What is a delay line?

A delay line is an additional speaker system set further from the stage and electronically delayed relative to the main PA. Its task is to serve the further part of the audience without the need for very loud playing from a single location.

The very name "delay" comes from the fact that the signal to the additional sets is intentionally delayed. The sound from the main PA needs a certain amount of time, to reach the back of the room. If the local sets played without delay, the listener would hear them earlier than the system at the stage.

A properly set delay should work with the main PA so that the audience perceives the entire system as cohesive.

So it's not just about simply "placing additional speakers at the back." It's a separate zone of the system that requires conscious design. 

What problem does the additional speaker line solve?

The delay line primarily solves the problem of a large difference in level between the front and the back of the audience.

If the main system is at the stage, the first part of the audience is very close to the source, while the last rows are much further away. With distance, the level drops, so a natural reaction may be to increase the volume of the main PA.

However, this causes it to get even louder at the front.

The delay line allows for a different approach to the problem. Instead of trying to deliver the entire required level from one place, part of the energy is generated closer to the further listeners.

This provides two important benefits:

• a smaller SPL difference between the front and the back,
• a greater proportion of direct sound in the back of the audience.

This is particularly important when planning sound reinforcement for large spaces and avoiding uneven sound coverage.. 

When does the main system stop being sufficient?

The main system stops being sufficient when it cannot provide an adequate level and quality in the back of the audience without creating a too high SPL closer to the stage.

There is no single distance at which you "have to add delay". The following are important:

• efficiency main PA,
• directionality,
• height of suspension,
• depth of the audience,
• required SPL,
• acoustics of the venue,
• presence of obstacles or stands.

In one hall, one main system can effectively serve all the audience, while in another venue with a similar number of participants, an additional zone will be needed much earlier.

Delay may be particularly justified when:

• the room is very long,
• the outdoor area has a large depth,
• the last sectors are noticeably quieter,
• increasing the level of the main PA causes discomfort in the front,
• the readability decreases in the back of the audience.

Therefore, it is worth making the decision based on geometry, rather than according to a simple meter conversion. 

Where to place the delay speakers?

Delay speakers should be located where the main system starts to lose sufficient level or clarity, but before the further audience completely falls out of its useful coverage.

This is important because the new zone must smoothly take over the task from the main PA.

If the delay is set too close to the stage, both systems may overlap significantly over a large area. This increases the risk of interference and makes alignment difficult.

If it is too far away, an insufficiently amplified zone may form in front of it.

It is also necessary to consider:

• the height of installation,
• angle of coverage,
• width of the audience,
• physical obstacles,
• safety of the audience,
• ability to conduct power and signal.

In a large outdoor setting, the delay system can be located on a separate tower. In a hall, it can be suspended under the structure or mounted at side points.

The most important thing is that its location is based on the needs of the specific sector.

How is delay calculated?

Delay primarily results from the time difference in which the sound from the main PA and the local system reaches the selected listening area.

Distance and propagation time

The speed of sound in air is approximately 343 m/s at a temperature of about 20°C.

This means roughly about 2.9 ms per meter.

So if the path difference is 10 m, the time difference of propagation is about 29 ms.

This gives a simple starting point:

delay [ms] ≈ path difference [m] × 2.9

However, this does not mean that for every system it is enough to enter the value from the calculator.

Why is the calculator alone not always sufficient?

The actual configuration is more complex.

The following are important:

• actual position of the acoustic centers of sources,
• processing of systems,
• filters,
• selected reference point,
• level relationship,
• intended perceptual location.

In a large area, there is no single place where all distances are identical. Therefore, an area or line is chosen where both systems should work together best.

Additionally, sometimes a small additional delay is used beyond the pure geometric value to maintain the perceptual dominance of the direction of the scene. However, this should not be done mechanically — the decision should arise from listening and measurements.

How to set the level of delay speakers?

The delay should not simply be as loud as the main PA. Its level should be set to complement the main system in the further part of the audience.

If it is too quiet, it will not solve the problem.

If it is too loud, the listener will start to clearly localize the sound in the local setup instead of towards the stage.

The level is therefore adjusted together with:

• delay,
• directionality,
• size of the area,
• level of the main PA reaching that area.

In the area directly under the delay, the local system can have a significant level advantage, but it should still maintain consistency with the overall system.

It is not about having all setups in the area at the same output level. It is about ensuring that the audience receives a as uniform result as possible.

What should the listener perceive as the main source?

In a typical configuration, the listener should still perceive the stage as the main direction of sound origin.

This is particularly important during concerts and events, where the stage image should align with the location of the sound.

