STC and Rw values: how to read a sound-insulation certificate

The moment you decide to buy a sound insulation product, you are confronted with a pile of technical values. STC 50, Rw 45 dB, Ctr -3, Dn,e,w 60 dB... Knowing what these figures listed on the product label mean, and which one is the real performance indicator, is the first condition of choosing the right product. Because even if two products side by side carry similar figures in their catalogues, they can perform very differently in the field. In this article we examine in detail the STC and Rw values, the standards on which they are based, what they mean, and what you need to pay attention to in order to understand real performance.

Why Are Standards Reduced to a Single Number?

The sound insulation performance of a wall or a panel shows large variations according to frequency. The same structure may reduce sound by 25 dB at 100 Hz while reducing it by 55 dB at 1000 Hz. Since it is impractical to explain this multi-dimensional performance with a graph every time, international standards define a single summary number for each product. STC and Rw are two different versions of these summary numbers.

STC (Sound Transmission Class) is the American standard, defined by ASTM E413. Rw (Weighted Sound Reduction Index), on the other hand, is based on the European and international standard ISO 717-1. Both follow the same basic logic: they compare the product's performance curve across the frequency bands in laboratory tests with a reference curve and produce an average score.

In practice, STC and Rw values come out very close to one another; for most products there is a 1-2 point difference between them. That is, a product with Rw 50 dB will also be measured at around 50-52 as STC. However, the two standards are not equivalent in every case; in particular, low-frequency behaviour is evaluated differently.

STC: Sound Insulation in the American Standard

The STC value is based on the sound transmission losses measured in 16 frequency bands between 125 Hz and 4000 Hz. The calculation method is as follows: the curve measured in the laboratory is compared with a reference curve defined by ASTM; the total deviation of the values falling below the curve is kept within a certain limit, and the reference curve's value at 500 Hz is taken as the score.

The practical interpretation of STC values:

• STC 25-30: Normal speech is easily heard. Standard interior partition walls.
• STC 35-40: Loud speech is heard but its intelligibility decreases. Acceptable for office partitions.
• STC 45-50: Loud speech can only just be heard; a sense of privacy begins. Reasonable for residential interior partitions.
• STC 50-55: Speech is practically inaudible. Ideal for separating neighbouring apartments and for hotel rooms.
• STC 60+: Even loud music is inaudible. Studio, cinema, recording room level.

Rw: Sound Insulation in the European Standard

The Rw value is measured according to the ISO 717-1 standard in 16 one-third octave bands between 100 Hz and 3150 Hz. The calculation logic is similar to STC; a reference curve and a deviation limit are defined. A wall with Rw 50 dB means it provides 50 dB of sound reduction on average; however, this value varies across the frequency bands.

An important advantage of Rw is that it is reported together with two adaptation values (C and Ctr). This way the product's performance can be evaluated not only with a single number but also against the real-world source type.

C and Ctr: Real Life's Two Critical Adjustments

STC and Rw are measured under laboratory conditions with a pink-noise source. But in real life sounds rarely resemble pink noise; traffic noise is low-frequency-weighted, while speech is concentrated in the mid-high frequencies. For this reason ISO 717-1 adds two adjustments alongside the Rw value:

• C (Pink-noise adjustment): The appropriate adjustment for speech, everyday domestic noise and office sounds. Generally -1 to -3 dB.
• Ctr (Traffic-noise adjustment): The adjustment for heavy traffic, diesel engines and low-frequency-weighted sources. Generally -3 to -8 dB.

So a product labelled Rw 50 (-1; -5) performs at 49 dB against speech and 45 dB against traffic noise. Even though the same product looks perfect at its laboratory value, it may not deliver the performance you expect in a home next to a motorway. That is why, in facade selections, Rw + Ctr must be evaluated together.

Typical Values of Building Elements

The typical STC/Rw values of the building elements we frequently encounter in our field practice:

• Standard 8 mm single glazing: Rw 28-30 dB
• Double glazing (4-12-4 mm): Rw 32-35 dB
• Asymmetric double glazing (6-16-4): Rw 38-40 dB
• Laminated acoustic glass (44.2 + cavity + 6.2): Rw 42-46 dB
• Standard plasterboard wall (100 mm, single profile, single layer): Rw 35-40 dB
• Double-layer plasterboard + rock wool (floating profile, 125 mm): Rw 50-55 dB
• Double-leaf floating wall system: Rw 60-68 dB
• Standard interior door: Rw 22-25 dB
• Acoustic door (certified): Rw 35-44 dB
• Brick partition (10 cm, plastered): Rw 38-42 dB
• 20 cm concrete shear wall: Rw 52-56 dB

Field vs. Laboratory: The Most Critical Difference

By definition, STC and Rw values are measured under laboratory conditions. The test rooms are ideal: the side walls, ceiling and floor are acoustically isolated; sound travels only across the building element being tested. The situation in the field is very different. A wall may be certified at Rw 55 dB in the laboratory; but if it is not correctly installed on site, if there is flanking transmission through the side walls, or if its door is Rw 25 dB, the real performance drops to 35-40 dB.

