Can I use two or more monitors next to each other (like in a PA system) or is it better to just use one large monitor on each channel?

This question has a simple answer: always use a single monitor for each reproduction channel. However, the explanation is a little more difficult to describe. Let us start with a typical application where loudspeaker arrays are used.

In PA systems for large concerts one needs to design a system that has very narrow directivity. This is so that the sound energy from one loudspeaker in an array does not interfere with the sound energy from the next loudspeaker in that array. The low frequencies have a long wavelength so mutual coupling effects are used to advantage to gain system efficiency. The mid and high frequencies are highly directional due to the horn-loading; for example, a 60/40 horn has 60° horizontal directivity and 40° vertical directivity. Comb filtering effects due to interference from adjacent sound sources are therefore avoided as the sound energies have little overlap. Also as the cabinets can be arrayed there is increased total SPL into the listening area and controlled coverage in the auditorium ensures that everyone can hear something.

The big disadvantage of most PA systems is that they do not have a smooth power response. This is because the low frequencies are omni-directional whereas the mid & high frequencies are highly directional. Also, the long and narrow horns, combined with the use of compression drivers, ensure high SPL's and high acoustic distortion. This is not an acceptable situation for studio monitoring.

The opposite is true for a monitoring system. GENELEC monitors are deliberately designed to have a wide directivity angle that is suitable for recording studios and other enclosed spaces specially designed for critical listening where absolute accuracy is required. The wide directivity is a function of the DCW™ shape and allows for a smooth transition between the on & off-axis sound energy thereby resulting in a smooth power response. This is critical for an accurate frequency response at the listening position and a wide horizontal listening window of consistent tonal characteristics along the length of the mixing console.

The disadvantage of this wide directivity is that if two monitors are used next to each other then severe comb filtering will result in the mid-range frequencies. Now, if you move around in front of the monitors the distance from each monitor varies and when two path lengths differ by a ½ wavelength there will be a full cancellation of that frequency - a spatially variable comb filter! This effect varies over distances as small as 1 or 2 cm and sounds little like a ‘phaser’ or ‘flanger’ effect! Also, the non-coherent summing of the two sources will only result in an average 3 dB increase of acoustical output at mid and high frequencies.

The exception to this rule is with subwoofers where there are advantages in positioning them close to each other. This is called mutual coupling.

So, in conclusion:

  • Use only one monitor for each reproduction channel for an accurate frequency response
  • To gain some SPL, reduce the listening distance or use a larger model in the GENELEC range