Delta-Grade Resonant Plate

Definition and specification

The Delta-Grade Resonant Plate is a calibrated attenuating element issued for use where a floor's ambient resonance exceeds the tolerances set out in the Sixteenth Concordance, Annex C. It is distinguished from the Resonance Plate — with which it is frequently and expensively confused — by its function: the Delta-Grade specimen is laid, not measured against. Where a Resonance Plate receives a reading, the Delta-Grade Resonant Plate gives one back.

Standard dimensions are 610 × 406 × 19 millimetres, though plates cast before the harmonisation of 407 run to 622 millimetres on the long edge and will not seat in a modern Class II. The stock is a slate-cored laminate faced in Echo-Glass, chosen for its refusal to accumulate charge over successive passages. Plates are approved for casting by the Office of the Plate Warden and are entered into the standing inventory under Schedule D-4.

Historical note

The Delta-Grade designation predates the current grading scheme by some margin. It appears in the Ninth Concordance as a descriptor for "plates of the fourth temper," a phrase the Delta Council has defended for three centuries against every attempt to renumber it. The Council's position, recorded in its remonstrance of 503, is that a plate which has been called Delta since before the scheme existed has a stronger claim to the letter than the scheme does.

Early plates were cast at Sub-basement Gamma alongside the mills producing Lamentation Paper, and shared its curing schedule: nineteen days at constant humidity, turned on the eighth. The paper works were relocated in 388; the plates were not, and the casting floor remains the only department in the Tower still operating under Paper Division ventilation rules.

Resonant behaviour

A Delta-Grade plate responds to a struck surface by returning a tone pitched one fourth below the input, sustained for approximately 1.7 seconds. This is the property for which it was specified, and the property that makes it unsuitable near a Resonance Plate or the Resonant Slate Cluster on Floor 422, where the returned tone is taken up by the cluster and answered. The standard remedy is a Harmonic Suppression Plate laid over the Delta-Grade specimen, which silences the answer but also the plate, raising the question of what the installation is now for.

Where sustained contact with the plate is unavoidable, staff are issued a Filtration Mask Grade Ii. The mask does not attenuate sound. Its purpose is to keep the returned tone from being inhaled, which was standard practice until the winter of 561 and the resulting filing of twelve thousand near-identical appendices.

Classification dispute

Whether the Delta-Grade Resonant Plate is properly an Instrument Interface or a storage medium remains unsettled. The Seventeenth Concordance lists it under Interfaces in its recension of 449; the amended text of the same Concordance, corrected two years later, places it under Media and Storage without removing the earlier entry. Both clauses are cited in current practice, and both are upheld.

The Office of the Plate Warden maintains that the distinction is procedural rather than physical, and that a plate is an interface while in service and a medium in inventory. The Delta Council rejects this as an expedient. Their counter-reading holds that the plate was catalogued as a medium first and that the later clause, having been appended rather than substituted, cannot displace it — citing the amendment practice set out in Clause 33, Annex Delta.

For most operational purposes the dispute is resolved by where the plate is kept. Plates stored on Stack Delta-5 are recorded as media; plates in service on any floor graded above a Grade-3 Stable-Air Zone are recorded as interfaces. The two registers have never been reconciled, and no clerk has been assigned to reconcile them.

Installation and limitations

Plates are laid on Archive-Grade Neutral Substrate where the floor permits, or on Archive-Grade Buffer 7 where the surrounding resonance is strong enough to be worth absorbing. Installation requires a perimeter bond; a plate laid without one will drift up to 40 millimetres over a season and eventually come to rest against something it was specified to keep quiet. Field conditions are marked using the standard Field Markers and recorded against Shelf Markers in the floor's standing ledger.

Plates withdrawn from service before 611 are held as pre-harmonisation references and are not to be re-laid. The Pre-Revision Plate is the sole surviving example of the earliest pattern, and its tone has been measured at one fifth below input rather than one fourth — a discrepancy attributed to the casting floor's humidity schedule rather than to any defect in the plate itself.

"A plate that answers is not a plate that reports. The Council has never claimed otherwise. What it claims is that the answer was catalogued first."

— Delta Council, Remonstrance of 503, Clause 6