What is STC (Sound Transmission Class) and how is it tested?

What is STC?

STC is a rating of airborne sound insulation. The element under test is placed between two rooms, sound is generated on one side, and the difference in level between the two rooms is measured in sixteen one-third-octave bands from 125 Hz to 4000 Hz. That curve of transmission loss is then fitted to a reference contour under the rules of ASTM E413, and the value of the contour at 500 Hz is the STC. One point of STC is roughly one decibel of insulation, so STC 50 lets through about 10 dB less sound than STC 40.

What does STC stand for?

STC stands for Sound Transmission Class. It is the North American (ASTM) rating. The ISO equivalent used in European and most international specifications is Rw, the weighted sound reduction index, measured to ISO 10140-2 and rated to ISO 717-1. The two are close in value for most constructions but are not interchangeable — use the one your specification names.

How an STC test is done

A laboratory STC test follows ASTM E90. The specimen — a wall panel, a door in its frame, a window, a partition system — is mounted in the opening between a source room and a receiving room that are structurally isolated from each other, so the only path for sound is through the specimen. The sequence is:

• Mount the specimen in the test opening and seal the perimeter; check the mounting for leaks (a 1 % open area caps transmission loss at around 20 dB whatever the panel is made of)
• Generate broadband noise in the source room and measure the average sound pressure level in both rooms in each one-third-octave band
• Measure the reverberation time of the receiving room, which gives its sound absorption
• Calculate the transmission loss per band from the level difference, the specimen area and the receiving-room absorption
• Fit the transmission-loss curve to the ASTM E413 reference contour: the sum of deficiencies below the contour may not exceed 32 dB and no single band may fall more than 8 dB below it
• Report the STC, the per-band transmission loss and the mounting conditions

Where STC tests are done in Thailand

Laboratory STC and Rw tests on building elements and products are carried out by Acoustic Laboratory Thailand (ALT) in Bangkok, which tests to ASTM E90 and ISO 10140-2 in its reverberation-room pair and reports both STC and Rw from one test. Geonoise does not run laboratory tests; Geonoise measures the installed result on site — see the field section below.

Should STC be high or low?

High. A higher STC means better sound insulation. The practical meaning of the number is easiest to see against speech from the room next door:

These descriptions are for speech-range sound. Bass-heavy sources — a subwoofer, a nightclub, a generator — are dominated by frequencies below 125 Hz that STC does not rate at all, which is why an element with a good STC can still fail against low-frequency noise. For those cases, the per-band transmission loss and the field measurement matter more than the single number.

Is an STC rating of 27 good?

Not for a wall or floor between two occupied rooms. STC 27 is the territory of a single-glazed window, a lightweight hollow-core door or a thin partition: loud speech next door is still understood, and normal speech is clearly heard. It can be acceptable for a window on a quiet street or an internal door where privacy does not matter. Typical values of common elements, for orientation only — the real value is whatever the test report says:

STC vs Rw vs DnT,w — which number applies?

The same question — how much sound does this wall stop — is answered by different ratings depending on the standard family and on whether the number describes the product in a laboratory or the wall as built. Specifications and brand standards name one of them; the test must report that one.

Rw and STC are usually within a few points of each other for the same element, but the fitting rules and the frequency ranges differ, so a product rated STC 50 is not automatically Rw 50. If a specification is written in Rw and the test report is in STC, ask for the per-band data and have the rating recalculated rather than converting the single number.

Laboratory STC vs field STC — why the installed wall scores lower

The laboratory value describes the element on its own, mounted and sealed by people whose job it is to mount and seal it. The field value describes the whole separation between two rooms as it was actually built. Sound in a building does not only go through the wall; it goes around it. The usual reasons a field result (ASTC or DnT,w) comes out 5–10 points below the laboratory STC are:

• Flanking transmission through the floor slab, the ceiling void, the façade or a continuous corridor wall that runs past the partition — measured separately to ISO 10848-2 when the path has to be found
• Gaps and penetrations: unsealed head and base tracks, back-to-back socket boxes, pipe and duct penetrations, a door undercut
• Doors and glazing in the partition, which set the ceiling for the whole element regardless of the wall's own rating
• Building services: ducts that connect the two rooms, or a plenum above a ceiling that both rooms share
• Workmanship: missing insulation, screws bridging resilient channels, boards not staggered
• Room size and furnishing, which is why DnT,w and ASTC are standardised to a reference reverberation time

This gap is the reason a specification should state which value is required and where it will be verified: a laboratory STC 50 product can be installed to give an ASTC 42 wall, and only a field test will show it. Geonoise measures the installed value as a service — see field sound insulation testing — and, when a wall underperforms, locates the path with an acoustic camera before anyone pays for the wrong fix.

What STC rating do you need?

Thailand has no general STC minimum in building law. Requirements come from the project specification, from hotel brand standards (commonly STC 50–55 between guest rooms, with a field verification clause), from green-building and wellness schemes, and from the owner's own expectations. Indicative design targets, laboratory values unless stated:

Set the target in the value that will be verified. If the contract is accepted on a field test, write the requirement as DnT,w or ASTC with the ISO 16283-1 or ASTM E336 method named, so that a laboratory product certificate cannot be offered as proof of the installed result. Geonoise reviews acoustic clauses before signature under its contract and guarantee review service.

How to prove the STC on your project

• Product selection: ask the supplier for the laboratory test report to ASTM E90 or ISO 10140-2, not a brochure value; check the mounting and the specimen size match your installation
• Design: have the flanking paths, doors, glazing and penetrations checked against the target, because they decide the field result
• Construction: inspect sealing and insulation before the boards close the wall — it is the cheapest moment to fix it
• Handover: measure the installed separation to ISO 16283-1 (DnT,w, R'w) or ASTM E336 (ASTC) with Class 1 instruments calibrated by an ISO/IEC 17025 accredited laboratory, and compare it with the contractual value
• Dispute: when a wall underperforms, a measurement plus an acoustic-camera scan shows whether the product, the installation or the flanking path is responsible

Laboratory tests on products are done by the ALT laboratory; on-site measurement, interpretation and the fix are Geonoise's work under building acoustics and field sound insulation testing. Geonoise sells neither the products nor the measures it recommends, so the number you get is the number the wall actually delivers.

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