Sound Reinforcement (PA) - A Guide to Making Live Sound Work For You

Discussion in 'Bad Dog Cafe' started by simoncroft, Aug 2, 2019.

  1. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    Diagram H is scary stuff at first glance, because I’ve used one diagram to illustrate several related points. There’s a lot of information in there, so let me break it all down into some manageable blocks for you, starting at the top and working downwards.

    Hopefully, you’ll recognize the gray block with the sliders on it as my attempt to draw a graphic equalizer. Even if you didn’t recognize my drawing, you’ll have seen similar devices everywhere from bass rigs to hi-fi systems. This is no ordinary graphic though: this one has the full 31 bands typically used to compensate, or ‘tune’ the output of a Sound Reinforcement rig to the acoustics of a venue.

    Because there are 31 bands, they are close enough together to allow problem frequencies to be tuned out without unduly compromising the overall sound balance. In fact, this format of equalizer is so universally recognized, the frequency centers from 20Hz-20kHz are set out in an ISO (International Standards Organization) specification. I’ve set those frequencies out in numbers below the graphic of the equalizer, with the octaves highlighted in yellow.

    Below this, there is another block showing where these frequencies sit in terms of ‘sub-bass’, ‘bass’ etc. In an advanced 5-way rig, these general labels would correspond directly to the speakers that these frequencies would be coming from (with some deliberate overlap at the crossover points).

    You might wonder, if this is such a useful format, why doesn’t every channel of a mixing console have a 31-band graphic? Well, one reason is that 31 bands take up an entire 19-inch rack case, so our channel strips would be unworkably big. As it turns out, for individual channel EQ, we don’t need access to all 31 bands. What we need is to be able to tune one or two bands to the frequencies we find useful on that particular channel.
     
  2. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    Screen Shot 2019-08-05 at 17.16.53.png

    Diagram I shows the EQ section of a mic/line channel on a Soundcraft GB2 analogue console. Soundcraft pioneered this type of EQ format, and it is well worth becoming familiar with, because you’ll find it appears over-and-over on analogue consoles of many brands. The short-form description for this type of EQ is: ‘four-band, with two sweep mids’. Let’s see how it works.

    The LF control at the bottom offers 15dB cut/boost of the Low Frequencies, while the HF control at the top does the same for the High Frequencies. Together they are the closest thing on a modern mixing console to those treble-and-bass controls on our original mixer/amp.

    The Lo Mid and Hi Mid controls can both be operated across a range of frequencies. These are set for each of the two ranges using the associated ‘sweep’ control. These are marked 80-1.9k and 550-13k respectively. Because the control ranges overlap, it is possible to address two different but closely spaced frequency centers within part of the frequency spectrum. This can be useful when trying to tune out unwanted ringing or resonances.
     
    beagle likes this.
  3. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    Screen Shot 2019-08-05 at 17.13.35.png

    Going back to Diagram H, the lower part is an idealized plot of how the four-band EQ works across the 20-20kHz spectrum. Because these plots, or graphs, can be a bit intimidating to anyone who isn’t familiar with them, I’ve color-coded mine to make it easier to refer to each band in the text.

    In my scheme, the frequency curves for the LF control are green and the corresponding ones for the HF control are purple. The top half of the graph shows how the frequency curves would be changed if the LF and HF controls were turned up the full 15dB. Obviously enough, the bottom half shows how the frequency curves would be changes if the controls were turned down to -15dB.

    The two curves have something important in common. They both start very gradually from the mid range, and only reach full cut/boost as they reach the ends of the 20Hz and 20kHz frequency spectrum. This is done with a ‘shelving’ circuit. You’ll find shelving circuits used a lot of LF and HF bands because the result they produce is musical and therefore useful as an enhancement tool.

    The red lines represent the action of the Lo Mid. The first thing you’ll probably notice is that the curves appear twice on the graph. This represents the lowest and highest points in the sweep control’s range. The horizontal lines with arrows indicate that this control is continuously variable between the two points.

    The Hi Mid section is represented by the blue lines. These show exactly the same thing as the Lo Mid, but operating over a different range of frequencies. Again, note how the graph indicates the overlap between the two, as well as the range over which each one can be swept.

    The curve described by the mid range circuits looks like a bell. It is actually known as a ‘bell curve’. It is very useful in mid range circuits, as the cut or boost has a definite ‘center’ but the result remains musical. Bell curves generally interact well, in the sense that if two or more are applied at closely spaced centers, the result is still pleasing to the ear. Referring back to the 31-band graphic equalizer’s centers – where there are three separate sliders for every octave of frequency spectrum – helps to reinforce how important it is that equalizer circuits interact in a pleasing manner. Remember that between the centers there are frequencies that still have to sound like music!
     
