Club touring presents many difficulties, one of which is the possibility that you can’t travel with your own P.A. system. Couple that with the fact that when the venue changes every day, mixing on a different sound system every night becomes a huge challenge. Your mix can sound totally different from show to show, and it’s tough to pinpoint the reasons why. Is it the console? The system processing? The speaker arrays? Here are some tips to help you stay out of the loony bin.
Look At It
Speaker placement in a venue is critical to the sound quality and coverage in a room, for a lot of reasons. Foremost is avoiding feedback from the stage. The front line (vocal mics) should be at least 5 feet behind the P.A. If the P.A. is hung (or stacked) too close to the front edge of the stage, feedback is inevitable. Line arrays may let you cheat this distance by a foot or so due to their directional control, but placing conventional boxes close to the performers is asking for trouble. In some situations you may be able to get the house crew to push the speaker stacks forward (i.e., toward the mix position) to help avoid this problem. If that’s not practical (or if the main rig is flown), pull the stage monitors and vocal mics upstage to place them further behind the house stacks.
People sitting or standing close to the stage generally don’t hear vocals from the P.A. because the speakers are throwing sound to areas behind them. In the old days, you could count on spill from vocal monitors covering these areas, but with the widespread use of in-ear monitors, there often are no vocal monitors to spill. That being the case, you need a front fill on the downstage edge. I’d suggest feeding the fill from an aux send so you can dial in vocals, effects and maybe a bit of guitar. No need for drums and bass in there, since they’ll usually be loud enough coming off the stage. During sound check, you may need to walk back and forth between the money seats and the console while adjusting the fills, or have someone assist you with the process (for more info on using fill speakers, visit T&P in the July 2012 issue of FRONT of HOUSE).
Listen To It
Especially in small P.A. systems, every driver counts — so make sure they all work! There are a lot of ways to do this. Here are a few suggestions:
Run pink noise into a channel (EQ set flat, no processing). Play the speakers at a very low volume and get up close and personal with it. Listen to every driver in each cabinet to make sure it is producing sound while someone babysits the console to make sure no one accidentally turns up the volume.
Next, turn up the P.A. so that your SPL meter registers around 100 dB. I suggest you set the meter for C-weighting. You can use A-weighting, but whatever you decide, be consistent. If there are separate faders for the left and right masters, set them the same. Pan the noise left and measure the SPL using a slow response time. Pan the signal to the right and again measure the SPL. A difference of more than a dB or two indicates that something is wrong. Possibilities include a power amplifier not being turned up all the way, a damaged speaker component, or mis-matched crossover settings (more about that below).
Analyze This!
Next, analyze the noise using a Real Time Analyzer. RTAs are no longer cost-prohibitive. Handheld RTAs that include SPL metering and noise generators are available for a few hundred dollars and are worth every cent — especially when you mix on different systems every day. Studio Six Digital (studiosixdigital.com) offers a plethora of hardware and software tools that facilitate audio analysis on your iPhone or iPad, while SmaartLive from Rational Acoustics (rationalacoustics.com) or Metric Halo’s SpectraFoo Complete (mhsecure.com) both include powerful audio diagnostic tools for your laptop.
With the signal panned to one side, set the RTA to a slow response and observe the RTA curve (your meter or software may let you store the measurement). Then pan to the opposite side. The RTA should show similar results within a few dB. If not, use a 1/3-octave EQ to match frequency response of the left and right channels of the P.A. If they don’t sound the same, you’ll go crazy trying to mix. As most sound systems also incorporate a compressor on the left/right bus, pay attention to the position of the compressor in the signal flow. Severe EQ’ing before the compressor can trigger compression.
Check each frequency range of a multi-amped system. Most P.A.’s employ a loudspeaker processor (such as a BSS Omnidrive, dbx DriveRack, etc.) Mute all outputs on both channels. One by one, turn on each output and listen to the sub, low, low-mid, mid and high ranges of the P.A., making sure that each range is working for left, right and — where applicable —center channels. Each range should sound consistent from left to right.
