What is STC? What does the wall STC rating tell us about acoustic performance?

What is STC? What do STC ratings tell us about soundproof walls?

: STC (Sound Transmission Class) is a single-number rating used to evaluate the airborne sound insulation performance of an entire wall system, rather than the property of any single material alone. The higher the STC rating, the better the wall blocks sound transmission. Generally, STC 50 is considered the baseline standard for residential spaces requiring basic privacy, STC 55 is suited for areas requiring higher privacy, and STC 60 and above is typically used in professional studio-grade environments. STC differs from dB (decibels) in that STC is a summary rating compared against standard reference contours, whereas dB is a unit of actual sound level measurement at specific frequencies.

STC is one of the most frequently discussed metrics in soundproofing, yet many still misunderstand it as a property belonging to a single "board" or material—such as believing that "this board has an STC of 60." In reality, STC is measured across the complete wall system as a finished assembly, not just a single material component. This article provides a clear explanation of STC along with the meanings of the most commonly requested rating levels.

• What is STC?

STC stands for Sound Transmission Class. It is a single-number standard derived from testing the sound transmission loss of a wall across various frequencies from 125 Hz to 4,000 Hz, and comparing the results against standard reference contours according to the ASTM E413 test method to summarize it into an easy-to-compare figure.

The most crucial point to understand is that STC is a rating for the entire assembled wall system, not for any individual material within the wall. Sound insulation performance results from the interaction of all components combined: board mass, framing, internal insulation, air cavities, and joint sealing. The exact same board material, when used in different wall systems (such as single vs. double framing), can yield significantly different system STC values.

• What do different STC levels indicate?

In general practice, different STC ratings typically correspond to the following perceived privacy levels:

What is STC 50? Simply put, it is the level at which normal conversation from the other side of the wall is barely audible. It is commonly used as a minimum requirement for partition walls in many condominiums and hotels.

Is STC 55 good? It is notably better than STC 50 in terms of privacy, making it ideal for spaces requiring more acoustic control than typical residential standards—such as important meeting rooms or bedrooms facing busy streets—though not yet at the highest level for extreme quietness.

What is STC 60? This rating enters the territory of specialized spaces, such as music practice rooms or studios, which require blocking louder activities than normal conversation from disturbing adjacent areas.

• What characteristics are required for a high-STC wall?

Achieving a high STC rating typically relies not just on increasing material thickness, but on advanced structural design tailored to the target level. Examples include decoupling (using two independent framing systems), adding sound-absorbing insulation within the wall cavity, or increasing mass by combining masonry with drywall systems. The higher the target STC, the thicker the wall and the more complex its components usually become.

• What is the difference between STC and dB?

dB (decibels) is a unit of actual sound pressure level measurement, used to indicate loudness or the amount of sound attenuation at a specific frequency. In contrast, STC is a single-number rating summarized from sound attenuation results across multiple frequencies compared against standard reference contours, meaning it is not a direct physical measurement unit like dB.

An additional point to note is that STC is calculated from the 125–4,000 Hz frequency range, which primarily covers human speech. It does not accurately reflect performance at very low frequencies, such as heavy bass or certain types of music. Therefore, a wall with a high STC rating may still allow heavy bass sounds to pass through more than the STC number suggests. This is why spaces like music studios or home theaters often need to evaluate transmission loss data broken down by frequency, rather than relying solely on the STC rating.

STC Value Examples for Sound Insulation Wall Systems from Trandar Acoustics

Trandar Acoustics has developed the Trandar Hitech Wall sound insulation wall system under the name TSIS (Trandar Sound Insulation System). The STC value specified for each model is measured from the complete, fully assembled wall system, not just a single material sheet. For example:

• System TSIS53 d95 provides an STC rating of 53 for the complete system, constructed with two 10 mm thick Trandar Zoundboard panels combined with Trandar Heplar Pro framing and Trandar dBphon S75 insulation in the cavity.

• System TSIS58 d105 provides an STC rating of 58 by increasing the Zoundboard thickness to 15 mm on both sides.

• System TSIS73 d200 provides an STC rating of 73 using a double independent framing system (Decoupling) combined with double-layer insulation.

As you can see, the Zoundboard panels, Heplar Pro framing, and dBphon insulation are all integral components that must work together to achieve the specified STC rating.

If you are considering sound insulation wall systems and want to know which STC level is suitable for your space, feel free to chat with the Trandar Acoustics team for consultation. We are happy to help recommend acoustic solutions using Trandar TSIS systems that precisely meet your actual operational requirements.