  4. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    Screen Shot 2019-08-05 at 17.24.59.png

    NB – 'Parametric EQ' is a term that often gets wrongly applied to the sweep-frequency circuits I described above. In order to be truly parametric, and EQ circuit has to give control over three 'parameters': gain, frequency and 'Q'. Q refers to how broad or narrow a frequency range the cut or boost is applied to. In the screen grab above, I've placed a boost centered on 150Hz with a broad Q, plus a cut centered on 1.2kHz with a very narrow Q. Narrow Qs can be very useful for 'notching out' unwanted resonances in snare drums, and for eliminating feedback at a specific frequency.

    Also on the screen grab, I've placed an HF (High Frequency) and a LF (Low Frequency) roll-off. You'll be lucky to get both of those on a live mixing console, so let's got back to the Soundcraft shown in Diagram I...
     
    24 track likes this.
  5. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    Neither of the following controls is part of the EQ circuit, strictly speaking, but they both have a radical effect on how it works in practice. Just below the LF control is the EQ Cut. This gives an instant answer to the all-important question: “Is this EQ I’ve set up actually an improvement on the original sound?”

    There is a second button, part of which you can just see above the top of the EQ section. This is a 100Hz bass cut (or 'rolloff'). It actually sits in the circuit ahead of the EQ and is part of the mic/line input section but we’ll talk about it here, because it has an important affect on the sound of each channel.

    Like the bass roll-off function on some microphones, the bass cut switch on a mixer channel imposes a progressive reduction in low frequencies, rather than a ‘brick wall’ cutoff at 100Hz. But you might wonder why we would even want such a function, given that the circuits of the mixing console have a bandwidth that theoretically goes all the way down to 20Hz.

    On most of the channels – unless they are specifically carrying bass instruments – there is no meaningful content below 100Hz, just a certain amount of ‘stage rumble’ and unwanted LF spill that will muddy the mix and cause the subwoofers to work extra hard reproducing sound we don’t even want. Given that many subwoofers have very little output down at 20Hz, it is important that we use the capabilities of the rig in the last octave wisely.

    These are issues we’ll return to in later posts, when we look at operating a mixing console. At that point, we’ll consider the practical aspects when adjusting EQ to suit different types of input, whether vocals or instruments.

    Next topic, the Auxiliary (Aux) mixes...
     
    peeweepete likes this.
  6. 24 track

    24 track Doctor of Teleocity Silver Supporter

    Posts:
    13,465
    Joined:
    Nov 6, 2014
    Location:
    kamloops bc

    you've put alot of thought and effort into this thread kudos to you!, I havent read the whole thing through yet but I will , there are alot of us sound guys on this site , and our accumulated experience is some thing we take for granted , so its great to see some take the time to explain it , ( sorry to hijack here ) carry on!
     
    simoncroft and beagle like this.
  7. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    Screen Shot 2019-08-06 at 17.32.31.png

    The Aux Mixes Explained

    So far in this thread, our mixer has evolved a more sophisticated series of input channels than the basic mixer/amp we started with, but it still has only one output (well, two if we assume the unit is stereo). In reality, most modern mixers have multiple outputs, some of which feed the on-stage monitor mixes, while others are for external effects. The levels to these additional outputs are fed by the Auxiliary, or Aux, mixes.

    There are basically two types of Aux mix: ‘Pre Fade’ and ‘Post Fade’. The pre fade auxes are used for monitoring, and the post fade auxes are used for effects. This is shown in Diagram J above.

    As you can see, the pre fade aux mixes sit in the signal chain before the main channel fader. This means that the monitor mixes are not affected by any changes to the main mix. (If they were, it would be very distracting for the musicians on stage.) Conversely, the post fade aux mixes are in the signal chain after the main fader. This means that these mixes stay in proportion to the main mix. (For instance, when you bring down the level of a vocal channel, the reverb level reduces with it. If it didn’t, there would be more and more reverb on the vocal as the level of the channel fader was reduced.)
     
  8. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    It might be helpful at this point to look at entire mixer strips on the Soundcraft GB2 desk, so we can see where each group of controls sits in the scheme of things.

    Screen Shot 2019-08-06 at 17.39.58.png
     
  9. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    You’ll see that the mixing console above is equipped with six aux mixes. Two of these are pre fade, two are post fade, and the two in the middle can be switched pre or post. This means the same Front of House desk can support either four monitor mixes and two effects mixes, or two monitor mixes and four effects mixes.

