Why are halls acoustically challenging?

Halls are often difficult to amplify because they have a large volume, significant distances, and many hard surfaces that reflect sound. Concrete, metal, glass, or large flat walls cause the energy not to end with the audience but to return to the room.

The listener then receives simultaneously:

• direct sound from the PA,
• the first reflections from the nearest surfaces,
• later reflections creating a reverberation field.

The greater the contribution of the reflected sound, the less precisely are the subsequent impulses and syllables perceived.

The problem is exacerbated by geometry. A large back wall can cause strong reflections returning towards the stage, and a high metal ceiling can create a complex reverberation field throughout the venue.

Therefore the acoustics of the space affect the choice of sound system as much as the maximum SPL or the number of speakers. 

What does reverberation do to music and speech?

Reverberation causes overlapping of subsequent sounds over time. In music, it reduces precision, and in speech, it can directly worsen comprehensibility.

With short impulses – e.g., snare, kick, or articulation of consonants – the listener should clearly hear the beginning and end of the event. If a large amount of energy remains in the room, the next sound appears before the previous one has properly faded.

In music, the effects can be:

• blurring of rhythm,
• less separation of instruments,
• poorer localization of sources,
• loss of dynamics perceived by the listener.

In speech, the problem is even more critical, as subsequent syllables can mask each other. Rapid speech, softly speaking presenters, and panels with many open microphones become particularly difficult to understand.

Therefore, a conference system in a hall may require a very precise design despite a significantly lower target SPL than a concert.

Why does increasing the volume usually not help?

 Increasing the PA level raises not only the level of direct sound but also the amount of energy sent into the room. If the system still hits the walls and ceiling, the reflections also become louder.

This leads to a vicious circle: a participant at the back says they do not understand the speaker, so the system is turned up. The direct sound increases, but the reverberant field does as well. At some point, further increasing the level stops improving intelligibility.

Similarly at a concert. A higher SPL can make the music more intense, but if the reflections dominate over the direct signal, the mix becomes even less precise.

Therefore, the question should not be "how to play louder?", but:

how to ensure that a larger portion of the energy reaches directly to the audience?

How to increase the share of direct sound?

 The most effective method is to reduce the distance between the source and the listener while simultaneously limiting the energy hitting reflective surfaces.

This can be achieved by:

• appropriate height of the system,
• proper orientation of the arrays,
• more controlled directivity,
• dividing the hall into several zones,
• using additional sources closer to the further audience.

If one system at the stage has to cover a very large distance, it must operate at a high level. The further part of the audience then receives relatively a lot of reverberant field.

However, if a properly set and delayed delay appears in the middle of the hall, the further audience can be served from a shorter distance.

As a result, the local source can operate more quietly, and the share of direct sound increases.

This is one of the basic principles of planning sound reinforcement for large spaces without uneven coverage..

Selection of system directivity.

Directivity is one of the most important parameters of the system in a reverberant hall. The setup should cover the audience, but as little surface area as possible where no one is present.

If the speaker has too wide a vertical pattern, some of the energy may hit the ceiling. With too wide a horizontal pattern, the system excites the side walls.

On the other hand, too narrow a dispersion may leave part of the audience outside the proper coverage.

Therefore, it is necessary to adjust:

• the width of the beam,
• the vertical dispersion,
• the height of the system,
• the distance from the audience,
• the number of required zones.

It is not about maximizing the narrowing of the system. The goal is to match its characteristics to the actual area of the audience.

In difficult acoustics, a controlled system with a lower maximum SPL can yield better results than a very powerful source with inappropriate directivity.

Line array in a reverberant hall – when does it help?

Line array can help when its length and geometry allow for effective control of the vertical coverage of the audience. However, it does not automatically solve the reverberation.

The greatest benefit may be the ability to direct energy:

• to further sectors,
• to the center of the audience,
• to the front rows,

while simultaneously limiting unnecessary radiation to the ceiling.

This is a real advantage in a high hall with a deep audience.

However, if the applied array is very short, suspended too low, or its geometry does not fit the object, the name of the technology will not fix anything.

In a small or medium hall, a well-chosen point source can give equally good or better results.

Therefore, the decision should be preceded by a design. The differences are described in more detail in the article Line array or point source – when to choose which sound system?

Distributed system and delay line

A distributed system can be very effective in a reverberant hall, as it allows placing sources closer to the listeners instead of trying to cover the entire space with a very high level from one location.

For example, the main PA can amplify the first part of the audience, and the delay line the next.

The additional system must be properly delayed, as the listener nearby still receives sound coming from the main PA. If the delay plays too early or too late, it can create:

• echo,
• change of source location,
• interference,
• worse intelligibility.

