The primary function of a reactive silencer is to reflect sound waves back to the source. Energy is dissipated in the extended flow path resulting from internal reflections and by absorption at the source. The operation principle of the reactive silencers is a combination of lambda/4- and Helmholtz-resonators acting as acoustic filters. Reactive silencers have tuned cavities or membranes and are designed to attenuate low frequency noise from machines.
The reactive silencer may have excellent low frequency performance, is non-fibrous and cleanable and has small or negligible pressure loss. The simplest kind of a reactive muffler is the expansion chamber. In general reactive silencers are used for fixed speed machinery producing pure tones. The reactive silencer is suitable for engines requiring very low exhaust system back pressures for a maximum engine performance.
Reactive silencers are rarely used in HVAC systems.
Diffuser type silencers have perforated pepper pots to slow down flow velocity and prevent the generation of low frequency noise and are mainly used for applications involving nozzles, control valves, jet engines etc.
The total pressure drop is divided in several stages across the nozzle, the valve and the diffuser. This allows a better pressure ratio between upstream and downstream and reduces the noise level.
Active noise control is sound field modification, particularly sound field cancellation, by electro-acoustical means. Active silencers use microphones and electronics to determine and attenuate noise.
In its simplest form, a control system drives a speaker to produce a sound field that is an exact mirror-image the offending sound (the "disturbance"). The speaker thus "cancels" the disturbance, and the net result is no sound at all. Such silencers can be effective at low frequencies under 300 Hz.
Active noise control is best suited for applications with relatively steady noise fields - like fans, engines or similar. Active silencers are not suitable for broadband noise reduction.
Room acoustics and acoustic properties, decibel A, B and C, Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation.
Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems.
Low frequency noise transferred from main duct to end terminals is reflected back to main duct.
Sound transmission from ducts to surrounding rooms.
Sound transmitted through duct walls, floors and ceilings.
Noise attenuation in unlined sheet-metal ducts.
Acoustic noise calculation procedure HVAC systems.
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