Acoustic Shadow and the Directivity of Line Array Systems in Practice
Reading time: 7 minutes
Quick summary
Most halls struggle with reverberation, or echo. Conventional loudspeakers can make this problem worse because they send sound towards the ceiling and walls. Line array technology directs sound towards the audience instead. But there is an important limitation: even the best loudspeaker cannot fix a hall that is acoustically bare and has no treatment panels.
Before discussing solutions, the problem needs to be defined precisely. Why do conventional loudspeakers fail in many spaces, and why is the cause not their quality but physics?
A standard point-source loudspeaker behaves somewhat like a light bulb. It distributes energy in many directions at once. The sound wave expands through the room and reaches every surface. Walls, ceilings, floors, glass partitions and marble reception surfaces all become reflective surfaces.
Every reflection creates a copy of the original signal. This copy reaches the listener several milliseconds after the direct sound. In a small room, the delay may be negligible. In a conference hall, auditorium or congress centre, however, these delayed copies accumulate and overlap.
The result is acoustic chaos. New words from the presenter mix with reflections of the words that came before them. The listener hears everything but understands very little.
Degradation of speech intelligibility caused by long reverberation
Reverberation is this accumulation of reflections. It is measured using the RT60 parameter, which represents the time in seconds required for sound energy in a space to decrease by 60 dB. In a well-designed meeting room, RT60 should be between 0.4 and 0.6 seconds. In many real spaces containing concrete, glass and stone, it reaches 1.5 seconds and sometimes more.
What does this mean in practice?
By the time the presenter says the second sentence, the first one is still sounding in the room. The listener hears a mixture of both. After twenty minutes, they are concentrating so intensely on understanding the speech that they begin to become tired.
The problem is not the loudspeaker itself. The problem is that the sound produced by the loudspeaker reaches the walls and returns to the listener. Simply increasing the quality of a conventional loudspeaker does not eliminate this phenomenon.
The physics of the space will not allow even the most expensive technology to perform correctly if the installation lacks acoustic design for reverberation and appropriate lighting.
How line array loudspeakers work
If the problem is a sound wave reaching every surface, the solution is to direct as much of that sound energy as possible towards one target: the listeners.
Generating a more controlled wavefront and a narrow vertical dispersion angle
A line array loudspeaker is not a single loudspeaker but a vertical array of many individual drivers, with premium systems sometimes using 32 or 64 elements, operating together as one coordinated source. Instead of dispersing sound broadly, the system concentrates acoustic energy into a narrow vertical beam, similar to the difference between a spotlight and a light bulb.
The principle is straightforward: when a large number of sources operate together in a controlled array, the resulting sound field can be concentrated within a narrower vertical area. The longer the array, the greater the potential control over vertical dispersion and the less sound energy reaches the ceiling and floor.
The result is that, in a space where a conventional loudspeaker distributes sound throughout the room, including towards reflective surfaces, a line array system can concentrate more energy on the seating area. The walls and ceiling remain closer to an acoustic shadow, reducing unwanted reflections.
Electronic steering of acoustic energy towards the audience area
Modern line array systems go further than passive column loudspeakers. Each driver can have its own amplifier and DSP channel. Software controls the delay and level of individual drivers, electronically shaping and steering the sound beam towards the required area.
This technology is known as beam steering. In practice, it means that a sound beam can be directed, for example, 15° downwards towards the front rows, while another beam from the same column can be directed 30° downwards towards the rear seating area. The audience receives more consistent and intelligible sound while the ceiling and rear wall remain less excited.
Some systems can create two independent beams simultaneously, one for the ground floor and another for a balcony. This is a situation where a conventional loudspeaker may struggle because optimising coverage for one audience area can compromise another or increase reflections throughout the room.
Measured results after implementing the technology in practice
Theory is one thing, but measurements are another. The following are specific results from real installations where line array technology demonstrably changed the acoustic comfort of the space.
Achieving consistent coverage in a large-capacity auditorium: STU Auditorium
The Aurela Stodolu Auditorium at the Slovak University of Technology has a capacity of 800 seats. For an educational space of this size, the requirement is clear: spoken content must remain intelligible at every seat.
By deploying line array technology, consistent sound intensity was achieved across the entire audience area. This means that the difference in perceived volume and clarity between the first and last rows is minimised. The directed beam covers the depth of the room with controlled attenuation.