After proper time alignment, both sources should be perceived as part of one system.

In some venues, it is possible to use more than one delay line or several smaller zones. However, it is not about random placement of multiple columns.

Each additional source should cover a specific area and work with as little overlap as possible on adjacent zones.

How to reduce reflections from the ceiling and back wall?

Reflections can primarily be reduced by changing the direction and range of the system's energy. If the problem arises from the PA shining directly at the ceiling or back wall, tonal correction will not solve the cause.

Helpful at the ceiling are:

• controlled vertical directivity,
• appropriate mounting height,
• correct tilt of the system.

The problem of the back wall can be mitigated by designing it so that the main system does not have to send a large amount of energy to it.

In a very deep hall, it is often better to end the useful coverage of the main PA earlier and serve the further audience with a delay system, rather than aiming the main system far behind the last listeners.

If the architecture allows, adapting the back wall with absorbing or diffusing materials may also be significant.

However, it is worth checking first how much of the problem is generated by the PA itself. Sometimes improving the system's directivity provides a significant change without interfering with the venue.

Can EQ shorten the reverb?

No. EQ does not shorten the physical reverb time of the room.

Correction can reduce the amount of energy sent in a selected frequency range. If the room has clearly excessive energy in a specific band, appropriate correction can improve the subjective balance.

However, this does not mean removing the reverb.

If the energy in the hall takes a long time to decay after stopping the source, EQ cannot make the reflections suddenly disappear faster.

It will also not fix:

• reflections from the back wall,
• poorly aimed speaker,
• large difference in distance,
• poorly aligned delay system.

Therefore, DSP should be used after solving geometric problems, not instead of solving them.

First the direction and position of the sources, then tuning.

When is acoustic adaptation needed?

Adaptation is needed when the properties of the room itself remain the main limitation even after properly designing the PA system.

It may include materials:

• absorbing,
• diffusing,
• appropriately designed low-frequency devices.

Not every soft surface works equally well across the entire range. A thin curtain may limit some reflections in the mid and high frequencies, but it will not solve serious problems in the lowest range.

For a one-time event, adaptation options are often limited. Existing curtains or set elements can be used, but the main tool remains the PA design.

In a venue regularly used for concerts, conferences, and events, permanent acoustic adaptation can provide benefits that cannot be achieved by simply changing the sound system.

The system and adaptation are therefore not competing solutions.A good PA controls the energy sent to the room, while adaptation controls what happens to the energy once it enters the room.

How to conduct measurements and tuning?

Tuning should verify the design at several representative points in the hall, not just at the FOH position.

Before proper correction, it is necessary to check:

• whether all system elements are working correctly,
• polarization,
• temporal relationships,
• connection of main PA and subwoofers,
• additional delays and fills,
• level differences between zones.

Time response measurements also help identify strong reflections. This allows distinguishing between problems arising from the system itself and problems generated by the object.

In a large hall, it is worth comparing at least:

• the first part of the audience,
• the center,
• the back,
• side sectors,
• zones of transition between main and delay.

One should not try to "straighten" every single EQ point. The hall will inherently have spatial differences.

The goal is to achieve a coherent system throughout the significant area, rather than an ideal graph in one spot.

Sample strategy for concert and conference

A concert and conference in the same reverberant hall may require a similar geometric approach, but different priorities.

For a concert, a broadband system with adequate headroom is needed. The strategy may include:

• directional main PA,
• limiting energy sent to the ceiling,
• frontfill for the first rows,
• delay for the further audience,
• controlled subwoofer section,
• appropriate stage level management.

It is also necessary to take into account the fact that drums, amplifiers, and monitors generate energy in the room themselves. The louder the stage, the less control FOH has over the overall sound field.

At conferences, the required maximum SPL is usually lower, but the priority becomes speech intelligibility. The system can utilize more sources working closer to the audience, allowing each of them to operate quieter.

Microphones working close to the mouth and limiting the number of simultaneously open channels are also very important.

In both cases, the starting point should be an analysis of the object. A site visit and sound system design for acoustically challenging spaces allow for deciding whether a directional system, line array, distributed system, or a combination of several solutions is needed.

Summary

Large reverberation in a hall is not primarily combated with greater power or aggressive EQ. The most important thing is to increase the proportion of direct sound and limit the amount of energy that hits the ceiling, walls, and other reflective surfaces.

Proper directivity, system height, a conscious division of the audience, and additional sources placed closer to the further listeners help with this. A line array can be very effective, but only if its geometry matches the object. In other situations, a point source or distributed system may be better.

Measurements and DSP are used to verify and tune a properly designed system. However, if the hall itself has very unfavorable properties, acoustic adaptation may also be necessary.

Frequently Asked Questions