How to Measure Sound Absorption Coefficient
Understanding how effectively a material absorbs sound is essential when designing spaces for noise control, speech clarity or reverberation management. The sound absorption coefficient is the standard metric used to quantify this performance, and it plays a critical role in selecting the right acoustic products for real-world applications.
This article explains what the sound absorption coefficient is, how it is measured, and why accurate testing matters when specifying acoustic materials.
What Is the Sound Absorption Coefficient?
The sound absorption coefficient (α) is a numerical value between 0 and 1 that indicates how much sound energy a material absorbs rather than reflects.
- 0.00 = total reflection (no absorption)
- 1.00 = total absorption (no reflection)
For example, a material with an absorption coefficient of 0.75 absorbs 75% of the incident sound energy and reflects the remaining 25%.
Because sound behaves differently at various frequencies, absorption coefficients are always measured across frequency bands, typically from 125 Hz to 4,000 Hz.
Why Measuring Sound Absorption Matters
Accurate absorption data allows acoustic professionals to:
- Control reverberation times in rooms
- Improve speech intelligibility
- Reduce unwanted noise build-up
- Specify compliant materials for building regulations and standards
- Compare acoustic products objectively
Without standardised measurement methods, absorption performance claims would be unreliable and difficult to verify.
Primary Methods Used to Measure Sound Absorption
There are two internationally recognised laboratory methods used to determine sound absorption coefficients.
1. Reverberation Chamber Method (ISO 354 / BS EN ISO 354)
This is the most widely used and trusted method for measuring sound absorption.
How It Works
- A sample of the material is placed inside a large reverberation chamber
- Broadband noise is generated within the room
- The reverberation time is measured with and without the test sample
- The difference in decay time is used to calculate absorption values
Key Advantages
- Represents real-world sound behaviour
- Suitable for wall panels, ceiling systems and absorptive linings
- Produces octave or third-octave band results
This method is commonly used for commercial acoustic products and provides the data seen on most professional specification sheets.
2. Impedance Tube Method (ISO 10534-2)
The impedance tube (or Kundt’s tube) method measures absorption under controlled, one-dimensional conditions.
How It Works
- A small material sample is placed at one end of a rigid tube
- A loudspeaker emits sound waves down the tube
- Microphones measure reflected sound waves
- Absorption is calculated at specific frequencies
Key Advantages
- Highly accurate for research and development
- Requires small sample sizes
- Useful for comparing material cores and finishes
Results can differ from real-world installations, so impedance tube data is often used alongside reverberation chamber testing rather than as a standalone metric.
How Results Are Presented
Absorption results are typically shown as:
- Frequency-based absorption coefficients
- Weighted sound absorption (αw)
- Noise Reduction Coefficient (NRC) (primarily used in North America)
For a clear comparison of typical materials and their performance values, you can view the absorption coefficient chart. This chart provides a practical reference point when selecting materials for specific acoustic challenges.