Fill-osophy: Proper Fill Goes Well Beyond Just Adding Loudspeakers
Looking at the possibilities and necessities in a wide range of SR situations...

August 13, 2014, by Tom Young


Front fill loudspeakers are required in many sound reinforcement situations because the main loudspeakers don’t cover the first few rows of seats, and in some productions, they’re also used because the mains (often far above the seats or positioned to the sides) result in skewed localization.

Another less frequently encountered need is to correct the mix balance as heard from the front rows, primarily in musical theater.

Even very well-designed, pattern-controlled loudspeakers, clusters, and arrays lose pattern control at middle and low frequencies, so if we attempt to cover the front rows with higher frequency energy from the loudspeakers above, we can end up washing the front of the stage or platform with (uncontrolled) middle and low frequency energy.

This results in muddy sound on the stage (which may force stage monitor levels up) plus increased potential for feedback from the front line microphones.

It’s also often the case that the loudspeakers we own (sound departments, rental companies) or which we have selected for other specific reasons (installed systems) simply do not cover from the front to rear rows, or they do, but with significant overlap or projecting sound onto ceiling surfaces.

Other factors which dictate where we locate the primary loudspeakers (and therefore impact how they cover) include architect/event planner/client influence (appearance) and structural needs.

Finally, on rock n’ roll stages (clubs, larger spaces and outdoors), the stage monitor and back line volume levels may, at times, be so loud (due to musician demands) that even if we have complete coverage from the primary loudspeakers, the quality of the mix heard directly in front of the stage has taken a severe hit. This problem is also present in many churches employing louder contemporary worship and/or modern gospel music.

Location, Location
For theatrical sound where the primary loudspeakers cover to the edge of the stage, those seated in the front rows have the experience of seeing the actors directly ahead but hearing their voices coming distinctly from above. Transparency takes a big hit!

There can also be audible comb filtering if the acoustic source and the reinforced sound from the loudspeakers are near equal in level because of the difference in their arrival times (location disparity). The need to prevent these distortions also occur in other events such as worship spaces and corporate work.

In these situations, we employ front fill loudspeakers that can be laid across the stage lip/apron, attached via yoke mounts (to stage or pit rail) or propped up in front of the stage. Often they must be visually masked or completely concealed. Front fills should be mounted as low as possible (for sight lines), but their depth of coverage is compromised when mounted too low.

Other than in large-scale concert productions, there is seldom a need for very large front fill loudspeakers with high output. Close proximity to the target seats/patrons plus the efficiency of modern-day loudspeakers allow us to employ compact short throw devices, and in most cases, the vertical coverage is not as big of a concern as the horizontal.

Given the mounting height we’re faced with, and the throw distance required, the vertical pattern typically needs to be in the 40-degree to 60-degree range.

There are two characteristics which effect what we need in the horizontal axis: wider coverage (so fewer devices are needed) and seamless coverage, with no holes nor and overlap across the target seats.

Figures 1 and 2 show two front fill systems in theater spaces. In Figure 1, the seats are far enough away that the 80-degree conical devices are well suited and few are required. In Figure 2, the front seats are much closer and require a larger quantity of wide coverage front fills for complete coverage across the front rows. This tendency for coverage to vary depending on seating versus loudspeaker locations exists in any other type of venue or event.

Figure 1: Three 80-degree (h) loudspeakers cover 17 seats across at front row. (Photo by Kai Harada)

Some stages and almost all platforms in worship spaces are low in elevation. When faced with this, along with the need to reach into several rows of seats, we must get the front fill loudspeakers as high as we can without encroaching into the line-of-sight of the audience.

Fortunately we can rely on the ability of middle and high frequencies to diffract, to some degree, around the listeners and often we can reach into 2-3 rows—“enough to get by”—when this condition exists.

Older and more traditional churches may provide platforms, pulpit design or other millwork at the front that can facilitate installing front fill loudspeakers at an appropriate height and without being conspicuously visible. In some cases we may even seek complete concealment which is great, provided that electro-acoustic performance is not compromised.

Figure 2: Five 100-degree (h) loudspeakers cover 13 seats across. (Photo by Tom Young)

Various Scenarios
Unlike the other classes of fill, front fills may need to be fed a different mix than that which is sent to the rest of the loudspeaker system. In musical theater, those who are seated close to the orchestra pit are often subjected to an imbalance of instruments over voices.

Correcting this with front fills is the sound designer’s responsibility and requires the cooperation of the music director (conductor) to rehearse and then control the pit band. In this case, the front fill mix (sent via a matrix or an aux mix bus) consists of voices, and the front fill levels are set so that these voices are blended with the instruments emanating from the pit. Localization is also improved as part of this process.

Figure 3: If the main loudspeakers have an A/B or A/B/C configuration, the approach also applies to fill loudspeakers. (Photo by Mark Frink)

Another loudspeaker-related requirement unique to some musical theater productions is the need for A/B or A/B/C loudspeakers in the primary loudspeaker system. Logically this will also be needed for front (and other) fill loudspeakers (Figure 3).

