Mechanical Room Sound Transmission and Flanking

Whether the wall is even the path. The relation is a subtraction: the source level, less the partition's transmission loss, plus a term for the partition area against the receiving room's absorption. Every trap is in what it leaves out. STC DOES NOT DESCRIBE THIS. It is a single-number rating weighted for speech frequencies, and mechanical equipment noise is concentrated far lower, so a wall with a high STC can perform poorly against a chiller or a fan. Octave-band transmission loss data rather than an STC number is what the calculation needs, and a partition selected on STC alone against a low-frequency source is selected on the wrong number. FLANKING IS WHY WALL UPGRADES DISAPPOINT. If the partition stops at the ceiling and the two rooms share a plenum, sound goes over the top and the wall's rating is nearly irrelevant; the same applies to a shared floor slab, to ducts penetrating both rooms, and to any unsealed penetration -- a few square inches of open penetration can undo an entire assembly, which is why sealing is not a detail. THE FLANKING TEST IS THE POINT OF DOING THIS AT ALL. If the calculation says the wall should deliver a given level and the room measures ten decibels higher, that is a factor of ten in energy and no wall constructed as specified underperforms by that much -- so there is a flanking path, and finding it is cheaper than any acoustic upgrade. If calculation and measurement agree, the wall is performing as built and the choice is between a better wall, a quieter machine, or vibration isolation. The absolute prediction carries real uncertainty because neither the source level nor the transmission loss is known precisely in the field; the DIFFERENCE is robust, and the difference is the finding. This is a single-band calculation on entered levels. It does not work in octave bands or compute an NC rating from a spectrum, predict transmission loss from a construction or convert an STC to band data, model flanking paths or estimate their contribution, address structure-borne transmission and vibration isolation, which is a separate and often dominant path, or determine what any standard requires. ASHRAE Applications, tested partition data, and the acoustical consultant govern.

L_p2 = L_p1 - TL + 10 log10(S / A); the flanking test compares that calculated level against a measured one, and the gap in decibels is a factor of 10^(gap/10) in energy.

Octave-band transmission loss is the right input. STC is weighted for SPEECH frequencies and mechanical noise is concentrated far lower, so an STC number is the wrong number here.

One subtraction and one logarithm.

Estimate. AHJ and licensed professional govern.

Field names used by the API: source_spl_db, partition_tl_db, partition_area_ft2, receiving_absorption_sabins, criterion_db, measured_spl_db, flanking_threshold_db, area_term_db, received_spl_db, over_criterion_db, tl_required_db, measured_excess_db, energy_factor

• Single band an NC rating needs the whole spectrumthe acoustical consultant
• Airborne path only structure-borne transmission is separate and often dominantASHRAE Applications
• Transmission loss entered by band an STC number is the wrong input for a low-frequency sourcetested partition data