What is the Sabine formula for reverberation time?

The Sabine formula is T60 = 0.161 * V / A, where V is room volume in cubic meters and A is total absorption in sabins. Total absorption A is the sum of each surface area multiplied by its absorption coefficient. Developed by Wallace Clement Sabine in the 1890s, it remains the fundamental equation for estimating reverberation time in rooms.

When should I use Eyring instead of Sabine?

Use Eyring when the average absorption coefficient exceeds 0.2 (well-treated rooms). Sabine overestimates RT60 in highly absorptive spaces because it assumes low absorption. Eyring uses the natural logarithm correction: T60 = 0.161V / (-S * ln(1-alpha_avg)), which converges to T60 = 0 when alpha = 1 (perfect absorption), while Sabine incorrectly predicts a non-zero RT60.

What is NRC and how is it calculated?

Noise Reduction Coefficient (NRC) is the arithmetic average of absorption coefficients at 250, 500, 1000, and 2000 Hz, rounded to the nearest 0.05. It provides a single-number rating for comparing materials. An NRC of 0.85 means the material absorbs 85% of sound energy on average across the speech frequencies. NRC is defined by ASTM C423.

What absorption coefficient values do common materials have?

Concrete: 0.01-0.02, glass: 0.03-0.05, drywall: 0.05-0.10, carpet on concrete: 0.15-0.35, 2-inch acoustic foam: 0.30-0.95, 4-inch fiberglass panel: 0.70-1.00, heavy curtain: 0.40-0.75. Values vary by frequency; most porous absorbers are more effective at higher frequencies.

How much absorption do people add to a room?

A seated person in upholstered theater seating provides approximately 0.4-0.6 sabins (depending on frequency). A standing person adds about 0.3-0.5 sabins. In a 500-seat venue, the audience adds 200-300 sabins of absorption, significantly reducing RT60. This is why rooms sound more reverberant when empty than when full.

What is the target RT60 for different room types?

Recording studios: 0.3-0.5s (dead), classrooms and offices: 0.6-0.8s per ANSI S12.60, conference rooms: 0.5-0.7s, churches (speech): 1.0-1.5s, concert halls: 1.6-2.2s (Romantic orchestral), opera houses: 1.2-1.6s. These targets assume the 500-1000 Hz average. Low frequencies typically have longer RT60 due to less absorption.

Why does RT60 vary with frequency?

Absorption coefficients of most materials change with frequency. Porous absorbers (foam, fiberglass) are more effective at high frequencies. Membrane absorbers (drywall, panels) absorb more at low frequencies. This causes RT60 to be frequency-dependent. A well-designed room has relatively uniform RT60 across the spectrum, which requires a mix of absorber types.

How does the Sabine formula relate to measured RT60?

The Sabine formula predicts RT60 from known room geometry and materials. SonaVyx RT60 measurement tool measures the actual RT60 from impulse response decay per ISO 3382-1. Comparing calculated versus measured values reveals whether your material absorption assumptions are correct and identifies hidden absorption or reflection sources not in the model.

What is the Sabine formula for reverberation time?

The Sabine formula is T60 = 0.161 * V / A, where V is room volume in cubic meters and A is total absorption in sabins. Total absorption A is the sum of each surface area multiplied by its absorption coefficient. Developed by Wallace Clement Sabine in the 1890s, it remains the fundamental equation for estimating reverberation time in rooms.

When should I use Eyring instead of Sabine?

Use Eyring when the average absorption coefficient exceeds 0.2 (well-treated rooms). Sabine overestimates RT60 in highly absorptive spaces because it assumes low absorption. Eyring uses the natural logarithm correction: T60 = 0.161V / (-S * ln(1-alpha_avg)), which converges to T60 = 0 when alpha = 1 (perfect absorption), while Sabine incorrectly predicts a non-zero RT60.

What is NRC and how is it calculated?

Noise Reduction Coefficient (NRC) is the arithmetic average of absorption coefficients at 250, 500, 1000, and 2000 Hz, rounded to the nearest 0.05. It provides a single-number rating for comparing materials. An NRC of 0.85 means the material absorbs 85% of sound energy on average across the speech frequencies. NRC is defined by ASTM C423.

What absorption coefficient values do common materials have?

Concrete: 0.01-0.02, glass: 0.03-0.05, drywall: 0.05-0.10, carpet on concrete: 0.15-0.35, 2-inch acoustic foam: 0.30-0.95, 4-inch fiberglass panel: 0.70-1.00, heavy curtain: 0.40-0.75. Values vary by frequency; most porous absorbers are more effective at higher frequencies.

How much absorption do people add to a room?

A seated person in upholstered theater seating provides approximately 0.4-0.6 sabins (depending on frequency). A standing person adds about 0.3-0.5 sabins. In a 500-seat venue, the audience adds 200-300 sabins of absorption, significantly reducing RT60. This is why rooms sound more reverberant when empty than when full.

What is the target RT60 for different room types?

Recording studios: 0.3-0.5s (dead), classrooms and offices: 0.6-0.8s per ANSI S12.60, conference rooms: 0.5-0.7s, churches (speech): 1.0-1.5s, concert halls: 1.6-2.2s (Romantic orchestral), opera houses: 1.2-1.6s. These targets assume the 500-1000 Hz average. Low frequencies typically have longer RT60 due to less absorption.

