Stereo: Comments from Bose
- Update 2021
- All the comments below are equally applicable to the Bose L1 Pro products (PRO8, PRO16, PRO32)
- An L1 system can project a wide stereo field on stage and into the audience -- particularly the Model I and the Model II. Because the I and II are essentially 14' line arrays (7' speaker with a 7' mirror image source created by the floor reflection) they project sound at much less loss with distance than a conventional speaker. This has the effect of greatly enhancing the size of the stereo field.
It's a mistake to place stereo speakers too closely together. Not only does the area over which listeners hear stereo sound shrink but you can get undesirable interference effects caused when the same (or very nearly the same) signal is played simultaneously through two or more like speakers. (Multi-source interference can cause audible dropouts in tone.)
For these reasons, I'd recommend spacing L1 systems used for stereo at least 10' apart. There are competing interests here. A player located too close to one of the L1 systems spaced too far apart won't hear stereo — but his or her audience will. On the other hand, space the L1 systems closer so that the player gets stereo and you won't project stereo to the audience.
A player located roughly between two spaced L1 systems will hear stereo well and many in the audience will too. 10-20' feels good to me for this kind of setup although I don't want to have this come across as more than a rule of thumb.
Our system has such a gradual change in sound pressure with distance that a pair of systems are clearly and more equally heard way off-axis. This is the quality that gives a pair of L1's such a wide stereo image "sweet spot". It's also a quality that leads to bad dual-mono. In a dual-mono setup where, say, the vocal is center-panned, if you are off-axis, you might hear both systems equally in level, but they will arrive at your ears at different times, corresponding to the difference in distances between the straight-line paths of the two L1's. At a half-wavelength path difference, you will get a complete cancellation where you are listening and above this frequency, you will get, alternatively, reinforcement, then cancellation and so on as frequency goes higher. You will also get a horizontal polar pattern that starts to look like one of those cartoon-character gloves. These are called "lobes" or "fingering" and the higher you go in frequency, more lobes (or fingers) will show up. And so, as you walk in an arc around the pair, you will hear sound get loud, soft, loud, soft and so on. In contrast, a single L1 has an almost perfect, arc-like polar pattern throughout most of its range.
The minor degradation of midrange interference off the centerline probably cannot be avoided when recorded stereo material is played on any speakers. This is one of the compromises of the stereo format itself.
More reading: more articles and references to Stereo
See also: Two L1®s in the Same Sound Field