Backline Alignment. With systems for music or theatre application, another potential alignment problem can occur when sources on stage are loud enough to be heard by the audience at the same (or nearly the same) level of these sources are heard through the loudspeaker system.
Because the loudspeaker system is almost always positioned in front of sources, the combined sound of the direct source with the reinforced source will be smeared to varying degrees.
This is most commonly encountered with a pop or rock band using a trap drum set. Snare and kick drums are typically loud enough to project into the house at fairly high volume levels. If a show is mixed at conservative to “reasonably loud” sound pressure levels, most of the audience hears both the direct source and the reinforced source.
Due to the percussive nature of these instruments and the degree of misalignment that can occur (as much as 30 milliseconds), the “smearing” that results has a very destructive effect on the clarity or definition of the source signals.
Add the typically overly reverberant nature of most performance spaces, and you have an all-too-familiar situation where it’s next to impossible to produce a distinct rhythmic sound. More melodic or sustained instruments, as well as those lower in source level, are less affected.
But spoken word—in particular that of upstage actors projecting well—will display a decrease in intelligibility from this misalignment. Localization of sources to the stage can also be compromised. Other musical sources that can be hindered include brass instruments, bass guitar, electric guitar and other percussion instruments.
Fortunately, it is relatively easy to minimize this problem by delaying the front-of-house loudspeaker system so that the arrival time of the direct sound is more closely in sync with the reinforced sound. Synchronizing to the loudest acoustic source on the stage, such as the snare drum, provides much improved alignment for the majority of sound sources located elsewhere on the stage.
Those “Pesky” Stage Monitors. In amplified pop music productions and some theatrical events, it is necessary to provide stage monitors for performers. When stage monitors are run at high volume levels in enclosed stage areas, sound from these loudspeakers is projected into the rear wall and may be reflected back into the house.
In this scenario even sources that are low in natural output (such as pop vocals) are reinforced in the stage monitors to a point that reflected sound becomes the source for misaligned combined energy in the audience area. Interestingly, there is also a misalignment problem for those using the stage monitors. The wrap-around of the front-of-house loudspeakers can arrive at the performers’ ears at a high enough level that it can effect their perception of things like timing cues.
On a crowded stage, performers also hear leakage from nearby stage monitors and/or stage instrument amplifiers. Almost unconsciously, they ask for more stage monitor level and without the guidance of a seasoned audio crew, this escalation of levels can easily reach a point where there is risk of hearing damage.
Further, the excess stage energy can impact front-of-house sound in two ways: stage monitor sound is reflected out into the house, and it can leak into open microphones. It can sometimes be helpful to delay signal feeding the stage monitors, so that the wrap-around from the front-of-house loudspeakers is in alignment with the stage monitors. However, one must be cautious not to apply so much delay that the timing of performers is affected.
The use of in-ear monitoring (IEM) systems has become far more common, and in general, it can help minimize many of the time-based problems that can be caused by stage monitors. But some issues, such as the aural conflict caused by front-of-house loudspeaker wrap around, will still be a hindrance and therefore remain likely candidates for the corrective measures outlined in this article.
Tom Young served for decades as a highly respected electro-acoustic consultant, sound designer, and mix engineer. He also moderated the Church Sound Forum on ProSoundWeb for several years. width=