A Duct silencer is a piece of ductwork that is designed and manufactured to attenuate sound transmitted through the ductwork. The common duct silencer is a dissipative silencer. It uses changes in duct cross section and sound absorbing materials to attenuate sound.
Dissipative duct silencers are available in a number of configurations including straight, elbow, tee, and transition. They are also available as a rectangular or round duct section. The illustration below shows a typical straight rectangular configuration.
In the illustration above, you can see the three stages of sound attenuation in the duct silencer.
1. A change in cross sectional area at the inlet causes a reflection of sound back towards the sound source.
2. Acoustic media in the baffles absorbs sound as it flows down the passage.
3. A change in cross sectional area at the outlet causes another reflection of sound back towards the source. This attenuation component is small compared to the other two.
Dissipative duct silencers have three measurable performance characteristics:
1. Acoustic Insertion Loss: This is a measure of the attenuation provided by the duct silencer. Insertion loss varies slightly (±2 dB) with air flow direction and velocity. It is expressed as a spectrum of dB values (insertion loss) for the octave bands from 63 Hz to 8 kHz.
2. Airflow Generated Noise: This is a measure of the noise generated by the silencer. The noise is generated by air turbulence caused by the silencer. This noise is a function of air velocity and silencer geometry. It is also expressed as a spectrum of dB values (noise levels) for the octave bands from 63 Hz to 8 kHz.
3. Air Pressure Drop: This is a measure of the total air pressure drop caused by air flow through the silencer. Just like all duct fittings, pressure drop is also affected by upstream airflow conditions (turbulence). Pressure drop is expressed as a single number in inches water column or kPa. ASHRAE recommends that this does not exceed 0.35 inch of water (90 kPa).
Increasing the insertion loss typically also increases the air pressure drop. Dissipative silencers must be selected to provide sufficient insertion loss while minimizing air pressure drop.