    However, it’s fair to point out that multiple monitor mixes, tailored for different members of the band, are a luxury. Most TDPRI members probably count themselves lucky if even one decent monitor mix is provided.

    We all know how frustrating it can be as a performer when you are on stage and can’t hear yourself – or the rest of the band – very well. Obviously, the first person we blame is “the monkey behind the desk”, or whatever pet name we have for the sound engineer.

    Let’s turn the tables for a minute and ask ourselves: “How does the sound engineer have the slightest idea what the sound is like on-stage?” The glib answer is: “By pushing a button on the mixing console.” The less reassuring answer is: “With extreme difficulty.” At this point, we need to introduce some acronyms: PFL (which got a mention earlier in the thread) and AFL.

    PFL stands for Pre Fade Listen, while AFL stands for After Fade Listen. Exactly what they do, and the difference between them, can wait until another time. For now, it’s enough to know that these switches give the engineer a way to monitor either an input channel (PFL), or a group of channels (AFL) in isolation. That’s where the ‘push of a button’ part comes in.

    But the audience doesn’t want to hear a mix ‘under construction’, it wants to hear the main Front of House mix as it is supposed to sound – ie, as if someone behind the desk knows what they are doing. That makes sticking the monitor mixes though the main rig a strict no-no at any time after the sound check.

    When the band is actually performing a set, the only way the engineer can check a monitor mix is through headphones, or through a small speaker kept for that purpose somewhere near the mix position (often at the engineer’s feet). Either way, those headphones or small monitor speaker are now competing with an entire band in mid flow, so will be difficult to hear with any certainty. That’s where the “extreme difficulty” part of the job comes in!
     
    beagle and 24 track like this.
  10. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    Even during the ‘window of opportunity’ presented by the sound check, it’s worth considering that monitor mixes will sound different from the Front of House position compared to the way they sound of stage. For instance, the engineer may be able to hear the guitarist only too well, because his stack is up loud and pointed straight at the mix position. But the bass player may have difficulty hearing the guitarist, due to the directional nature of the guitar cabs.

    This means that the mix engineer’s idea of a good monitor mix is not at all the bass player’s idea of a good monitor mix. The best way for the bass player to resolve this situation is to ask the engineer: “Can I have more guitar in the monitors, please?” A polite, but specific, request is more likely to produce a usable response than yelling: “Hey monkey, how come I can’t hear anything up here?”

    Often, the sound through the stage monitors is unlike the sound through the main rig. This can be a frequent source of complaint from musicians: the sound we’re hearing is nothing like the sound out front. This disparity can be down to the fact that the main rig has been ‘tuned’ using graphic equalizers, whereas the monitors are not equipped with the same equalization for cost reasons.

    Another significant factor is down to the design of many traditional ‘wedge’ stage monitors. Often, these have increased mid-range emphasis to make the vocal range more prominent.
     
  11. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    At the risk of boggling your mind with more detail, there is a constant risk the engineer has to take into account. Stage monitor speakers are just as capable of causing feedback (howl-round) as the speakers in the main rig. If that happens, it can be a lot harder to pinpoint than when one of the mic channels causes feedback through the main system.

    Most mixing consoles have at least a ‘peak’ LED – and sometimes a complete bargraph meter – for each mic channel. This is useful when setting the initial input gains, but will also indicate if a channel is causing feedback. However, there is no similar provision on the pre fade aux monitor mixes. That means the mix engineer has to be very aware of possible causes of feedback, and to keep stage levels under control as much as possible.

    Diagram K (below) illustrates the kind of monitor situation that can cause trouble. Acoustic instruments are designed to amplify sound in their own right. For reasons I’ll happily detail in a later post, this can make them really prone to feedback. (Basically, that sax bell messes with the directional characteristic of the mic, turning it closer to omni.) The mix engineer’s nightmare begins when the player announces in a plaintive voice: “I can’t really hear myself, can you turn me up in the monitor please?” No prizes for guessing what happens when the volume from the monitor speaker increases…


    Screen Shot 2019-08-06 at 17.48.02.png
     
    beagle likes this.
  12. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    Of course, not all stage monitoring involves the traditional ‘wedge’ speakers. IEM (In Ear Monitoring) has become very popular, not least because the close-fitting earpieces help to isolate the performers from high levels of sound on stage, rather than simply adding to it. That’s another interesting topic that can wait for another day.

