Window Acoustic Ratings: How to Compare Rw, STC and OITC Evidence

1. Begin with the rating name and the underlying measurement

A single-number rating is a classification output, not the full acoustic evidence. ISO 717-1:2020 defines single-number quantities for airborne sound insulation of building elements including windows and says they are determined from octave- or one-third-octave-band measurement results. ASTM E413 likewise describes a method for calculating single-number ratings from laboratory and field sound-attenuation measurements. Before comparing any number, obtain the full frequency-band data, the measurement method, the specimen description and the classification standard invoked.

For an ASTM laboratory route, ASTM E90 covers laboratory measurement of airborne sound transmission loss for windows and other building elements. Its public scope makes an important boundary explicit: laboratory rooms are arranged so the specimen is the primary sound path, whereas buildings can have flanking paths. ASTM says laboratory ratings do not relate directly to field sound isolation and represent an upper limit to what field testing might measure. That is why a supplier’s laboratory rating cannot be rewritten as a promise for the finished hotel room or façade.

Use the rating name exactly as reported. Rw belongs to the ISO 717-1 rating framework. STC is the single-number rating named by ASTM E90 when Classification E413 is invoked. OITC is the outdoor-indoor transmission class defined by ASTM E1332. Similar-looking values are not a licence to relabel a report.

2. Match the rating route to the actual noise question

A quiet office adjacent to speech has a different acoustic question from a bedroom facing road, rail or aircraft noise. ASTM E413 says its single-number ratings correlate in a general way with subjective impressions for speech, radio, television and similar sources, and that the method is not appropriate for sources with markedly different spectra such as transportation noise. ASTM E1332 is designed around a reference spectrum derived from typical transportation sources and describes OITC as a ranking device for façade elements. The correct lesson is not that OITC is always ‘better’; it is that a requirement needs a stated source/noise context.

ISO 717-1 says its framework takes account of different spectra including noise inside a building and traffic outside. It also references spectrum adaptation terms in its rating process. When a project specifies an ISO route, ask the acoustic professional exactly which reported quantities and adaptation information are required and how the project will use them. Do not ask a supplier to manufacture a conversion because a comparison table needs one column.

This is especially important for low-frequency content. ASTM E1332 notes that its calculation requires one-third-octave sound-transmission-loss measurements over a stated frequency range and cautions that specimen size and edge restraint can influence lower-frequency measurements, particularly for modular specimens such as doors or windows. A single score without the report and specimen boundary hides that sensitivity.

3. Require evidence for the complete intended element

For a window package, the relevant test item must be named precisely. A centre-of-glass or insulating-glass-unit report may be valuable component information, but it is not automatically evidence for a finished frame, sash, seals, operable joints, mullions, hardware, vents, panels or installation perimeter. ASTM E90 includes windows within its scope, but that does not make a glass-only report a tested window. Record what was actually mounted between the test rooms.

Build a specimen identity table: fixed or operable; dimensions; frame/sash or curtain-wall system; glazing build-up; gaps, seals and gaskets; hardware and locks; vents or trickle devices; number and layout of panels; mullions/transoms; infill; and the test opening/mounting condition. Compare it with the scheduled project unit. Differences should be visible to the acoustic reviewer, not buried in a catalogue family designation.

Do not overpromise from a closed specimen. A laboratory test can have a fully closed and controlled condition; the actual project may have other operations, adjacent façade elements, interfaces and maintenance states. A buyer should ask whether the specified evidence corresponds to the required operating condition, then refer the conclusion to the responsible acoustic and project process.

4. Keep laboratory product evidence separate from field building evidence

ASTM E90 is clear that laboratory rooms are intended to make the specimen the significant sound path. In a completed building, flanking transmission through surrounding construction and other paths can affect isolation. ASTM E413 distinguishes laboratory and field rating names, while the actual test method identifies what was measured. Therefore, a procurement specification should separately state the product evidence requested before award and any field verification route required after installation.

This separation protects both buyer and supplier. The supplier can offer a configuration-specific laboratory record. The project team can plan field sampling, room selection, adjacent-construction checks, ventilation status and acceptance interpretation if the project requires them. Neither party should pretend that one document resolves both scopes.

• State whether the requirement seeks laboratory product evidence, field building evidence, or both.
• Name the method and classification standard used for each evidence stream.
• Keep a full report with band data, not only a rating on a quotation.
• Document the specimen and the installed configuration separately.
• Record adjacent construction, penetrations and other potential sound paths in the project field plan.
• Assign the acoustic reviewer who decides whether a difference is acceptable.

5. Make the quotation comparison honest

Create one row per proposed unit or unit group. The row should preserve the rating label exactly, the measurement and classification route, laboratory identity, report date, specimen/configuration, frequency-band attachment, test context and differences from the project. If one bid reports Rw and another reports STC, show them in separate columns rather than forcing an apparent conversion. If a bid supplies an OITC rating for a traffic question, still confirm that its test specimen and source context match the required project evidence.

The decision record may conclude ‘not comparable from supplied evidence’. That is useful information. It prevents a procurement team from awarding on a score that looks favourable but answers a different question. Ask for matching evidence or obtain the responsible acoustic review; do not repair a missing report by changing the title on a supplier declaration.

Compare three acoustic offers for a traffic-facing bedroom

In a hypothetical hotel enquiry, bidder A supplies glass-only Rw evidence, bidder B supplies a complete operable-window STC report, and bidder C supplies a complete-window OITC report. Do not sort the bids by headline score. Ask the acoustic designer which evidence route answers the source and room brief, then identify what each bidder has actually submitted.

For A, request evidence for the finished assembly rather than a promised conversion. For B, ask for the source-specific review required for the traffic question instead of treating STC as a room-noise prediction. For C, compare the specimen's operation, layout and accessories with the proposed window. The OITC label alone does not close that comparison. Retain required frequency-band attachments with the original reports.

Mark any ventilation opening on the elevation used for the review. A closed assembly in one offer and a different operating condition in another are not a common comparison basis. Agree ventilation conditions with the project team instead of promising quietness that depends on occupants using the room differently.

Make the quotation useful to the room designer

Request separately identified base and alternative configurations, with glass build-up, frame and sash identity, seals, operation, ventilation provision and evidence reference. Ask what each price includes for the perimeter interface and any specified field verification. This reveals scope differences without pretending the cheapest or heaviest assembly is automatically the best acoustic choice.

Close the bid review with three questions: which offers address the required source context, which configuration differences remain, and which room or façade paths belong to other trades? Preserve that division of responsibility in the order. A laboratory product rating should not become a guaranteed indoor sound level in the commercial summary.

Use the installation checklist together with the performance report guide to prepare the return. Send your acoustic brief with the opening schedule and the evidence you want compared.

FAQ

Continue the project review

Connect this guide with the proposed window and door systems, then review how Cavorlen approaches manufacturing and quality control. When your schedule is ready, send it through the project review form.

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