Factors include speaker placement, symmetry and room size
Accurately monitoring 5.1 surround sound involves more than just adding four more loudspeakers to a stereo system.
Creating a successful 5.1 critical monitoring environment involves the right synergistic combination of such key elements as the room (location, dimensions, construction), acoustical treatment, noise control, loudspeakers (type, placement, mounting), electronic equipment, furniture, and more. Let's concentrate here on loudspeaker placement and some aspects of room design. It may sound obvious, but the room needs to be large enough to comfortably accommodate all the gear and people who need to work in it.
What is the size of the mixing console? Mixing consoles for 5.1 can be rather large. What outboard processing and patching is needed? How much work surface is needed for controllers, computer display monitors (for the audio console, digital audio workstations, other computers), telephone, keyboard/mouse, and even for the audio mixer's notes?
Don't forget an HDTV display or projector and screen, and of course, the five loudspeakers plus the subwoofer.
A good guide for loudspeaker placement, critical listening position, and room dimensions is offered by the International Telecommunications Union (ITU) Radiocommunications Assembly in the documents ITU-R BS.775-1 and ITU-R BS.1116-1.
The recommendations suggest placing the five loudspeakers (front left, center, right, and rear surround left and right) around the circumference of an imaginary circle at specific locations, with the key listening position in the center of this circle. The recommended radius of the circle (or the distance from the key listener to the loudspeakers) is between 6.5 to 13 feet.
(click thumbnail)ITU's recommended layout for 5-channel surround scheme. The subwoofer is not included here. (From the book "Total Recording," by Dave Moulton, KIQ Productions).
The center loudspeaker is placed along the centerline of the room aimed directly at the key listening position. (This is the 0 degree reference line.) The front left and right speakers are positioned 30 degrees on either side of this reference line (to the left and right of the center loudspeaker).
The ITU guidelines give a little more leeway in positioning the rear left and right loudspeakers as they suggest placement anywhere from 100 degrees to 120 degrees from the reference line. In the literature, 110 degrees seems to be a common starting point, but some designs have placed the rear surround loudspeakers at 125 degrees.
Since the loudspeakers are all at the same distance to the key listener, the sound emanating from each should reach the listener's ears at the same time. This is one important factor for properly perceiving which direction sound is coming from, something you'd want for 5.1 mixing.
Real room design involves compromise, and the ideal isn't always practical. The center loudspeaker may have to be placed above or below the front video monitor or screen. All the front loudspeakers may have to be placed in a straight line rather than along the circumference of the circle. In these types of cases, sound arrival times to the listener won't be the same from each loudspeaker. Audio signal delays in the shorter acoustical monitor paths are helpful to correct this.
It should go without saying--but it's surprising how many times this doesn't happen--that loudspeakers should be aimed at the listener's ears. The ITU guidelines also recommend that the acoustical axis of the loudspeakers should be positioned at the same vertical height as ear level, defined as 4 feet above the floor for a person seated.
This leaves some room open for manipulation for optimum response. How is the acoustical axis defined for each loudspeaker? Acoustical axis is not necessarily the geometric center of the loudspeaker box, and usually isn't. Other factors to take into account are: how large the loudspeakers are, their directional characteristics (polar pattern), and the method of mounting them.
Large loudspeakers with large bass drivers may need to be placed higher above the floor to avoid detrimental acoustic cancellations at the lower frequencies due to the interaction between the direct sound and the reflections from the floor. If this is the case, then check where the higher frequency drivers are aimed. The loudspeaker unit may now need to be tilted so that the listener is on-axis with the mid- and high-frequency drivers. Tilt angles, if needed shouldn't be too severe, no more than 15 degrees.
Moving on to the sixth loudspeaker, the low frequency subwoofer or .1 channel of the 5.1 mix. This unit doesn't necessarily need to be placed around the circumference of the circle. A single subwoofer is typically placed on the floor against the wall in the center of the room, or nearly so. It is sometimes placed in the corner of a room.
Genelec, the monitor loudspeaker manufacturer, recommends a distance of less than 3 feet between a subwoofer (set for the reproduction of low frequencies 85 Hz and below) and a wall that provides part of the low frequency loading.
How the loudspeakers are mounted within the room affect not only room size, but factors like room geometry, surface construction, and acoustical treatment.
Loudspeakers can be flush mounted into the walls, mounted on or near the walls, or positioned some distance (3-1/2 to 4 feet) away from the wall. The last option takes up the most room.
Symmetry around the critical listening position is very important for accurate surround sound reproduction.
The loudspeaker positioning as recommended by the ITU is symmetrical around the key listener. The five loudspeakers themselves should be the same and of good quality designed for professional use. (What constitutes a good quality loudspeaker is a topic for another article.) However, smaller-sized rear loudspeakers, ideally from the same family of products as the larger ones, are often used due to space considerations. Their mounting may differ from the front speakers as well, and they may also need to be tilted toward the key listener.
The room dimensions and shape should also be symmetrical. So should the acoustical treatment of the room, and the shape, size, positioning, acoustical properties of room furniture, cabinets, rack consoles, etc.
Room dimensions should not be integer multiples of each other (or even close to integer multiples) to avoid a bumpy low frequency response inside the room due to standing waves reinforcing each other rather than being more spread out.
A review of the literature gives some guidelines.
Where W=width (smaller floor dimension), L=length (longer floor dimension), and H=height of room:
1.1W/H² L/H² 4.5W/H -4
L/H <3 and W/H <3
At the same time, the room floor area should be greater than 320 square feet, and the total room volume less than 10,500 cubic feet.
As you work through the calculations, you will probably find the need for ceiling heights greater than 8 feet.
Some other tips regarding the room: avoid parallel surfaces, through room geometry and with acoustic treatment; and avoid early reflections from wall, ceiling, floor, console and other surfaces to the key listening position.
Another factor to consider is the "back deck" or the producer's seating area. Modifications to the ITU loudspeaker positioning recommendations may have to be made to accommodate the listening needs for people working in this area of the room.
This brief discussion shows that successful audio control room design is not a cookie cutter process. There are myriad intricate interactions among all the elements, and good designs find the best-fit solutions for each particular case, user goals, and applications.
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