What’s the Difference Between Fully and Semi-Anechoic Chambers? — Key Selection Criteria and Common Misconceptions Based on Test Objectives and Site Conditions —
08/01/2025
MFAC / MSAC
Introduction: Two Distinct Types of Anechoic Chambers
In acoustic testing and environmental noise evaluation, the term “anechoic chamber” generally refers to one of two types:
- Fully Anechoic Chamber (FAC)
- Semi-Anechoic Chamber (SAC)
Each has distinct design goals, structural features, and applicable use cases.
Choosing the wrong type can lead to significant issues in measurement accuracy, cost, and facility requirements.
What Is a Fully Anechoic Chamber (FAC)?
A FAC eliminates sound reflections from all six surfaces: walls, ceiling, and floor, creating a near-perfect free-field environment.
It is ideal for:
- Free-field response testing of speakers and microphones
- Acoustic directivity measurements of noise sources
- Near-field audio testing of headsets, hearing aids, and audio devices
Note, however, that achieving this environment requires a floating floor structure, which increases both cost and ceiling height requirements.
What Is a Semi-Anechoic Chamber (SAC)?
A SAC uses sound-absorbing treatment on the walls and ceiling, but retains a reflective floor surface (usually concrete).
It is suited for testing systems that rest directly on the ground, such as:
- Sound power measurements for vehicles and machinery (e.g., ISO 3744)
- Operational noise testing for appliances and industrial equipment
- AVAS/EV sound testing, where floor reflection is part of real-world behavior
For large and heavy equipment, a SAC is often the more practical and robust solution.
Common Misconceptions and Realities
| Misconception | Reality |
|---|---|
| “FAC always provides better accuracy” | SAC may be more appropriate depending on the objective |
| “FAC has superior absorption performance” | SAC can meet ISO free-field criteria when well-designed |
| “SAC causes unstable measurements” | Proper floor modeling ensures consistent results |
| “Adding floor absorbers makes it equivalent to a FAC” | Interactions between floor and ceiling must still be controlled |
Sonora’s Approach: Application-Based Design
We tailor our chamber designs to match each application and relevant standards:
| FAC | Modular FAC (MFAC) combining high isolation and ease of installation |
| SAC | Vehicle SAC (VSAC) and general-purpose SAC (MSAC) compliant with ISO 3744/26101 |
| Custom FACs with walkable floors | Hybrid designs maintaining floating structure integrity with floor accessibility |
Conclusion: Select the Right Chamber for the Right Purpose
The decision between FAC and SAC is not about superiority, but about measurement intent.
Frequency range, reflection control, floor conditions, and available space must all be considered.
Choosing the appropriate chamber type ensures optimal accuracy and cost-effectiveness.