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Microphones


Dynamic, directional mics are most common. This is to reduce feedback possibilities. Close miking is standard and the lack of sensitivity will aid in off-axis rejection. Knowledge of the null point of the polar pattern will also help.

FIGURE 2



Mic Splitter Box/Junction Box

This splits the signal to go to both F.O.H. (Front of House) and monitor consoles. A simple paralel split is not suitable as this could induce earth loops between F.O.H. and monitor console.

FIGURE 3

Transformer splitter box:

FIGURE 4

The feed to the F.O.H. console is a paralel split, so that phanton power would not be blocked. Active mic splitter boxes also exist (transistor based). These can often provide phantom power.

Tip: Keep lightning system as far as possible from sound system. Some concerts have a second generator only for lightning.



Monitor Console

Typically matrix. This makes easier to perform simultaneious mixes.

FIGURE 5



GEQ (mon)

Used to achive maximum levels before feedback (ringing out a room).



Feedback Buster

This identifies the envelope of feedback and automatically atenuates the offending frequencies.

Tips to avoid feedback:

i. Try phase reversing the input signal.
ii. Make sure the singer holds the mic properly. Holding the microphone on the head closes its rear port resulting into a omnidirectional polar pattern microphone.


Limiter

A compressor, with very high ratio and fast attack time. This is to protect the equipment (especially from amplifier clipping).



Monitors


FIGURE 6

 
Backline
    Name given to the amps that are on stage, in other words, the amps/cabinets of the band (e.g. 'Marshall stack').
 
Drum fill
    A full-bandwidth monitoring system (woofer, mid-range & tweeter) for the drummer.
 
Wedges
    These are wedge-shaped monitors, with a narrow dispersion angle. Used for individual on-stage mixer.
 
Side fills
    These fill the stage with sound so that the musicians can move about. These are the greatest cause of feed back.
 
In-ear monitors
    These enable high fidelity, discrete mixer, with total freedom of movement. They can almost eliminate the chances of feedback.
   
Problems associated with these include:
   
limited number of legal radio frequencies.
   
radio interference.
   
sense of 'isolation'.
   
Future possibilities:
   
Extra high frequency telephone signals for in-ear monitors.
   
'Audio spotlite' system


F.O.H. Console

This can be any type of console, but a dedicated live console will have:
- Phase reverse buttons,
- Lots of auxes (12 or more),
- grouping capabilities (and VCA grouping),
- non-destructive solo capabilities (PFL, Pre Fader Listening) - the F.O.H is fed via the stereo output, not the monitoring output.

Digital consoles are becoming more popular, as features such as recall are very useful.

Tips for mixing live sound:

i. Put vocal slightly higher than you would in a 'studio mix'.
ii. When mixing drums, start with the overheads.
iii. In a small venue, start with the F.O.H., then the monitors.
iv. Don't be afraid of using 'drastic' EQ.

Should you mix in stereo? Easier in small venues. Sometimes used in larger venues for effects.

 
F.O.H. console position
    A critical listening position is not desired. A 'common' listening position is better.
   
Front/Back placement:
   

No closer to the stage than the distance between the speakers. No further than twice the distance between the speakers from the stage.


FIGURE 7

 

   
Left/Right placement:
   
Slightly off-centre will give the comb filter FX that the audience will hear. However, this is a matter of personal opinion.


GEQ (F.O.H.)

This is used to obtain a flat frequency response (tuning a system room).



Cross-over

Splits the input signal into different frequency ranges (e.g. hi, mid, lo).


 
Active cross-over
   

Deals with the level signals (before amps)

FIGURE 8

 
Passive cross-over
   

Deals with speaker level signals (after amps)

FIGURE 9

There are many advantages to using an active cross-over:

Each amp can be chosen for its specific frequency range (power capabilities, slew rate etc).
Inter-modulation distortion will not occur between the frequency ranges, e.g. with a passive x-over, when more bass power is needed, it can take its power from the high frequency. This will not happen with an active x-over.
Passive x-overs are inneficient, as the components will drop voltage across them.
The damping factor is reduced when passive x-over are used, as this effectively increases the output Z of the amplifier.


