D4 - Live Sound

8/7/2019 D4 - Live Sound

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D4: LIVE SOUND ENGINEERING... THEORYThe role of an audio engineer is no longer exclusive to the recording studio. The skill set of a modern engineer extends

to video, radio, graphics and many more. One of the key growth areas for finding employment opportunities in thesector, is that of live sound engineering.

The theory of live sound is one which combines the two things present in most

musical engineering, technical ability and intuition. Each one is as important as

the other, a live sound engineer has to be able to be a master of both their

equipment/set up, and of the live sound mix which can differ vastly from an in

studio mix, applying different techniques and principles.

At the core of the live sound set up are 2 things, the mixing desk and thespeaker system/rig. The applications of the mixing desk in a live situation

show many differences compared to its use in a studio, even in the earliest

stages such as choosing the type of desk! For instance, in most instances a

live engineer would lean towards using a digital mixing desk as apposed to the

generally more desired analogue desk in the studio situation. One of the prime

reasons for this is that a digital desk can contain audio processing equipment

within its interface, such as gating and compression. These are available at

the touch of a button, as apposed to the outboard equipment used in a stu-

dio. This feature can help save valuable time when mixing a live event, this

example shows one of many advantages of using a digital desk for live.

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Live in Cotgrave...

An image capturing the digital

yamaha desk, courtesy of MerlinPAs live sound engineers.

LIVE SOUND

Tim Price FdSc 2A 15th January 2010


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As well as there being 2 types of mixing desks, in the live sound sector

there are 2 main uses for a desk. The most obvious is for bringing all the

signals to the main speakers and creating the mix, this is known as front of

house (FOH). The other is for stage monitoring, creating a mix or separate

mixes for the musicians particular taste(s). At bigger events, often there are 2

desks in play, one for the front of house and one specifically for the on stage

monitoring.

FOH mixers

Front of house mixers usually come with a set number of input channels;

12, 16, 24, 32, 40 and 48 channels are what is usually found, specialized

mixers used for bigger rigs can be sizably larger. The use of this large num-

ber of channels can range from the close micing of a drum kit and guitar

amps, to the music used between acts and pre recorded tracks used in a

set. Another use for having so many channels is if a show with many acts on

the bill is taking place, and bands settings need to be set. However, with the

advancement of digital desks with recall systems and flying fades, needingto manually save a live mix has almost become a thing of the past.

The features on a mixing console vary with the expense of the desk, but

the regular features include: - Input sockets (typically XLRs for mic and DI signals)

- Phantom power, phase inversion (important for sorting out phase issues in

a live mix, especially is the mix contains identical signals, e.g. the mix and

line signals from a bass guitar)

- Pad (reduces the signal by a set amount, typically -10db, this is useful for

controlling over loud signals, such as a over eager guitarists amp)

- Gain control (to set each signal at a sensible level before mixing)

- Equalisation (this comes on all desks, both live and studio based, however

in live the tendency is to use an external graphic Eq in order to meticulously

notch out those problem frequencies)

- High and Low pass filters

- Panning

- Auxiliary send (used for effects and vastly used later for the monitor desks

One feature of most, if not all desks, that is utilised greatly in live sound

are the sub group faders. This means that once a sub mix (e.g. the drum kit)

is mixed, the faders can be assigned to a sub group with a singular fader to

bring the whole mixed up or down as one. This is a great time saving device

when mixing on your feet! Also on more sophisticated desks, other groups

such as solo and mute groups are present so certain parts

can be muted together if needs be.

Monitor Mixes.

In a smaller set up, the monitor mixes are usually

done from using the Aux sends. This will work but

there are not usually enough of these to create a

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EM LIVE LOUNGE

ELECTRIC MAYHEM:

LIVE LOUNGE

Pictures from group E1s

live lounge session

recording ...

.1 - Setting up the drums

.2 - Crate guitar amp

.3 - Neve VR analogue! mixing desk


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THE MONITOR SET UP IS MORE PRONE TOFEEDBACK......so several types of eq may be used live sound manual

comprehensive monitor mix, this is generally because if there

are 4 auxs on a small desk, 3 and 4 will be dedicated to usingeffects, and therefore will be set as post-fader.

Whilst there are many large scale gigs around, so the majority

of mixers manufactured and sold are of a budget and mid

ranged level. Therefore dedicated monitor desks are quite

rare. Because the application of these desks are different, they

obviously have difference features. One thing to note is that

they contain many auxiliary sends, enough to house one mix for each

musician, there are also many groups, usually ranging from 8 to 24.

