What is STC and how is it measured?

Sound Transmission Class (STC) is a single-number rating per ASTM E413 that indicates how well a building partition blocks airborne sound. It is derived from transmission loss measurements at 16 one-third octave bands (125-4000 Hz). The STC value equals the reference contour position at 500 Hz after fitting to the measured TL data with deficiency limits applied.

What is the 8 dB deficiency rule?

ASTM E413 requires that no single one-third octave band transmission loss value falls more than 8 dB below the reference contour. If any single band exceeds this limit, the contour must be shifted down until the 8 dB condition is met. This prevents a partition from getting a high STC rating while having a severe weakness at one frequency.

What STC rating is required by building codes?

The International Building Code (IBC Section 1207) requires STC 50 for walls and floor-ceiling assemblies between dwelling units. Many states and municipalities require STC 55 or higher. Luxury residential typically targets STC 60-65. Healthcare facilities often require STC 45-50 for patient room walls per FGI Guidelines.

What is the difference between STC and Rw?

STC (ASTM E413) and Rw (ISO 717-1) are both single-number ratings for sound insulation, but they use different reference contours and calculation procedures. For most constructions, STC and Rw are within 1-3 points of each other. The main difference is that ISO 717-1 also provides spectrum adaptation terms C (for living noise) and Ctr (for traffic noise).

What STC rating blocks conversation?

STC 25: normal speech easily understood through wall. STC 30: loud speech understood. STC 35: loud speech heard but not understood. STC 40: loud speech audible as murmur. STC 45: loud speech barely audible. STC 50: loud speech inaudible. STC 55+: most sounds blocked. For privacy, target STC 50 minimum between residential units.

Why is low-frequency insulation harder to achieve?

Low-frequency sound has long wavelengths that pass through partitions more easily. The mass law states that doubling the partition mass adds only 6 dB of transmission loss. At 125 Hz, a standard drywall partition may provide only 25 dB TL versus 45 dB at 4 kHz. This low-frequency weakness is the most common reason for noise complaints despite adequate STC ratings.

What is FSTC versus STC?

FSTC (Field Sound Transmission Class) is measured in an actual building per ASTM E336, while STC is measured in a laboratory per ASTM E90. FSTC is typically 5-8 points lower than STC because field installations have flanking paths (sound traveling through shared structures), gaps, and construction imperfections that laboratory conditions eliminate.

How do I improve the STC of an existing wall?

Add mass (extra layer of drywall), add an air gap (resilient channel or staggered studs), add absorption in the cavity (mineral wool), and seal all gaps. A basic single-stud drywall wall starts at STC 33. Adding resilient channels raises it to STC 44. Filling the cavity with insulation and adding a second drywall layer can reach STC 52-56.

What is STC and how is it measured?

Sound Transmission Class (STC) is a single-number rating per ASTM E413 that indicates how well a building partition blocks airborne sound. It is derived from transmission loss measurements at 16 one-third octave bands (125-4000 Hz). The STC value equals the reference contour position at 500 Hz after fitting to the measured TL data with deficiency limits applied.

What is the 8 dB deficiency rule?

ASTM E413 requires that no single one-third octave band transmission loss value falls more than 8 dB below the reference contour. If any single band exceeds this limit, the contour must be shifted down until the 8 dB condition is met. This prevents a partition from getting a high STC rating while having a severe weakness at one frequency.

What STC rating is required by building codes?

The International Building Code (IBC Section 1207) requires STC 50 for walls and floor-ceiling assemblies between dwelling units. Many states and municipalities require STC 55 or higher. Luxury residential typically targets STC 60-65. Healthcare facilities often require STC 45-50 for patient room walls per FGI Guidelines.

What is the difference between STC and Rw?

STC (ASTM E413) and Rw (ISO 717-1) are both single-number ratings for sound insulation, but they use different reference contours and calculation procedures. For most constructions, STC and Rw are within 1-3 points of each other. The main difference is that ISO 717-1 also provides spectrum adaptation terms C (for living noise) and Ctr (for traffic noise).

What STC rating blocks conversation?

STC 25: normal speech easily understood through wall. STC 30: loud speech understood. STC 35: loud speech heard but not understood. STC 40: loud speech audible as murmur. STC 45: loud speech barely audible. STC 50: loud speech inaudible. STC 55+: most sounds blocked. For privacy, target STC 50 minimum between residential units.

Why is low-frequency insulation harder to achieve?

Low-frequency sound has long wavelengths that pass through partitions more easily. The mass law states that doubling the partition mass adds only 6 dB of transmission loss. At 125 Hz, a standard drywall partition may provide only 25 dB TL versus 45 dB at 4 kHz. This low-frequency weakness is the most common reason for noise complaints despite adequate STC ratings.

What is FSTC versus STC?

FSTC (Field Sound Transmission Class) is measured in an actual building per ASTM E336, while STC is measured in a laboratory per ASTM E90. FSTC is typically 5-8 points lower than STC because field installations have flanking paths (sound traveling through shared structures), gaps, and construction imperfections that laboratory conditions eliminate.

How do I improve the STC of an existing wall?