If, while standing by the delay tower, the listener begins to have the impression that the vocalist "is" above their head or behind them, the local system is probably too dominant in time or level.

The appropriate relationship of delay and level can utilize the precedence effect: an earlier signal has a very strong impact on the perception of the direction of the source.

In practice, it is about finding a compromise. The delay must be strong enough to ensure clarity, but it should not unnecessarily distract the sound image from the stage.

How to avoid echo between the main system and the delay?

Echo occurs when two similar signals arrive with a time difference large enough that the listener begins to perceive them as separate events.

To avoid this, three things need to be controlled:

• time,
• level,
• the area of overlapping sources.

If the delay is set correctly, the signal from the local system appears in the appropriate relation to the sound of the main PA.

However, problems may still occur if:

• the systems are aimed too widely,
• the local system is playing too loudly,
• the delay is set for the wrong point,
• the audience is in a very extensive transition zone.

In such cases, it is necessary to adjust not only the time but also the geometry and level.

Therefore, good alignment is not about finding a single number in milliseconds. It is a combination of several decisions.

How does the delay system affect the uniformity of SPL?

The delay line helps to reduce the level difference between the front and back of the audience.

Without additional sources, the main PA must deliver power over a long distance. This usually means a very high level at the front and a significantly lower level at the back.

After adding delay, the further part of the audience is closer to the local source. Therefore, you can:

• reduce the required level of the main PA,
• improve clarity at the back,
• equalize SPL over a larger area.

This does not mean that the system becomes perfectly even.

There are still:

• differences in geometry,
• interferences in transition zones,
• differences in directionality,
• the influence of acoustics.

However, a properly designed multi-zone system provides significantly greater control than trying to amplify an entire very long space with a single source.

Delay line in a room vs outdoors

The principle of operation is similar, but the design conditions are different.

In a room, reflections and reverberation come into play. The main problem may not only be the drop in SPL but also that in the further part of the room, the direct sound becomes weaker compared to the reverberant field.

The delay line then allows for an increase in the contribution of direct sound.

This is particularly important when amplifying a hall with a large reverberation..

Outdoors, there is no typical room reverberation, but there are:

• greater distances,
• wind,
• temperature,
• differences in propagation,
• noise limitations.

System delay can then help not only to equalize SPL, but also to reduce the need to send a very high level from the main PA to the entire area.

In both cases, the goal is similar: to deliver energy closer to the place where it is needed.

How to verify alignment with measurements?

The system alignment should be checked with measurements in representative points of the transition zone.

Before setting the delay, you must first ensure that:

• both systems are functioning correctly,
• the polarization is consistent,
• the processing is correct,
• the routing does not introduce additional, unknown delays.

Next, you can compare the arrival time of both sources.

Measurement helps to determine the actual time difference, instead of relying solely on the distance measured by tape or from a plan.

After setting the time, you still need to check:

• the summation in the overlap range,
• level,
• the tone,
• the consistency at several points,
• the localization of sound.

You should not tune the system in just one location.

In a large area, there is always a compromise, which is why several representative positions are measured.

For larger implementations the design and tuning of a multi-zone sound system should be treated as one process: prediction before installation, and then measurement verification on site.

The most common configuration errors

The most common mistake is treating the delay line as an additional pair of speakers that just needs to be set up in the back and "input a few milliseconds."

The problems are particularly caused by:

• setting the delay without analyzing the coverage of the main PA,
• too large an overlap area of the systems,
• too high a level of the local source,
• delay calculated solely from one distance,
• lack of measurements,
• improper directionality,
• setting the arrays too low,
• lack of tonal alignment,
• attempting to fix poor geometry with just DSP.

It is also a mistake to use a single fixed conversion like "delay every 30 meters."

There is no such universal rule.

The distance between subsequent zones should result from:

• the capabilities of the main system,
• the geometry of the audience,
• the required SPL,
• the height of the sources,
• the characteristics of the specific system.

The most important principle is: delay is added because it is meant to solve a specific coverage problem, not because the space has exceeded an arbitrary number of meters.

Summary

The delay line allows for sound reinforcement in very long spaces without forcing an extreme level from the main PA. Additional sources are located closer to the further audience, thereby improving the uniformity of SPL and the contribution of direct sound.

Simply positioning additional speakers is not enough. You need to select their position, directionality, level, and delay relative to the main PA. In practice, a simple calculation of distance to milliseconds is just a starting point — the final alignment should be verified with measurements.

A well-designed delay should not draw attention to itself. The audience should simply hear a more even, clearer system, maintaining the perceptual connection of sound to the stage.

Frequently Asked Questions