ASTM E413 • Knowledge Share

Understanding the STC Reference Contour

Interactive guide to Sound Transmission Class rating methodology

Why This Shape?

The STC contour isn't arbitrary—it's weighted for speech intelligibility.

Steeper slope (9 dB/octave) at low frequencies = more "forgiveness" for bass because it's physically harder to block long wavelengths.

Moderate slope (3 dB/octave) through speech frequencies where privacy matters most.

Flat plateau at high frequencies = stricter expectations because mass blocks these easily.

Reading This Chart

X-Axis 1/3-octave band center frequencies (125–4000 Hz) — the 16 standard ASTM test frequencies
Y-Axis Transmission Loss in decibels — how much sound energy is blocked at each frequency
STC # The contour value at 500 Hz when properly fitted. STC 50 = 50 dB at 500 Hz.

STC Contour vs. Measured Transmission Loss

Test Data:

STC Reference Contour

Measured TL (Select Test)

Deficiency (contour − measured)

Low Frequency

125 – 315 Hz

Bass, HVAC, traffic rumble

Speech Range

400 – 1250 Hz

Conversation, TV dialogue

High Frequency

1600 – 4000 Hz

"S" & "T" sounds, alarms

How to Fix Deficiencies

Low Frequency Solutions

Problem: Mass-air-mass resonance, long wavelengths

Add Mass — More gypsum layers
Widen Cavity — Pushes resonance lower
Decouple — Staggered studs, resilient channel
Insulation — Damps cavity resonance

Mid Frequency Solutions

Problem: Stud transmission, standing waves

Break Bridges — Resilient channel, sound clips
Cavity Insulation — Absorbs standing waves
Add Mass — Helps across all frequencies
Seal Gaps — Flanking paths hurt mid-range

High Frequency Solutions

Problem: Coincidence effect (bending = sound wave)

Damping Compounds — Converts vibration to heat
Vary Thickness — Misaligns coincidence frequencies
Multiple Panel Types — Different materials, different behavior
Constrained Layer — Factory-damped boards

The Two Critical Rules

The contour "fits" when both rules are satisfied:

8 dB

Max single-frequency deficiency

32 dB

Max total sum of all deficiencies

Select a test to see fit status

Slide the Contour

The STC rating = contour value at 500 Hz. Drag to see how the contour shifts:

Current STC

42

Contour at 500 Hz = 42 dB

STC 35 STC 57 STC 80

Max Single Deficiency: — Select test —

Total Deficiencies: —

NOAL 25-12001 - Interactive Analysis

Click layers for acoustic properties | Hover over heatmap for frequency insights | Adjust mass to explore physics

Frequency Performance Map - Transmission Loss by Frequency

Excellent (0 dB deficiency)

Good (1-2 dB)

Fair (3 dB)

Poor (4+ dB)

Click on any frequency bar to see detailed transmission loss data

💡 Click any layer for acoustic properties

Quick Reference

Gypsum Layers (Side 1)

Total:

1-7/8" (3 layers)

Type:

5/8" Type X each

Cavity Configuration

Studs:

2-1/2", 18 mil

Spacing:

24" o.c.

Insulation:

2-1/2" Glass Fiber

Gypsum Layers (Side 2)

Total:

None / Open Framing

Type:

N/A

Physical Properties

Mass:

666 lb (302 kg)

Density:

6-15/16 PSF

Thickness:

4-3/8" (11.1 cm)

Acoustic Performance

STC 42

Sound Transmission Class

Mass Law Calculator

Explore the physics of sound blocking - estimated based on mass law principles

Total Mass: 666 lbs (6.92 PSF)

Light (200 lbs) Heavy (1000 lbs)

Mass Law Prediction:

40

Theoretical STC

+2

vs Actual (42)

💡 Why the difference?
Mass Law assumes perfect limp behavior. Real walls have resonances and mechanical coupling that reduce performance at specific frequencies.