An independent study by SynAudCon (Synergetic Audio Concepts), led by Pat Brown, one of the recognised specialists in speech intelligibility in the AV industry, examined this principle. Ten different sound sources, including seven actively steered line array systems, were tested in an environment with a reverberation time of approximately two seconds.
The measurements showed that actively steered arrays created a strong direct sound field while minimising excitation of the room. The result was significantly higher speech intelligibility compared with conventional point-source loudspeakers in the same environment.
In a supplementary comparison test, Pat Brown compared a directional line array system with a conventional coaxial loudspeaker in a room with an RT60 of approximately two seconds.
The result was that the directional source maintained a positive direct-to-reverberant sound ratio across the measurement positions, while the point source degraded as distance increased.
This is the type of result achieved in the STU auditorium and the reason why conventional loudspeakers can be insufficient in spaces of this type.
Improving intelligibility in video conferencing meeting rooms
Line array technology is not used only in large halls. In a meeting room at the Faculty of Mechanical Engineering of the Slovak University of Technology, we deployed it together with ceiling beamforming microphones that automatically focus their pickup towards the active speaker and reduce unwanted room reflections and background noise.
This combination, a directional loudspeaker covering the room and a directional microphone focusing on the speaker, creates an acoustic corridor between the technology and the user. Sound travels more directly between the loudspeaker and the listener and between the speaker’s mouth and the microphone, reducing the influence of the walls and ceiling.
This is particularly important for video conferencing. The remote participant hears more of the person speaking and less of the room, while echo-control processing prevents the speaker from hearing their own voice returned through the system.
The limits of electronic compensation and the need for physical acoustics
It would be misleading to claim that line array technology solves every acoustic problem. It does not. Transparency on this point is one of the differences between a good integrator and a supplier focused only on selling equipment.
Combining line array directivity with acoustic absorption panels
A line array minimises excitation of reflective surfaces but does not eliminate it completely. In extremely reverberant spaces, such as churches, historic halls or industrial halls with metal surfaces, even a carefully directed beam produces some reflections. Those reflections accumulate.
This is why a good sound system design should not rely only on loudspeakers. It may also include physical acoustic treatment such as:
- absorption panels on critical surfaces, usually the rear wall and the ceiling above the stage,
- bass traps in corners to reduce low-frequency build-up,
- and diffusers on side walls to scatter reflections without absorbing them completely.
The ideal result combines both approaches. The line array reduces the amount of sound energy reaching reflective surfaces, while the acoustic treatment absorbs or disperses the energy that still reaches them. Either approach can help on its own, but together they can be considerably more effective.
Reference: Distance Learning at STU
Connecting sound reproduction with digital signal processing and AEC
The loudspeaker is only one part of the signal chain. On the capture side, ceiling or tabletop microphones pick up the speaker’s voice.
Between the microphone and a remote video-call participant is a digital signal processor, or DSP, which performs several critical tasks:
- AEC (Acoustic Echo Cancellation) prevents the remote participant from hearing their own voice returning from the loudspeakers.
- Noise reduction removes background noise such as ventilation, projectors or equipment in adjacent rooms.
- Automatic gain control balances the level between a quiet speaker in the rear row and a louder speaker in the front row.
Biamp, one of the major manufacturers of DSP processors for conference systems, states in its technical documentation that arguably the most important component of a conferencing system is the room itself.
High reverberation cannot simply be corrected electronically. Physical treatment of the room is required to reduce it.
This is the key message for anyone considering an investment in conferencing technology: a DSP processor can make the most of good acoustics, but it cannot replace them. Hardware, software and the room need to operate as one tuned system rather than as three separate investments.
Summary: when to choose a line array and when it is not enough
Line array technology is most effective in spaces where the combination of large volume and hard surfaces creates long reverberation times, including auditoriums, congress halls, historic buildings and industrial spaces converted into conference centres.
In these conditions, conventional point-source loudspeakers may not provide the required intelligibility because a significant proportion of their energy excites reflective surfaces.
At the same time, a line array is not a magic solution that works without context. The best results are achieved when it is combined with appropriate acoustic treatment and correctly configured digital signal processing. A professional integrator designs and deploys these three layers together rather than treating them as separate systems.