Live Mixing (Part 1)

How does live mixing differ from studio mixing. It’s a question I hear a lot, and there are a lot of different aspects to the answer. It’s not a one sentence reply.

There is a lot to take into consideration. Everything affects it, from the fact that the entire band is playing and singing at the same time, with no overdubs, iso-booths, etc., how the PA speakers interact with the room, and how monitor and stage volume affects everything.

Consider the room as a closed system. Every speaker or sound source feeds acoustic power into that system. Some of that acoustic power is heard directly by the audience. Some, however, enters the system and bounces around off of various surfaces (ceiling, walls, etc), and only reaches the audience through an indirect route, resulting in “echoes” and “reverb.”

There are three “zones” in which a listener can be located:

Near Field” – This is an area located in front of a speaker, where almost all of the sound heard by a listener is directly from the speaker. The room has little influence on the sound, since most of what the listener hears comes directly from the speaker.

Free Field” – The zone or area where most of the audience will be located, in which the listener still hears a lot of direct sound from the speaker, but also hears a substantial amount of the reverberant field.. There is a definition which quotes what percentage of “room” vs. “direct sound” is heard in a free field, but I don’t remember what it is, and can’t find it on Google.

Far Field” – Area where very little direct sound from the speaker is heard. Nearly all sound heard by a listener in this ‘zone’ is indirect or the ‘reverberant field.’

Obviously, the near field is the very best place to be, since you are hearing ‘the speakers’ and not ‘the room.’ The far field is the absolutely worst. There will be no intelligibility, no dynamic range, and the frequency response will depend totally on the room’s reverb field. Just solo a reverb return, and put an entire mix through your reverb unit, including bass, keys, drums, and you’ll get an idea of just how undesirable it is to have any audience members be located in the far field.

So, it would be ideal to sit everyone in the near field. Everyone would hear the same mix, and it would be like listening to a really high quality stereo, just really loudly. In some purpose-built theaters with a proscenium arch, where all seats are on the floor, in front of the speakers, or only slightly off-axis, it is possible to approach this ideal.

In the real world, not so much.

What would be required to achieve this ideal? Well, obviously the first thing is to have every audience member hear only direct sound. Second, you have to reduce the amount of acoustic power being put into the reverberant field. While you, as FOH dude, don’t have any influence over the monitors, hopefully the monitor mixer is understanding and will try to help you out. The rest of the problem you DO have control over, but mostly during speaker selection when you are building your system, or during speaker placement when you are setting up the system. It should be clear that you don’t want speakers aimed at places where there won’t be any audience members – that would only add acoustic power to the reverberant field, which is exactly what you don’t want.

What else affects this issue? Oddly enough, your choice of crossover frequencies can have a huge impact. To understand why, consider what happens to the dispersion angle of a horn as frequency changes: as frequency increases, the dispersion angle narrows. Some horns “beam” more than others. It is one of the major selection criteria you should look at before you choose. Conversely, as frequency decreases, the dispersion angle increases. Below the horn’s cutoff frequency, the driver essentially becomes ‘unloaded,’ and the horn provides no dispersion control whatsoever. (also, being unloaded, the driver is much more likely to exceed its excursion limits and fail.) What does this all mean? If you remember that dispersion varies with frequency, you’ll see that while an array’s on-axis response may be acceptably flat, if you depend on each driver for a wide frequency range, its off-axis response is going to be pretty miserable. The dispersion of horn which you are using for a frequency range of 1K – 10K is going to vary a LOT more than if you use that same horn/driver for 5K – 10K. Keep that in mind as you are shopping for horns, or horn-loaded speaker cabinets. Dispersion is different for different frequency ranges. If the spec sheet only gives dispersion at one frequency (or worse, has no reference to what frequency the measurement was taken), there’s no easy way to tell how it will perform. This ‘dispersion varying with frequency’ factor is what inspired the ‘Constant-Q’ horns, where, through varying throat wall angles, the manufacturer attempts to make the horn perform more consistently over a wider range of frequencies. What it boils down to is that, since dispersion varies with frequency, anyone sitting off-axis to a speaker array hears a different frequency response than those sitting more on-axis. Also, the reverberant field is fed an incorrect frequency distribution.

Also, remember that ALL sound that is being put out by any source is feeding the reverberant field, which decreases the size of the near and free fields. As an example, consider a stage monitor that is pumping out keyboards. NONE of its direct sound is heard by the audience, but it is adding a considerable amount to the reverberant field, muddying up your mix, and in general, making life difficult for you. The only way to get your mix back under control is to have the monitor mixer pull down the keyboard in that monitor, or maybe pull only certain frequencies down a bit. Hopefully, the keyboard player and monitor mixer understand the situation, and are able to work out a compromise that everyone can live with.

People are sometimes tempted to use noise gates on vocal mics in an effort to clean up a mix by reducing the amount of stage noise coming through the mics. My recommendation: don’t. If you set the threshold low enough that they ‘open’ when someone sings into the mic, it will also open if they step away from the mic so the mic is picking up drums and guitar amps. This results in a mix which is clean and tight when that person stands in front of his mic, but loosens up considerably, and changes the complete mix, as soon as he moves. This is more distracting that having a steady amount of stage noise coming through the mic.

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