What are the different beam parameters and how do I apply them in my design?
Which Coverage Beam parameters can I adjust and what do they do?
HOLOPLOT Plan provides users with a comprehensive set of adjustable parameters, extending beyond the automated optimization process of Coverage Beams. This feature empowers designers to fine-tune beam optimization to suit specific room conditions. However, this level of flexibility necessitates a thorough understanding of these parameters and their impact on the final performance of the beam/system.
This chapter will explain what each parameter does and how best to use them.
Environmental parameters: Temperature and humidity
Temperature significantly influences the speed of sound, as both heat and sound are forms of kinetic energy. When temperatures increase, the molecules within a medium gain energy, which leads to faster vibration rates. This increase in molecular vibration allows sound waves to propagate more swiftly through the medium. As such, understanding the relationship between temperature and sound propagation is crucial for accurate sound design and engineering, especially in environments where temperature variations are significant. There is also a significant impact on air absorption at extreme temperatures (both high and low). This impact is greater at higher frequencies. This understanding helps to predict how sound will travel in different conditions, ensuring sound quality and consistency in diverse settings. A HOLOPLOT system adjusts to different environmental conditions by modifying the speed of sound in the optimization calculations. System designers can optimize beams for various temperature conditions, such as daytime and nighttime shows. These settings can be activated by a third-party controller either manually or at a predefined time or temperature threshold, ensuring that the system is always optimized for the specific environmental condition of interest.
Humidity also has an impact on the speed of sound but is almost deemed negligible, more importantly, it has an impact on the amount of air absorption losses (the higher the humidity, the greater the losses). The HOLOPLOT system compensates for these losses in two stages. First, the humidity percentage setting determines the amount of air absorption and attenuation between the array and the receivers. In the second stage, the air absorption compensation parameter (described in more detail in the section below) allows the user to set the maximum amount of compensation (in dB) that will be applied. For increasingly distant receivers and/or increasing frequency, the compensation may not be sufficient and will eventually be kept constant to ensure driver integrity and system headroom.
Target response
In the design process, a target response or house curve is tailored to each individual beam to optimize the system for specific applications. For instance, beams optimized for speech may be configured with a different target curve than those intended for music playback.
The target response is a predefined system response that configures the internal set of filters. It guides the optimization algorithm to achieve the desired acoustic curve over as much of the audience area as possible. In simpler words, the target response is the beam's average response across the targeted audience area. This optimization is subject to the constraints imposed by the array size and the designated coverage area.
The below charts show the target curves that can be applied to each individual beam "Low-shelf 7dB
Air absorption compensation
When sound travels through air, it gradually gets quieter because it loses energy. This happens due to two main reasons:
Classical absorption: This is where the air molecules bump into each other, creating friction that uses up some of the sound's energy. The amount of energy lost depends on the air's temperature and how high-pitched (frequency) the sound is.
Relaxation processes: This involves the molecules of nitrogen and oxygen (which make up most of the air) absorbing energy from the sound to move and rotate. Each type of molecule absorbs energy differently.
The key factors that affect how much sound is absorbed by air include humidity and temperature. These factors change how much energy the sound loses as it moves through the air, especially for sounds of different pitches.
In long-throw applications, high-frequency propagation is strongly affected by air absorption. This effect can be counter-balanced by HOLOPLOT's air absorption compensation parameter. This feature is not a simple signal EQ but rather a 3D spatial EQ that compensates HF air absorption effectively in direction and distance-dependent ways. To avoid excessive compensation levels at high frequencies, the maximum compensation can be set to realize an optimum trade-off between spectral uniformity and maximum obtainable SPL. For very high frequencies and/or receivers at very large distances for which the level can never be completely restored (i.e., lost cause), the compensation level drops to guarantee driver integrity and system headroom.
Application example
In the next example, the effect of different compensation levels is compared in a venue with a depth of ~40m. Array 1 (A1) and Array2 (A2), made of 4x2 MD96, contain Coverage Beams optimized with a maximum air absorption compensation of 0 and 12dB, respectively. Both beams have a flat target response. The results are shown below. The following can be observed:
The broadband SPL of A1 is about 8dB higher than that of A2.
In the back of the venue, the frequency response of A1 shows a roll-off at high frequencies, while A2 has an almost flat response (as expected).
At the front of the audience, the frequency response shows a bump at high frequencies (>5kHz) due to spatial aliasing. The height of this bump is stronger for A2 than for A1. This means that we cannot compensate the HF in the rear of the venue without creating stronger grating lobes in the front.
It is worth noting that the spatial variation at low frequencies (400Hz) is caused by the limited low-frequency control using a 4x2 array.
SPL drop over distance
Level drop over distance. This parameter sets the desired SPL drop across the audience in the range from 0 to 6dB per distance doubling.
This parameter is useful in applications with small arrays, low mounting height or long throw distance. Allowing some level drop over distance improves the spectral uniformity at the cost of spatial uniformity.
What’s the 'best' option? It depends :
For music reproduction a consistent, i.e., spectrally uniform (not necessarily flat), frequency response across the audience is preferred in most cases. Some level drop over distance is acceptable.
In PA or VA systems a uniform SPL distribution and a high STI (≥0.5) are required for good speech intelligibility. Some spectral variation over distance is acceptable.
To assess the performance of a PA/VA system, not only the average STI is important, but also the variation (i.e., standard deviation) across the audience.
As a rule of thumb, the mean STI minus the standard deviation, should be larger or equal than 0.5: -std≥0.5.
Phase response
Adjusting the phase response of a beam enables the designer to decide whether to prioritize phase linearity or phase coherence over factors such as beam latency and increased arrival time. This decision-making process involves carefully evaluating the trade-offs associated with each parameter, allowing for a more refined control over the acoustic performance.
By optimizing the phase characteristics, the designer can significantly influence system performance. This is critical in ensuring that the system meets specific auditory requirements, whether reducing latency for live inputs like speech or live performers or ensuring the highest level of phase coherence for audio playback like cinema or immersive applications. The table below outlines the expected latency of Coverage Beams with and without MD80-S Audio Modules.
Phase alignments and delays for arrays that contain only MD-96
| Name | Latency (ms) |
|---|---|
Optimized linear phase | 11.9 |
Optimized mixed phase | 7.1 |
Optimized near-minimum phase | 3.3 |
Optimized minimum phase | 3.1 |
Parametric | 1.2 |
Phase alignments and delays for arrays that contain MD-80s and MD96 or just MD-80s
| Name | Latency (ms) |
|---|---|
Optimized linear phase | 51.1 |
Optimized mixed phase | 31.1 |
Optimized near-minimum phase | 9.3 |
Optimized minimum phase | 4.7 |
Parametric | 4.7 |
It is worth noting that a significantly higher latency will be introduced when a subwoofer element is included in the array.
Input signal
The input signal has no effect on the shape of the filters or on the shape of the system's transfer function (to change that, the target response should be modified). The only thing that is affected is the scaling of the internal filters to reach the maximum driver's rated voltage (maximum input voltage - MIV) for the selected Input signal spectrum.
For an optimum scaling of the beam filters, it's recommended to choose an input signal that matches the real (live) input as closely as possible. If you don't know what the input signal will be, AES2 is usually a good start.
If you use a different test input signal during simulation than what was used during optimization, the power headroom can either be positive, meaning that MIV is not reached yet, or negative, indicating that MIV is exceeded.
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