If you play live regularly, or plan to, or if you ever engineer for live acts, you should be aware that many different miking and DI'ing scenarios can occur on stage. As an example, for small gigs you may get away with miking up only the vocals and perhaps acoustic instruments such as guitars, but for medium to large venues, you really need to mic up the drums and backline as well. This article aims to examine effective ways of tackling the common situations you might come across.
Mic Choice: When you're miking vocals on stage you'll probably find that there's seldom room for a pop shield. To avoid spill problems, the singer will invariably work very close to the microphone, so studio capacitor models tend to be less than ideal, as without a pop shield the level of popping with this type of mic would be unacceptable — and the moisture from the singer's breath would also probably incapacitate the microphone within minutes. Either a dynamic cardioid (directional) or a capacitor cardioid model designed specifically for stage use will produce the best results, while being less susceptible to popping.
Mic Positioning: Ideally, the mic should be mounted on a stand, as a fixed location gives you more scope for dealing with feedback problems, but if the singer wants to move with the mic, all you can do is keep your hand on the mixer fader and anticipate feedback problems. Keep an eye open for singers who insist on holding the mic around the bottom of the basket, as this can block the rear sound ports and make feedback problems worse.
To avoid excessive spill, try to position the vocal mic so that there are no loud amplifiers or drum kits directly in its path, and so that any stage monitors in its vicinity are correctly placed to capitalise on the mic's 'dead zone', where it picks up sound much less efficiently (in the case of a mic with a cardioid pickup pattern, towards the rear of the microphone).
Feedback Suppression: In very difficult cases, where the singer does not have a particularly loud voice or great mic technique, a dedicated feedback suppressor can help gain you a few extra decibels before feedback sets in. These systems work by recognising frequency spikes in the signal and then assigning deep, narrow notch filters to those frequencies. Most such systems have multiple filters, some of which are set and locked during the soundcheck and the rest of which roam during the performance to deal with trouble as it occurs. Of course, the feedback has to be audible before the system can recognise it as such, but the response time of a good feedback destroyer is impressively fast.
In the studio, you can spend as long as you want experimenting with mic positions, but at a gig you normally have to choose a mic position fairly quickly and sort out any tonal issues on the console using EQ.
Mic Positioning: One thing you should definitely avoid is hanging a dynamic mic by its cable in front of the speaker, as this presents the sound to the side of the mic and usually compromises the tone. Instead, pick a dynamic cardioid (Shure SM57s or similar are good in this role) and use a stand to aim it directly at the centre of the speaker, with the mic almost touching the grille cloth. Do a quick test to see if the sound is right, and if you find it's too bright move the mic sideways towards the edge of the speaker cone, where it should sound warmer and fuller.
If the cabinet has more than one speaker in it, see if any one speaker sounds better than the others, but only if you find you have the time. If you work with the same amp on a regular basis, make a note of the best mic position.
DI Instead: Another approach is to DI the guitar. However, in most cases the sound taken direct from the preamp output is disappointing, as it bypasses the tonal contributions of the power amplifier and speaker cabinet. A far better option is to use a dedicated guitar DI box that includes filters to emulate speaker response and can also be connected directly to the extension speaker outlet of the amplifier. This type of DI box is available in both active and passive versions and provides a balanced output that can be connected to the stage box in the same way as a regular microphone. I find the passive Palmer Junction box effective in this role but there are many other options.
DI'ing in this way may be a good option where the guitar amplifier is physically small and produces a thin or nasal sound, as the DI feed will make it sound as though a larger cabinet has been used. Speaker-simulator filters are particularly important for overdriven guitar sounds, which can be harsh and raspy when DI'd with no filtering.
If the amp has two speaker jacks (wired in parallel), and only one has a speaker plugged in, you could simply plug the DI device into the spare speaker jack. Although the impedance of your DI or speaker-sim box can effectively be ignored in calculating the total load, watch out for amps where self-switching jacks are used for the speaker outputs; plugging in a second jack will sometimes activate a different tap on the output transformer. With combos, also make sure that plugging anything into the extension speaker jack doesn't disconnect the internal speaker! Most of the classic Fender valve amps, however, and their many design derivatives, have a simple parallel connection for their extension speaker sockets, actually forcing the amp to run into a mismatched load when used with both internal and external speakers, but creating the ideal connection for a speaker-sim DI.
If no other satisfactory option is available, you can solder or clip a spare jack socket across the speaker terminals in a typical open-backed combo. Having said that, many simulator/DI boxes have a speaker Thru feed, so all you need do is insert the DI box between the amplifier output and the speaker, then use the balanced DI output to feed the PA mixer. Where a ground-lift switch is fitted to the box, it may be useful to use it, to reduce ground-loop hum.
Bass Guitars: Bass guitar amplifiers can be miked up in a similar way to conventional guitar amplifiers, though the mic is usually set up a few inches from the speaker grille, rather than being hard up against it. Many general-purpose dynamic mics have a built-in low-frequency roll-off to compensate for the proximity effect when used with vocals, so pick a mic with a good bass response, such as a Sennheiser MD421.
