Search Gear
 

Master Class: Live Sound 101

February 1, 2012
share
Bon Iver performs in Nashville.

Bon Iver performs in Nashville.

There are certain live sound goals that every band should seek to accomplish, no matter where you are performing. First and foremost is making the lead instrument clearly audible to as much of the audience as possible. I don''t care how great your band is, if the audience can''t hear the main instrument—in most cases, the vocal—they won''t enjoy your performance and you won''t get a positive reaction. Next, make sure that you can accomplish the first goal without subjecting the audience to feedback. Nothing runs a crowd out of the room faster than a loud, high-frequency squeal coming through the P.A. system. You also want to present a coherent balance of instruments so that the audience “gets” your band, and it''d be great if you could do so at a volume level that doesn''t send musicians or audience members running for cover. Let''s take a look at some of the ways you can address these issues and get an edge on your live sound.

Fig. 1. Basic Layout of house speakers and mix position relative to stage and audience.

Fig. 1. Basic Layout of house speakers and mix position relative to stage and audience.

NUTS AND BOLTS
There aren''t many rules in the audio world, but here''s one: The house speakers (i.e. the P.A. speakers that the audience hears, as opposed to the stage monitors) must be placed in front of the band. A band member facing the audience should be able to see the rear panels of the house speaker cabinets (see Figure 1). I''m amazed by the number of times that I see a band with a P.A. placed behind them. This setup is just begging for feedback, because the speakers will have a direct path into the microphones. Even when you are setting up on the floor in a club that doesn''t have a proper stage, adhere to this rule. It will enhance your quality of life.

The second rule: Make sure that the audience can see the house P.A. speakers. With the exception of subwoofer cabinets, if the audience can''t see the P.A., they won''t hear it very well. This means that sitting full-range speaker cabinets on the floor is a no-no. Low frequencies produce long wavelengths that easily pass through and around objects, including human beings, so people standing in front of a subwoofer cabinet do not present significant obstacles. On the other hand, midrange and high frequencies have shorter wavelengths that beam, like a flashlight. A person standing in front of a high- or midrange speaker will block that sound just as they''d block the beam from a flashlight. As a result, overall volume is reduced and (since high-frequency sounds suffer the worst) the audience hears a muffled mix. One way around this issue is to put your full-range speakers on top of the subwoofer cabinets or on pole-type stands. Raising the full-range cabinets above the heads of the audience avoids beaming sound directly into someone''s ears and allows the speaker to “throw” the high frequencies deep into the room so that patrons in the back can hear a crisp mix. (That''s why concert sound systems are typically flown.)

That said, it''d probably be a bad idea to raise a subwoofer into the air because subs couple with the floor, enabling the cabinet to be more efficient, produce deeper bass, and throw that bass farther into the room. Stacking subs and/or full-range cabinets on the front of the stage is common but can promote feedback. Club stages are usually big, undamped, empty boxes, and boxes resonate. When your microphone stands are resting on the same surface as the house speaker cabinets, vibrations transmit from the speaker cabinet through the floor to the mic stand, through the stand and to the microphone(s). This is one reason that most mixers provide a high-pass filter (HPF) or low-cut switch on the input channel. An HPF is designed to remove unwanted low-frequency sound from microphones without butchering the bottom end. Getting cabinets off the stage also helps reduce vibration-borne feedback.

It''s important to orient P.A. speakers in the manner intended by the manufacturer because high-frequency dispersion will be affected by speaker position. For example, let''s say we are using a JBL SRX725 speaker. This cabinet houses two 15-inch woofers plus one high-frequency horn driver. The dispersion pattern of the SRX725 is spec''d at 75x50 degrees, meaning that the box covers an angle of 75 degrees in the horizontal plane and 50 degrees in the vertical. If you place this box on its side, you change the dispersion to 75 degrees vertical and 50 degrees horizontal. Why do you care? Dispersion angles define the “coverage” of the audience. We want wide horizontal coverage, to spread sound across the audience, and narrow vertical coverage, to avoid bouncing sound off the ceiling. Rooms that are very wide may require more than one full-range cabinet per side to improve horizontal dispersion—even if you don''t need to play the P.A. loud. If that''s the case, the cabinets should match in brand and model and should be butted up against each other so that their outputs couple in a constructive manner.

DON'T CROSS YOUR WIRES
Make sure that your cables are correct for the purpose and are clearly labeled. Mic cables are easy because they typically use XLR connectors, but 1/4-inch tip-sleeve (TS) cables can deceive you. A TS cable intended for guitar or keyboard should not be used to connect a speaker to a power amp (although TS jacks on speakers and power amps are becoming scarce in favor of the SpeakOn connector). Such cables—especially when used in lengths greater than 20 feet—can load down a power amp, making it work harder than it needs to, and in severe cases, shutting it down. Speaker cable for live sound use should be no thinner than 16-gauge, preferably 14-gauge. Given the possibility that your gear has balanced inputs and outputs, use those wherever possible, with tip-ring-sleeve (TRS) and XLR cables between them. Balanced lines can be run farther without loss of audio quality and are less susceptible to RFI (Radio Frequency Interference) from cell phones, two-way radios, nearby radio or TV stations, etc. While a discussion of balanced and unbalanced lines is beyond the scope of this article, it''s worth noting that just because you use a balanced cable does not mean you are achieving a balanced connection. Consult your manual for details.

