Silencers, attenuators and duct equipment

Silencers for HVAC systems, industrial plant and pressure venting, selected on attenuation together with pressure drop and self-generated noise — not on length alone.

• The selection figure is Dynamic Insertion Loss, measured with air actually flowing, paired with Self-Noise, the sound the silencer itself generates. Selecting on a no-flow attenuation figure gives an answer that does not match the site.
• Attenuation in the first five octave bands is generally higher in reverse flow than in forward flow, so return-air systems can often take a shorter and cheaper selection.
• Low-frequency ranges are engineered specifically for the 63 Hz, 125 Hz and 250 Hz octave bands, which are usually the bands that set the length of the unit in practice.
• Pack-less silencers are reactive units with no infill or lining at all, for systems where fibre migration cannot be tolerated or where the airstream would attack a conventional lining.
• Clean-Flow units encapsulate the fill completely and add a honeycomb standoff, so exposed surfaces can be cleaned. That is the range for operating theatres, recovery rooms and clean rooms.
• Acoustic louvres exist for openings where airflow has to be preserved, not where maximum attenuation is wanted. Those two things trade against each other every time.
• This is project work. For small quantities there is usually a better-value route than us, and we will say so.

Where this is specified

• Air conditioning to high-rise buildings, hospitals and schools
• Clean rooms, operating suites and laboratories
• Food, electronics and semiconductor manufacturing
• Chemical plants, refineries and areas handling flammable vapour
• Tunnel ventilation and large axial fan systems
• Engine test cells and outdoor plant compounds
• Steam and pressurised gas discharge to atmosphere

Standards that define the requirement

• ASTM E477, ISO 7235 and BS 4718 are the test methods that produce Dynamic Insertion Loss and Self-Noise ratings for duct silencers. The manufacturer states that catalogued ratings are determined under the applicable sections of these standards.
• ASTM E84, NFPA 255 and UL 723 are surface burning characteristic tests, applied to the acoustic fill inside a silencer.
• HVAC DW 144 and the ASHRAE guidance cover construction of medium pressure rectangular ductwork, which is what the outer casing of a rectangular silencer is built to.

These are the documents that set the requirement. Naming one is not a claim that a particular supplied item is certified to it; test evidence is issued per project.

Product lines in this family

• Rectangular attenuators, in standard, low pressure drop and low frequency ranges
• Circular attenuators
• Clean-Flow rectangular attenuators for clean and hygienic systems
• Conic-Flow circular attenuators
• Pack-less attenuators, rectangular and circular
• D-Duct diffuser attenuators for axial fan discharges
• Acoustic louvres, curved and linear blade
• Acoustic louvre doors for plant room walls
• Pipe insulation and pipework vibration damping pads
• Steam vent and pressure relief silencers
• Engine and gas turbine exhaust silencers
• Intake and discharge silencers for large industrial fans