What is the role of a subwoofer in a concert system?

The subwoofer is responsible for reproducing the lowest part of the spectrum, which the main system cannot reproduce with adequate efficiency or dynamics. It is not just about "more bass," but relieving the main sets and creating a complete full-range system.

In practice, subwoofers reproduce, among other things, the fundamental energy:

• kick drums,
• bass guitar,
• synthesizers,
• electronic kick,
• low frequency effects.

The boundary between subwoofers and main PA depends on specific system and its DSP configuration.

A well-designed low-frequency section should be seen as part of the whole PA, not an independent system playing "alongside." If the bass is clearly localized in a different place than the rest of the mix or has a completely different dynamics in different sectors, the problem usually lies in the configuration, and not just in the number of subwoofers. 

How
low should the system really play?

The lower range of the system should result from the musical material and the expected effect. Not every concert requires the reproduction of extremely low frequencies at full level.

In acoustic music, the naturalness and control of the upper bass may be much more important than a very extensive subsonic range. In rock or pop, the energy of the kick and bass is important, while electronic music can contain significant content much lower.

Therefore, one should not evaluate a subwoofer solely based on the lowest frequency stated in the specifications. The level at which it can operate with distortion and how well the system can work in that range also matters..

A subwoofer that nominally goes lower but quickly loses usable output at high levels may not be a better concert solution than a design with a slightly higher low range but greater efficiency.

Therefore, the selection should include not only the question of "how low?" but also "how loud and with what headroom?". 

Why does the music genre affect the required LF section?

The music genre influences the amount of energy in low frequencies, the dynamics of the signal, and the audience's expectations.

A jazz band concert and a large EDM event can take place in the same venue, but the requirements for the subwoofer section will be completely different.

In electronic music, sub-bass is often one of the key elements of the arrangement. The system must maintain a large headroom in the low range for a long time.

In rock and pop, the transients of the kick and the relationship between the bass and the entire system are very important. The subwoofer must respond dynamically, and not just generate a high average level.

At events primarily based on speech, an extensive LF section may even be unnecessary.

This shows why there is no universal configuration "for 1000 people." A system for the same number of participants may need a completely different low-frequency section depending on the repertoire. 

Maximum SPL and headroom of subwoofers

Maximum SPL says more about the usability of the subwoofer than the amplifier power or the diameter of the driver itself, but it should also not be analyzed as a single detached number.

It is important:

• in which range the high level is achieved,
• how long the system can maintain the required output,
• how the limiter behaves,
• what the distortions are,
• how many sets will cooperate.

Headroom means the margin between the typical operating level and the limits of the system's capabilities.

If the subwoofer section operates on limiters throughout the concert, there is no room for dynamic peaks. The bass may then seem loud, but at the same time flattened and less precise.

A well-chosen system should not require continuous operation at maximum level.

This is especially true for large outdoor events, where the distances are greater and the LF section must cover a large area.

Is there a correct ratio of the number of subwoofers to
the main system?

There is no universal ratio like "one subwoofer for two line array modules" or "two subs for one top."

Such proportions may work in a specific system, but they are not a rule of electroacoustics.

Different sets:

• have different maximum SPL,
• operate in different frequency ranges,
• differ in efficiency,
• have different power compression,
• are designed for different applications.

In addition, there is the repertoire and the way of placement.

Four efficient subwoofers in a favorable configuration can provide a better result than six weaker sets placed randomly. On the other hand, even a very powerful single subwoofer cannot replace the appropriate number of sources if the project requires controlling coverage over a large area.

That is why when choosing a sound system for a large outdoor event the LF section is analyzed as part of the whole project, not as a simple addition to the number of mains.

Where to place the subwoofers?

The position of the subwoofers affects the distribution of low frequencies as much as their number. The same sets placed in different locations can give a completely different result in the audience.

Left/right

Placing subwoofers on both sides of the stage is simple logistically and often convenient, but it can cause characteristic interference between the left and right groups.

On the axis of symmetry, signals can sum, while outside of it the difference in propagation paths causes frequency maxima and minima.

A listener moving across the audience may therefore hear clear changes in bass levels.

The greater the distance between the groups, the more noticeable these phenomena can be in the low-frequency range.

Center

A central cluster of subwoofers eliminates the problem of interference between the widely spaced left and right sections.

It can provide a more symmetrical level distribution in the horizontal plane, especially with a compact audience.

However, it is not always possible for stage, visual, or structural reasons. The center in front of the stage may be needed for a passage, barriers, photo pit, or other infrastructure elements.

Central placement does not automatically mean ideal evenness — the behavior of the entire system still needs to be analyzed.

Line in front of the stage

Arranging subwoofers in a line across the width of the stage allows for the use of interference from multiple sources to shape the characteristics.

However, the spacing between the sets affects the directivity depending on the frequency. If they are too large relative to the wavelength, additional lobes of radiation and uneven coverage may occur.

Therefore, the subwoofer line should be designed as an array, not as sets placed at random intervals.

Flown systems

Some designs use subwoofers suspended together with the main system or in separate clusters.

This can help, among other things, in better integrating the low-frequency part with the main system and changing the energy distribution vertically.

However, a flown sub is not automatically better than sets placed on the ground. There are issues of weight, rigging, performance at the lowest range, and differences in behavior resulting from the lack of direct contact with the ground surface.

