How do I decide on the position of my arrays?
Find essential insights into positioning arrays effectively and explore the unique aspects of positioning HOLOPLOT arrays compared to conventional systems.
Correct positioning of arrays is crucial for optimal acoustic performance, impacting sound levels, the direct-to-reverberant ratio, coverage, and sound localization. While HOLOPLOT systems can adjust for less-than-ideal speaker placements, designers should aim for the best possible locations to reduce the need for extensive processing to achieve desired performance levels.
Often, the geometry or architectural characteristics of a space dictate speaker placement. This could be influenced by an architect's initial design, space limitations, or the load-bearing capacity of existing structures, which might result in compromises.
When compromises are necessary, it’s important to evaluate the following parameters carefully:
Is there sufficient weight-loading and space allocation for the specified arrays?
See the product manuals in Product Family for specific weight loading specifications. Make an assessment of space allocation based on the architectural model and consider visiting the site where applicable.
What are the required beam opening angles, and can the specified array sizes achieve this across a suitable bandwidth?
See the chart in Array Right Sizing to make an assessment of the arrays steering capabilities
What are the beam steering angles, and can these be achieved across a suitable bandwidth?
See the chart in Array Right Sizing to make an assessment of the arrays steering capabilities
Are the steering angles likely to create problematic side lobes, potentially causing audible echoes or reflections off hard surfaces?
Making an assessment of the potential lobing that is inherently caused when electronically steering a beam can be assessed by calculating the the expected spatial aliasing frequency and simulating the performance to asses the impact. It's important to understand this as a designer to ensure any unwanted reflections or artefacts won't be present in the final system deployment. This will also provide the designer with an understanding of the expected amount of edge defraction causing smaller side lobes outside off axis of the main lobe. When taking this into consideration the following rules apply.
At high frequencies, the directivity control is defined by the spacing between the loudspeaker drivers. In particular, reducing the spacing between drivers will improve (increase) the maximum frequency that can be controlled for beam steering or beam shaping.
Above a certain frequency, grating lobes (i.e., replicas of the main lobe that radiate in a different direction) will occur due to spatial undersampling of the array (too few samples or low loudspeaker driver density) during acquisition. This frequency is known as aliasing frequency.
A typical directivity pattern of an array is illustrated in the polar diagram below. Apart from the main lobe, also smaller side lobes are shown. These side lobes occur due to array diffraction. Tapering (gradually reducing) of the amplitude of the loudspeaker drivers near the edges of the array, decreases the level of these side lobes, compromising on the beam width (effectively using a narrower array) This is implemented into the HOLOPLOT optimization algorithm to reduce the level of side lobes.
To avoid spatial aliasing the spacing d should be smaller than half the wavelength λ (Nyquist criterion) of the maximum frequency to be controlled.
If arrays are flush mounted into walls, is there sufficient ventilation to maintain a healthy operating temperature?
See the product manuals in Product Family for specific weight-loading specifications.
How do I ensure the correct localization of the acoustic source or visual content for the majority of the audience members?
To make an assessment of correct localization, we have to consider 3 main criteria: angle, level, and arrival time of the first wavefront. The aim of the designer is to achieve a plausible localization of the acoustic source. In a simple frontal mono system (single source), this is a simple concept where the position of the array should be within approximately 30 degrees of separation from the acoustic source to ensure the psychoacoustic phenomena where the listener makes up the difference between the source position and the first wavefront produced by the loudspeaker take effect. This often becomes problematic to achieve when an array needs to be located in a high position above a stage opening, causing the localization to break down for the front rows of the audience. This will often require front fills (multiple sources) to correct the localization.
Auditory events with multiple sources
Now consider two sound sources emitting the same signal. Several auditory events can occur depending on the location and delay between the two sources:
Summing localization (phantom source) For very short time differences (≤1 ms), summing localization occurs, where the location of the perceived sound depends on the level and time difference of the two sources at the listener position.
Precedence effect or 'law of the first wave front' Increasing the time delay between the two sources to more than 1 ms, the perceived direction no longer changes. It sticks closely to the direction of the leading source.
Echoes Further increasing the time delay to more than 50-80ms, the sound perception transits from one (wide) source to two sources separated in time and location as a primary auditory event and its echo.
Echoes
The echo threshold is defined as the delay at which the secondary source is barely perceptible. The echo threshold is not a single value, but also depends on the level difference between the two arriving signals as well as their temporal and spectral structure. It varies roughly from 1 ms for short impulses to around 80 ms for more continuous sounds. The probability of hearing an echo with a speech signal is around 50 ms when the level of the lagging signal is 10 dB lower than the leading (Blauert, 1974). The figure below shows the different thresholds as measured using the standard stereophonic loudspeaker arrangement, speech presented at a rate of approximately 5 syllables per second, and the level of the primary sound approximately 50 dB at the position of the subject.
The lowest threshold curve represents the the masked threshold.
The next curve upwards is the echo threshold. If the delay time is less than approximately 32 ms, the level of the lagging sound can even be as much as 5 dB higher than that of the primary sound without the echo becoming audible.