In large-scale concert sound, where there is an effort made to provide better sound at the very front, front fills need to have more horsepower than what we would need for smaller venues.

In this scale of production, we can almost always achieve what is needed with available loudspeakers such as stage monitors or medium size “full range” models that are yoked or propped up at the rear so they are aimed correctly, provided that these devices exhibit appropriate coverage patterns when in this orientation.

Figure 4: Front fills deployed on (left to right) large, medium and small concert stages.

Loudspeakers with rotatable high-frequency horns may be of benefit here (Figure 4). Although they may not be the best candidate, multiband line array elements may be used for front fill duty in this scale of venue—provided, of course, that the HF vertical coverage is not excessively narrow, mounting height is appropriate, and aiming is done correctly.

Finally; in large concert stages we more often see center subwoofer arrays at floor level which can place more demands on the front fills which often sit on, or directly above, the subwoofers.

Under & Over
Almost all point source and line array systems used in performance venues and churches are trimmed at a height that facilitates even coverage to all exposed seats, and in as compact a manner as possible.

In venues with balconies, there will be seats that are shadowed by the balcony overhang. and in many venues the ceiling, or the acoustic reflectors suspended over the audience, obstruct sound from reaching the rear-most and highest seating.

The devices used for under balcony fill are likely to vary from small (i.e., dual-5 or dual-6, 2-way) to medium small (single-8 or -10, 2-way) and installed as close to the ceiling as possible. Although multipurpose loudspeakers are often deployed for this, there are purpose-built fill devices available from a wide range of manufacturers.

Over balcony devices vary from medium-small (single-8 to single-10, 2-way)  up to mid-size 2- and 3-way horn-loaded designs. Most often, we use multipurpose products adapted to this application (Figure 5).

For portable sound applications such as Broadway show bus and truck tours, under and over balcony loudspeakers are likely to be hung from lighting pipes and/or box trusses that are already installed or brought in for lighting.

Rock n’ roll shows in road houses seldom carry their own fill loudspeakers and will tie into the installed fill systems. When these are aligned and equalized to the primary system, they can work very well. However, not taking the time to measure and correct the alignment, etc. can result in severely compromised sound for those who are seated in the “wrong” seats.

Figure 5: Under balcony and over balcony fill deployment.

With installed systems, we have the opportunity (and often the need) to position under and over balcony devices in a more streamlined manner. Yoked fill loudspeakers are most often used, provided there are structural elements behind the ceiling surfaces to anchor them to. In some cases we can work with the architect/client to embed the loudspeakers into the ceiling and color-match them.

Placement & Coverage
In general, it’s good practice to position overhead fill loudspeakers forward of the target seats so that, along with delay, the acoustic energy from these is localized toward the stage.

More often than not, under balcony ceilings do not work well with recessed ceiling loudspeakers due to the direction that the sound is projected as well as the resulting limited area that these will cover.  However, acceptable results can be achieved with such loudspeakers when the ceiling is curved or angled upwards, from front-to-back.

Where we place these devices and where their coverage begins are determined primarily by intuition and experience. It’s been my experience that sighting where the balcony occlusion begins to obstruct where one sees (while seated) the HF elements in the primary loudspeaker(s) simply doesn’t work and locating the fill devices 2-3 rows ahead of this point is necessary.

From this point to several feet above standing height at the rear wall determines the required vertical coverage. We should avoid reflections from the ceiling, especially when it is flat and low. Ceilings that are curved up, from front-to-back, are less of a concern. Reflections off untreated rear walls are seldom an issue because we typically angle the fill loudspeakers down and therefore the reflections are “grounded” (absorbed) by the audience.

Figure 6: Polar graphs showing coverage, in both axes, of a typical small-format 90-degree (h) by 40-degree (v) loudspeaker.

The depth of the under balcony seating determines whether one or two delay “rings” are required as well as the coverage required from these devices. Unless complex computer models are constructed (and you trust them), when determining the best-suited fill loudspeaker and how many are needed, one should (at the very least) utilize the manufacturer’s polar response graphs or beamwidth chart and pay particular attention to the coverage pattern over the 2 kHz -12.5 kHz range.

This frequency range significantly affects clarity/intelligibility as well as timing cues (localization). Note that in most cases the specified coverage is less in both axes at these higher frequencies (Figure 6).

Tom Young is the principal consultant at Electroacoustic Design Services (EDS) with both worship and performance space projects in and around New York City and throughout New England. EDS specializes in sound reinforcement system design, loudspeaker system measurement and optimization, acoustic design and noise reduction. He’s also the moderator of the Church Sound Forum here on PSW.


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Fill-osophy: Proper Fill Goes Well Beyond Just Adding Loudspeakers