Music doesn’t always reveal problems with fatigued drivers (particularly in the low-end), so play sine wave tones varying from 50 Hz to 16 kHz through the system. Be sure to not use square waves, since they mask distortion. A 100 Hz tone easily uncovers a woofer with a rubbing voice coil or rattling grills. Third-octave tones can help reveal “hot spots” in a P.A., which often show up at resonant horn frequencies of 1.6 kHz, 3.15 kHz and 6.3 kHz, and can be easily corrected using a 31-band, 1/3-octave equalizer. If a particular frequency range is especially hot, turn your attention toward the output controls on the crossover or loudspeaker processor and adjust (for example) the drive to the mid- or high-frequency amplifiers before reaching for the EQ.
Please Not Again, Donald
A lot of engineers use a favorite CD to tune a P.A. system. Once you intimately know a recording, you know how it should sound over a variety of different playback systems. By listening to that music through the system, you can apply EQ until it “sounds right.” This is a valid approach, but keep in mind that a CD — with its processing, compression and mastering — is not representative of the transients characteristic of amplifying a live band. Also be aware that Steely Dan’s Aja and Donald Fagen’s The Nightfly have been officially retired and may no longer be used for tuning P.A.s. Some engineers prefer to use a vocal microphone for the purpose. Carry your own for consistency and hygiene. Talk into the mic to excite the resonant frequencies of the P.A. and then use EQ to correct them. In this case, you are using the same tools as you are using in your live mix: compressors, equalizers and mics — as opposed to a CD.
Aux To Subs
Most P.A. rigs feed the L/R output of the mixing desk to a system processor (or crossover), which divides the audio signal into bands and routes those bands to various amps in a multi-way system. When audio is routed in this manner, it’s possible to inadvertently send a signal to the subwoofers that has no business being there (a hi-hat or triangle microphone, for example). High-pass filters remove low-frequency garbage that might otherwise sneak into a hi-hat channel, thus preventing that signal from ever reaching the subwoofer, but using an aux send to feed the subwoofer(s) provides more concise control. Feeding the subwoofers from an aux totally removes unwanted audio from the subs simply by virtue of the fact that you turn up that aux only for channels that need to be in the subwoofer. Channels such as kick drum, floor tom, bass guitar and synth get fed to the subwoofer aux, while channels such as lead vocals, hi-hat and that triangle microphone do not.
The aux output feeding the subwoofer is usually routed to a crossover or low-pass filter, from the filter to a compressor, and then to the subwoofer amps. In lieu of a “proper” crossover, I’ve seen engineers run the subwoofer aux out from the console to a single-channel, 31-band EQ with all of the high-frequency sliders (say, those 125 Hz and above) pulled down all the way — thus acting as a low-pass filter. This aux send needs to be post-fader or your low-end will become disproportionate every time you move a channel fader. Be aware of the fact that when you mute the main L/R outputs of the system, the subwoofer aux send will not be muted, and your audience will hear low-frequency rumblings from the subwoofers.
While we’re on the topic of low-end, don’t forget to take advantage of the high-pass filters provided on each channel. While the band is sound checking, audition each input over the P.A. (not headphones) one at a time, turn the high-pass filter “on” and bring up the cutoff frequency until you can hear the low-end start to thin. Then back it off a bit. This will keep unwanted sounds (such as mic stand rumble) from ever reaching your low-frequency amps, preserving clarity in the bottom-end and maintaining system headroom.
Use Your Resources
The house or systems engineer at a venue is your friend — your greatest resource when you are visiting. They have night after night experience, know how the P.A. and room interact, and are typically more than happy to share their knowledge. Ask a question like ‘is the bass response at the mix position consistent with the rest of the room?” Then listen to the answer. It astonishes me how often I see a house engineer offer advice to a visiting engineer, the visiting engineer completely blows it off — then wonders why the mix sucks come showtime. Duh….
