Duct Breakout Noise and Lagging

Sound leaving a duct SIDEWAYS through its wall, into whatever room the duct passes through.

Run the calculator

Example

You enter

You get

Details, formula, and sources

This is the noise a silencer does not touch, and that is geometric rather than a matter of degree: a silencer attenuates sound travelling ALONG the duct to a downstream outlet, while breakout leaves through the wall wherever the duct runs, and arrives in the room without ever passing through a diffuser. Rumble that does not change when the outlets are blanked off is breakout. RECTANGULAR DUCT IS THE PROBLEM. A large flat sheet-metal panel is an efficient radiator at low frequency, and the wider and flatter the panel the worse it is -- so a wide shallow duct is close to the worst case, and it is also the shape a tight ceiling forces. Round duct of the same area is enormously better because a cylinder is stiff and has no flat panels to flex, commonly 15 to 25 dB better in the low bands where fan sound power is concentrated. The equal-area round diameter is computed here for exactly that comparison. LAGGING WORKS ONLY IF IT IS DONE CORRECTLY: a limp mass layer decoupled from the duct by a soft layer adds transmission loss, while mass strapped directly to the metal couples to it and does much less. And because the complaint is low-frequency rumble, thin materials do almost nothing -- the required surface mass is substantial. THE CHEAPEST FIX IS ROUTING AND THE SECOND IS ROUND DUCT, and both are design decisions rather than field ones. By the time a balancer is measuring the complaint the duct is in and the ceiling is closed, which is the argument for identifying breakout on the drawings. This computes radiating area, panel geometry and the room level from an ENTERED transmission loss and sound power. It does not predict breakout transmission loss from the duct construction, work in octave bands (breakout is a low-frequency problem and a single number hides it), estimate fan sound power, model duct-borne noise or the outlets, predict a lagging system's performance, or determine what any standard requires. ASHRAE Applications, the duct and lagging manufacturers' data, and the acoustical consultant govern.

radiating area = duct perimeter x exposed length; the room level is Lp = Lw - TL + 10 log10(S / A); the equal-area round diameter is sqrt(4 x duct area / pi).

The breakout transmission loss and the duct sound power are ENTERED. Breakout TL depends on the duct construction and gauge and is a low-frequency problem a single number hides.

Perimeter geometry and the standard room equation.

Estimate. AHJ and licensed professional govern.

Field names used by the API: duct_width_in, duct_height_in, exposed_length_ft, sound_power_db, breakout_tl_db, room_absorption_sabins, room_criterion_db, lagging_improvement_db, exposed_area_ft2, aspect_ratio, round_diameter_in, round_area_ft2, room_spl_db, over_criterion_db, tl_required_db

Related tools