    Without sounding too cynical, the reality from most sound engineer’s point of view is fairly simple. The audience hears the Front of House sound, not the monitor mix(es). The venue owner is mainly guided by the reaction of the audience, as measured by sales of a) tickets, b) beer, or c) both. Pleasing those parties is how the engineer keeps his job. That leaves a small bunch of contracted labor on stage who require a mix all of their own… Although any professional sound engineer (by which I mean both caring and competent) will do their best to provide an act with a usable monitor mix, most of that thinking has to be done before the audience arrives.

    This is not just about dialing in monitor mixes at the desk: only lazy soundpersons leave the monitor wedges in the same place on the stage, night-after-night regardless of the line-up of the acts.

    NB – In concert touring rigs, the roles of Front of House Engineer and Monitor Engineer are separate, as are the consoles they operate. A Monitor console has a different format to a Front of House console, because the aux mixes are the whole point to the design, whereas there is no need for ‘main faders’, for the simple reason there is no ‘main mix’.
     
  13. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    Compared to monitor mixing, sending a post fade aux mix to an effects unit such as a reverb or delay is easy. Because it’s an integral part of the main mix, all you really need to do is turn up the effects sends for each channel until it sounds good. (Guidelines for what is likely to work will come in a later thread.)

    Providing the overall level of the aux mix is not so high it overloads the outboard unit’s input, all should be fine. Fortunately, there’s a master level for each aux mix that makes it easy to manage that.

    The effects still have to be returned to the mix however, which brings us to the next topic: routing, grouping and aux returns.
     
  14. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    At the risk of making this thread messier than it has to be, the is what Max Porras has come back to us with from E-V's Customer Support:

    1. Have you had dynamic mics returned for repair that have been damaged by connection to a 48V Phantom Power source?

    No, dynamic microphones don’t recognize phantom power as there is no “load” for the power, hence nowhere for it to go.​

    2. As a manufacturer, what are you findings regarding the possibility of 48V PP causing damage to a dynamic microphone?

    The only dynamic microphones that phantom power is known to cause damage to are ribbon microphones (although phantom powered versions of them do exist).​

    3. If there is a possibility of damage, is there a design feature within the mic that can protect it?

    There is not the possibility of damage to dynamic microphones with the exception of the aforementioned ribbon microphones.​

    Although Sennheiser won't respond until their marketing lady is back from vacation, all the informed opinion thus far says: phantom power does not damage dynamic mics, unless the circumstances are exceptional.
     
    Last edited: Aug 7, 2019
    PastorJay and beagle like this.
  15. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    Routing, Grouping and Aux Returns
    (Is it just me, or does that sound like something Bill Haley would have written?)

    In the last post, I explained how a Post Fade Aux Mix could be used to create an affect mix for a reverb or delay. Some mixing consoles offer internal digital effects – and often have an aux mix labeled FX to indicate that it sends to an internal processor – but it other instances, the mix will go to an external processor located in the outboard rack.

    The question you might want answering at this point is: “Where do external effects come back into the signal chain on the desk?” I’ll answer that question, as part of the bigger subject of today’s post, which is essentially how signals are routed around the system and how groups of channels can be controlled to make the process of mixing easier to manage.
     
  16. beyer160

    beyer160 Friend of Leo's

    Posts:
    2,663
    Joined:
    Aug 11, 2010
    Location:
    On Location
    I'm actually impressed that EV and Shure would come right out and say this, instead of hiding behind corporate doublespeak to avoid any possible liability of someone using their products improperly.
     
    beagle and simoncroft like this.
  17. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    Effects returns

    A lot of mixing consoles have dedicated ‘Effects Return’ or ‘Aux Input’ channels that introduce the signal from external units into the mix. On smaller desks, these often have little or no facilities beyond a line input, although some may offer an input level and possibly an EQ section.

    An important thing to understand about ‘effects return’ inputs is that they are not linked in any way to the signal path from an aux mix unless you chose them to be. For instance, if you take the output of Aux 4 to a reverb unit and connect the output of the reverb to Aux Input 1, they form what is called an ‘effects loop’ on a guitar amp. But if you didn’t need a return input for the reverb, you could equally use that Aux Input for any line level device you want. This is worth remembering when the main input channels on a mixer are all in use and you need somewhere to connect, say, an MP3 player that will only be used to provide music between the acts.

    Similarly, if you’d like a full set of EQs and a set of faders for the levels, there is nothing to stop you using the regular input channels for effects returns, if you have some to spare. If you have any stereo input channels, even better, but an adjacent pair of mono channels is almost as good. (You just need to pan them hard Left and Right… ah yes, we’ll cover the Pan control as part of our discussion on routing below.)

