Why are there level differences between the front and
the back?

Level differences arise primarily from the different distances of listeners from the system. A person standing a few meters from the PA receives significantly more energy than a listener located several dozen meters away.

Let's imagine a system set up by the stage. The first section of the audience is 5 m from the speakers, and the last is 40 m further away. This is a very large difference in distance and cannot be completely compensated for by just adjusting the volume.

If we increase the main PA level so that the last row receives sufficient SPL, the front rows will get even more energy. The problem does not go away — we are just increasing the level of the entire system.

Therefore, the primary goal of the project is to limit level differences resulting from geometry, not to mask them with greater power. 

Distance from the source and SPL

For an ideal point source in free field, every doubling of the distance results in a decrease of about 6 dB.

So if we have the following under certain conditions:

• a specified SPL at 5 m,
• at 10 m it will theoretically be about 6 dB lower,
• at 20 m it will be another approximately 6 dB lower,
• at 40 m it will again be about 6 dB lower.

A real system is not, of course, an ideal point source. The directivity of the speakers, their configuration, line array, reflections, and weather conditions affect the outcome.

The principle does, however, clearly illustrate the fundamental problem: distance has a huge impact on level.

Therefore, instead of trying to serve a very distant audience with one source at the stage, it is often better to provide another source closer to that part of the audience.

This is exactly how delay systems work. 

How does the height of suspension change the level distribution?

Raising the system can reduce the distance difference between the nearest and farthest listeners.

If the set is low, a person in the front row may be only a few meters away from it, while the last audience member is often much farther away.

After suspending the PA higher, the distance to the first row increases, but the distance to the back of the audience increases proportionally much less. The ratio of these two distances becomes more favorable.

This is one of the reasons why large systems are often suspended high.

However, this does not mean that "the higher, the better." Too much height can:

• leave the front rows out of coverage,
• increase the amount of energy hitting the ceiling,
• make it difficult to achieve the required SPL,
• create mechanical and rigging problems.

Therefore, the height should result from a specific design, and the missing coverage of the front rows can be supplemented with frontfills.

How does the directionality of the system help control energy?

Directionality determines where the speaker sends energy. In a large space, it is often more important than the maximum loudness of the set itself.

A well-chosen system should direct energy:

• toward the audience,
• to the appropriate width,
• to the proper range of distances.

At the same time, it should limit the energy hitting as much as possible:

• beyond the audience,
• into the walls,
• into the ceiling,
• onto the stage,
• into areas served by other sources.

If the system is too wide, a large part of its energy may go beyond the audience. If it is too narrow, the side sectors remain insufficiently amplified.

In larger productions, this is why different systems are often used for different zones instead of one source trying to cover everything.

The article describes the differences in directivity more broadly Linear system or classic PA setups – what to choose for the event?

When does one main system stop being sufficient?

One main system stops being sufficient when it cannot provide acceptable coverage for the entire audience without creating too large differences in level.

Typical situations include:

• very deep audience,
• wide audience,
• side sectors,
• front rows located under the system,
• balcony or mezzanine,
• spaces behind obstacles.

This does not automatically mean that a larger main PA needs to be used.

Often a better solution is to divide the space.

Instead of one very loud system, several sources can be used operating at lower levels and serving specific sectors.

Such a solution can improve both the uniformity of SPL and intelligibility.

Frontfill, outfill, and delay – three different tasks

Frontfill, outfill, and delay are not three names for the same solution. Each of these systems addresses a different coverage issue.

Frontfill serves the front rows. It is needed especially when the main system is high and its lower boundary coverage starts further from the stage.

Outfill expands coverage to the sides. It is used when the audience is wider than the useful characteristics of the main PA.

Delay line serves the further part of the deep space. Instead of increasing the level of the main system, additional sets are placed closer to distant listeners.

The most important thing is that each zone has a specific task. Adding more speakers without a plan can worsen the coherence of the system.

How to properly time-align additional systems?

Additional sources should be time-aligned so that their signal works with the sound coming from the main system.

