There are recommendations from various organizations on the correct placement of loudspeakers for a 5.1 surround sound installation. In general, they all agree on the following positioning in the horizontal plane:
- The front left and right loudspeaker angles can be pushed out slightly (5° max) as there is a real source (centre channel) and not a phantom image formed by a stereo pair. However 30° is the preferred angle
- The rear loudspeakers can be position will less restriction as human source positional perception is not so good to the rear
Vertically all the monitors should be at the same height, however, if there is a viewing screen or window in the way, the centre channel can be raised by up to 7° - this is allowed due physchoacoustic limitations in the ability of humans to resolve vertical directions. Lowering the centre channel is not preferred as the negative effects of the floor reflection are increased. The monitors' acoustical axis should be placed at ear height or the monitors can be raised slightly to help reduce floor reflection effects and to give a less obstructed path from the monitor to the listening position. Remember to angle them down so that they still point towards the listening position.
The rear channel angles can be raised by more than the front monitors as humans have poor poisitional perception of sounds from the rear but the standards do not agree with one another on this: 15º is perhaps sensible maximum here.
In all cases the acoustical axis of the monitor must be angled both horizontally AND vertically towards the listening position to give the best on axis frequency response.
Finally, all of the monitors should be positioned the same distance from the listening position, i.e. the monitors should be placed in a circle with the listening position at the centre of the circle. Where this is not possible time delays must be used toensure that sound from each of the channels arrives at the same time. Approximately 1ms is required for every 30 cm positional difference so a delay unit with a 10 µsec resolution is the minimum requirement to give a 3 mm positional resolution accuracy. If the rear monitors are further away from the listening position then all 3 front channels must be delayed by the same amount. If the centre monitor is closer to the listening position then only a single channel delay unit is required. If the monitors are not aligned properly so that the sound arrives at the same time then ripples will be experienced in the midrange range at the listening position.
Time delay formula
tdelay = (dmax - dmon) / c
tdelay is the time delay needed to ensure that the monitor sound arrives at the same time as the others
dmax is the maximum distance of any of the monitors to the listening position
dmon is the distance of the closer monitor to the listening position
c is the speed of sound in air at 20°C at sea level = 344m/s
The centre channel monitor is 2.12 m from the listening position. The left and right monitors are both 2.46 m from the listening position The time delay required on the centre channel is:tdelay = (2.46 - 2.12) / 344 = 988 µs (so set the delay unit to 1ms)