting the microphones front-to back position. Depending on the room, player, and kit the best sound may come from over (sometimes even behind/above) the drummer or slightly in front of the kit.
Some engineers prefer to use a spaced pair, with one mic above the right-hand side of the kit and the other above the left. By adjusting the angling and exact locations of the pair, a very good picture of the kit can be created. This is especially true if they are hard-panned to opposite sides of the stereo image. Unfortunately, this technique often proves to be less than mono compatible.
Well, the rest of the kit will have to wait for TCRM 12. A thorough outline of the acoustics and methods for recording the separate pieces of the trap set is more than can fit here. Next time we will also discuss methods for recording that most fickle creature… the singer.
John Shirley is a recording engineer, composer, programmer and producer. He’s also on faculty in the Sound Recording Technology Program at the University of Massachusetts Lowell.
Supplemental Audio Examples
TCRM 11 discusses miking techniques for piano and drums, and focusses heavily on stereo techniques, so the following examples are demonstrations of how mic choice and placement affect some common examples of these. Special thanks to Connor Smith for helping run the sessions as well as playing the piano. Thanks also to Kevin Martin who played who played the drums for these recordings.
Piano - Stereo Techniques
First, let's compare two recordings of an electric guitar miked with an SM57 cardioid dynamic microphone. First right in front of the center of the speaker cone and very close to the grill; then at 20-inches back from that same spot.
Piano recorded in XY with SM81s at about 1' back from the open lid (Pic 1): TCRM11_1.wav
Next, a close xy pair of TLM103s over the hammers (TCRM11_pic1): TCRM11_2.wav
Now, the ORTF pair of AT4041s a bit higher and about 4' back (Pic 1): TCRM11_3.wav
A Blumlein pair is created using KSM44s (Pic 1): TCRM11_4.wav
Finally, U89s are used to do MS (Pic 1). In MS, decoding allows variable image width and room tone. (see pic 2)
MS with Side channel at -12 dB and middle at unity: TCRM11_5.wav
MS with Side channel at -6 dB and middle at unity: TCRM11_6.wav
MS with Side channel at -3 dB and middle at unity: TCRM11_7.wav
MS with Side channel at 0 dB and middle at unity: TCRM11_8.wav
MS with Side channel at -6 dB again, but not decoded properly. Compare this to TCRM11_6.wav: TCRM11_9.wav
Piano - Mono Techniques
Next, we will listen to a few mono techniques. Some were recorded at the same time as the stereo examples, a few are from a different session.
First, a PZM mic is taped to the lid of the piano. (see Pic 1. Pic 3 is easier to see, but the other mics are not set up correctly yet, so ignore them.): TCRM11_10.wav
A TLM103 smack in the center of the open lid, inside the piano. (Pic 1): TCRM11_11.wav
A DPA4006 omni-directional mic used as a room mic: TCRM11_12.wav
A TLM103 angled towards the lower strings, over where the strings cross. (Pic 4): TCRM11_13.wav
A TLM103 in the middle register closer to, and angled towards, the strikers. (Pic 4): TCRM11_14.wav
A TLM103 about 4' back from the open lid. (Pic 5): TCRM11_15.wav
A TLM103 about 15' away used as a "room mic": TCRM11_16.wav
Drums - Stereo Techniques
(see Pic 6)
TLM 103s are now used to capture an XY image of drums: TCRM11_17.wav
A near-coincident is captured using SM81s: TCRM11_18.wav
A spaced pair technique using unidirectional TLM 103s: TCRM11_19.wav
A spaced pair technique using omnidirectional dpa4006s: TCRM11_20.wav
Blumlein is created using KSM44s: TCRM11_21.wav
ORTF is captured using AT4041s: TCRM11_22.wav
Again, variants of an MS image using U89s...
MS with Side channel at -15 dB and middle at unity: TCRM11_23.wav
MS with Side channel at -9 dB and middle at unity: TCRM11_24.wav
MS with Side channel at -3 dB and middle at unity: TCRM11_25.wav