Audio Purists

March 20, 2009

There is a group of sound guys I call “audio purists.” These are people who eschew anything which colors the sound, such as EQ. To them, “purity” of sound is the ultimate goal, above all else.

All EQ colors the sound, not only by varying the frequency response, but also, as a side effect, affecting the phase angle at certain frequencies. This is most apparent when boosting frequencies, and, in extreme amounts causes “ringing,” or resonance at the boost frequency. While this is clearly heard at extreme levels of boost, it is present to some degree at more moderate amounts of boost. This is what purists object to.

Because of this, a “purist” will, when using the 31 band EQ to tune the system, only use it to cut frequencies. In fact, there are a few graphic EQs on the market that are “cut-only.” They do not boost at all. In theory, this should be a good idea. But, what of a situation, where the system sounds pretty good, but has one frequency area that is slightly lower in response to the rest of the spectrum? Most people would simply use the 1/3 octave to boost those few frequencies the 1 or 2 dB needed to smooth things out. The “purist,” however, would prefer to cut all other frequencies, to avoid boosting any band. However, one thing often overlooked is that cutting adjacent bands does NOT result in a flat response. For example, if you cut every band on a graphic EQ by 3dB, the resulting curve would not be a flat response which was simply 3dB lower than the input. Each band has the most affect at its center frequency, and gradual shoulders which boost or cut less and less the further from the center frequency you get. Also, phase shift problems are most apparent in these “shoulder” areas. These shoulder areas are additive, which means that the cut (or boost) of two adjacent bands combine where the shoulders of the filters for those two bands overlap. Therefore, the result from pulling every band down 3dB would be a response which was down 3dB at each center frequency, with ‘ripples’ between bands of lesser or more attenuation. In addition, there would be phase shifts across the entire spectrum. The end result is that, to avoid boosting in one small area of the frequency range, you would be introducing an odd frequency response in the entire spectrum which would be filled with phase anomalies. This in the name of “audio purity?”

A related thought is that they will refrain from using any (ANY!) channel EQ. In fact, they will switch it out of the circuit. This idea actually has some merit, of not carried to the extreme. Their thinking is that EQ is bad, for the reasons I stated above, and since they aren’t going to use the channel EQ, they might as well take it out of the signal path. Since every circuit adds some small amount of noise, you can avoid adding it by not having that circuit in the path at all. Consider that each EQ adds some noise, if you remove the EQs from all 32, 48, 64, or however many channels you are using, this can add up to quite a bit of noise you are avoiding. The thought goes that you should get your sound solely from mic choice and placement. In a situation where you want the most natural sounds possible, and if you are working with acoustic instruments where a “natural sound” is desired, this may be possible. I agree that you should do everything you can to get the sounds you need with mic choice and placement, but in a live situation, it’s not always possible, and you are most likely not working with cellos, violins, violas, etc. So, what is a “natural” sound for a synthesizer, electric guitar, or bass? Odds are, you are going to want to do some EQ to each one, or you will end up with a lifeless and unexciting mix. Can you imagine the average kick drum in a rock mix if you couldn’t have any EQ on it? Cheap EQ sections can sound pretty bad, but if you have good channel EQ, there’s no reason not to use it. Every board sounds different, and the EQ is a big part of this. It is one of the major things you should listen carefully to when shopping for a new mixer.

Another technique that is rather common, or at least was, among the purists,  is to put all of the channel faders at the +/-0 line, and do all of the mixing with the mic trim pots. I’m not real sure where this technique came from, other than the “purists” see a point on the fader where it is neither attenuating or boosting, and they figure that is the “natural” (or “neutral”) spot. However, if you read one of my earlier posts, you may remember that, on instruments that need to be quieter in the mix, this can result in added noise, since you are turning it down at the trim pot, and then running the fader at 0. Whatever noise is added by the channel’s electronics would be better reduced by getting a good strong signal through the channel, and then running the fader at -15dB, or where ever you need it. In my opinion, it is far better to get as hot of a signal as you can coming in to the channel. During sound check, have the player go through his loudest part, and use the PFL meter to set the input level to 0VU and do your mixing with the channel fader. That way, you have plenty of signal to work with, and if you are also sending monitor mixes from the FOH console, makes it MUCH easier to deal with.