As well as these advanced features, monitor mixes also contain many controls identical so the FOH desks, thought

they can still often be used in a different manner. These features include: Phantom power, PFL/AFL, FX sends and

returns (used for effects such as vanity reverb), and of course, Eq. On monitor set ups, Eq are used more because it

is more prone to feedback, being closer to the microphones on stage.

Power amps

These are essential to a live sound set up, they work as power converters as the signals in the PA up to the point of

the power amps are too lightweight in current and voltage for the speaker system to recognize and the sound to be

loud enough for a gig. They work the same way as pre-amps in the actual mixing desk.

The way a power amp works is that the currents signal swing increased to raise it to the optimum level for thespeakers, and amplifiers voltage swing can be elevated by between 10 and 50 times (compared to the line levels

which can be as low as 2 volts). Power amps have many classes (A through to H) and generally come with balanced

XLR inputs.

Signal & Effects Processing

Processes to effect the live sound signals come both internally in the mixing desk and also as outboard equipment.

This is usually referred as the FX rack. These effects can used to control, clean up, blend and add to a live mix,

including compression, gating, reverbs, equalisation and delay, these processors are connected to the desk using

auxiliary sends or channel inserts...

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Gating can clean up the FOH sound,stopping unnecessary spill...live sound manualEQ - This comes in different, versatile forms. From the simple

3-band eqs found on budget mixing desks with hi, mid and

low eq pots, to the more elaborate forms such as graphic and

parametric.

Graphic equalisation is generally found when using monitors,

this is because, due to the fact that monitors are near to, and

have so many mics pointing towards them, the eq needs to be

very advanced in order to notch out those problem frequen-

cies. It is called a graphic eq because the notches used give u

an accurate representation of the curve that is formed when

changing the frequencies, they generally have 28 to 31 bands.

GEQs are usually outboard equipment, as they need to be big

enough to give quick access.

Parametric equalisation is the most powerful form of Eq. This is because not only will it allow an engineer to notch outthose pesky problem frequencies, but because it contains a Q control, this helps control the spread of a frequency

peak, this means that one is able to precisely effect the narrowest of areas if needs be.

Now that most desks used in live are digital, any engineers now use the built-in equalisation features that come

with the more high end models. These are usually coupled with a digital representation of the Eq. The advantages of

using digital as apposed to G and PEQs are that the eqs can in theory be perfectly matched channel by channel, and

these settings can be saved in an internal system. Also, in higher end systems, one is

able to see a visual of the sound coming through the channel, therefore seeing

which problem frequencies to notch out and how they are being affected in real

time.

Effects- such as reverb and delaycan be found as both internal, and outboard

equipment. These are essential to create a cohesive mix and comes even in most

budget mixers big enough to put on a gig. It can add different kinds of space to a

mix, pushing instruments back and forward in a mix, and generally blending the

sounds together as a whole, especially in outdoor events, where there is nothing for

the sound to bounce off. Reverb is generally coupled with some kind of Eq on an

Aux send channel, this can help to brighten the sound of a room artificially.

Delay, or echo, has been used in different forms in live sound since rock n roll

slap-back in the mid 50s. It has since been used on almost everything to create its

own sense of space and ambience. Most delays, be it outboard or internal, runsonce again through an aux send, and comes with features like delay time, decay

and usually a tap tempo button to adjust the speed.

Gates & compressors - these are essential in a live sound set up. Gates help to

cut out spill on single hit channels such as the snare and sometimes backing vo-

cals, this helps cutout spill which could otherwise make a mix seem muddy. They

work by setting a volume threshold at which the sound will be let through when

reached. Gates can come in outboard or onboard formats, and once again can be

useful as onboard equipment because visual representation can be a feature on

higher end desks. A more advanced feature on some gates is side-chain, this

means the gate can be control by things such as Eq, so instead of just a volume

threshold, the gate can be worked by a certain frequency (e.g. for feedback).

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AN EXAMPLE OF MERLIN PAS

SET-UP IN ACTION!


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Compressors are also very important pieces of equipment in a live sound set up. It is essential for controlling sounds,

tightening them up and making them more punchy. Compression can also be used to make sounds louder, bringing

up the quieter parts of the sound and raising it as a whole to make it more useable in a mix (this could be useful on

quieter instruments such as flutes). Another example of this type of compression being useful is in vocals, most vo-

calist, however experienced, have unexpected jumps of volume in their singing, so compression is pretty much the

only way to deal with this. Compressors work in a similar way to gates. They have a threshold which sets the level

above which the compression takes effect. The ratio tells the compressor how abruptly it will work (if a compressor is

above around 15:1 it becomes a limiter), and it features attack and release functions, to dictate how fast the

compressor will work and how quickly it will cease to have an

effect. Also, more advanced compressors again feature side-

chain functions.

