Mufflers are like attenuating waveguides which operate in a wide frequency range. When wavelengths are long compared to the size of the muffler we use “lumped parameter” acoustic theory and when the wavelengths are short compared to the size, we use “distributed impedance” acoustic theory. It is the transition zone between these two areas that proprietary techniques, developed over many years of working with acoustics, are implemented.
Mufflers are intended to quiet intruding noise and not generate a new noise. Air flow must be smooth, with low surface friction so that eddy currents in the air flow are not created. In some applications adding tuned turbulence generators raises the internal noise floor inside the air duct which reduces the signal to noise ratio and improves privacy, particularly in the bass range.
Sound in the ducts should not escape or break-out through mufflers and sound outside the muffler should not break-into the muffler. Flexible, flat metal ductwork and the triangle flats of round to rectangle transitions should be avoided or if used, reinforced, Sound absorbing Duct Board is very sound absorbing but not soundproof. Round metal ducts are soundproof ducts.
Vocal isolation is the most common, and the frequency range is between 80 Hz and 4k Hz. For musical applications the range of 45 Hz through 12k Hz applies and for powered musical audio and recording applications, the muffler system needs to address the full audible range 20 H through 20k Hz. It is difficult and expensive to absorb low frequency sound and it is much more cost effective to let the low frequency pressure fluctuations leak it out of the duct in some area where the deep bass sound does no harm.
A number of accessories are used in low noise HVAC projects. One of the most common is a sound absorbing air deflecting baffle plate attached in front of the opening of the duct into the room. The technique of employing a two stage HVAC system is particularly useful for very low noise applications. HVAC cools an equipment or entry room and an air recirculation system exchanges that air with the air in the quiet room. In-line booster fans are often needed. Paramount in any system is to have it balanced after all is installed. Only after balancing can the quality of the quiet achieved be evaluated.