Add mass (extra layer of drywall), add an air gap (resilient channel or staggered studs), add absorption in the cavity (mineral wool), and seal all gaps. A basic single-stud drywall wall starts at STC 33. Adding resilient channels raises it to STC 44. Filling the cavity with insulation and adding a second drywall layer can reach STC 52-56.

STC Rating Calculator

Sound Transmission Class (STC) is a single-number rating that indicates how well a wall, floor, or ceiling partition blocks airborne sound. SonaVyx calculates STC per ASTM E413 from 1/3 octave band transmission loss measurements, applying the 8 dB single deficiency rule and 32 dB total deficiency limit, and compares results against building code requirements for residential and commercial construction.

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Measure sound insulation in the field and calculate STC from your measurements.

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Technical Specifications

Parameter	Value	Standard
Standard	ASTM E413-22	STC classification
Frequency Range	125 Hz - 4 kHz (16 bands)	1/3 octave bands
Reference Contour	ASTM E413 Table 1	Shifted in 1 dB steps
Single Deficiency	8 dB maximum below contour	At any single band
Total Deficiency	32 dB maximum sum	Sum of all deficiencies
ISO 717-1 Rw	Also calculated	With C and Ctr terms
Building Codes	IBC, UBC, SBC, residential	Code compliance check

How to Calculate STC

Enter Transmission Loss Data

Input the measured transmission loss (TL) values at each 1/3 octave band from 125 Hz to 4000 Hz (16 bands). These values come from laboratory testing per ASTM E90 or field testing per ASTM E336. Higher TL values mean better sound blocking.

Apply ASTM E413 Procedure

The calculator shifts the STC reference contour upward in 1 dB increments until two conditions are met: no single TL value falls more than 8 dB below the contour, and the total sum of all deficiencies (where TL is below the contour) does not exceed 32 dB.

Review the STC Rating

The STC value is the TL of the reference contour at 500 Hz after the final shift. The calculator also shows the deficiency at each frequency band, highlighting any bands that triggered the 8 dB single deficiency rule or contributed significantly to the 32 dB total.

Check Code Compliance

Compare the calculated STC against building code requirements. The International Building Code (IBC) requires STC 50 for party walls and floors between dwelling units. Many jurisdictions require STC 55 or higher. The calculator flags whether the partition meets common code thresholds.

Compare with ISO Rw

For international projects, the calculator also provides the ISO 717-1 weighted sound reduction index (Rw) with spectrum adaptation terms C (for pink noise) and Ctr (for traffic noise). Rw is approximately equal to STC for most constructions but can differ by 1-3 points.

Understanding Sound Transmission Class

Sound transmission through building partitions is one of the most common sources of noise complaints in residential and commercial construction. The STC rating system, standardized by ASTM E413, provides a consistent method for comparing the sound-blocking ability of different constructions. The higher the STC number, the better the partition blocks airborne sound. However, STC has important limitations: it covers only 125 Hz to 4000 Hz and does not adequately represent low-frequency performance.

The Mass Law

The mass law states that doubling the surface mass of a partition increases transmission loss by approximately 6 dB. For a single-leaf partition without structural resonances, TL increases at approximately 6 dB per octave with frequency. This means heavier walls block more sound, but the law of diminishing returns applies: each additional doubling of mass adds only 6 more dB, making extremely heavy constructions impractical.

Double-Leaf Construction

Double-leaf constructions (two separate leaves with an air gap) outperform single-leaf constructions of the same total mass by 10-15 dB or more. The air gap decouples the two leaves, preventing direct transmission. Adding absorption (mineral wool) in the cavity further improves performance by 5-8 dB. Resilient channels between the leaves and the structure add another 5-10 dB by reducing structural coupling.

STC Limitations and the Low-Frequency Problem

STC only covers frequencies from 125 Hz to 4000 Hz, yet many noise complaints involve bass from music, subwoofers, and traffic below 125 Hz. A wall with STC 55 may still transmit significant bass energy. The OITC (Outdoor-Indoor Transmission Class, per ASTM E1332) addresses this by including frequencies down to 80 Hz with a weighting that emphasizes low frequencies. For applications involving bass noise, consider OITC alongside STC.

STC Calculator Comparison

Feature	SonaVyx	Smaart v9	REW	OSM
STC calculation	Yes (ASTM E413)	No	No	No
ISO 717-1 Rw	Yes (with C, Ctr)	No	No	No
Deficiency analysis	Yes (per-band)	No	No	No
Code compliance	Yes (IBC, UBC)	No	No	No
Browser-based	Yes	No	No	No
Field test integration	Yes (measure + calculate)	Measure only	Measure only	Measure only
Price	Free	$898	Free	Free

Frequently Asked Questions

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Standards References

ASTM E413-22 — Classification for rating sound insulation (STC calculation procedure)
ASTM E90-22 — Laboratory measurement of airborne sound transmission loss (lab TL measurement)
ASTM E336-20 — Measurement of airborne sound attenuation between rooms in buildings (field FSTC)
ISO 717-1:2020 — Rating of sound insulation: Airborne sound insulation (Rw, C, Ctr)
IBC Section 1207 — Sound transmission requirements for dwelling units