Audio Tech In the Real World

NOTE: This material is also available on video via the "Audio Tech in the Real World (Cloidt lecture video)" page

MICROPHONES

Microphone types:

* CONDENSER mics require phantom power. (Phantom is a 48 volt signal which is provided either by the mixer through the mic cable, or by an internal battery in the mic). Plusses: accurate, excellent high-frequency response, sensitive. Minuses: somewhat delicate, can be harsh, can be overloaded, expensive.

* DYNAMIC mics (no phantom power required). Plusses: physically robust, can handle high SPL; tend to not be harsh, but more “rounded” in sound, generally less expensive than condensers. Minuses: Not as accurate across the entire frequency spectrum as condensers, less sensitive to high frequencies, tend to be larger than condensers.

* RIBBON mics use an extremely thin and fragile metal ribbon to pick up vibration. Plusses: sound is “flattering”, “smooth”, “warm”. Minuses: Physically delicate (ribbon can easily be damaged by rough handling), Electrically delicate (can be ruined by normal phantom power), not as accurate across the entire frequency spectrum as condensers,

* PICKUPS (piezo) are usually fixed directly to an instrument body, often using double-sided foam tape or non-residue putty. Plusses: high gain before feedback, excellent high frequency response. Minuses: unnatural sound, can be harsh, frequency response may be tilted strongly toward high frequencies.

Microphone Patterns:

* OMNIDIRECTIONAL (AKA "OMNI") mics pick up sound in all directions equally. This is the flattest microphone pattern, and the most “accurate”. However, this pattern also picks up the sound of the entire environment indiscriminately, so it often reveals too much ambient sound. Omnis are generally useless for PA work because of susceptibility to feedback (though omnis mounted directly on an instrument or dropped inside a bass or cello, etc., may have enough gain to overcome this).

* CARDIOID mics exhibit a “heart-shaped” pickup pattern. Cardioids are never perfectly directional, but will exhibit some rejection of signals from in back of the mic, and less so to the sides (e.g. “off-axis” positions). This directionality works best at higher frequencies and less well at lower frequencies. The directional pattern is achieved at the expensive of a less-flat frequency response. Cardioids are generally used for general purpose amplification and recording, where user wishes to emphasize a single instrument or area, rather than picking up the entire stage/room. Using a cardioid reduces the amount of ambience picked up by the mic.

* HYPERCARDIOID mics are an even tighter pattern than the cardioids. This is achieved by a tradeoff resulting in an even less flat frequency response than the cardioid pattern. Useful for controlling feedback or for situations where the mic can’t be physically near the source, such as theatrical “foot mics”, etc. Extremely directional hypercardioids are known as “shotgun mics”, and usually sound pretty bad.

* FIGURE-8 mics are most sensitive to sounds coming from directly in front of or behind the microphone (a drawing of the pattern resembles a figure-eight). Frequency response is nearly as flat as omni patterns.

* GENERAL MICROPHONE NOTES are mostly just good sense. Microphones are delicate and should be treated as fragile. Don’t force a clip or fitting onto the screw threads of a microphone–if it doesn’t go on smoothly, it’s either the wrong clip or needs to be re-started. You can lubricate sticky threads (on mics, or anything metal) by rubbing some graphite from the end of a pencil on them. Make sure that mic cables are wound on stands in such a way that if the clip fails, the mic will be caught by the cable in midair, rather than falling to the stage. Don’t over-extend mic stand booms–use the shortest extension you need for each mic; this keeps the center of gravity close and makes the stand less likely to tip. Gaff mic cables at the base of stands so that anyone tripping over the mic cable will not tip over the mic stand. It’s a good idea to put a mic down while you extend the boom, screw on the clip, and route the cable, then pick it up to finish–it’s awfully easy to drop a mic if you’re trying to hold it in your hand while you do all that.

AUDIO CABLES
* Balanced cables have 3 connectors: plus, minus, and ground. They are designed to reduce hum (ask for the long explanation of this amazing technology, if you want it). They are good for long runs, up to hundreds of feet without problems (though mic signals may suffer high-end losses from impedance on very long runs, if cable quality is not good; it’s best to preamplify mics to line levels before really long runs). Common microphone cords use XLR connectors; these are the most common balanced cables.

* The recommended limit for (noise-free) use of unbalanced (2-conductor) cables is 15 feet. Beware output cables from laptops which run through 25-foot or longer cables to the PA input. These are particularly vulnerable to noise in halls which have full theatrical lighting systems using dimmers.

