Design Requirements for Anechoic and Hemi-Anechoic Chambers
06/02/2025
Room Size, Absorption Performance, and Sound Source Placement According to ISO Standards
Anechoic and hemi-anechoic chambers are essential environments for accurately evaluating the acoustic characteristics of products. In particular, compliance with international standards such as ISO 3745:2012 and ISO 3744:2010 is critical.
This article provides a clear overview of the latest ISO-based design requirements for:
- Room size
- Absorption performance
- Sound source placement
What Are Anechoic and Hemi-Anechoic Chambers?
| Anechoic Chamber | A space in which sound reflections are virtually eliminated, providing a fully free-field environment. |
| Hemi-Anechoic Chamber | A room where only the floor remains reflective, while all other surfaces are acoustically treated. |
These chambers are defined based on the condition that they reproduce a free field or free field over a reflecting plane, where the inverse square law (sound pressure level decreases in proportion to the square of the distance) is satisfied.
Room Size Requirement: At Least 200 Times the Source Volume
According to the now-superseded ISO 3745:2003 Annex K, the volume of the test room should be at least 200 times the volume of the sound source. Although this annex has been removed in the 2012 version, it remains a useful design reference.
In addition, at least 0.5 meters of space should be secured on all sides of the virtual source box.
Example
- Sound source volume: 0.1 m³
- Required room volume: ≥ 20 m³
- Suggested dimensions: approx. 2.5 m × 3.5 m × 2.5 m
Absorption Requirement: Priority on Inverse Square Law, Not Absorption Rate
According to ISO 3745:2012:
The specific numerical requirements such as normal incidence sound absorption coefficient ≥ 0.99 or wedge thickness ≥ λ/4 have been removed.
Instead, the focus has shifted to:
- Ensuring that the inverse square law is satisfied (i.e., a valid free-field condition)
This allows for flexible chamber design using non-conventional absorption structures and materials such as:
- Glass wool covered with perforated metal
- Flat panel or melamine foam absorbers
- Sonora’s proprietary BFW (Broadband Fractal Wedge), a non-wedge structure
These materials reduce visual and spatial pressure while ensuring compliance with the inverse square law.
Sound Source Placement: Centralized and Near the Reflective Plane
ISO 3745:2012 recommends placing the sound source at the center of the chamber. In hemi-anechoic chambers, the source must be positioned within 150 mm of the reflective surface (floor).
Recommended Configurations by Frequency Band
| Frequency Band | Sound Source Type | Placement (Hemi-Anechoic) |
|---|---|---|
| ~400 Hz | φ25 cm speaker | ≤ 8 cm from reflective floor |
| 400–2000 Hz | Face-to-face speaker setup | ≤ 2 cm from reflective floor |
| >2000 Hz | Narrow pipe speaker | ≤ 0.5 cm from reflective floor |
The K2 Value: A Key Index of Environmental Accuracy
K2, or the environmental correction factor, quantifies the effect of room reflections on the acoustic field.
| Method | Allowable K2 (dB) |
|---|---|
| ISO 3745 (Precision) | ≤ 0.5 dB |
| ISO 3744 (Engineering) | ≤ 4.0 dB |
In practical applications, a target of K2 ≤ 4.0 dB is often adopted. This avoids excessive absorption requirements while maintaining reliable measurement accuracy.
Conclusion: The Inverse Square Law Is the Key to Compliance
ISO 3745:2012 emphasizes performance-based requirements—particularly the fulfillment of the inverse square law—over strict structural or material specifications. This shift allows for more flexible and innovative designs.
At Sonora, we design and construct anechoic and hemi-anechoic chambers using our proprietary BF Series absorbers to achieve both compliance with international standards and superior user comfort.