Loudspeakers

A loudspeaker is a transducer. They convert electrical energy into acoustic energy.

Different types:

A.
Electro Dynamic - coil or ribbon based.
B.
Piezo-Electric - crystal based.
C.
Electro-Static - capacitor based.


Categories:

1. Sub-woofer
    Used to extend the frequency range of a system as low as 20Hz to 30Hz (very rarely above 100Hz). Typically 'cone drivers' 12" to 60".
2. Woofer
    Low to lo-mid frequency range (below 500Hz). Typically 'cone drivers' 8" to 15".
3. Mid-range
    500Hz to 6000Hz, cone driver 4" to 12", compression driver 2.5" to 5".
4. Tweeter
    1000Hz and above, cone driver 2" to 5", compression driver 1.5" to 4".
5. Super-tweeter
    Used to extend the frequency range up to and above 20kHz, compression driver or piezo-driver.

A. Electro Dynamic loudspeaker

Coil Based

FIGURE 10

These work by electro-motive force. The coil is wrapped around the former. This sits in the air gap. The air gap is in a permanent magnetic field. When current passes through the coil this induces a magnetic field. This field reacts with the permanent magnetic field, moving the former in one direction or the other. This causes the cone to move, producing sound.

 
Cone
    Usually made of paper, though other materials (e.g. PVC, polypropelemne, Keular, aluminium etc) can be used. The material must be light, ridgid and easy to produce for consistancy.

Folded cone: Cut from a sheet of paper, folded and bonded.

FIGURE 11

Moulded cone: Paper pulp is sieved into a mould. This gives no seam.

 
Dome
    Acts as a dust cover for the voice coil.

 
Suspension
    Spider and surround. These:
   
Centralise the cone.
   
Ensure the coil can only move in the front/back axis.
   
Keep the voice coil centered in the air gap.
    The suspension will determine the compliance of the speaker - how easy it is to move the cone. High compliance, hard to move (and vice-versa).

 
Voice coil
   
A tightly wound coil of wire. Wrapped around the former, which is made of paper, card or mylar. The more coils, the stroger the magnetic force produced, but also the higher the Z and the greater the weight.

 
Magnet
    Made of:
   
1.
Ferrite - most common
   
2.
Almico - uncommon.
   
3.
Samarium Cobalt - very light, used in quality high frequency drivers and headphones.
   
4.
Neodymium - similar to Samarium Cobalt, but can loose its magnetic properties if sustained in high temperatures.
   
The more expensive materials give a more uniform and stronger magnetic field.

Design options:

i. Equal hung
    Voice coil and air gap are the same length. This means that part of the voice coil can leave the air gap in extreme excursions. This can lead to distortion. Only formed on hi-fi's and tweeters.
ii. Under hung
    Voice coil is shorter than the air gap. This means that the voice coil never leaves the air gap under normal operation. Found on studio quality monitors.
iii. Over hung
    The voice coil is longer than the air gap. This ensures that the same amount of the voice coil is always in the air gap, allowing for greater excursion. Found on woofers.

Ribbon Based

FIGURE 12

Uses the reverse action of a ribbon mic. Noy suitable for low frequencies, or high SPL. Used for tweeters, mostly in expensive domestic hi-fi. It gives a nice warm sound.



B. Piezo-Electric loudspeaker

FIGURE 13

Uses the same principle as a piezo-electric pick-up. If a voltage es applied across the plates of the bimorph, it will 'flex' in one direction or the other. The phase plug is to ensure phase coherency of hi frequency, due to distances travelled.

FIGURE 14



C. Electro-Static loudspeaker

FIGURE 15

If a negative charge is applied to the diaphragm, the diaphragm will move to the bad plate, and vice-versa the two fixed plates create a push-pull effect, giving an even response between them.

These 'panel' speakers are very expensive. They give very good quality sound (not much bass response). They have a 'figure of 8' dispersion, and should be placed at a distance from the walls.