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PA HIRE IN & AROUND GLOUCESTERSHIRE |
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Hiring A PA System: What to Look For
If you need to hire a PA system for an event, you have to be sure it will meet all your requirements and - if you are promoting the event - the technical rider(s) of the performer(s).
Whilst the quality or the PA system and its components shouldn't be overlooked (see the PA System Design for more about this), most of the technical concerns for PA Hire are with quantity (which is easier to evaluate):
• How loud does the PA system need to be?
• How many microphones, stands, and DI boxes are required?
• How many mixer channels are required?
• How many monitors (speakers) are required?
• How many monitor mixes (auxiliary sends and amplifier channels) are required?
• How many (and which) processors are required?
How loud does the PA system need to be?
The required overall volume will ultimately be determined by two things: the programme content (you will need more output capability for Heavy Metal than you would for a conference); and the venue size.
The programme content determines how loud it needs to be at a typical audience position (from around 85db SPL for speech, to 110dB+ SPL for very loud music styles).
The venue size determines the distance from loudspeaker cabinets to a typical audience position. For small to medium-sized venues you can calculate the capability you require either by multiplying speaker cabinets of known peak output level, or hiring speaker cabinets with greater output capability.
To calculate the audience position level for a known cabinet output (dB SPL @ 1 metre), reduce the level by 6dB for every doubling of distance, or by 20dB for every tenfold increase of distance. For example:
a. Speaker cabinet output (Peak SPL @ 1m): 126dB
b. Distance from speaker to audience position: 20m
c. Level (Peak SPL) @ 20m: 100dB*
*20 metres represents 2 x 10 x 1 metre, so one doubling of distance loses 6dB, and a further tenfold increase loses a further 20dB.
To calculate the number of speaker cabinets required to reach a higher audience level, add 3dB for every doubling of speaker cabinets. For example:
a. Single speaker cabinet at 20m: 100dB
b. Required level: 106dB
b. Number of speaker cabinets required: 4†
†2 x 1 cabinets = 100 + 3dB = 103dB; 2 x 2 cabinets = 103 + 3dB = 106dB.
This method has some drawbacks, the most significant of which is poor phase alignment between separate speaker cabinets (which can produce directly audible effects to audience members if they move around, and produce other artefacts - tonal colouration and reduction in intelligibility - if the audience is stationary.
Generally, therefore, it is better to use louder speaker cabinets (or cabinets which are specifically designed to be arrayed in multiples) where the overall level requires more than two cabinets - or two stacks if a multi-enclosure PA system is used - per side.
With larger audience areas the practical choices are limited: using only clustered speakers in front of the stage results in very uneven levels between front and back positions in the audience (it is much too loud near the stage, and not loud enough at the back).
Before the advent of line-array systems the solution was to have secondary (delayed) speakers or clusters part of the way into the audience area. This method brings several headaches, principally finding safe positions for secondary systems in the audience area that do not block audience sight-lines to the stage, and running cable to them. Currently, for audience sizes of more than a couple of thousand, line-array speaker systems are the most straightforward option, and the best solution in acoustic terms.
Generally, you should hire a PA system that will comfortably manage the required output level (i.e. with several dB of headroom), but you can see from the calculations above that if you want 3dB of headroom you need twice as many cabinets. This won't double the budget, but will certainly increase it substantially.
How many microphones, stands, and DI boxes are required?
This will be fairly easy to work out if the programme content is known, and if each contributor to the programme has provided Technical Specifications, a Channel List, and a Stage Plan. Generally you should hire at least a couple of spare microphones and DI boxes on top of whatever is actually specified (and if you are hiring microphones separately, make sure you get clips with them and hire enough cables as well as stands).
If tech specs, channel list and stage plan are missing, your best bet is to chase the participants or their agents for them.
How many mixer channels are required?
If the production only involves a single act, the channel requirements will be determined by their technical rider. Otherwise, if you know about the number of inputs (microphones and DI boxes) you will be more or less OK if you add four more channels (or two stereo channels) to allow for effects returns, plus a couple more for possible changes on the night.
If there is more than one act you should ideally hire a mixer with enough channels for at least two acts (both of them if there are only two), so that channel settings do not have to be changed between any two acts. This allows each act to retain their soundcheck channel settings.
What mixer you need to hire is not only informed by the number of channels: the number of monitor mixes required will determine the number of pre-fade auxiliaries needed.
How many monitors (speakers) are required?
This question should be asked in conjunction with the next. The number of monitors required will always be at least equal to the number of monitor channels required, but can often be more if:
• One or more performers require a pair of monitors (lead vocalists often require two);
• Some performers will be sharing the same monitor channel.
You should assume that each individual performer will require his/her own monitor, and if possible their own mix as well.
How many monitor mixes (auxiliary sends and amplifier channels) are required?
This question has two parts: auxiliary sends (does the mixer have enough?), and amplifier channels. As most PA amplifiers are two-channel, you need to hire at least one amplifier for every two monitor mixes. I say at least because if the monitors are bi-amped (some are) you will need to hire one two-channel amplifier per mix.
The mixer should also have at least two post-fade auxiliary sends for reverb and effects (see below).
How many (and which) processors are required?
Again, this will usually be determined by the performers' technical rider or technical specifications, but as a default you should hire:
1. Enough (minimum 2U) graphic equalisers to provide one EQ channel for Mix Left, Mix Right, and each individual monitor mix;
2. Two reasonably good multi-effects units;
2. 3. If there is a drum-kit, two noise gates (i.e. four channels), or at least compressors that include basic noise gates.
You should also look for clear information about what the PA Hire company will be providing (see our suggested defaults on the PA Hire checklist page).
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