📚 Understanding Mass Law

What it calculates: The theoretical STC for an idealized single-leaf wall based only on mass per square foot. This is an empirical formula (STC ≈ 17.4 × log₁₀(mass) + 23) derived from testing many simple panels.

What "perfect limp panel" means: An imaginary panel that's infinitely flexible (no stiffness), has no internal resonances, and blocks sound purely through its mass. Think of it like a heavy rubber sheet - it moves with the sound wave but its mass prevents transmission.

Why real walls differ: Actual assemblies have stiffness (causing bending waves and resonances), mechanical coupling through framing (creating transmission paths), cavity effects (mass-air-mass resonance), and insulation (which helps dampen resonances). These factors cause real performance to deviate from the pure-mass prediction.

"What If?" Scenario Builder

Modify wall parameters to see estimated STC impact

Stud Spacing

Side 2 Finish

Insulation Type

ESTIMATED STC

42

Baseline Configuration

Current: Standard asymmetric configuration. All mass on Side 1 limits mid-frequency performance.

Component Details

Frequency: 125 Hz

Wavelength: 9' 0"

Source Side → Wall Assembly → Receiver Side

📊 Real-Time Frequency Response

🎚️ Frequency Control

125 500 1000 2000 4000

25

Transmission Loss (dB)

1

Deficiency (dB)

✓ Low Frequency - Performing Well

At 125 Hz, despite long wavelengths, the wall shows only 1 dB deficiency. The triple-layer gypsum mass (6.43 lb/ft² total) provides effective blocking at this bass frequency range.

🔊 Real-World Sounds at This Frequency

Bass sounds in music, traffic rumble, HVAC systems, large machinery, thunder, building vibrations

📐 Acoustic Field Explanation

At low frequencies near resonance, the wall's mass and the air cavity form a coupled system. Sound pressure waves bounce between gypsum faces, creating standing wave patterns that efficiently transfer energy through the assembly.

Animation Speed

Incident

Transmitted

Reflected

Absorbed

NOAL 25-12001 • STC 42

Sound Demo Experience

Experience how sound energy is reduced as it passes through the wall assembly. Select a frequency, then listen to the source and transmitted tones.

Source Room

Receiving Room

Transmission Loss

26 dB

STC 42 Assembly

80

dB Source

44

dB Received

125 Hz — Bass / Low Frequency

Think: bass guitar, HVAC rumble, traffic noise. This wall reduces this frequency by 26 dB.

Click to Begin
Audio Experience

Sound Transmission Lab

Understanding the STC Reference Contour

Why This Shape?

Reading This Chart

STC Contour vs. Measured Transmission Loss

The Two Critical Rules

Slide the Contour

NOAL 25-12001

Gypsum Layers (Side 1)

Cavity Configuration

Gypsum Layers (Side 2)

Physical Properties

Acoustic Performance

Mass Law Calculator

"What If?" Scenario Builder

Component Details

🔊 Sound Wave Propagation Simulation

🎚️ Frequency Control

✓ Low Frequency - Performing Well

🔊 Real-World Sounds at This Frequency

📐 Acoustic Field Explanation

Sound Demo Experience

125 Hz — Bass / Low Frequency

Click to BeginAudio Experience

Sound Transmission Lab

Why This Shape?

Reading This Chart

STC Contour vs. Measured Transmission Loss

The Two Critical Rules

Slide the Contour

NOAL 25-12001

Gypsum Layers (Side 1)

Cavity Configuration

Gypsum Layers (Side 2)

Physical Properties

Acoustic Performance

Mass Law Calculator

"What If?" Scenario Builder

Component Details

🔊 Sound Wave Propagation Simulation

🎚️ Frequency Control

✓ Low Frequency - Performing Well

🔊 Real-World Sounds at This Frequency

📐 Acoustic Field Explanation

Sound Demo Experience

125 Hz — Bass / Low Frequency

Click to Begin
Audio Experience