Introduction to sound system design

In this section, you will explore the key considerations for designing with a HOLOPLOT system and delve into the main performance indicators of a well-designed system.

The design process starts with a consultation with the client, allowing the designer to gain a thorough understanding of the project's requirements and intended applications. From this initial discussion, a list of design assumptions, performance targets, and engineering constraints can be compiled.

Following this, a detailed assessment should be conducted, focusing on key acoustic performance targets such as:

• Loudness

• Fidelity

• Uniformity

• Intelligibility

• Localization

Loudness

• Signal-to-noise ratio

• Artistic requirements

• Adequate levels even at the last seat

• Loudness homogeneity across all audience areas

• Overall values highly dependent on application

Many might assume that the louder a sound system, particularly in live entertainment, the better it performs. However, system designers must consider several other crucial factors.

• Uniformity of sound level is often a priority for clients wanting to ensure consistent coverage and quality across all seating areas within a venue.

• It's also vital to have a distortion-free signal reproduction. To achieve this, it's necessary to specify a system that provides ample headroom to handle all anticipated uses.

• Moreover, even a system with adequate headroom must achieve the necessary gain before feedback onstage to be fully effective. During the design phase, it is essential to evaluate onstage sound levels to ensure they meet this criterion.

The specific level requirements vary based on the application and can be influenced by factors such as:

• High ambient noise levels as a result of crowd noise or environmental noise at locations like transport hubs.

• Preferences of the artists and engineers.

• Regulations set by local authorities concerning health and safety or noise limitations at the venue's perimeter.

Fidelity

• No coloration

• No distortion

• Faithful copy of the original signal delivered to all seats

Achieving sound reproduction that remains true to the original signal is the primary objective for system engineers. This fidelity allows front-of-house (FoH) engineers to create a mix that closely resembles the studio recording. When a system introduces significant coloration, extensive post-processing becomes necessary to attain the desired sound. This aspect is particularly crucial for touring shows that aim for a consistent sound quality at every performance, irrespective of the venue's acoustical influence. The less a system is affected by room acoustics, the less need for equalization and dynamic processing.

Uniformity

• Spectral homogeneity across all receivers

• Small standard deviation

• Typically +/- 3dB

Uniformity in sound systems isn't just about consistent levels or even distribution across different areas; it also involves ensuring spectral uniformity so that all audience members have a similar auditory experience. Evaluating both uniformity and the sound level per octave band is crucial, intending to minimize the standard deviation among all listeners to within a specified tolerance, commonly ±3 dB. However, this tolerance might be broader for higher frequencies, particularly when speaker arrays must cover long distances or when environmental conditions, such as temperature and humidity, come into play.

Intelligibility

• Intelligible speech across all listeners

• Speech Transmission Index (STI)

• ISO 60268-16

Intelligibility hinges on three critical factors:

1. Signal to noise ratio (SNR): This measures the signal level against the background noise.

2. Direct to reverberant ratio: This assesses the clarity of direct sounds compared to reflected sounds.

3. Signal masking: The upward spread of masking is an acoustic phenomenon where lower-frequency sounds (maskers) make it harder to hear higher-frequency sounds (signals). This effect is more pronounced when the lower-frequency sounds increase in volume or intensity.

For optimal intelligibility, the system should be designed to maintain the sound level at each octave band significantly above the background noise—typically 10-15 dB higher. However, achieving this can be challenging in environments with inherently high background noise, such as transport hubs, stadiums, or road tunnels. In these cases, the cost and size of the system must be balanced against the potential improvement of intelligibility, keeping in mind the diminishing returns as the ear's self-protection mechanism can reduce signal intelligibility at very high volumes.

A direct sound pressure level (SPL) of 70-80 dB is generally considered ideal in quieter settings.