Many years ago, not long after I got serious about recording, a friend and fellow musician played me a song that she had just recorded. I remember being knocked out by the sound of her acoustic guitar. I had heard her guitar before because I had recorded it myself, but I had never been able to make it sound like that.
What was the secret? Did the engineer have a special microphone that worked magic on acoustic guitar? Was it something he'd done in the mix? What was it that made that guitar track sound so delicious?
A few months after hearing that recording, I got a call to play drums for a session. As luck would have it, the same engineer who had recorded my friend's song was behind the board. After the session, I asked him about what he'd done to get that amazing sound from her guitar. To my surprise, he said he'd used a microphone that I owned at the time and compressed the track with a compressor that I also happened to own — and it wasn't a particularly notable one, either. He insisted that the mic preamp was nothing special, nor was the recording space.
Now I was really stumped. What had he done to get such a great sound?
Then he let me in on a little trade secret. “The trick,” he said, “is in finding the sweet spot. You have to fiddle around with mic placement. Sometimes a small change can make a big difference.”
A big difference, indeed. Since that day, “find the sweet spot” has been my guiding principle in recording. I've explored that principle for many years now, and I'm still exploring it. During the course of that exploration, my recording chops have improved considerably. What follows are some thoughts, tips, and techniques based on what I've learned about finding the sweet spot.
WHAT IT MEANS
Let's start by identifying terms. What exactly is a sweet spot?
The definition is fairly self-evident: the sweet spot is the spot where the sound is the sweetest, or the best. More specifically, it's the location from which the ear can hear the most favorable balance of sound elements: lows, highs, and mids, as well as ambience and dynamic qualities.
The sweet spot is not merely a fixed location awaiting discovery — if that were the case it could be found by a machine. The sweet spot represents an artistic choice. By finding the sweet spot, you are defining what you consider to be the most desirable balance among multiple interrelated ranges of sonic elements. In fact, that is something you might intentionally vary from song to song, depending on the arrangement and the mood you're trying to create.
Every mic has its own way of picking up sound: what sounds sweet to one might not sound sweet to another. It helps to be familiar with your tools, particularly any special qualities that they may have that will affect finding the sweet spot.
A key factor is the mic's polar pattern, which determines the shape and size of its pickup area, as well as its null-point zone (the region where the mic rejects sound). In general, the more directional the mic, the more finicky it will be about close positioning. When I say that a small change in mic positioning can cause a big change in sound, I'm mainly referring to directional (cardioid, supercardioid, and hypercardioid) and bidirectional (figure-8) mics. That's partly because directional and bidirectional polar patterns naturally exhibit bass boosting from the proximity effect. Within a certain range (usually less than a couple of feet), the proportion of low-frequency content increases — often dramatically — as the mic is moved closer to the source. Bidirectional mics are the most prone to the proximity effect, followed by hypercardioid, supercardioid, and cardioid.
The proximity effect can be both a blessing and a curse: on the one hand, it can be used to enhance low end, which is helpful in the case of a thin-sounding source; on the other hand, it might hamper your efforts, for example, at close-miking an especially boomy-sounding acoustic guitar. A directional mic at close range acts as an equalizer, allowing you to fine-tune the proportion of low-frequency content in the signal. The mic's null area is a potential ally, allowing you to block out, or at least minimize, sound coming from other parts of the recording space.
Another potential aid in directional mic positioning is off-axis coloration (see Fig. 1). Although it's generally regarded as a shortcoming in a microphone, off-axis coloration is commonly exploited to equalize the sound. When you turn a directional mic just a little bit in different directions, you're making adjustments to equalize or improve the sound. Such incremental moves are the heart of mic positioning. Some manufacturers have even been inspired to offer rotating-head mics (see Fig. 2). Given the combined power of proximity effect and off-axis coloration, it's not far off the mark to think of a directional mic as a giant tone knob.
By definition, omnidirectional mics pick up sound in the same way in any given space. The capsule's pickup area is all around the cap, much like a human ear's; it has not been “shaped” to pick up sound from only one direction while blocking out the rest. Because omnis don't distort the surrounding sound field, there's typically little or no off-axis coloration and proximity effect. (For more about omnis, see the sidebar “Know Thine Omni.”)
The result is that omnis tend to be less finicky about positioning than directional mics: sweeping an omni in small increments from side to side yields less tonal change than what you get from a directional mic. You can position an omni as close as you want to the source with little or no unwanted bass buildup. I have captured stunning acoustic-guitar tracks using an Earthworks QTC1 omni (see Fig. 3) positioned only an inch or two from the strings and aimed directly into the sound hole of an acoustic guitar — a position that would hardly qualify as “sweet” for any directional mic.
Although a mic's polar pattern generally exerts the greatest influence, other factors can also affect finding the sweet spot, such as whether the microphone is a dynamic or a condenser. Dynamic and condenser mics don't behave so differently in terms of positioning. Given equivalent polar patterns, a directional dynamic and a directional condenser will track the same way in close-miking situations, providing the same kind of tone control as you move and turn either mic.
But there are other differences that can figure into the equation. Dynamics are less sensitive than condensers and have more limited frequency responses: they can pick up sound neither as fast nor as far as their condenser counterparts. But sometimes those limitations can be advantageous. For example, the pickup patterns on dynamic mics don't extend as far as those on condensers, and dynamics provide better rear rejection, which is a godsend when you're attempting to record several musicians playing at once in the same space. In such cases, finding the sweet spots becomes as much about strategy as it does about tone: you listen not only for where each mic sounds best, but also for where each mic picks up the least amount of sound from the other instruments.
