Using Loudspeakers as Stage Monitors in PA Systems
A Monitor is often a multi-angled full-range loudspeaker lying on its side. However, speakers designed solely as monitors are also widely available, and choosing and using one involves different criteria.
Physically, it is a loudspeaker enclosure designed so that when it is placed on the floor the drivers point upwards at an angle. This enables a monitor to be aimed at an individual performer. Because it is often wedge-shaped, a monitor is sometimes called a wedge.
It is a loudspeaker used to enable performers to hear themselves and each other. The equalisation and mix used in a monitor will usually differ from what is used in the front-of-house system, and (if there are enough auxiliary sends available) will usually differ from the EQ and mix in any other monitor.
For information on loudspeakers and drivers, please see the section on full-range cabinets.
Most monitors are similar to small full-range cabinets, and many manufacturers now make multi-angled loudspeaker cabinets so that the same speaker can be used in either role. However, dedicated monitors are also available and these often have different dispersion characteristics (many dedicated monitors have a conical pattern): a monitor's main purpose is to achieve moderate to high sound pressure levels in a small area. If the sound spreads too widely or too far it can spill into vocal (and other) microphones.
Much as with any other speaker, you send the mix for each monitor to its own amplifier (or crossover and amplifiers, if it needs to be bi-amplified), and connect the amplifier to the monitor using loudspeaker cable.
Vocalists often want (and depending on the backline levels also sometimes need) to hear their own voices at high volume. Also, vocal mics frequently need relatively high channel gain. Finally, a vocalist's ears are inevitably very close to his or her microphone. This means that feedback from vocal monitors is a constant threat, and frequently an actual problem.
Because of this, positioning and equalisation of the monitors can be much more difficult than with front-of-house speakers. Ideally, the sound from a monitor should get to a particular performer's ears, and nowhere else (for this reason, in-ear systems are becoming increasingly common, and have a lot to recommend them). If a cardioid microphone (e.g. Shure SM58) is being used for vocals, position the microphone and monitor so that the microphone is pointing directly away from the monitor. This places the monitor where the microphone is least sensitive, and will give that monitor several extra decibels of valuable feedback rejection. If the lead singer (or any vocalist) has to have two monitors - the real need for this is debatable - it is better to use a hypercardioid mic (e.g. Shure Beta 58A) if one is available. With hypercardioid mics, try to position the monitors so that the back of the microphone is about 60 degrees off axis from the high-frequency horn (also, try to angle them so the high frequencies miss the microphone but get to the vocalist's ears). The chances are the vocalist will move the microphone or monitor(s) the second your back is turned, but at least you tried.
Reflection of sound can also be a problem: sound from a monitor can reflect from nearby surfaces (including the singer) into the vocal mic. Try to minimise this by keeping vocalists away from walls or other surfaces (if the vocalist's head is on the floor, you may have other problems!). Sound from the monitor can also reflect off the back wall of the stage, so if monitor levels are too high the reflected sound will interfere with the front-of-house mix.
Repositioning is often more effective than equalisation in reducing feedback. However, if repositioning doesn't work (or if the singer usually unclips the mic and moves around on stage anyway), EQ is all you have left: a 31-band graphic on each monitor send is essential (although a parametric EQ or notch filter can be as useful). As a quick fix - WAIT UNTIL NOBODY IS ON STAGE - use a variation of the feedback control method detailed on the Graphic EQ or Parametric EQ pages for each monitor send. The method of dealing with the EQ is the same, but close all channels except the mics nearest the monitor you wish to equalise. The two things that most increase the danger of feedback are proximity and high gain. If feedback is occurring in a particular monitor, look at the mics with the highest gain that are nearest to it first.
A monitor mix does not usually require substantial low-frequency output (drum fills may be an exception) as low-frequency sound is less directional, and performers will normally be able to hear the low frequencies from front-of-house cabinets and backline amplifiers. You can often get a little more useful level in vocal monitors if you roll off the lowest frequencies.
If you are using a PA system for after-dinner speeches, probably not. For most live music performances, monitors are more or less essential.
Monitors themselves are more important than many people realise. To perform at their best, musicians need to be able to hear themselves and each other clearly and coherently. Because of some of the difficulties in achieving a usable monitor sound (see above), speakers with poor pattern control and/or an uneven frequency response (on or off axis) are probably more trouble than they are worth. Get the best you can afford. It's also worth looking at mirrored pairs (or symmetrical multi-angled cabinets), if you're likely to meet vocalists who demand them.
Where more than one monitor is used on a single amplifier channel, all monitors on that channel should be of the same type (you can't EQ for two different types of speaker with a single EQ channel). In practice it is generally best if all the monitors are the same (with the possible exception of the drum monitor). If an on-stage monitor desk is being used, it is useful for the monitor engineer to have his own monitor (AKA Listen Wedge), and if all the monitors are the same he will then be able to hear any individual mix in his own monitor exactly as it sounds to the performer. If the loudspeakers themselves are suitable for the purpose, consider using the same type of speaker throughout your system (i.e. for monitors and front-of-house speakers), as this will increase system flexibility, as well as maintaining consistency between what the audience and the performers hear.
Separate mixers for front-of-house and monitors are ideal, but most smaller productions will run monitor and front-of-house mixes from the main (front-of-house) desk. This introduces the question of how many separate mixes are available. Smaller and/or older desks tend to have fewer auxiliaries available for monitors (two is fairly common). Most modern full-scale concert desks have at least four, while dedicated monitor consoles usually have more than six (the Soundcraft SM12 has - you guessed! - 12). Obviously you will need at least as many monitors (and graphic EQ and amplifier channels) as you have monitor mixes if you want to use all of them. Conversely, your desk will need at least as many available mixes as you have monitors if you want a different mix in each. If you are planning to run more than one monitor from each send, avoid 4Ω monitors unless you want to double up on amplifiers unnecessarily.