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Definition
STC Rating
What sound transmission class measures, what the numbers mean for privacy, and where the measurement falls down.
STC stands for Sound Transmission Class. It is a single number describing how well a building element — a wall, floor, door or window — blocks airborne sound from passing through it. Higher is better. It is measured under ASTM E90 and calculated under ASTM E413 across sixteen frequency bands from 125 Hz to 4000 Hz. An STC of 25 means normal speech is intelligible through the wall; an STC of 50 means loud speech is barely audible.
How STC is measured
ASTM E90 and E413The element being tested is installed between two rooms. Sound is generated on one side and measured on the other across sixteen one-third-octave bands from 125 Hz to 4000 Hz. The resulting transmission loss curve is fitted against a standard contour, and the value at 500 Hz gives the STC number.
The important consequence is that STC ignores everything below 125 Hz. Low-frequency noise — traffic, plant, bass from music, the thump of a nearby door — passes largely unmeasured. Two walls with identical STC ratings can behave completely differently against low-frequency sources.
What the numbers mean
For speech privacySTC 25 — normal speech clearly intelligible through the wall.
STC 30 — loud speech intelligible, normal speech audible but not clear.
STC 35 — loud speech audible but not intelligible. Often cited as the minimum for basic office privacy.
STC 40 — loud speech faintly audible. Reasonable privacy for most workplace conversations.
STC 45 to 50 — loud speech barely audible or inaudible. The range typically specified for confidential spaces.
STC 55 and above — effectively no speech transmission. Usually requires substantial construction.
Why booths are not rated in STC
ISO 23351 insteadSTC applies to a single building element such as a wall. An acoustic booth is a complete enclosure with a floor, roof, four walls, glazing and a door, and its performance is dominated by the weakest point in that assembly — almost always the door seal.
For that reason, freestanding booths are measured under ISO 23351-1, which tests the whole enclosure and reports speech level reduction in decibels, sorted into classes. Class A is the highest band and corresponds to roughly 30 dB of reduction.
If a supplier quotes an STC figure for a booth rather than a speech level reduction class, ask what was actually tested. An STC rating for a wall panel tells you very little about the assembled product you are buying.
STC and NRC are different things
Frequently confusedSTC measures sound passing through an element. NRC measures sound absorbed within a space. They are unrelated, and a material can score well on one and poorly on the other.
Acoustic foam has a high NRC and almost no STC — it absorbs reflections but blocks nothing. A sheet of lead has essentially no NRC and an excellent STC. Most real assemblies combine mass for transmission loss with absorption for internal comfort.
Common questions
What does STC rating mean?
STC stands for Sound Transmission Class, a single number describing how well a wall, floor, door or window blocks airborne sound. It is measured under ASTM E90 across sixteen frequency bands from 125 Hz to 4000 Hz. Higher numbers block more sound.
What is a good STC rating for an office?
STC 35 is often cited as the minimum for basic speech privacy, where loud speech is audible but not intelligible. STC 40 to 45 is a reasonable target for meeting rooms, and 45 to 50 for spaces where confidential conversations happen.
What is the difference between STC and NRC?
STC measures sound passing through a barrier. NRC measures sound absorbed within a space. They are unrelated measurements. Acoustic foam has a high NRC and almost no STC — it reduces echo inside a room but blocks nothing from leaving it.
What are the limitations of STC?
STC ignores frequencies below 125 Hz, so low-frequency noise such as traffic, plant equipment and bass passes largely unmeasured. Two assemblies with the same STC can perform very differently against low-frequency sources. It also tests a single element rather than a complete enclosure.
Do office pods have an STC rating?
Generally not, and a supplier quoting one should be asked what was tested. Freestanding booths are complete enclosures whose performance is limited by the door seal, so they are measured under ISO 23351-1 as speech level reduction and sorted into classes, with Class A the highest.
Related: NRC rating · reverberation time · how to read an acoustic claim · full glossary
Specifying an acoustic booth?
We publish airflow, speech level reduction and weight figures for every model, and we will send the test report references if you ask.