PA Systems

Disclaimer: this was written in 1998 so some of the
concepts and terminology are “Old-School”.


Rough rule of thumb: 5 watts per person in audience


Vocal PA        6 - 12        300 - 500 watts    up to 100 people

Single 3-Way   12 - 16       1000 - 2000 w       around 200 people

Double 3-way   16 - 32       2000 - 5000 w       200 - 800 people

Triple or quad
3,4, 5-way     24 - 32       4000 - 10 kw        500 - 4,000 people

Size and Type of system:
The Size refers to the number of sets of speakers on each side:
The Type refers to how many different sorts of speakers are used

For example:
a single 3-way may have: 1 W (bass) Bin, 1 mid bin & 1 Horn (treble), on each side of the stage.
a double 4-way may have: 2 Subs (bass), 2 mids, 2 Horns & 2 sets of Rings (ultra-high) on each side.
There are many variations on this, ie;
a double 3-way might consist of a total of 4 Sub-lows and 4 Composite 2- way boxes. or a double 2-way might be 8 4560 Bins (low) and 4 horns (hi) The term ?-way refers to how the active crossover splits the sound before the amplifiers feed the speakers.
A further example; you could have a system described as being active 3-way/passive 4-way where an active 3-way crossover feeds 3 sets of amps driving a speaker system that contains subs, mids, horns and rings, within which there is a passive crossover which further splits the hi component into hi and ultra hi between the horns & rings.

A Vocal P A is generally used for vocals only, ie: the room or crowd is small enough that the instrument amps and drums are loud enough without additional re-inforcement. Except the Kick drum is usually also miked due to its poor position (close to
the floor) and its importance in most music forms.
Usualy the speaker system would be a 15″ speaker and 90° horn combination, but this is not regarded as 2-way as there is no active crossover involved and only 1 amp drives the speakers.
(the term 1-way is not used, the usual descriptive term is full-range box as the speakers handle both low and hi) These speaker boxes would contain a passive crossover which splits the sound low & hi between the speaker and horn.
A system such as this could be described as passive 2-way.

Number of channels relates to the number of inputs of the mixing desk. This is usually the maximum number of microphones. In a full mic up you will consider every vocal,
number of instruments and size of drum kit. For example the following is a typical mic list:

desk ch.  instrument          microphone

 1        kick drum           Beyer 88
 2        snare drum          Shure 57
 3        hi hats             condensor
 4        rack tom            Sennheiser 421
 5        floor tom           Sennheiser 421
 6        over head           condensor
 7        bass guitar         D I Box
 8        bass guitar amp     Shure 57
 9        left guitar amp     Shure 57
10        right guitar amp    Shure 57
11        keyboard            D I Box
12        sax                 bug
13        left vocal          Shure 58
14        centre vocal        Shure 58
15        right vocal         Shure 58
16        delay return
aux1      left reverb return
aux2      right reverb return


These figures are the most used and abused specifications that you will come across. Since the advent of composite speakers the conventions of classifying in terms of double, single, 3-way, 4-way etc have been going by the wayside. It seems very
clear cut to use a number to express the power of a system but you must realise that the Dynamics of a system will be a function of its Efficiency versus its Power. As yet there is no clear measurement of efficiency. Surprisingly the more modern composite systems are less efficient than the larger old style speaker systems, so a 4 kw composite system may be equal to a 3 kw double 3-way in terms of Sound Pressure Level.

S P L:

Sound pressure level is measured in deci-Bells. this is an even worse can of worms as dB s are measured in a logarithmic scale and the value of SPL measured in dB is also a function of distance from the source.


Another way of describing systems and the way in which the sound is divided between the different types of speakers. A Passive system might have one power amp driving a passive speaker box containing a high frequency Horn, low frequency Speaker and a Passive Crossover which divides the signal within the speaker box. An Active system will incorporate an Active crossover, which is an electronic processor, wired in
before a number of power amps enabling separate amps to be used for each type of speaker.


Capacitors pass high frequencies and resist low frequencies.
Inductors/Coils pass low frequencies and resist high frequencies.

An RLC circuit has a resonant frequency, the resistance of the speaker is a determining factor in the behaviour of a passive crossover.

The crossover frequency is a point 3dB below where the frequency attenuation drops away or comes back up. This is because it is not possible to stop dead at a particular
frequency, the frequency attenuation curves away according to a slope measured in deci-Bell per Octave. It is difficult to measure the point where the frequency starts to curve away as it starts as a gentle slope, it is easier to measure where it has dropped by 3dB. This point is of significance to us as a drop of 3dB is usually about half the sound power. If both your high and low speakers use the same crossover point you
should have a fairly even response as both speakers are each taking half the power for frequencies surrounding the crossover point.

A steeper attenuation slope is more desirable as this means that less of the low frequencies go to the high speaker and less of the high frequencies go to the low speaker. A steeper slope is achieved by a more complex crossover circuit. For high powered speaker systems it is more desirable to use an active crossover which is an electronic signal processor operating at line level before the power amps (separate amps for low and high frequencies).