Literature

In 1972, IAC developed silencer Dynamic Insertion Loss and Self-Noise ratings both under FORWARD FLOW (+) and REVERSE FLOW (-) conditions for Rectangular and Cylindrical Duct Attenuators. Since attenuation values are generally higher in the first five octave bands in the Reverse Flow (-) mode compared to the Forward Flow (+) mode, more economical silencer selections can often be made on return air systems. These phenomena are illustrated below.

All of the duct attenuators manufactured by IAC Acoustics are of the passive design because they do not require mechanical or electrical means to function. They do their job very simply by providing a trouble free static means for the dissipation of sound energy by converting it into very minute quantities of heat. Many of the original air conditioning attenuators developed by IAC in 1950 are still in use today. Active duct attenuators operate electronically by means of microprocessors, loudspeakers and microphones. They cancel sound by feeding back an additional noise source that is 180 degrees out of phase with the original noise. In theory, noise reduction is effective for low frequencies, usually below 300, Hz. Initial research to develop a commercial product was particularly strong in Great Britain but, today, research goes on throughout the world. HVAC noise control requirements are rarely confined to a narrow low frequency range. The broad band low and high frequency attenuation capabilities of typical passive attenuators are practically always required..

Die-formed single-piece splitter constructed throughout Shell-noise radiation minimised by double-skin or splitter construction in most models Acoustic splitters designed for maximum attenuation at low frequencies, the toughest job of all Straight-through air passages designed for maximum pressure drop Solid, rounded noses that increase noise reduction Bell-mouth entrance and exit to minimise turbulence, pressure drop and Self-Noise No protruding fastener heads to case turbulence or Self-Noise Solid air-impingement surfaces and self-cleaning air passages to minimise dirt entrapment Acoustic fill protected against erosion by perforated metal containments