Now that we've considered key mic variables that affect mic selection and positioning, let's get back to finding the sweet spot.
My big revelation about mic positioning was slow in coming, due in part to my reliance on a variety of “instructional” photographs that presumably depict the best place to put the mic. For longer than I care to admit, I dutifully positioned mics as these photos instructed and lived with the results.
But there are a couple of problems with instructive photographs. One stems from the nature of photography: it's difficult to take photographs that accurately convey distances and angles. In my opinion, spatial exactitude can't be properly represented in a photograph. While photos are still useful in showing the general idea, don't rely on them for the critical task of finding the sweet spot.
The other problem is that microphones cannot see. That may seem obvious, but this fact led me to a profound discovery. During my formative years of information gathering — watching other engineers position mics, reading descriptions of mic placement, and poring over photos of miked instruments — I developed a visual orientation to microphone placement. In my experience, most people position mics using visual cues, based on the direction in which the mic is aimed.
The problem is that even if you're monitoring a mic while positioning it, the visual information interferes with the auditory information, obfuscating what you're hearing. I experienced more gratifying results when I closed my eyes and relied solely on my ears to position a mic. That's the secret: use your ears, not your eyes. Only then can you really concentrate and let the sound guide your hand.
If you're working alone, you can monitor the mic signal using a pair of high-quality closed headphones. After the preamp gain is set, loosen the mic stand or boom a bit so that the mic can be moved freely. Then close your eyes and listen for the sweet spot as you move the mic around in small increments. The perfect sound can sometimes come from the most unexpected place.
After you lock the mic into that magic position, note that it may visually appear odd or wrong. But don't mind how it looks, all that matters is how it sounds.
Although finding the sweet spot often comes down to moving the mic in tiny, precise increments, that's not the whole picture. The recording space has a sweet spot as well — the “instrument-room sweet spot.”
Unless I'm familiar with the space, I always start a recording session by trying to find the best-sounding spot for the instrument. To do that, have the musician play in different parts of the room while you both listen for the location that brings out the best from the instrument. Note that this is a relationship: the room's sweet spots exist only in relation to the sound coming from the instrument.
After finding the instrument-room sweet spot, start listening for the instrument-mic sweet spot. Some engineers recommend covering one ear so that your other ear hears a more monolike sound. I often listen first with both ears and then with one. The idea is to find the area (or areas) from which the instrument sounds best. That will be your starting point when placing the mics.
Once you've found the sweet-spot area, fixed the mic to the stand, set the gain, and monitored the sound using headphones (or monitors if you have a control room and an assistant), make certain that the musician plays the exact part that's going to be recorded and at the proper volume level. Sweet spots are contextual not only to the instrument, the mic, and the space, but also to the mood and arrangement of the song. If you're recording a cello part for a song, and the part requires only a few quietly sustained, closely grouped notes, then the last thing you need is for the cellist to be whipping through scales from low to high while you're trying to make subtle sweet-spot tweaks.
The sweet spot can be elusive, but it's always worth your best effort to find. It's not simply a location awaiting discovery; it exists at an intersection of relationships — between the room and the instrument, the instrument and the mic, and even the vision and the capabilities of the recordist. You have an active hand in shaping the sound.
Tools are best used in accordance with their capabilities. When it comes to fine-tuning mic positions, close your eyes — the sweet spot is much easier to find in the dark.
Brian Knave,former EM senior associate editor, has found some really sweet spots in his time.
FOUR STEPS TO LOCATE SWEET SPOTS
- Find the spot where the instrument sounds best in the room (the instrument-room sweet spot).
- Make sure the musician you're recording is playing the same (or similar) part — and at a similar volume — as what will be recorded.
- With the instrument in place and the musician playing, find the spot in the room where it sounds best (the instrument-mic sweet spot). Listen with both ears, and then cover one up to get a more monolike sound.
- Using headphones to monitor the sound, set up the mic in that area, and then close your eyes and move the mic around on its stand until you find the best-sounding spot.
KNOW THINE OMNI
If you're trying to find the most favorable positioning for an omni mic, it's good to be aware of certain strategic differences between the two common types: dual-diaphragm and single-point (one diaphragm) mics. Technically speaking, omnidirectional mics do not exhibit null responses. However, in practice they do, if only because of structural interferences from the mic bodies. A single-point omni (such as the Earthworks QTC1) will exhibit a bit of nulling directly behind the mic body. Likewise, a dual-diaphragm omni (such as an AKG C414 in omni mode) will exhibit nulling below the mic body, as well as around the imaginary plane separating the two capsules. (That is due in part to interference from the piece of metal joining the two sides of the grille cage.)
If you want to reduce sound coming from behind the mic — for example, when miking an acoustic guitar — a single-point omni would be a good choice. On the other hand, if you want to capture lots of room sound along with the guitar, a dual-diaphragm omni will capture sound equally from front and back.
Another point about single-point omnis is that they are the closest, both structurally and operationally, to the human ear. That makes them a natural choice for room miking — especially when you're after a natural sound. It's a subtle yet audible difference.