Hi All,
I’d like to add a few points about sound systems.
A line array is simply another type of array—a group of speakers designed to cover a larger area than a single speaker (cabinet) could on its own. There is no intrinsic characteristic about it that dictates mono or stereo. The popularity of line arrays for touring systems resurfaced in the past few years with the advent of V-DOSC from L’Acoustics and many other manufacturers (Meyersound—M series, JBL VerTec, EAW KF760 and 730 series, McCauley, SLS, etc…) joining on the trend.
Please understand that this is a grossly simplified description of a line array (and just about everything else). Many white papers for both theory and application can be found online. The L’Acoustics and Meyersound (see the “Can Line Arrays Form Cylindrical Waves” for an interesting read) websites are great for such materials.
www.prosoundweb.com has recently started manufacturers’ forums to showcase their equipment.
Unlike a traditional array of cabinets, line arrays are designed to operate with a single column of boxes that each include low, mid, and high frequency devices. Subwoofers may also be added in another flown column or stacked on the stage or floor as deemed appropriate from rigging points and system designer choices.
Typically, most line array cabinets boast a 90 degree (-6dB down) horizontal (left to right) coverage and a narrow (5-10 degree) vertical (front to back) angle. Conventional cabinets are usually at least 30 x 30 degrees. Most line array enclosures place the LF drivers on the outside edges and the mid and HF drivers toward the middle since the distance between the smaller drivers must be quite small to keep the interaction between the devices constructive. If the distance between the drivers is too great, the line array is effectively broken and the drivers begin interacting in destructive and spurious manners.
As is true with many speaker enclosures, tightening the splay angle between boxes will extend the throw distance of the combined cabinets. Consequently, we frequently see the top of the array tightly packed to throw the long distances to the back of the room while the lower portion is curved since those cabinets will only be throwing a significantly shorter distance. While it may be a matter of infinite hope, the concept of the curved line-array is that the SPL will be very similar in the back of the room as they are in the front. While the inverse square law still remains in effect, as do all the laws of physics, the line array may see a less substantial level roll-off (around 3dB per doubling of distance as opposed to the traditional 6dB per doubling). It is heavily debated in the audio industry whether this truly results from the line array principles or is achieved through good system design, which includes the tight-packing of the upper boxes and can include some level tapering (setting the amp levels higher on the top than on the lower boxes).
Standard arrays are designed to behave quite similarly. Typically the larger, long-throw boxes are placed at the top and have as many cabinets per side as is needed to cover that side and any overlap necessary depending on mono or stereo format. Below that, there will be a row of shorter throw speakers and then a set of downfills to cover the shorter distance—the front of the venue. The very front of the room, since we are discussing large systems here, will typically have some side fills on the stage sides and front fills across the lip of the stage.
For those considering recording very close to a single cabinet, you may get lucky and have beautiful results. However, as can be seen when measuring a cabinet (using analysis equipment) the frequency response changes radically with mic position. Remember, a single cabinet is composed oftwo or usually more devices. If you are in the path of the HF or MF horn, it can sound very ugly (bright, harsh, shrill) until you are at least several feet away and the other drivers have integrated into the path of the horn.
When it comes to the issue of mono versus stereo this is typically the choice of the mix engineer since most major touring companies will provide a system that is set up for left/right (stereo) operation. While stereo by definition would imply differences in time arrival, in live sound, like most studio recorded material, we are really hearing amplitude differences achieved through left/right panning of instruments into various positions in the mix.
As a parallel to this, near-coincident (like ORTF) microphone techniques take advantage of time differences whereas coincident (x/y) utilize amplitude (level) differences. Still, these are both considered by most to be legitimate stereo microphone techniques.
For live sound applications, mix engineers have various approaches to creating a stereo image. Some common practices utilized by most touring acts are keyboards panned to at least the 10 and 2 O’Clock positions, toms spaced at least that far as well, with guitar effects being spread similarly by way of the stereo returns from the reverbs and other effects processors. Lead vocal and kick drum are typically straight up the middle (ie. mono).
The Grateful Dead, as many of us know, placed the musicians in the P.A. as they were seen on stage. String Cheese Incident is mixed with each instrument in stereo (ie. keyboards split left to right, guitar left to right, and drums panned into various positions). When Andy Meyer was mixing Widespread Panic (I can’t speak of what is done today), he used effects that sent sounds into different pan positions almost randomly. For example, a given conga did not always show up in the same left, right, or center position all the time, but nonetheless, it was not always in the center (ie. not mono).
While it is regularly argued that a stereo mix neglects people on the extremes of the venue, the general sentiment is that it is giving the best possible sound to the people who got the good seats. Fortunately, the region of “good seats” can extend well beyond the dead center position.
With a mono system, each side of the PA should be kept to its own respective side to prevent severe combing (destructive effects of common signal arriving out of time but at similar level). Comparatively, a system run in mono will usually have considerable overlap (ie. The left side covering the right side of the room and its own and vice versa). The effects are far less destructive since the content is different and it is possible to hear each side of the PA from spots off center.
An extension of this is a 270 degree seating configuration, arena style, where there are frequently outer boxes covering the extreme sides that can be reversed . Although the cabinets are actually far left or far right, they will be assigned the opposite feed so those sitting between them will still get a stereo mix, albeit reversed from what is being heard in the center of the room. Dylan’s engineer does this very effectively as did the Shaped Music crew (same folks as SCI uses) in their system for Bonnaroo (2nd outside stage).
To clarify our ongoing matter of sounding the same in all positions, this is impossible but no one here has actually said that. The concept expressed is that sound pressure levels should be maintained from front to back (vertical coverage of the P.A.). While the inverse square law is always in effect, the effective sound system designer and operator will set up a system that provides similar levels throughout the room. This does not imply that the usually favored positions, front and center, are not preferable to being out on the wings, under a balcony, or any other obviously undesirable position. While room anomalies are going to have significant impact on the sound, many people will have a similar experience insofar as perception of stereo and SPL.
As for a list of bands mixing in stereo, it is the standard. Generally, small rooms and large rooms utilize left/right mixes. In small houses, you will hear a lot from the stage, usually considered detrimental by system designers and mix engineers, since they will put less of those instruments in the PA in order to prevent detrimental interaction from the PA with the already loud stage source. You will find the exceptions but ask the engineers what they are doing. If they aren’t busy, many love to share more than you would ever expect.
Happy Recording Everybody,
Marc
Topic: Taping in Mono rooms (Read 19126 times)