For this reason the ISO 717-1 standard separately defines field performance as R'w (with an apostrophe). The value obtained in field measurement is generally 5-10 dB lower than the laboratory value. Professional acoustic engineering accounts for this difference between the product specification and the field reality from the outset.

If in our field work there is an Rw 55 dB target for a wall, we prefer a construction certified at Rw 60-62 dB in the laboratory. The 5-7 dB in between is the safety margin left for installation losses, flanking transmission and structural connection details.

Flanking Transmission: The Enemy You Don't See

No matter how high a wall's STC value is, if there are surfaces that transmit sound from the side or from below, the total insulation performance drops. This is called flanking transmission. The connection detail of the side wall, a break over the suspended ceiling, a shared screed between floor slabs, a ventilation duct, even an electrical socket penetration - all of these can create a flanking transmission path.

A scenario we frequently see in hotel room insulation: the partition wall has been built certified at Rw 55 dB; but the screed between the two rooms runs across the whole floor. The result: the sound heard between rooms is 35 dB. Because the sound passes not through the wall, but through the floor slab. Such problems should be foreseen at the design stage by carrying out a flanking analysis.

Dn,e,w and Other Special Standards

On sound insulation certificates you will sometimes also see special values such as Dn,e,w. This value means "the element's normalised sound level difference" and is used especially for prefabricated building elements, modular cabins and acoustic doors. The Dn,e,w value is similar to Rw, but the measurement context is different; the two values should not be compared directly.

When it comes to facade insulation for housing, the DnT,w + Ctr standard applies; this value gives the traffic-noise performance normalised according to the room's volume. In facade product selection, looking not only at Rw but at the DnT,w + Ctr expectation gives a more realistic result.

The Hidden Trap of STC and Rw: Low Frequency

A performance value summarised in a single number naturally conceals the detail. The best-known trap of STC and Rw is low-frequency behaviour. Because the standard STC value does not measure frequencies below 125 Hz, it may not reflect real performance against bass-heavy sounds (the outdoor unit of an air conditioner, a generator, a sub-woofer, traffic). The same Rw 50 dB value can be 35 dB at 100 Hz on one wall and 48 dB at 100 Hz on another.

For this reason, in spaces with a low-frequency problem (cinema, nightclub, generator room, dance studio) the octave-band graph should be examined as much as the Rw + Ctr value. In our field practice, the 63 Hz, 80 Hz and 100 Hz performance is critical in construction selection for these spaces; if these values do not appear on the certificate, the product may fall short in a low-frequency application even though it is the right one for another space.

Side Facade and Window: The Determinant of Total Performance

A facade's sound insulation performance is limited by its weakest component. A 50 cm thick brick wall on a residential facade may be certified at Rw 55 dB; but if the window ratio on the same facade is 30% and the window is Rw 30 dB, the total facade performance plateaus at around 35 dB. Because sound transmits through the place it most easily passes.

That is why, in facade acoustics projects, not only the wall but also the window, the shutter, the ventilation grille, the shutter box, and even the air conditioner penetration must be calculated together. Acoustic engineering calculates the Rw of the total surface as a weighted value based on the area and individual performance of each component. Budgeting done without this calculation often results in overspending on the wall and insufficient spending on the window.

Do Not Buy a Product Without a Certificate

STC and Rw values may be written on the product label; but the real value is not proven without the test report of an accredited laboratory. In Turkey, TÜBİTAK, the ITU Acoustics Laboratory and private accredited laboratories carry out these tests. In Europe, organisations with NOTIFIED BODY status are recognised. Points to be considered in test reports:

• The name of the laboratory where the measurement was made and its accreditation number
• The full description of the tested product (thickness, density, manufacturer)
• The measurement date (reports older than 2-3 years may have lost their currency)
• The frequency-band table (not just a single number, but octave-band data)
• The test method reference (ISO 10140-2, ASTM E90, etc.)

A supplier who says "our production is STC 50" but cannot provide documentation may, in reality, deliver a product with 30-35 dB performance. In acoustic engineering, there is no product ahead of the document.

Verification in the Field: Measurement

In important projects, a field measurement should be carried out after the application. This measurement is made with an accredited sound measurement device and a reference source (a dodecahedron loudspeaker); the sound transmission difference between two rooms is evaluated according to the ISO 16283 standard. The measurement report documents the post-application performance and shows whether the target value has been reached in the real world.

The cost of a field measurement remains at around 1-2% of the total investment in most projects; yet it is the only tool that makes performance visible. Certified material + meticulous application + field measurement are the three pillars of a sound insulation project.

Conclusion

STC and Rw values are the two basic concepts needed to learn the language of sound insulation. But on their own they are not sufficient for choosing the right product. The Ctr adjustment added alongside the figure, the certification document, the test laboratory, the octave-band data and the field conditions must all be evaluated together. Two products with the same Rw value can perform very differently under different conditions of use.

In sound insulation projects, closing the gulf between "on-paper performance" and "field performance" is possible only with the right design and meticulous application. At project surveys, our team looks at not only the STC/Rw value but also the Ctr adjustment, the octave-band data and the certification document together when selecting a product, and presents the solution proposal that suits the real conditions of your site.