A lot of the ideas that the “audio purists” have are based on situations in the mythical ‘ideal situation,’ but don’t often translate well to the real world of mixing a rock band. As always, use your ears and judge for yourself, but keep these things in mind when some “helpful” purist starts making suggestions. Try everything, and keep what works for you – just always consider every aspect and consequence of every technique you try. Otherwise, you may not know what is causing other, seemingly unrelated problems.

Wrestling With Monitors

November 18, 2008

Monitors are most people’s worst nightmare, and sometimes that fear is justified, but there are some tricks, issues, and considerations to keep in mind that may, or may not, keep you from pulling your hair out during a particularly difficult soundcheck.

These are going to be in no particular order, and have no real relationship to each other.

Any spike in frequency response of a mic or monitor can cause feedback at or near that frequency, since the mic is going to pick up more of that frequency from the monitor than the other frequencies. This is true of off-axis frequency response too, so even a mic that is relatively flat on-axis can have a problematic response curve off-axis. And, to make it even more fun, that off-axis response curve will change dramatically at different off-axis angles. A certain mic may have a sharp spike at 2KHz at 180 degrees off-axis (directly aimed at the back of the mic), but that spike may go away at 120 degrees off-axis (or 100 degrees, or virtually anything). If a particular vocal mic (for instance) wants to ring at a frequency you know is not a hot one in your monitor, the first thing I would try would be changing to a different mic of the same type. That’s right – if you’re using an SM58 (for example) and are having trouble, before you do almost anything else, I would swap it with another SM-58. SM-58’s have pretty lousy consistency from mic to mic.

If it’s still a problem, move the monitor to either side, and aim the monitor toward the back of the singer’s head. Typically, 120 degree off-axis provides the best feedback rejection for a lot of cardioid mics, because there is a slight “lobe” of increased sensitivity at 180 degrees.

Only then would I even touch the EQ. EQs introduce phase shift, which can sometimes cause more trouble than they cure. I would hardly ever boost anything on a channel EQ on a monitor system, especially on lesser-expensive boards. The EQ itself can introduce a slight ‘ringing,’ which only complicates your life further. A good graphics EQ is an absolute MUST. Cheap ones introduce a lot of nasty side-effects at the edges of each band, and you need a very reliable way to boost or cut a particular band without having bad interactions with adjacent bands.

If you are using any compressors on vocal mics in the monitor system, you are asking for trouble. When a singer really hits a note hard, he expects it to be a LOT louder than when he is singing quietly. If it’s not, he will think the monitors need to be turned up. The more you turn it up, the closer you will get to the ‘feedback threshold.’ You may be able to turn his mic up in the monitors, but you have raised the feedback threshold much higher than it would have been without the compressor, with all the problems that can cause.

If you are having problems in the 200-800Hz range, check for obvious things such as a wedge touching the base of a mic stand.  If you are having problems with ringing in the 40-200Hz range, it may be low frequency spill-over from the FOH system.

If, during soundcheck, one of the musicians complains that there’s not enough low end in one of the instruments in his monitor, don’t just automatically boost the lows on that instrument.  First, call the FOH mixer and have him bring that instrument up in the FOH system.  There likely will be more than enough low end spilling onto the stage to please anyone.  If there’s still not enough, put more of that instrument in the side-fills.  Side-fills are far more capable of delivering low frequencies than wedges, which are usually extremely inefficient below 100Hz.  Plus, an abundance of low frequency in the wedges simply eats up power, of which you desperately need all you can get in the wedges.

Also, keep in mind that you have enough power on stage to cause a lot of problems for the FOH guy, so be cautious.  If he complains that one of the instruments in the monitors is muddying up his mix, you may have to cut the problem frequency range on that instrument.

There are two clues that the singer’s monitor is too loud (regardless of what he says): He is consistently singing flat, and/or he is not singing as loudly as you know he should.  Slowly turn his mix down slightly.  He will probably not notice, it should fix both of those problems, and will vastly improve his performance.