Echo-Sounding

Echo-Sounding is the general term for the systematic emission of a sound into a bounded volume and the registration of what returns, together with the whole body of doctrine, apparatus, and dispute that has accumulated around that practice. It is distinguished in Tower usage from Echo Sourcing, which concerns the procurement of the original sound, and from the mere hearing of an echo, which is not catalogued. Echo-sounding produces a return, and a return is a document. The Sounding Gallery on Floor 173 maintains that everything which can be sounded can be known, and that everything which cannot be sounded is not, for administrative purposes, there.

Origin and the First Recension

The practice is conventionally dated to the winter of 204-AF, when surveyors working the lower vent shafts of the Seventh Spire began calling into the dark and recording the intervals before reply. The intervals were inconsistent, and inconsistency demanded a standard. The first recension of the sounding tables was entered into the Ventral Registry in 206-AF, and it fixed the unit of measure still used: the span, being the distance a sound travels in the interval between its emission and its return, corrected to the throat temperature of the shaft under examination.

Early echo-sounding was crude and, by modern standards, impolite. Sounders called into occupied floors without notice. The Concordance on Unannounced Sonation of 231-AF required, for the first time, that a floor be warned before it was sounded, and that the warning itself be sounded. Later amendments permit the warning to be written, provided it is written loudly.

The Sounding Apparatus

Modern echo-sounding is conducted with calibrated apparatus issued under seal by the Office of Acoustic Consistency. The principal instruments are the throat-horn, the damped bell, and the graded whistle; each carries a strike-plate stamped with its span tolerance and the date of its last recalibration, which must fall within eleven years. Instruments out of tolerance are not destroyed but retired to the Cabinet of Unreliable Voices on Floor 219, where they are sounded once a year for comparison and otherwise left alone.

Registration is by hand, in the sounding ledger, in the presence of a second clerk. Echo-Sand is used where a return is required to persist after the sounding body has departed; the sand is spread, sounded, and sealed, and its delay of four to eleven seconds is measured against a certified clepsydra rather than against the sounder's own counting, a requirement introduced after the Scandal of the Counting Clerks in 388-AF.

Echo-Lines and the Reading of Returns

The return from a bounded volume is not a single sound but a train, and the train is read as an echo-line: a sequence of intervals, each interval corresponding to a surface and its distance. A trained reader can recover the shape of a room from its line alone, and an untrained one cannot, which is the whole difficulty of the discipline. Sounders are examined on blind lines. The examination is conducted in a darkened chamber on Floor 174, and candidates are permitted one lamp, which they are asked not to use.

Where the line cannot be read — where it bends, doubles, or returns before it was sent — the return is classed as anomalous and referred upward. Anomalous returns are the ordinary business of the Gallery's technicians and the extraordinary business of its committees.

The Primacy Dispute

Echo-sounding's relationship to the other echo disciplines is unsettled. The Echo Primacy Debate turns on whether sounding is the parent practice, from which sourcing and certification descend, or merely the instrument by which the others are verified. The Ninth Concordance, Annex 2, holds that "the sound precedes the source, as the question precedes the answer," and is cited by the sounders. The Fourteenth Concordance, Annex 2, holds precisely the reverse, and is cited by everyone else. Both annexes are numbered 2. Neither has been withdrawn.

A sub-committee of the Ad Hoc Committee on Acoustic Validation was asked to resolve the matter in Cycle 12701 and returned a finding, in eleven pages, that the question was "not susceptible of acoustic settlement." The finding was itself sonated, at the request of the committee, and returned an anomalous line. It has not been re-sounded.

Incidents and Restrictions

Echo-sounding is prohibited in the Silent Cabinet without a warrant countersigned by two Librarians, a restriction dating from the Quelling of the Unnumbered Echo in 441-AF and never since relaxed. Librarian-Surveyor Maren Ost applied to sound the Cabinet for cavities in Cycle 12701; the application was received, stamped, and is presently in its third decade of consideration. Sounders are reminded that the prohibition attaches to the emission and not to the return, and that a return arriving unbidden must be logged like any other.

The Gallery's standing notice on the matter is brief: "If a floor sounds you first, do not answer it. Record the interval and withdraw." It is posted at every stairhead on Floor 173 and, by a Concordance of 512-AF whose number is disputed, at every stairhead on Floor 172 as well.

Scholarship

Agreement on the history is thin. The traditional account credits the surveyors of 204-AF with the first deliberate sounding; the Revisionist School of the Lower Shafts argues that the vent shafts had been answering for centuries and that the surveyors merely began writing it down. The Aggregate Echo Map is held by the traditionalists as proof of systematic method and by the revisionists as proof only that someone, at some point, drew a map.

What is not disputed is the discipline's founding difficulty, recorded in the margin of the first sounding ledger and copied forward in every recension since: a sound that returns is a fact, and a sound that does not return is a question the Tower has never learned to file.