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People who are first introduced to Active Noise Control - creating sound in order to cancel another undesired sound - greet the demonstration with either unabashed wonder or complete skepticism. Many people unrealistically believe that Active Noise Control (ANC) can solve all of the noise problems on Earth. Others believe that it is far too complex or expensive to be practical.
The reality, is that ANC is a device that can reduce the noise emitted from a variety of sources that are present in our daily lives. For a handful of those sources, the device is both a cost-effective and practical noise control measure. The commercial applications include but are not limited to the following:
Active mufflers for automobiles, trucks and light machinery. Automotive and aircraft cabin silencing.
Active noise-reducing headphones. Mufflers for environmental pollution control equipment.
Active noise cancellation for HVAC equipment in commercial buildings and clean rooms.
This article will identify the capabilities and limitations of ANC in its application to HVAC noise control. ANC can be used in ducted HVAC systems to cancel ductborne, low-frequency fan noise by injecting sound waves of equal amplitude and opposite phase into an air duct, as close as possible to the source of the unwanted noise. Destructive interference of the fan noise and injected noise results in sound cancellation. The noise problems that it solves are typically described as "rumble", "roar" or "throb," all of which are difficult to address using traditional noise control methods.
This article will also contrast the use of active against passive noise control techniques. The main differences between the two noise control measures are acoustic performance, energy consumption, and design flexibility.
The article will first present the fundamentals and basic physics of ANC. The application to real HVAC systems will follow.
What ANC Is and How It Works
"Active noise control" is a general expression that describes any control system where a secondary sound source is used to destructively interfere with, and therefore control, the unwanted noise of a primary sound source. For active control of HVAC noise, the primary noise source is usually an air movement device - a fan. The secondary source, which cancels the unwanted noise, is typically a movingcoil loudspeaker.
Sensors, Actuators and System
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