Fig. 2. Graphic EQ patched between mixer outputs and power amp inputs.

Fig. 2. Graphic EQ patched between mixer outputs and power amp inputs.

GET A P.A. TUNER
After you have the P.A. placed properly, check that the entire system is working. In addition to using your ears, you''ll need a graphic equalizer (preferably 31-band stereo), a CD player, and a familiar CD of high recording quality. MP3s are not acceptable. The graphic EQ should be patched in between the outputs of your mixer and the inputs of your power amplifier (or powered speakers). Some powered mixers may incorporate a graphic EQ or may provide patch points labeled “preamp out” and “power amp in.” In the case of Mackie''s PPM series of powered mixers you''ll find “main out” and “power amp in” patch points on the front panel designed for insertion of an external processor such as a graphic EQ or compressor between the mixer and amp (see Figure 2).

Connect the CD player to two input channels of the mixer. Set the channel EQ and graphic EQ flat or bypassed. Play the CD at a low volume, walk up to each loudspeaker, and put your ear against the grill. Listen to each component (driver) inside the speaker cabinets and make sure that they are all producing clear, undistorted sound. Pan the two channels of the CD player center to create a mono mix and turn up the volume. Listen for consistency between the left and right sides of the P.A. You can pan channels hard left and hard right to hear each side separately as well. Sometimes one side might sound a bit brighter than the other, particularly if it''s close to glass or a mirrored wall, but there should be no major differences in sound quality or volume between the left and right sides of the P.A.

There are a million ways to tune a P.A. system. Some engineers will speak through a vocal mic into the P.A., try to excite resonant frequencies of the P.A./room combination, and then use EQ to correct them. Personally, I don''t find this effective. Many engineers use a favorite CD to tune their P.A. system. They become very familiar with a certain piece of music and know how it should sound over a variety of systems. By listening to the CD through the P.A. they apply EQ until it “sounds right.” There''s nothing wrong with this approach, but keep in mind that a CD—with its processing, compression, and mastering—is not representative of the transients encountered when amplifying a live band. Alongside playing a music CD, I find it useful to play a CD of test tones across the frequency range (you can use a car stereo test disk for this) and listen for “hot spots.” You might hear certain tones that are louder than others, indicating a nonlinearity in the P.A./room combination. Find the corresponding frequencies on the graphic EQ and pull them down to make the level of the tones more consistent across the frequency range. The optimum way to tune a P.A. system is to use realtime spectrum analysis (“RTA”) and a measurement microphone (a topic we''ll save for another time). In all cases, the idea is to carefully adjust the graphic equalizer to compensate for room issues. You will not be able to fix acoustic issues this way, but you can make the overall sound of the P.A. work better in a particular room.

When listening to a CD, ask yourself if it sounds the way it does on a high-fidelity home or car system. If there are parts of the frequency range that are lacking or overemphasized, use the EQ to make adjustments. You cannot EQ something into your system that it is incapable of producing, so attempting to get earth-rattling bass out of a P.A. speaker that uses a 10-inch woofer ain''t gonna happen. Boosting very low frequencies outside of a system''s capabilities can overwork the power amp and/or woofer, possibly blowing up one or both. I always feel a lot better about using a graphic EQ to cut what I don''t want. For example, some P.A. speakers are inherently hot in the upper mids at frequencies like 1.6kHz, 3.15kHz, and 6.3kHz, so cutting a few dB at these frequencies can smooth out the upper vocal range and keep sibilants from taking your head off. If the P.A. sounds like it “has a cold,” try cutting a bit at 630Hz and 800Hz. If you need to boost or cut a frequency more than a few dB, you have a more serious problem, or the room may have acoustic issues.

EVERYTHING TO GAIN
A big challenge facing anyone who operates a P.A. system is understanding the concept of gain structure. Gain structure (sometimes called “gain staging”) refers to the manner in which signal levels are set in (and between) the various devices in an audio system—for example, between a mixing console and a power amplifier. Regardless of whether you have a brand-new digital mixer or a 30-year-old analog dinosaur, proper gain structure is crucial to avoid problems and keep the various components of your P.A. system interacting happily. Poor gain structure results in high noise levels, increased distortion, and decreased headroom. When you understand gain structure, you''ll be able to easily recognize when a piece of gear is malfunctioning, or when you have a defective cable in the chain.

