Noise control

A noise-control problem is a budget, not a component choice. Between a machine and the person who hears it there is a path, and each element on that path removes a known number of decibels per band; the design question is which combination closes the gap between what the source emits and what the criterion allows. Machinery noise is attacked at the source, the path and the receiver in turn, and this section holds the path — both halves of it: the element models, and the two composed chains that spend their output against a criterion.

Everything here is a prediction from declared geometry and declared material data. That matters when a catalogue is open beside the screen: the figure a supplier publishes for the same device is a measured insertion loss, obtained under the conditions of a measurement standard — ISO 7235 for a ducted silencer on a laboratory rig with and without airflow, which also gives the regenerated flow noise and the pressure loss, ISO 11691 for the survey method without flow, ISO 11820 for a silencer in situ, and ISO 11546-1 and -2 for an enclosure in the laboratory and in situ. A computed transmission loss and a catalogue insertion loss are not the same quantity. Neither is wrong; they answer different questions, and a design that mixes them without saying so is not defensible.

Duct-Borne Noise: Fan to Room follows an airborne path from the fan through the duct run into the room: attenuation in straight duct, at bends and takeoffs, end reflection at the terminal, regenerated flow noise added back, the room effect at the receiver, and the result laid against the room criterion. It also states the limit every element model in this section shares — the frequency above which higher-order modes cut on and the plane-wave assumption stops holding. Room to Room: Partition, Receiving Room, Criterion follows the airborne room-to-room path instead: a source-room level built from a sound power and the room constant, a partition with its transmission loss, a receiving room with its absorption, the received spectrum and its verdict — and the inverse problem, the transmission loss a partition or a lined enclosure must have for the receiving room to meet its criterion, solved backwards.

The two element pages supply what those chains call. Silencers covers the reactive four-pole elements (expansion chambers, Helmholtz, quarter-wave and extended-tube resonators) with their transmission and insertion loss, and the choice between reflection and dissipation, while Industrial Noise Control keeps the HVAC duct attenuation and flow noise of an installation and the insertion loss of a machine enclosure.

If the noise travels in a duct, start at Duct-Borne Noise; if it travels through a wall, start at Room to Room; open the element pages when a chain asks for a number you do not have.

Both ends of the problem are settled outside this section, and a path calculation with either end missing has no verdict. At the source end, what a quieting measure is judged against is the emission of the machine itself, determined by the Sound power and intensity pages — and reducing it there is almost always cheaper than treating a path. At the receiver end sit the criteria: the NC and RC Mark II families of Room noise criteria, plus whatever occupational limit applies, in Occupational exposure (ISO 9612).

Pages in this section

• Silencers: reactive silencers by the four-pole method and the reactive-versus-dissipative choice.
• Duct-Borne Noise: Fan to Room: the end-to-end fan-to-room calculation against a room criterion, and the higher-order-mode cut-on that limits every plane-wave method.
• Room to Room: Partition, Receiving Room, Criterion: the composed source-room to receiving-room chain and the transmission loss a partition or an enclosure needs to meet a noise criterion.
• Industrial Noise Control: HVAC and Enclosures: duct attenuation, flow noise and machine-enclosure insertion loss.

What this section does not cover

• Nothing here is a measurement: every number is predicted from geometry and declared data, and the measurement standards named above are cited as the source of a supplier’s figures, not implemented. Within the predictions, three limits are structural. Only reactive silencer elements are computed — dissipative duct-lining silencers are discussed for selection but are not modelled from liner properties anywhere in the library, and on the HVAC page the lined-elbow figure is a table lookup (Bies Table 8.11) and the plenum attenuation is Wells’ closed form driven by a declared mean absorption — neither is a liner model. Mean flow is outside the element matrices: convection, temperature gradients and the flow-dependent impedance of perforates do not appear, so a silencer carrying significant flow is predicted as though it were not. And enclosure_insertion_loss never predicts the panel’s transmission loss: you supply R measured or from another model, and the module combines it with the interior correction — predicting R itself is Insulation design. Above the higher-order-mode cut-on frequency the plane-wave assumption every duct model rests on stops holding, which the duct-path page states and which no method here works around.