    The one thing you definitely DON’T want to do if you are using input channels for aux returns is open up the aux levels for that mix on those channels! Diagram L hopefully makes this clear. It’s a simplified view of a mixing console, and I’ve grayed out the controls that are not directly concerned with the signal path to and from the digital reverb.


    Screen Shot 2019-08-07 at 17.09.31.png
     
  18. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    The aux mix colored blue and labeled ‘post’ goes via the master level control for that mix to the input of the outboard unit, a reverb in this case. It’s being returned to the mix via the last two input channels. If we turned up the aux levels on those last two channels, the result would be a feedback loop, which is to be avoided at all costs. (Let’s put it this way, people might forget to thank you at the end of the concert.)

    To be absolutely clear about this, if we had multiple aux mixes (which is often the case) we only have to worry about sending a specific aux mix back on itself. So if our effect is on Aux 2, it’s opening up the Aux 2 mix on the return channels that would cause a feedback loop.

    (At this point, you may be starting to understand why smaller mixing desks tend to have dedicated effects return input that are hard wired to the stereo output, so that they can be used to add effects to the mix, with no possibility of sending the signal somewhere it isn’t wanted. Unfortunately, to get the maximum flexibility from any mix system, you also have to take responsibility for incorrectly routing signals. Many years ago, I attended a lecture by the producer/engineer Alan Parsons, where he played an example of a Pink Floyd take he’d ruined by sending the reverb that was only supposed to be in the monitor mix also to multitrack. As a result, it was printed across all the tracks and couldn’t be removed.)
     
  19. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    Why are there buses in my mixing console?

    So far, we’ve assumed (or maybe just glossed over) that the faders from all our input channels go straight to a main mix, or more likely, main the left and right Mix Buses. You’ll find the term ‘buses’, or ‘busses’ used a lot in any discussion of mixing console architecture. That’s because they are fundamental to the way signals are routed and combined in the mixer.

    In everyday life, a ‘bus’ is something we can all jump on and ride if we all want to go on the same route. We might not all get on at the same place, but we’re on a particular bus because we all need to go on the same route. The ‘buses’ in mix systems are exactly the same. In fact, if you open up a traditional analogue mixing console, you can actually see the ‘bus bars’ linking each channel like routes on a transport map.

    Diagram M shows detail from the block diagram of the output section of a Soundcraft GB mixing console. After you’ve looked at it for a bit, you might think: “That’s just like a train map for a foreign city.” You’d be exactly right, and if you are interested enough, I’m sure we can get to the stage where you can read that map with complete confidence. For now, let’s use the diagram to illustrate one fundamental concept. See those parallel horizontal lines at the bottom? They’re the buses.


    Screen Shot 2019-08-07 at 17.15.22.png
     
  20. simoncroft

    simoncroft Tele-Meister

    Age:
    63
    Posts:
    341
    Joined:
    May 29, 2011
    Location:
    Worthing, SE England
    The lowest bus is labeled ‘Aux 1-6’. If we took the actual console apart, that would be six separate conductive ‘bus bars’ stretching across the channels. The diagram only shows one because it is generally understood that this is simply ‘shorthand’ for the six separate routes a signal could take.

    Move up a couple of lines and you’ll see ‘Mix L-R’. You’ve probably immediately grasped that there are the buses that go to the main stereo output of the mixing console. On the simplest of mixers, every input channel is permanently routed to the Left & Right buses via the Pan controls, which enable the relative L/R levels to be set. Matching the position of the pan controls to the physical position of musicians on stage gives a more realistic stereo field to the mix.

    So far, simple enough, but if we move down a line, what is ‘GRP 1-4’ all about?

    The purpose of subgroups

    When we have more input channels to control than we have fingers on our hands, it’s time to consider whether we could manage the process more efficiently. Of course we can, and one approach is to ‘assign’ channels that have something in common to a ‘subgroup’ fader that will act as a master volume for them. So for instance, once we are happy with the drum mix, we could assign all the drums to a drum subgroup. Then we could do the same with all the vocals.

    This makes the job of mixing a lot easier, especially if we need to make quick decisions (such as the drummer seems to have got really loud since he had a beer break…)

    I’ll continue this theme on the next post. (For readers who are following this but already know what subgroups are, I’ll also get into VCAs, virtual hierarchical assignments on digital console and all of that, but some way down the line.)
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.