Audio gear features meters to help manage gain structure. If you observe that the meters on interconnected components of your P.A. system display widely different levels, that''s an indication of poor gain structure. The first step in achieving proper gain structure is plugging gear into the correct holes(!) This warning is not as silly as it sounds. We typically deal with three distinct categories of signal level: microphone level, line level (the output level delivered by devices such as CD players, outboard reverbs, delays, compressors, equalizers, DJ mixers, etc.), and instrument level (signals from electric guitar and bass, as well as many keyboards and drum machines). Mixers provide a variety of inputs along with gain or trim controls to accommodate these signals. Line signals are much stronger than microphone signals, so a mic input incorporates a preamp designed to raise the mic''s output up to line level so that it can pass through a mixer on to other gear. Instrument level falls somewhere between mic and line level, so neither a mic input or line input will make your bass happy when you plug it directly into the mixer. That''s why some mixers provide a few channels that also feature an instrument input. If your mixer does not have an instrument input, use a DI to interface the instrument with a microphone input.

As you''re aware, mic inputs typically use XLR connectors, while line inputs are usually 1/4-inch TS (unbalanced) or TRS (balanced) connectors. Since an instrument cable uses a 1/4-inch TS connector, it''s tempting to plug a guitar or bass into a line input. However, a line input is not sensitive enough for an instrument, so you''ll end up cranking the gain knob to the point where noise becomes an issue. Also, a line input does not have the correct impedance for guitar or bass pickups, so mismatching can also cause a loss in fidelity. Since line inputs are less sensitive than mic inputs, using an XLR-to-1/4-inch adapter cable to plug a mic into a line input is another no-no. You''ll have to crank the gain way high just to hear the mic, and this process will add noise. Conversely if you plug a keyboard into a mic input, you''re probably going to hear distortion even when the gain is turned all the way down because the signal from the keyboard is strong enough to overload the mic preamp. These situations are all examples of poor gain structure.

Some mixers have a mic/line switch on each channel so you can select between inputs (or possibly a mic/line/instrument switch), but mixers without a mic/line switch may leave both jacks connected at the same time. It''s important that you connect a source to only one of these. You probably won''t break anything, but you may cause an increase in noise or distortion to both signals.

SHOW YOUR MICS SOME LOVE
Setting the gain or trim on a mic input is very important because it''s the first step in the signal path. It''s like a main water valve: Screw it up and you won''t get proper water flow through the building. In our water analogy, you need to set the valve so that you get sufficient flow (good signal-to-noise ratio), yet not set it so high that you create too much pressure (distortion). You can boost the fader up as high as you want but if the trim is off, you''ll get nothing but noise. On the other hand, if you set the trim way up and the fader way down, chances for distortion are much higher. (For details on how to set gain, see the sidebar “Setting Proper Input Gain.”)

Once the gain is set, you can bring up the channel fader to hear the signal. At least some of the channel faders should be at or near the “0” mark; if all the faders are very low or very high, something is wrong. Other “valves” affect the audio signal, such as the main mix fader(s)—which should also be set at or near “0.” If setting the master at “0” makes the volume in the room too loud, turn down the level controls on the power amp(s). If you need to bring the master fader all the way up to get adequate volume in the room, either the power amps are set too low or your system is under-powered.

A P.A. system facilitates a positive sonic experience for your audience. When that happens, it''s a win-win situation. The audience gets to hear you at your best and by providing you with good feedback, you enjoy performing. We''ll take a look at other aspects of live sound management in future issues of Electronic Musician.

SETTING PROPER INPUT GAIN
There are several ways to measure input level. Unfortunately most compact mixing consoles do not provide a meter for each channel, but they usually feature a switch called PFL, or Pre-Fade Listen. Generally, pressing this button on the channel temporarily switches the mixer''s main meter to show the selected channel''s level before that channel''s fader. In other words, it''s letting you measure the water pressure at the main valve, before the kitchen faucet. If you set the level incorrectly here, you''re practically doomed to distortion or noise. Adjust the gain or trim knob while watching the meter. You can raise the level until the meter reads “0,” but remember that other microphone signals must make it into the audio “plumbing” during the mix—so leave a bit of headroom by PFL''ing the signal at roughly –7 to –5. This way, when you start combining signals, you won''t overflow the main mix pipe. Adding EQ will likely change the signal level, so you want to leave room for that as well. If you have the trim all the way down and the PFL signal is still way over “0,” use the channel''s Pad switch; this will lower the sensitivity of the mic preamp by a fixed amount, reducing the possibility of distorting the signal (sort of like narrowing the water main).

Variations on this type of metering include Solo, as implemented on most Mackie consoles. The trick here is knowing that this type of solo does NOT show pre-fader level, so the fader must be set at “unity” while you make the adjustment or you won''t get an accurate reading. (After you adjust the gain setting, you can put the fader wherever you want.) This is the spot where the fader is putting out exactly what it is receiving—it''s neither boosting nor cutting the signal. On some consoles, unity is marked with a “0” or a small arrow. Other consoles offer a two-color LED with green indicating “signal present” and red indicating overload. Adjust the trim until the LED blinks red briefly and then back it off by about 10 to 15 percent. Since some consoles have more headroom than others, you''ll have to experiment to see how far you can push the trim before distortion occurs.

 



Steve La Cerra is the front-of-house engineer and tour manager for Blue Öyster Cult.

 

Show Comments

These are my comments.

Reader Poll

Do you side with Taylor Swift or Spotify?



See results without voting »