For example, a listener standing by the delay tower still receives sound from the main PA, but it reaches them later due to the greater distance.

If the local delay set plays immediately, its signal may arrive before the sound from the stage.

Therefore, appropriate electronic delay is introduced.

The goal is not to mechanically enter values resulting solely from distance. It is necessary to consider:

• the actual location of the sources,
• the place of the transition zone,
• propagation,
• the character of both systems,
• the intended localization effect.

A properly configured delay should create the impression that the sound still comes from the direction of the stage.

The topic is detailed in the article How to amplify a long room or large space using a delay line?

Why can a poorly set delay worsen the sound?

A poorly set delay system can create a distinct echo, a loss of localization, or interference in the zone where both sources have similar levels.

If the delay plays too early, the listener may start localizing the sound in an additional speaker instead of on stage.

If it plays too late, it may be heard as a separate repeat.

The problem occurs especially in transition zones, where the level of the main PA and the delay system is similar.

Therefore, proper setup requires not only delay, but also:

• adjusting the level,
• controlling directionality,
• possible tonal correction,
• measuring the time relationship.

Delay should complement the main system, not compete with it.

How is SPL prediction used?

SPL prediction allows you to see the expected level distribution before physically building the system.

The model includes, among other things:

• geometry of the audience,
• position of the systems,
• height of suspension,
• angles of the arrays,
• configuration of the clusters,
• additional zones.

The software can then show a map of the expected level for different areas.

This makes it easy to notice:

• too high level in the front,
• drop at the back of the audience,
• lack of coverage from the side,
• areas of system overlap.

Prediction also allows for comparing variants.

You can check if a better result is achieved by:

• different main PA suspension,
• changing the angle of the arrays,
• adding outfills,
• additional delay,
• different type of system.

For large projects the sound system design based on the facility plan is much more predictable than choosing the system only during installation.

What should be verified with measurements during tuning?

Prediction describes the design, but after installation, the actual system needs to be checked.

During tuning, it is worth verifying primarily:

• levels in different places of the audience,
• temporal relationships between systems,
• frequency response,
• connection main PA and subwoofers,
• front fill,
• out fill,
• delay line,
• transition zones.

It is not enough to take measurements only at FOH.

If the goal is even coverage, you need to check several representative points:

• front,
• middle,
• back,
• left and right side,
• system overlap zones.

Measurements allow you to confirm whether the physical implementation matches the model and whether there were any differences from the design during installation.

However, DSP should not be treated as a tool for correcting geometry errors. If a speaker is aimed in the wrong direction, EQ will not change its physical directionality.

Example of a design for a wide and for a deep audience

Two spaces of similar area may require completely different systems.

Let's imagine the first audience: wide, but relatively shallow. Most of the audience is at a similar distance from the stage, but the extreme sections go beyond the coverage of the main PA.

In such a case, the solution may be:

• main PA,
• out fill,
• front fill at the stage.

Delay may not be needed at all, as the depth of the area is small.

The second case is a narrow but very deep audience. The main system can easily cover it horizontally, but the distance difference between the first and last row is huge.

Then the solution may be:

• main PA with controlled vertical directivity,
• frontfill,
• one or several delay systems.

In the first case, the main problem is width. In the second — distance.

To pokazuje, dlaczego the acoustics of the venue and the geometry of the audience affect the choice of sound system more than the number of participants.

Summary

Even sound coverage of a large space requires primarily control of the system's geometry. Attempting to compensate for large distances by simply increasing the SPL makes the front rows even louder, while the problem of the back of the audience remains unresolved effectively.

The height of suspension, directivity, and division of the system into zones help reduce level differences. Frontfill serves the front, outfill covers the side sectors, and delay addresses the back of the audience. All sources should be properly aligned in time and level.

For larger productions, it is advisable to prepare an SPL prediction before installation, and then verify the design with measurements at several points in the audience. This way, the system can be evaluated as a whole, rather than solely based on the level measured at FOH.

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