In practice, hybrid systems are also encountered: some flown subwoofers, some ground stacked.

Why does bass propagate differently than high
frequencies?

Low frequencies have a significantly longer wavelength than high frequencies, which is why a single subwoofer is usually much less directional than a set reproducing mid and high frequencies.

That is why you can stand behind a single subwoofer and still hear a lot of bass, while the high frequencies from the main PA are definitely more directed towards the audience.

The long wavelength also means that the physical distances between sources become significant.

With several subwoofers, differences in propagation paths can lead to:

• summation,
• cancellation,
• creating directionality,
• the emergence of side lobes.

These phenomena are not a mistake in themselves. In directional configurations, they are used intentionally.

The problem arises when interference is the result of random placement rather than design.

How do directional configurations work?

Directional configurations utilize the placement of subwoofers, delays, polarity, and level in such a way as to achieve greater energy in the desired direction and less in another.

There is no one universal cardioid or end-fire configuration suitable for all setups. Parameters should derive from the specific model, geometry, and target frequency range.

Cardioid

The cardioid configuration aims to achieve a higher level in front and reduction behind the system.

It can be implemented in various ways, including by properly positioning the setups directed in opposite directions and applying delays and appropriate polarity.

The result is directional interference: in one direction, signals sum, while in the other, they partially cancel.

Cardioid can be particularly useful when we want to limit bass hitting the stage.

However, it should not be treated as a magic wall. The degree of attenuation behind the system depends on the frequency and specific configuration.

End-fire

End-fire uses several subwoofers positioned one behind the other along the radiation axis.

Individual sources receive appropriately selected delays, allowing signals to sum towards the audience, while in the opposite direction, partial cancellation occurs.

The distances between the setups and the delay are related to the wavelength and frequency range for which we design the configuration.

End-fire can provide high directivity, but requires a sufficient amount of space in front of or under the stage.

Gradient

Gradient configurations also utilize differences in position and processing between sources to achieve directional characteristics.

In practice, many ready-made cardioid configurations used by manufacturers are indeed based on gradient principles.

The most important thing from the organizer's perspective is that the directionality of subwoofers is a result of the entire system, not a feature resulting solely from the number of sets..

For larger events, the design of the directional LF section should be part of a complete concert sound system, and not a decision made only during the setup of the equipment.

How to limit bass on stage and outside the event area?

Bass on stage can primarily be limited by controlling the directionality of the LF section. This is important for both musicians and the monitor engineer as well as the microphones on stage.

A large amount of low frequencies behind the system can:

• worsen the comfort of musicians,
• affect the microphones,
• increase the level on the monitor world,
• cause vibrations of stage elements.

At outdoor events, there is the problem of emissions outside the event area.

Low frequencies propagate effectively over long distances and are harder to limit by simply directing a single set.

Helpful are:

• cardioid configurations,
• end-fire,
• appropriate positioning of the entire array,
• proper orientation of the main axis of the system,
• control of the level in the LF range.

This does not mean that bass can be completely stopped at the boundary of the audience. The goal is to limit unnecessary energy where it is possible.

How to time-align subwoofers with the main system?

Subwoofers and main PA should work together in the range of frequencies where their bands overlap. If the time and phase relationship is incorrect, there may be a dip in the crossover region instead of the expected summation.

It is not enough to automatically set the delay based on the physical distance of the enclosures.

The following are also important:

• internal processing of the sets,
• crossover filters,
• their phase characteristics,
• position of the acoustic source,
• measurement point.

Therefore, alignment is done through measurement.

The goal is to achieve the correct phase relationship in the range where the main system and subwoofers work together.

This does not mean that there is one correct delay for the entire audience. The spatial relationship changes with the listener's position, so the design is always a compromise regarding the target area.

Most common mistakes in designing the low-frequency section

The most common mistake is treating subwoofers as a simple quantitative addition to the main system.

Problems are particularly caused by:

• selection based solely on the diameter of the driver,
• comparing only the power of the amplifier,
• assuming a fixed ratio of the number of subwoofers to main PA,
• random left/right placement,
• too large gaps in the line,
• copying delays from another system,
• lack of directional configuration prediction,
• lack of measurement alignment with the main PA,
• designing bass exclusively for one audience point,
• ignoring the level on stage and outside the event area.

A significant mistake is also the assumption that a larger number of subwoofers always means better bass.

Additional sources provide greater possibilities, but at the same time increase the complexity of interference. More subwoofers only help when their mutual operation has been designed.

Summary

Choosing subwoofers for a concert or outdoor event requires analysis of the bandwidth, required SPL, repertoire, audience size, and capabilities of specific sets. The diameter of the driver, amplifier power, or the number of mains alone are not sufficient to determine the correct LF section.

As important as the number of subwoofers is their placement. Left/right positioning, central cluster, line, or directional configuration provide different coverage and different consequences for the stage and surroundings.

Cardioid, end-fire, and gradient arrays allow for conscious shaping of bass directionality, but require correct geometry and processing. In the end, the low-frequency section must also be time and phase aligned with the main system. Therefore, well-designed bass is the result of the entire PA project, not just simply adding more subwoofers.

Frequently Asked Questions