Microphones

There are many kinds of microphones used in audio engineer-

ing. In live, those mics which are robust and durable are gen-

erally favoured for obvious reasons. The type of microphones

that are used in live work are mostly dynamic mics. The twomain reasons for this is because they posses the durability

previously mentioned (the other main type, condenser mics,

are quite delicate and can be damaged easily if for instance

they are used on a tour) and because they have a very direc-

tional quality, perfect for close micing which is used for the most part in live as the engineer is usually dealing with

micing guitar amps, kick/snare drums and of course vocals. Condenser mics are used, but only when needs be, such

as with drum overhead micing, where a bigger audio spread is needed. Mics with switchable polar patterns are also

desirable in live sound work. An example of this is using a figure of 8 pattern, if two vocalists were using the same

microphone, this pattern is equally sensitive at the front

and back of the mic.

A mics response patternchanges as soon as someonepicks it up, even if its designed tobe handheld... live sound manualSpeaker Systems

The speakers are the face of a PA system, they are

final part in puzzle that is live sound. This means they

have to be of a high quality, carefully matched to the

power amps to create the best sound for the audi-

ence. A speaker can be broken down into individual

parts. The cabinet, which encloses the speaker as a

whole, this can be shaped to emit the sound in a cer-

tain ways, usually to put out the best sound of

course! These cabs, like the microphones used in live

are built very solidly. This shows a vast difference to their studio equivalents, which are often catered much more to-

wards being aesthetically pleasing or compact.Inside this casing lies the heart of the speaker, usually referred to as the drive-unit.

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A drive unit, or driver, is a transducer like that found in a microphone, but with the difference that it works in reverse,

this means it takes electrical currents and transforms it into sound waves. Like in microphones, the most commonly

used transducer using moving coil principles (just like dynamic mics!), this is again because they are the most reliable

and economic. The diaphragm in a speaker is what the coil connects to, it used usually made from a paper pulp and

vibrates when the signal is passed through it. Most speakers contain two of these diaphragms, both held lightly in

place by a metal chassis. One for bass frequencies (which takes its name from this chassis) and another for the

higher range, referred to as tweeters, these tweeters require a light but firmer cone. Basically, bass frequencies

equals more vibration, which equals a bigger cone needed to cope with this.

The driver units are the real worker parts of the PA system. For most systems, a singular driver unit can cover a

chunk of audible range, around 3 octaves worth. Most high quality sound systems use multiple drivers to cover the

10 to 12 octaves that are present in human hearing, these are divided up by means of a crossover.

FOR MANY ARTISTS & AUDIENCE MEMBERS, THE SPEAKER CABS ARE THE PA!live sound manual

Common techniques & practices

Voicing the PA -Before the band or act arrives to soundcheck, a common practice is to make sure the PA sound

good on its own, this is known as voicing the PA. This is generally done by playing a song (it helps if you know it

well!) and blast it out through the speakers to make sure is sounds correct. This is a good way of sorting out the FOH

sound even before the audio signals are put in place, and can be used to identify problem frequencies in the room.

This way, when the band is in place, any problems that are occurring are more likely to be a problem with the sound

behind the PA, such as mic positions.

Pinking the room - Pinking is the process in which live sound engineers blast pink noise through the PA system

at a high volume level. Whilst the well known white noise is a concoction of every frequency, its pink equivalent is a

blast of random frequencies. This helps to identify problem frequencies in the pre-sound check.

RT60 management- RT60 is a reference to how long the reverb takes to decay below 60 decibels. For RT60

management in live sound, a loud noise is produced and the RT60 time calculated. This helps the engineers figure

out how much natural reverb is present in the room, and therefore how much artificial reverb to add later.

Inverse square law- Testing this process is much more relevant when mixing at bigger venues. The inverse

square law states that a quantities size is directly proportional to the distance it has travelled. This helps to decipher

how loud the PA should be, depending on how far the sound has to travel.

Mic placement- Mic placement is important as a practice, it helps to improve the mix and save time! Many live

engineers spend ours Eqing channels during soundcheck, when the simplest solution could be to spend more time

on getting the best direct sound with placing the mic on the sound source.

Avoiding monitor feedback- As mentioned earlier, because the on stage monitors are very near to, or have mi-

crophones pointing at them regularly, there is a tendency for feedback to occur. Therefore careful Eq using a graphic

or ideally a parametric eq, to identify the problem frequencies in the monitor mix before working on the FOH sound.

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D4 - Live Sound

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