* Unbalanced cables can be turned into balanced cables by using a DI box (DI = Direct Injection, but nobody calls them that). This takes an unbalanced input, usually a ¼” TS (tip-sleeve only) phone cable, and outputs a balanced XLR signal. DIs can be passive or active (meaning they are powered by a battery or by phantom power); active DIs are better. Some of them include additional isolated XLR outputs (this makes them “splitters”), which can be used to create two outputs so as to be able to route to both a recorder and a PA mixer with the two outputs safely isolated from each other. Some DI boxes also contain a ground lift switch. DIs are also sometimes combined in units of 2, 4, or 8.

* All cables which run through a public/audience area must be carefully gaffed. One must assume that there will be audience members who are blind, mobility-impaired, or in wheelchairs, and make sure the hall is safe for them. There is a cable cover unit at stage left in Hertz which can be used to cover snakes or other large cable bundles which cross the aisle.

* Audio cables must be coiled using the “hand over hand” method, otherwise they will become a kinked and tangled mess. Proper coiling extends the life of the cables and saves hours of frustrating detangling and frequent loud cursing when loading in. The method is a bit difficult to describe, but there are good demonstration videos at: http://www.youtube.com/watch?v=3j1Wdc-ymbI and at: http://www.youtube.com/watch?v=lLwwB29uQRg&feature=related

* Concerts in Hertz Hall are checked by a safety monitor. Large numbers of un-gaffed cables in a concert setup will be noticed and reported; this is a legal issue, not an esthetic one.

* Gaff (tape down) cables sideways, never lengthwise. The latter are a tremendous headache to un-tape.

* Use no tape on cables, ever, except gaffers’ tape (temporary) and white board tape for labeling (temporary); remove all tape when project is finished and gear is returned.

* Duct tape is not gaffers’ tape; and the quality of gaffers tape varies; good-quality gaffers tape costs $15-20 per roll. Cheap tape ruins cables.

* Don’t use plastic cable ties on cables. They’re difficult (and slow) to remove. Use only tie-line (black cord) or Velcro cable ties, please.

* Cables onstage should always be gaffed to the stage. No cables should cross the stage “in the air”. Seeing your equipment pulled off the table because someone can’t see your patch cable and walks through it will cure you of this forever, if this warning doesn’t.

* Any cable in a performance setup which can be kicked, stepped on, or accidentally pulled out will be, eventually.

A.C. POWER
* Hum in PAs is usually caused by too many ground paths in the audio system. (A brief explanation: AC from one outlet in a building may have a DC offset from power from another outlet. The difference in potential between the two manifests as a 60-cycle hum). Beware of power pulled from two or more points in a building. Beware built-in house snake patching (in most halls), which may have additional ground points. To fix a hum, try a ground-lift AC plug first. XLR ground-lifts are also sometimes useful (but not usually).

* Having said that, all of the AC in Hertz Hall seems to be free from hums and buzzes. You can plug gear in all over the stage (and throughout the house) without hum problems.

* Most buzzes (especially buzzes that change as you physically move modules or wiring) are caused by not enough ground paths. This occurs most often when cabling audio using unbalanced cables and AC plugs with ground pin lifted or missing. Look for 2-pin AC plugs and replace them with a 3-pin grounded plugs.

* Never cut a ground pin on a 3-pin plug. Ever. If you need to remove the ground, use a ground-lift adapter plug.

MIXERS
* Power-up and power-down order: Always turn speakers on last, and turn them off first. This will protect them from pops and transients from cable patching and equipment turn-on thumps, etc.

* Initial mixer setup (before turning on amps):
- Turn phantom power on (or off), as needed
- Flatten out all EQ and hi-pass filters, pans, assigns, etc.
- Turn output inputs and outputs down to zero.

* Test outputs: It is a good idea to test your amp and speaker system by plugging in CD playback into a pair of line inputs and making sure that the mixer and outputs are functioning, before plugging in input sources and testing them.

* Test inputs: Plug in mics and line inputs to the board and use the solo button to listen and make sure that audio coming in is undistorted, before moving on to the next step.

* Gain staging: For live sound, it is advisable to have both the master faders and the input faders set so you can use their full travel. With the input preamps turned down, bring up the master fader and then the channel fader. Then, slowly bring up the mic preamp level until the signal is approximately loud enough (using the solo button to make sure you don’t turn it up past the point where the input clips). Switch in mic pads if necessary (and available on the mixer). While you are “giving away” some usable gain at the inputs, you will end up with board settings that are practical. (You don’t want to have the whole dynamic range of a channel controlled by 1 inch of travel on a fader).

