How to Read Standards Related to Anechoic Chambers ― Organizing Sound Power Measurement Standards and Qualification of Test Environments ―

Introduction

Anechoic chambers and hemi-anechoic chambers are widely used for product noise evaluation and sound power level measurements. However, the reliability of a measurement is not determined simply by the fact that it was conducted “in an anechoic chamber.” What matters is whether the test space satisfies the prerequisite conditions of the applicable standard and whether the required accuracy and operational conditions have been properly defined in accordance with the measurement objective. ISO 3745 is positioned as a precision method for anechoic and hemi-anechoic rooms, ISO 3744 as an engineering method for an essentially free field over a reflecting plane, ISO 3746 as a survey method, and ISO 3741 as a precision method for reverberation test rooms.

In practice, measurement methods should not be selected based only on the standard name. The entire measurement system must be designed in consideration of the size of the test object, the required accuracy, the available test facility, the operational workload, and even the need for automation. At Moritani, there are many occasions where we examine solutions that combine HBK measurement systems with SONORA test environments. HBK also provides sound power software supporting ISO 3744, ISO 3745, and ISO 3746.

There Is No Single Standalone Standard for an Anechoic Chamber

The term “anechoic chamber” is widely used, but in practice, simply having a room called an anechoic chamber does not guarantee compliance with any standard. What matters is whether that test room satisfies the sound field conditions required by the measurement standard being applied. For example, ISO 3745 specifies precision methods for measurements in anechoic rooms or hemi-anechoic rooms, positioning the anechoic chamber as a test environment required within the framework of the measurement standard.

This way of thinking is especially important when introducing or upgrading facilities. What is required is not merely “having an anechoic chamber,” but ensuring that the space is valid for the standard being adopted. Therefore, it is important not to treat room performance verification and measurement system consistency as separate matters.

Basic Terms for Understanding the Standards

Free Field

A free field is a sound field in which the effects of reflections from boundary surfaces can be neglected. Anechoic chambers are used as test rooms to create conditions approximating this free field. ISO 3745 applies to precision methods in such anechoic and hemi-anechoic environments.

Free Field Over a Reflecting Plane

In a hemi-anechoic chamber, a reflecting plane such as the floor is retained, while conditions approximating a free field are created in the half-space above it. ISO 3744 is an engineering method based on this concept of an “essentially free field over a reflecting plane.”

Inverse Square Law

In a free field, the inverse square law applies: when the distance from a point sound source is doubled, the sound pressure level decreases by 6 dB. In considering the performance of anechoic and hemi-anechoic chambers, an important verification point is the degree to which this relationship is satisfied. ISO 26101-1 is the standard dealing with qualification of free-field environments.

Positioning Within Sound Power Measurement Standards

Standards for sound power determination by the sound pressure method are selected according to the measurement environment and the level of accuracy required. Under the current ISO framework, ISO 3745:2012 is positioned as the precision method for anechoic and hemi-anechoic rooms, ISO 3744:2025 as the engineering method for an essentially free field over a reflecting plane, ISO 3746:2010 as the survey method using an enveloping measurement surface, and ISO 3741:2010 as the precision method for reverberation test rooms.

These distinctions are not simply a matter of “precision,” “practical,” or “simplified” categories. They differ in the sound field conditions required of the measurement space, the treatment of reflections, the concept of environmental corrections, and the handling of measurement uncertainty. For this reason, standard selection must be considered together with the validity conditions of the test room, not only the measurement objective.

ISO 3745

Precision Measurements in Anechoic and Hemi-Anechoic Rooms

ISO 3745:2012 is a precision method for determining sound power levels and sound energy levels from sound pressure measurements in anechoic or hemi-anechoic rooms. On the official ISO page, it is described as Precision methods for anechoic rooms and hemi-anechoic rooms, and the 2012 edition was confirmed as the current edition in 2022.

This standard is well suited to applications where reproducibility and high accuracy are strongly required, such as research and development, certification testing, and comparative evaluation of low-noise products. On the other hand, it also places stricter requirements on test room performance, microphone arrangement, measurement point setup, and background noise control. HBK’s sound power software supports ISO 3745, making it easier to maintain consistency between instrumentation and analysis.

Practical considerations

When adopting ISO 3745, it is important not only to describe the specifications of the sound-absorbing materials, but also to verify in the end that the space is sufficiently close to a free field or a free field over a reflecting plane. From Moritani’s perspective, the key is to determine the required level of accuracy and then align SONORA’s test environment with HBK’s measurement system accordingly.

ISO 3744

Engineering Method in an Essentially Free Field Over a Reflecting Plane

ISO 3744 is an engineering method for determining sound power levels from sound pressure measurements in an essentially free field over a reflecting plane. In the latest ISO catalog, ISO 3744:2010 is listed as withdrawn, while the current published edition is ISO 3744:2025.

