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Today, I came across an old article by Wu Zongrong titled *"Hey! Sound People, Let’s Talk About Gain."* The piece discusses a fundamental yet often overlooked aspect of audio system design: the relationship between amplifier gain and speaker configuration. In many cases, people rely on price or basic rules of thumb to decide how many speakers are needed or what gain is required, but this approach lacks depth and precision. This article aims to provide a more systematic understanding of how sound systems should be designed based on real-world acoustic principles.
In the early days of public address (PA) systems, achieving sufficient sound pressure was a challenge. Amplifiers with 100W or even 150W were considered powerful, and once connected to speakers, the sound would reach maximum levels quickly—essentially, the number of watts dictated the performance. However, there was little serious discussion about the actual gain or sound pressure levels required for different environments.
Now, with modern technology, 100W amplifiers are no longer the norm. High-performance speakers are now widely available, and amplifiers can easily reach 1000W or more. Despite this, many still use outdated methods to determine system requirements, often focusing on cost rather than performance. This gap in knowledge leads to frequent misunderstandings and inefficiencies in system design.
The key question is: *How much gain do we really need?* A skilled sound engineer knows how to balance the sound pressure levels (SPL) across the entire system to ensure clarity and consistency. Without proper gain structure, you might end up with either underpowered or overdriven systems, both of which can ruin the listening experience.
Let’s consider a typical scenario: an indoor medium-sized venue where we want 95dB of sound pressure at the listener’s position during normal music playback, with a dynamic peak of 101dB. Adding a 10dB headroom for unexpected peaks brings the total requirement to 139dB. But sound pressure decreases as it travels through the air, following the inverse square law. At 80 feet (about 24 meters), the loss is approximately 28dB. So, the sound source must produce 139dB at the speaker to achieve the desired level at the audience.
This means that the speakers must be capable of producing at least 139dB at 1 meter under 1W of power. Most modern speakers have higher ratings, but they still need to be properly driven. For example, a horn with a sensitivity of 112dB at 1W (for high frequencies) can reach 141dB when driven with 200W. Similarly, midrange and low-frequency components have their own power and SPL requirements.
Calculating the exact power needed for each frequency range involves using logarithmic equations. For instance, the high-frequency component requires 500W, midrange needs 1000W, and the low-frequency section may require as much as 4000W. These numbers highlight why large-scale sound systems often use multiple speakers and powerful amplifiers.
In real-world setups, impedance also plays a role. Using 4-ohm speakers instead of 8-ohm ones changes the power distribution and can affect the overall system performance. Engineers must account for these factors to avoid overheating, distortion, or system failure.
Beyond the technical aspects, it's also important to understand the signal flow from the mixer to the amplifiers. The output levels from mixers and processors must be carefully managed to ensure the amplifiers are operating within their optimal range. This helps prevent clipping and ensures the best possible sound quality.
Ultimately, designing a professional audio system isn’t just about buying the most powerful gear—it’s about understanding the physics of sound, the limitations of equipment, and how to balance all elements for the best performance. This article provides a practical guide to help engineers make informed decisions and avoid common pitfalls in system design.