Why does RT60 vary with frequency?

Absorption coefficients of most materials change with frequency. Porous absorbers (foam, fiberglass) are more effective at high frequencies. Membrane absorbers (drywall, panels) absorb more at low frequencies. This causes RT60 to be frequency-dependent. A well-designed room has relatively uniform RT60 across the spectrum, which requires a mix of absorber types.

How does the Sabine formula relate to measured RT60?

The Sabine formula predicts RT60 from known room geometry and materials. SonaVyx RT60 measurement tool measures the actual RT60 from impulse response decay per ISO 3382-1. Comparing calculated versus measured values reveals whether your material absorption assumptions are correct and identifies hidden absorption or reflection sources not in the model.

Sound Absorption Calculator

Sound absorption determines how quickly acoustic energy decays in a room, directly affecting reverberation time, speech intelligibility, and music clarity. SonaVyx calculates total absorption using both the Sabine formula (best for live rooms) and the Eyring formula (more accurate for well-treated rooms), incorporating a database of 55 materials with per-octave-band absorption coefficients from 125 Hz to 4 kHz per ISO 354 measurements.

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Technical Specifications

Parameter	Value	Standard
Sabine Formula	T60 = 0.161 * V / A	ISO 3382-2
Eyring Formula	T60 = 0.161V / (-S * ln(1-alpha))	More accurate high-alpha
Frequency Bands	125, 250, 500, 1k, 2k, 4k Hz	ISO 354 octave bands
Material Database	55+ materials, 8 categories	Published alpha coefficients
NRC Calculation	Average of 250-2000 Hz	ASTM C423
SAA Calculation	Average of 200-2500 Hz (12 bands)	ASTM C423-17
Room Volume	L x W x H (metric or imperial)	User input

How to Calculate Absorption

Enter Room Dimensions

Input the room length, width, and height. The calculator computes volume (V) and total surface area (S) for a rectangular room. For irregular rooms, you can manually enter volume and surface area.

Define Surface Materials

Assign materials to each surface (floor, ceiling, four walls). Select from the built-in database of 55 materials including concrete, drywall, carpet, acoustic panels, glass, and specialty treatments. Each material has per-octave absorption coefficients.

Add Furnishings and People

Add absorption from furniture, curtains, seats, and expected audience. People contribute significant absorption, especially in the 500-4000 Hz speech range. A seated person provides approximately 0.45 sabins of absorption across the speech bands.

Review RT60 Results

The calculator displays RT60 at each octave band using both Sabine and Eyring formulas. Compare against target values for your room type: studios (0.3-0.5s), classrooms (0.6-0.8s), concert halls (1.5-2.5s). Identify frequency bands that need more treatment.

Iterate and Optimize

Swap materials on surfaces to see how RT60 changes. Add acoustic panels or bass traps to specific surfaces. The calculator instantly updates, letting you optimize the treatment plan before purchasing materials.

Understanding Sound Absorption

Sound absorption is the process by which acoustic energy is converted to heat as sound waves interact with materials. The absorption coefficient (alpha) ranges from 0 (perfect reflection) to 1.0 (perfect absorption). Porous materials like fiberglass and acoustic foam absorb sound through viscous friction as air molecules oscillate in the material pores. Membrane absorbers like drywall panels flex and convert sound energy through internal damping.

Sabine vs Eyring: When Accuracy Matters

Wallace Sabine derived his formula by assuming that sound energy decreases uniformly with each reflection. This assumption breaks down when absorption is high because the remaining energy after each reflection is no longer proportional to the original. Carl Eyring corrected this by using the natural logarithm of (1 - alpha_avg), which properly accounts for the exponential decay of energy in absorptive rooms. For rooms with alpha_avg below 0.2, both formulas give similar results. Above 0.3, Eyring is significantly more accurate.

Frequency-Dependent Treatment Design

Effective room treatment requires addressing absorption across all frequency bands. Thin porous absorbers (1-2 inches) primarily absorb mid and high frequencies. Thick porous absorbers (4+ inches) or absorbers spaced away from the wall extend effectiveness to lower frequencies. Bass traps in corners target room modes below 200 Hz. Helmholtz resonators target specific narrow frequency bands. A balanced approach uses a mix of absorber types to achieve uniform RT60 across the spectrum.

Absorption Calculator Comparison

Feature	SonaVyx	Smaart v9	REW	OSM
Sabine RT60	Yes	No	Yes (room sim)	No
Eyring RT60	Yes	No	Yes	No
Material database	55+ materials	No	Yes (limited)	No
Per-band calculation	Yes (6 bands)	No	Yes (6 bands)	No
Browser-based	Yes	No	No	No
Measurement integration	Yes (compare to RT60)	N/A	Yes	N/A
Price	Free	$898	Free	Free

Frequently Asked Questions

Related Tools & Resources

RT60 Measurement

Measure actual reverberation time

Treatment Calculator

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Room Mode Calculator

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Absorption Coefficient (Glossary)

Definition and measurement methods

Standards References

ISO 354:2003 — Measurement of sound absorption in a reverberation room
ASTM C423-17 — Standard test method for sound absorption and sound absorption coefficients (NRC, SAA)
ISO 3382-2:2008 — Room acoustics: Reverberation time in ordinary rooms
ISO 11654:1997 — Sound absorbers for use in buildings: Rating of sound absorption (alpha_w)