In many cases, this standard becomes the core method for practical evaluations using hemi-anechoic chambers. While it is less stringent than ISO 3745, it generally provides higher reliability than ISO 3746, making it a well-balanced method for development evaluation, product comparison, and pre-production verification. HBK’s sound power software also includes ISO 3744 among its supported standards.

Practical considerations

Although ISO 3744 is user-friendly, it does not automatically become valid simply because a hemi-anechoic chamber is available. It is important to determine the configuration by considering the dimensions of the test object, the definition of the measurement surface, the number of microphone positions, and even future automation requirements.

ISO 3746

Survey Measurements Using an Enveloping Measurement Surface

ISO 3746:2010 is a survey measurement standard for determining sound power levels or sound energy levels from sound pressure measurements using an enveloping measurement surface over a reflecting plane. In the ISO catalog, it is described as Survey method using an enveloping measurement surface over a reflecting plane.

This standard is better suited to applications where simplicity is prioritized over highly accurate certification evaluation, such as preliminary assessment, in-process verification, and early-stage development screening. However, uncertainty tends to be greater than with the higher-level standards, so it is necessary to define in advance the intended use of the measurement results. HBK’s sound power software also supports ISO 3746.

Practical considerations

It is not uncommon for a measurement system initially introduced as a survey method to later require expansion to the ISO 3744 or ISO 3745 level. For this reason, it is often more efficient in the long run to design the measurement configuration with future expandability in mind from the initial stage.

ISO 3741

Precision Measurements Using a Reverberation Test Room

ISO 3741:2010 is a precision measurement standard for determining sound power levels and sound energy levels from sound pressure measured in a reverberation test room. In the ISO catalog, it is described as Precision methods for reverberation test rooms.

Whereas standards using anechoic or hemi-anechoic rooms assume environments in which the effects of reflections are minimized, ISO 3741 is fundamentally different in that it utilizes a sufficiently diffuse sound field. As a result, depending on the characteristics of the product under test, it may sometimes be a more rational choice than an anechoic-room-based method.

Practical considerations

Even when existing facilities are centered around anechoic chambers, the reverberation room method may be more advantageous depending on the test object. Rather than working backward from the facilities already owned, it is important to choose the method based on the measurement objective. HBK also provides product-noise software for reverberation room applications.

ISO 26101

The Standard for Verifying Whether a Test Environment Is Valid

ISO 26101 is not a standard for determining sound power itself. Rather, it is a standard for verifying whether acoustic environments such as anechoic and hemi-anechoic rooms satisfy the prerequisites for standard-compliant measurements. In the current ISO catalog, ISO 26101-1:2021 covers qualification of free-field environments, and ISO 26101-2:2024 covers the determination of environmental corrections. The former ISO 26101:2017 has been withdrawn.

ISO 26101-1 includes test methods using discrete-frequency and broadband methods, qualification of omnidirectional sound sources suitable for free-field qualification assessment, expression of results, and uncertainty. In other words, it is positioned as an important prerequisite for ensuring the reliability of measurements based on ISO 3745 or ISO 3744.

Practical considerations

Even if measurement instruments and analysis software are all in place, the overall reliability of the measurement remains unclear if qualification of the test environment is insufficient. It is therefore important not to treat instrument installation and test room preparation as separate themes, but instead to regard them as the creation of a measurement environment based on standards compliance.

How to Think About Standard Selection

For high-accuracy R&D and certification evaluation	ISO 3745 should be the primary candidate. Verification of the performance of the anechoic or hemi-anechoic room is a prerequisite.
For a well-balanced practical evaluation system	ISO 3744 is central. A combination of a hemi-anechoic chamber and a standards-compliant measurement system is realistic.
For simplified evaluation or process verification	ISO 3746 is effective. However, future expandability should be considered.
For evaluation using a diffuse sound field	ISO 3741 should be considered. Suitability should be judged based on compatibility with the product under test.
For test room validity verification	ISO 26101 should be treated as a prerequisite. This is especially important for new construction, renovation, or relocation.

Summary

The key to understanding standards related to anechoic chambers is to regard the anechoic chamber not as an isolated facility name, but as a test environment that must be valid within the framework of sound power measurement standards. ISO 3745, ISO 3744, ISO 3746, and ISO 3741 should be selected according to the measurement objective and sound field conditions, while ISO 26101 should be understood as the prerequisite standard for test environment qualification.

From Moritani’s perspective, the most important practical point is not to consider HBK’s measurement systems and SONORA’s test environments separately, but to configure standards, space, instrumentation, and operation as one integrated measurement system. HBK’s product guidance is also structured in line with this standards-based approach.

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