The next curve is the equal loudness curve for the primary auditory event and the echo. At a delay time of 15 ms, the reflection must be more than 10 dB stronger than the primary sound to lead to an equally loud auditory event.
At delay times less than 50 ms, echoes are no longer perceived as annoying even if the reflection is considerably stronger than the primary sound. This is known as the "Haas effect," due to its description by Haas (1951).
The level above which the primary auditory event disappears is shown in the uppermost curve.
These fundamental principles need to be taken into consideration when designing with a HOLOPLOT system. The initial positioning of arrays can ensure that both correct time and level alignment are achievable for as large a proportion of the audience as possible.
What is different about how you position HOLOPLOT arrays compared to conventional systems?
HOLOPLOT arrays differ from conventional speaker technology because they can be electronically steered and optimized. While it remains crucial to mount them as close to optimal positions as possible, designers enjoy greater flexibility with HOLOPLOT arrays. In contrast, conventional speaker technology is often limited by fixed dispersion patterns, where the radiation pattern is dictated by mechanical components such as horns, baffles, or waveguides that physically direct sound dispersion.
Line array technology has addressed these limitations by enabling designers to control sound in the vertical axis, creating a cylindrical wavefront. This advancement reduces energy loss over distance from the typical 6dB per doubling of distance of a point source to just 3dB per doubling of distance. This technology allows for sound to be projected over greater distances, reducing the need for multiple mounting positions and enabling larger and more varied venue shapes to be efficiently covered with sound.
The major drawback to this technology, however, is the lack of control over the horizontal axis, which results in unwanted reflections off side walls and a loss of energy over distance.
HOLOPLOT Matrix Array technology has overcome this, enabling control in both the horizontal and vertical axis. This, coupled with HOLOPLOT's proprietary optimization algorithms, has granted the ability to control the loss of energy over distance; energy loss is now primarily down to air absorption impacting the high frequencies
See Beam design best practices for more information on how and when to compensate for these losses and reducing reflections which drives up the direct to reflected ratio improving overall intelligibility.
Example layouts for multi-array designs
Although a single HOLOPLOT array can cover a large area, a complete sound system often requires multiple arrays because of the size and shape of the audience areas or the presence of architectural elements blocking the sound, such as pillars, kiosks on train platforms, etc.
Using multiple, distributed sound sources in a space emitting the same signal increases the risk of echoes due to the time differences in the arrival of the direct sound. Hard reflective surfaces might also add echoes, which are ignored for now.
Elementary design concepts
In large spaces often, multiple arrays are needed. To speed up the design process, different elementary design concepts and beam options are available:
| Design concepts and beam options | Criteria |
|---|---|
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Central: Two arrays back-to-back, shooting in opposite directions.
Note that the arrival time delay increases from the center of the setup to either end. This is important if this setup is acoustically coupled to another area. Late echoes might be audible in the transition zone between the two areas.
Transversal: Multiple arrays in a row along a wall, shooting across the space
The spacing between the arrays depends on the horizontal opening angle. For column loudspeakers having a triangular coverage pattern the spacing shouldn’t be much larger than 10 m. For Matrix Arrays, the spacing can be increased by creating ‘virtual sound corridors’, minimizing the overlap between the adjacent sections.
An economical solution can be obtained by alternating larger and smaller arrays, having a longer and shorter throw, respectively.
Face-to-face: Two arrays shooting towards each other
To avoid echoes the distance between the arrays should be smaller than ~20m. Even if each beam covers only half the area, the floor reflection indirectly causes overlap and potentially delay issues.
Longitudinal (time-aligned): Multiple arrays along a line, shooting in the same direction.
The arrays must be time-aligned to create a coherent wavefront along the longitudinal direction. Due to the backward radiated energy (stronger for LF), a weak echo might be audible a couple of meters behind each delayed array. To minimize this effect, the delay could be set 5-10 ms shorter than what’s calculated based on the distance. The auto-delay tool in (the soon-to-be-released) HOLOPLOT Plan 2.01 will automatically take the effect of backward radiated energy into account.
To further improve the performance, high-passing or low shelving the delayed arrays can be considered. This will reduce the LF level behind the delayed arrays.
Note that the arrival time delay increases from one end of the setup to the other. This is important if this setup is acoustically coupled to another area. In the transition zone between the two areas late echoes might be audible.
Combinations of elementary design blocks
Central & longitudinal setup
This setup is particularly useful on train station platforms. The position of the two back-to-back arrays defines the ‘zero-time’ point of the setup. In case of a cross connecting tunnel or bridge between the platforms, covered by zero-delay loudspeakers, the zero-time on the platforms should be placed near the cross passage. This minimizes the risk of late echoes in the transition area between the two (weakly) acoustically coupled areas.
Multiple transversal
This setup is useful for covering a large area such as a mosque or baggage claim area at an airport. The size of the arrays mainly depends on the available ceiling height. The spacing depends on the type of array (see transversal setup).
Multiple face-to-face
In general, this setup is less favorable than the time-aligned longitudinal setup. However, it still has a use case for example if no delays can be applied due to a acoustically coupled neighboring area covered by zero-delay loudspeakers.
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