Live Sound vs Studio Sound
If you’re just starting out in sound engineering, there are a few basic concepts that are helpful starting points. For almost a decade I’ve been jumping back and forth between large scale sound system design, and hi-fi studio design and studio mixing. I felt it would be good to shed some light on exactly what “tuning” a sound system really means. In the case of a home stereo system during a recent visit to a friends house, it was a mere 5 minutes of basic equalization on his Mac Mini using Apples AU Lab, and some improvements in speaker placement.
When it comes to large stadium arrays, dance stacks and live venues things get a little more hairy. My favorite environment is an outdoor stack where there is little or no reverberant surfaces or “room bounce” to deal with. Outside I can let the system breath fully, with all frequencies nicely uniform.
How does this relate to tuning a live sound system? Let me break it down into the following four categories: Flat Frequency Response, Crossover, Time Alignment & Power Handling
Flat Frequency Response
Pink noise, when heard over a speaker sounds like static. Beyond just being an annoying sound, it’s actually a useful sound tuning test tone where all frequencies are played at an equal sound pressure level. If you look at a spectrograph of pink noise, you should see a fairly flat line or slope with minimal peaks and valleys for any given frequency.
When you play pink noise through a loudspeaker, you can use real time analysis tools and a microphone to see how accurately all frequencies are represented. Bass heavy systems will have a bump in the low end while systems that are poorly crossed over will have dips or peaks. If the transducers in your speaker just can’t handle a certain frequency, or the cabinet causes unwanted resonance, you may also see other anomoles. Companies like Funktion One, Turbo Sound, EAW and Meyer spend countless hours in R&D figuring out how to get high performance drivers to deliver smooth frequency responses over their operational range. Touring sound engineers that operate these systems have been provided reference EQ and crossover settings from the manufacturer. These settings are based on the manufacturers in depth understanding of how their speaker were designed.
These recommended settings only gets you part way. The rest is having good ears and/or good measurement equipment like SMAART.
Back to the recording studio environment for a moment… Earlier I mentioned the single cone Avantone mixcube. These speakers have one cone, and intentionally sound a little crappy. They have no tweeter, no subwoofer, no sizzle, which is exactly what I want when I am doing mixing and mastering for clients, they are my “alternate” speakers used for comparison with my main monitors. The masses don’t listen to your latest release on $2000 pro studio near fields. They listen in their car, on their iPod, their bose home stereo, in the club, or on a portable boom box. They also don’t always sit exactly in the middle of the speakers, so they might be hearing a more “mono” sound.
What’s so special about these minature Avatone speakers? The mixcubes have no crossover which means all frequencies are trying to come out of a single cone. This gives you the opportunity to hear how your mix will translate across a variety of crossover points. Not every speaker is designed the same. The Focal CMS 65 crossover their tweeter around 2K (they are bi-amped). A Funktion One Resolution 4 Touring horn is Tri-Amped and has 2 crossover points – at 445Hz and 5.77Khz.
Whether you are tuning sound for a live concert or setting up in-studio near-field monitors, getting the crossover points right is key. The goal is to avoid dips in frequency response or causing overlapping frequencies which lead to a “smearing” sound.
There are two general areas of alignment to focus on: transducer alignment and placement alignment. You’ve likely heard time mis-alignment, but not sure “why” you don’t like how it sounds. If the high frequencies, mid frequencies and low frequencies hit you at different times, it’s a bit slappy feeling. An extreme example is a drummer sitting on stage thwacking the kick drum. If you’re sitting in the audience, you hear the thud of the kick drum coming from the loudspeaker, then a split second later you hear actual sound coming from the drummer. This is more about placement alignment, which is a system wide adjustment versus transducer alignment. We need to get two alignments correct – the speakers stack in relation to all the drivers, and the front of house in relation to the band (although less important for DJ sets, unless their on stage monitors are bleeding into the audience).
Have you ever been to an event where the organizer decided that dumping a bunch of mismatched speakers on the stage was the answer to their volume and sound quality problems? Not only does it look like a mess, it certainly sounds like a mess too. They are likely not time or phase aligned, and are canceling out frequencies all over the place.
The Mackie SRM450 has done a decent job trying to get nicely timed speakers to the masses (although its easy to argue we have all cursed the overdriven distorted Mackie SRM speakers found out in the wild). They describe it as achieving simultaneous arrival of the High Frequency and Low Frequency.
We want this in “all” our systems, whether in the studio, at home or on large stadium array systems. If you are setting up a surround sound system at home, don’t put your center channel speaker 10 feet behind the TV, and the subwoofer 20 feet off to the side of the room, it will sound a mess.
When I tune touring grade sound systems, I usually have a starting point from the manufacturer. This includes time delay compensation and polarity settings. Polarity is another type of time alignment, or more specifically “phase” alignment. If you take two sine waves, and invert one of them – played back together they would cancel each other out. This can happen at any frequency, with any driver for a multitude of reasons (this is where the art of sound engineering comes in – it takes time to learn how to suss all these issues out).
The Funktion One Resolution 4 has a recommended list of time delays and crossover adjustments as a starting point.
Much like the home stereo surround sound comment above, this also applies to any satellite subwoofer stacks. The goal is to get the HF, MF, LF and Sub to all be in alignment, regardless of where they are stacked.
When I go to general admission live events, you will likely see me standing near the mix engineer tent. It’s almost always the “sweet spot” in terms of time alignment.
I admit it, I’m the engineer that helped Burning Man craft their “sound policy” for mutant vehicles. It got to the point that everyone wanted to be a superstar DJ and turn up their sound system as loud as technically possible. This lead to either blown out ear drums of participants trying to look at art or crappy sound. Louder does not equal better sounding.
A rule of thumb I use when designing speaker stacks is to have 1.5 to 2 times the power needed to drive all transducers powered by the amp channel. This means if you have a 1000 watt subwoofer, you need a 1500 or 2000 watt amp to run it. If you run two subs off the same amp channel (usually in bridge mono), you need 3000 to 4000 watts. This give you headroom! You can run the amps full, and keep your levels low on the mixer or digital signal processor. When you have headroom (more power than you need), the amps just crank along without complaining (i.e. distorting). Keep everything out of the red throughout your input chain (on stage mixer, artist gear, front of house mixer, crossover, limiters and amps) and it will sound amazing!
What hurts peoples ears is distortion and high sound pressure levels. You can talk to the person next to you because the sound is “clean” and only run at a level required to allow you to “feel” the music in your chest, while not destroying your hearing (I still recommend ear plugs!).
- Posted by Jeff Steinmetz, Voting Grammy Member // Producer, Engineer & Founder of Urge Productions