* Using computer audio interfaces without a mixer: It is tempting, for those of us who work mostly in studios, to try to patch from a laptop with audio interface and then directly into powered speakers for live performance. This is not a good idea, because: 1) If a mic feeds back or an audio source starts pumping out white noise at full volume, it is safer to have a physical fader which can be grabbed quickly to mute it. And 2) powered speakers like the Meyer UPL-1s in Hertz Hall have no input volume control. This means that you may have to turn the output of your interface ‘way down in order to adjust for the level you need. That can lead to a noisy output; and it also means that if the audio cable is unplugged, the signal pop in the PA speakers at full volume, which can blow drivers.

* General mixer notes: find out where the feedback point of the system (that is the PA with all the line and microphone channels open) is during soundcheck. (Open all the faders and gradually and carefully bring up the master until you reach the ring point; note it). While a full house may give you some extra headroom before feedback at the actual performance, it is best to know at what point the system becomes unstable before the show, not during. This exercise is also useful for checking for potential feedback trouble spots (a monitor pointed directly at a microphone, for instance).

POWERED SPEAKERS
* Power-up and power-down order: Always turn speakers on last, after all equipment is patched and powered up; and turn them off first, before any equipment is powered down or unpatched. This will protect them from pops and transients from cable patching and equipment turn-on thumps, etc. which can damage the speakers or shorten their lives.

* UPL-1 stands: The Meyer UPL-1s have Ultimate speaker stands which are stored in the stage left area of Hertz Hall. Always use two people when mounting or dismounting the speakers from the stands. Tighten the eyebolts securing the speakers to the mounting plates, and make sure all the stand fittings are tight (this is an earthquake precaution, and just good sense). The stabilizer bars at the bottom of the stand should be set so they are parallel with the stage floor, not tilted up or down.

* UPL-1 power: Turn the Meyer power switches off before mounting them, and then plug them into an AC floor pocket in the stage. IMPORTANT: when tearing down the speakers, ALWAYS flip the covers to the stage AC pockets back down flush with the floor. These can trip people, and have done so in the past.

* UPL-1 volume: note that the UPL-1s have no volume control, so they should always be used with a mixer.

* UPL-1 angle of coverage: The angle of coverage of the UPL-1s is 90° horizontal by 40° vertical. Keep this angle in mind when positioning and aiming the speakers. Generally, the farther onstage (and closer to the performers) the speakers can be placed, the better the coverage and the audio imaging. Sightlines may require a compromise with the best audio position, especially if the house is heavily sold.

* UPL-1s on carts: There are 2 additional UPL-1 powered speakers chained up backstage which can be used onstage as PA speakers or sidefills. They are the same as the free-floating UPLs, except that they are locked onto (quite large) rolling carts.

* UPM-1 free speakers: Hertz Hall has 3 floating Meyer UPM-1 speakers, which can be used either as monitors, or on stands as a main PA (with or without a subwoofer). The angle of coverage of the UPM-1s is 90° horizontal by 40° vertical–keep these angles in mind when positioning and aiming the UPMs for use as main PA speakers. The UPM-1s have neither volume controls nor power switches, and they use special Meyer power cables with blue locking connectors on one end and regular Edison AC plugs on the other.

* UPM-1 proscenium speakers: Hertz Hall has an additional 2 Meyer UPM-1s mounted on the side walls of the stage, left & right. These are wired to the audio rack at stage right, which is used for CD playback in rehearsals and classes, the “welcome to Hertz Hall” CD played at beginnings of concerts, etc. But they can also be used for concert playback. They can be accessed via jumper XLR cables (on the deck right below the speakers).

* UPM-1s as monitors: The UPM speakers can be used with the board aux sends as monitors. Monitors are usually run on pre-fader aux settings so that monitor levels don’t go up and down as the mixer changes levels during performance; but sometimes it is useful to use post-fader sends (for things like effects returns, CD or computer playback that needs to be faded out all the way, etc.) The UPMs each have 2 input/output XLR connectors, one male, one female. This can be used to chain multiple monitors together on a single send, etc. (Note: these XLRs are parallel connections; that is they are wired directly together, and either one can be used as either input or output.)

* Multi-speaker playback: Because of the raked seating, the need to keep fire aisles clear, and the lack of suitable speaker locations, it is very difficult and labor-intensive (though not impossible) to do surround speaker setups in Hertz Hall. It is quite easy to set up multiple speaker locations on stage, however.