Again, listen to each speaker in turn and pick the best-sounding one, specifically avoiding any that rattle. As with guitar-amp miking, moving the microphone off-axis will produce a more mellow tone.
Because bass speakers can sound less than optimal when being driven very hard, taking a DI feed is often a safer bet. You don't have to use speaker emulation in this case, though it can help to avoid fret noise and finger squeaks. Many bass amps now have a balanced DI feed; if yours does you can plug it into your stage box. Where there is no direct output, a DI box must be used.
Drum miking can be a problem, not least because many drummers don't spend enough time learning to tune their kits properly. A good drum sound starts with a decent-sounding drum kit. There's little you can do about it if the kit you have to work with doesn't sound as good as it could, but at least drums are generally so loud that you never need to worry about feedback.
Miking The Kit: At one time, miking a drum kit was an expensive business, but now there are complete drum-mic kits made in the Far East that deliver a very acceptable level of performance for astonishingly little money. The most convenient type to use for live performance provides similar clip-on mics for the snare and toms, a boom-mounted kick-drum mic, and a pair of back-electret or capacitor mics to use as 'overheads'. The clip-on mics need to be small enough not to be in the drummer's way. When looking at a set, consider how much space they will take up when the mic cable connectors are plugged in. The clips, which attach to the drum rims, usually put the mic close to the optimum position, which is roughly 5cm above the drum and 5cm in from its edge.
Hi-hats: Separate hi-hat mics are rarely necessary, but where one is needed, use a small capacitor mic and aim it just above or below the hi-hats, about 10-20cm away, so that the air forced out when the cymbals close doesn't rush straight into the front of the mic.
Kick Drum: The kick-drum mic is invariably a dynamic cardioid, but with extended bass response and a frequency-response curve tailored for kicks. This mic should be placed inside the drum shell, pointing towards the spot where the beater hits, but some adjustment of position may be necessary to obtain the best tone. Most kick drums used for pop work have a hole cut in the front head, so a short boom stand can be used to position the mic through this hole. A damping blanket is usually placed on the bottom of the drum to reduce unwanted ringing.
Cymbals: To capture cymbals we use the overhead stereo mics (capacitors, to capture the high end), but being further from the kit than the other mics they may pick up sounds from other sources on stage, so the distance arrived at will be a compromise between the best sound and adequate separation. A spacing of as little as 60cm above the cymbals can yield acceptable results, though a slightly higher position is best if spill conditions allow (see photo, right).
* Getting The Balance Right: Some studio engineers base their drum sound on the overheads and then use the close mics to achieve balance, but live engineers are more likely to use the close-miked sound as the basis of the kit mix, then add the overheads to bring out the cymbals. A low-end EQ cut on overhead mics can clarify a mix by reducing the level of LF spill.
In my experience, the biggest problem when miking drum kits is that the lower tom heads ring excessively when any other drum is hit, which tends to cloud the overall kit sound. This can be improved upon by using gates on the tom and kick mics, but you have to be very careful, when setting them up, not to exclude intended quiet drum hits.
* Percussion: Congas and similar hand percussion instruments are best miked in stereo from above, 30-60cm away, again depending on the instrument and the spill situation. A small-diaphragm capacitor mic will produce good results on most types of percussion. Suitable models are now no more expensive than decent dynamic mics.
It always helps to set a vocal level that gives you five or six decibels of headroom before feedback, then balance the rest of the band to the vocals. If you try to do it the other way round, you may find that the vocals get totally swamped.
Except for all-acoustic gigs, miking acoustic guitar is usually disastrous, because if you get the mic close enough to keep spill to an acceptable level, the player's movements cause a significant change in tone — and you probably won't capture an accurate sound anyway. It's better to DI, though the quality of the end result depends very much on the quality of the pickup system fitted to the guitar. Most acoustic pickup systems rely on piezo-electric transducers under the bridge saddle. These feed an onboard preamp with a balanced output and an unbalanced output, so that you can feed it both to an on-stage acoustic guitar amp and the PA mixer. These pickups can sound hard and scratchy when compared with the natural guitar sound, so you may need to apply fairly radical EQ to get them sounding good. A number of specific acoustic-guitar effects pedals are available that provide suitable EQ treatments and compression, as well as effects such as reverb and delay. These can significantly improve the sound of a basic piezo pickup system.
The better pickup systems combine either a piezo or a magnetic pickup system with a miniature microphone mounted inside the sound-hole of the guitar. There's usually a control to blend the contributions of the pickup and the microphone. Judging by the ones of this type that I've worked with, far less 'salvage' EQ is needed to get a good sound out of them. Incidentally, where EQ is needed with acoustic guitars, it's usually low mid-range cut (150-250Hz) to tame the 'boxy' element of the sound.