Secondary Acoustic Baffle

The Secondary Acoustic Baffle is a class of sound-attenuating structure specified in Third Concordance Annex 9-D for installation wherever a primary baffle is judged insufficient to prevent the propagation of resonance between adjacent collection chambers. Unlike its primary counterpart, which is sized against the originating noise, a secondary baffle is sized against the noise the primary baffle was unable to absorb — a distinction that has generated more paperwork than any other clause in the Annex. The Great Stratigraphy Schism is, at root, a dispute over whether the secondary baffle is doctrinally necessary or merely prudent.

Definition and construction

Annex 9-D defines the secondary baffle as "any baffle installed downstream of a certified primary baffle and upstream of a measured boundary," and specifies a minimum void-cushioning depth of 14 centimetres, rising to 22 where the adjacent floor carries an acoustic designation of Class II or higher. Lament Marble is the preferred facing material, as its secondary acoustic resonances are well documented and can be predicted rather than discovered.

Installation is supervised by surveyors of Secondary Instrumentals where the baffle bears on a plumb-line measurement, and otherwise by attending officers of the Department of Acoustic Phenomena. Siting is recorded in the Registry of Acoustic Properties within nine days of mounting. Baffles are not to be leaned against. This is stated on every fascia in the same type size as the clause number, and is stated because of Floor 88.

Floor 88 and the Sighing Stair case

In the winter of the year recorded as 44-Ω, a service stair on Floor 88 was observed to sigh. The sighing was traced to a secondary baffle that had been mounted against the stairwell wall rather than into it, conducting resonance directly into the masonry. The matter was adjudicated as Case 44-Ω and produced no ruling on the stair's acoustic properties at all, the assessors having divided on whether the baffle or the stair was the phenomenon under review. What survives from the case is a maintenance instruction, appended without comment: baffles are to be fitted by bracket, and the brackets are not to be reused. The stair is now catalogued as The Sighing Stair, and visitors are asked not to reply to it.

The Great Stratigraphy Schism

The Schism's adherents hold that the secondary baffle is a redundancy invented by overcautious surveyors and given false authority by an Annex amended more often than it was read. Their opponents cite the Plinth Report, which recommended the retrofitting of the Tier Beta Whisper Shafts with baffle arrays, and which informed the later installation of baffle arrays and Helmholtz resonators throughout that tier. The Schismatics answer that the Plinth Report concerns whisper shafts and not collection chambers, and that an argument from one to the other is an argument from depth to depth.

The Third Concordance itself is unhelpful. Annex 9-D, in its fourth recension, requires a secondary baffle "wherever resonance is audible at the boundary"; Annex 9-D(b), in the same recension, permits its omission "where resonance is not audible at the boundary." The two clauses have never been reconciled. Clerks settle disputes by citing whichever appears first in the index, an index that has been recopied in different orders since the sixth recension.

"The secondary baffle is not a second baffle. It is the first baffle, heard again."

— Surveyor Meridian Ashe, Notes on Downstream Absorption, folio 41

Notable applications

During the Grey Emanation, a temporary baffle chamber was constructed from Ignorance Marble and succeeded in attenuating the postulate leakage to within measured tolerance. The chamber was dismantled afterwards, its constituents being required elsewhere. On Floor 301, the eastern baffle array hosts Catalogua silentii, which is cultivated there for illumination and which is held to improve absorption by no more than three per cent, though the cultivation is maintained regardless. Aperture 511, officially an annex to that spiral, carries a secondary baffle of unusual section, and its figure is reprinted in the Acoustic Registry of the Tower with a note that it should not be taken as a standard.

Where a chamber is judged beyond baffling, officers may request a Certified Acoustic Nullifier in place of an array. The Guild of Acoustic Dampening has repeatedly objected that Nullifiers are baffles by another name and ought to fall under Annex 9-D; the Secondary Vestibules have declined to rule, and the Department of Acoustic Phenomena has recorded the objection and taken no position.

Disputed status and later commentary

The most persistent objection is taxonomic. Arlo Vex, formerly Correspondent of the Empty Shelf, argued that a secondary baffle is not a structure but a description of an effect, and that the Annex had catalogued a consequence as though it were a fixture. Later writers have extended the argument: officers of the Hall of Provisional Whispers note that fragments too acoustically awkward for immediate archiving are held there precisely because no baffle, primary or secondary, has yet been devised to receive them.

The Vault of Secondary Imperatives maintains a standing file of unresolved baffle citations, currently 311 entries, none of which has been withdrawn. The Acoustic Inversion has been proposed as an alternative to secondary baffling in at least four memoranda, and rejected in three.