Vestibule of Acoustic Permeability

The Vestibule of Acoustic Permeability is a small antechamber on Floor 9,602, eight paces by five, whose walls admit sound at a rate that has never been agreed upon. It is the only vestibule in the Tower whose Acoustic Permeability (a) is required by standing order to be measured rather than assumed, and consequently the only one for which a continuous instrumental record exists. The record is 411 years long. It has not yet produced a value.

The Vestibule is administered jointly by the Office of Acoustic Integrity, which supplies the instruments, and the Department of Acoustic Anomalies, which supplies the floor and the keys. Neither office claims responsibility for the door, which is original.

Description and measurement

The Vestibule has one entrance, one recess in the north wall of unknown purpose, and a barrel vault at 3.4 metres. Its walls are of dressed Greycourse Ashlar, a material catalogued elsewhere in the Tower as acoustically ordinary. Within the Vestibule it is not ordinary. A word spoken at the recess is heard at the entrance at a delay of 0.9 seconds, which corresponds to no path through the room and to a distance, if one assumes open air, of some 310 metres.

Measurement follows the Manual of Acoustic Assessment without variation. The Tri-Vector Caliper is mounted at the recess in the standard three-vector configuration; the Acoustic Permeability Index (API) is computed from the resulting readings and entered in the Register by the attending clerk. The Forty-Seventh Concordance, Annex Ψ, defines the quantity being measured, and does so in terms that the Office of Acoustic Integrity has twice asked to have clarified.

"Acoustic Permeability is the disposition of a boundary to be persuaded by sound. It is not a rate, and shall not be reported as a rate."

— Forty-Seventh Concordance, Annex Ψ, clause 2

It is reported as a rate. The Manual requires a scalar.

The record

Instrumental readings from the Vestibule occupy four hundred and eleven bound volumes in the Registry of Acoustic Properties. The API has fallen between 0.41 and 0.88 in every year but six. In those six years — Second Winter of the Recess, Year of the Ninth Silence, and four others — the instrument returned a value the clerks described as "the reading before the reading," by which they mean a figure that appeared on the dial before any measurement was begun and could not subsequently be altered.

The figure was 0.618. It is the same figure in all six years. The Acoustic Reconciliation Bureau has ruled that these entries are to be retained but not averaged, on the ground that "a value antecedent to measurement cannot be said to agree or disagree with measurement" (Twenty-Third Concordance, Annex 7, marginal note). The marginal note is not part of the Annex. It has nonetheless been cited in eleven subsequent disputes.

The naming dispute

The Vestibule was not always called what it is called. The earliest survey of Floor 9,602, conducted under the Fourth Concordance and surviving in a single recension at Aperture 112, names it the Vestibule of the Recess. The Vestibule Nomenclature Office replaced this with the present name in the Revisions of the Eleventh Cohort, on the reasoning that a vestibule should be named for what it is for rather than what it contains.

The Acoustic Reconciliation Bureau objected. Its position, set out at length in On the Naming of Vestibules, is that the Vestibule is not for acoustic permeability — it merely has it — and that naming it thus has caused three separate expeditions to arrive expecting to find equipment installed. The Nomenclature Office replied that all vestibules are named for what they have, and that the Bureau had confused a name with a promise. Both papers remain in force.

The clerk's grievance

In the Winter of Small Corrections, the attending clerk Enna Voss-Ridley submitted a memorandum observing that she had entered the same six anomalous figures by hand in three separate ledgers, and that the Manual provided no box for them. Her memorandum was referred to the Committee on Unprovided Boxes, which sat for eleven sessions and returned it unread on the ground that it was not addressed to them. Voss-Ridley re-addressed it. The Committee dissolved before receiving it. The memorandum is now catalogued as a holding in its own right.

Access

The Vestibule is open to measurement parties by written application to the Office of Acoustic Integrity, countersigned by the Department. Visitors are asked to speak in a normal voice and to leave the recess empty. Nothing is to be stored in the recess, including instruments, coats, and the small folding stool kept on the landing for the use of the attendant.

Parties of more than three are not admitted. The reason is given in the standing order as acoustic, though the Department has confirmed in writing that it is not.

See also

• Vestibule of Unanswered Premises
• The Hume-Locke Vestibule
• Manual of Acoustic Assessment
• Material Permeability
• On the Permeability of Archival Borders