When discussing the specifications of an anechoic room, the figure \u201csound absorption coefficient of 0.99\u201d can sometimes take on a life of its own. Seeing this value listed in a catalog for wedge-shaped sound absorbers, some people may assume that the space automatically qualifies as an anechoic room.<\/p>\n\n\n\n
However, a sound absorption coefficient of 0.99 is only an indicator of the performance of the absorbing material itself. Whether the space satisfies the sound field conditions required by sound power measurement standards is a separate matter.<\/p>\n\n\n\n
This article explains the relationship between sound absorption coefficients and the qualification of an anechoic room, and reviews the concept that has been clarified since ISO 3745:2012: evaluation based on whether the inverse-square law is satisfied.<\/p>\n\n\n\n
As a starting point, the sound absorption coefficient shown in catalogs is, in most cases, a value measured for the material itself in a laboratory. Typical measurement methods include the following:<\/p>\n\n\n\n
For wedge-shaped absorbers, measurements may also be performed using a free-field method in an anechoic room. In any case, the resulting value represents the absorption characteristics of that material under specific incidence conditions. It does not represent the sound field performance of the entire room.<\/p>\n\n\n\n
In other words, the value 0.99 simply means that, under specific measurement conditions, 99% of the incident acoustic energy did not return as reflected sound. This value alone cannot determine whether the space satisfies the requirements of an anechoic room under measurement standards.<\/p>\n\n\n\n
Even if high-absorption wedges are installed on all surfaces, this does not necessarily mean that the space satisfies the free-field conditions required by ISO 3745. In practice, sound field performance is affected by factors such as the following:<\/p>\n\n\n\n
As discussed in previous technical columns such as \u201cDesign Theory of the Inverse-Square Law Valid Region\u201d and \u201cThe Geometry of Designing an Anechoic Room,\u201d the performance of an anechoic room is not determined solely by the absorption coefficient of its wedges. It is a matter of comprehensive design: how the sound field is established as a room.<\/p>\n\n\n\n
It is also possible that even if the sound absorption coefficient of an absorber measured in a laboratory is below 0.99, the inverse-square law may still be satisfied in the room as a three-dimensional space.<\/p>\n\n\n\n
The sound power measurement standard ISO 3745 specifies precision methods for anechoic rooms and hemi-anechoic rooms. In the ISO catalog, it is described as Precision methods for anechoic rooms and hemi-anechoic rooms<\/em>. The current version is ISO 3745:2012, which was confirmed as current in 2022.<\/p>\n\n\n\n One of the major points clarified in the 2012 edition is the approach to qualifying anechoic and hemi-anechoic rooms. The key points are as follows.<\/p>\n\n\n\n In the past, the evaluation of anechoic room performance was included within ISO 3745 itself. However, the qualification of free-field environments has moved toward being organized separately under the ISO 26101 series. Currently, ISO 26101-1:2021 covers the qualification of free-field environments, while ISO 26101-2:2024 addresses the determination of environmental corrections.<\/p>\n\n\n\n The basis for determining compliance has shifted from absorber specifications, such as wedge absorption coefficient or wedge length, to the actual sound field established inside the room.<\/p>\n\n\n\n The concept of placing a sound source, measuring sound pressure levels at different distances, and evaluating how much the attenuation deviates from the theoretical value of \u22126 dB per doubling of distance has been more clearly positioned. This can be considered an evaluation equivalent to K\u2082.<\/p>\n\n\n\n In other words, the direction of the standard has shifted toward verifying an anechoic room by measuring it as a room, rather than by simply accumulating material specifications.<\/p>\n\n\n\n According to the concepts of ISO 3745:2012 and the ISO 26101 series, the essential evaluation criterion for an anechoic room can be summarized as follows:<\/p>\n\n\n\n A sufficiently large region must be secured within the test room where the inverse-square law is satisfied.<\/strong><\/p>\n<\/div>\n<\/div>\n\n\n\n In a free field, the sound pressure level decreases by 6 dB each time the distance from a point source doubles. If this relationship can be confirmed through measurement within the target frequency range, measurement distances, and allowable deviation range, the space can be judged to function as a free-field environment under the standard.<\/p>\n\n\n\n As a guideline in ISO 3745, the allowable deviation is approximately:<\/p>\n\n\n\n Conversely:<\/p>\n\n\n\n The \u201csound absorption coefficient\u201d is one element of the specification. Not confusing these two concepts is the starting point for properly considering anechoic room specifications.<\/p>\n\n\n\n When planning a new facility or upgrading an existing one, if the specification only states \u201csound absorption coefficient of 0.99 above XX Hz,\u201d this should be understood as a declaration of material performance, not a guarantee of compliance with a measurement standard.<\/p>\n\n\n\n The points that should be confirmed are instead the following:<\/p>\n\n\n\n From Moritani\u2019s perspective, when combining SONORA\u2019s test space design with HBK\u2019s measurement systems, organizing requirements based on \u201csound field performance\u201d rather than \u201cwedge specifications\u201d leads to more reliable final measurements.<\/p>\n\n\n\n \u201cBecause the sound absorption coefficient is 0.99, it is an anechoic room.\u201d<\/p>\n<\/div>\n<\/div>\n\n\n\n This understanding is one step removed from the philosophy of current international standards.<\/p>\n\n\n\n Since ISO 3745:2012, the evaluation axis for anechoic rooms has shifted from material specifications to sound field performance. More specifically, the focus has been placed on the establishment of a free field, namely the deviation from the inverse-square law.<\/p>\n\n\n\n The sound absorption coefficient remains an important design parameter. However, it cannot, by itself, demonstrate compliance with a measurement standard.<\/p>\n\n\n\n When considering facility specifications, the correct starting point is to ask:<\/p>\n\n\n\n Which standard, which class, and which frequency range must the room comply with?<\/strong><\/p>\n\n\n\n Starting from this question is the most reliable path toward creating a proper measurement environment.<\/p>\n","protected":false},"excerpt":{"rendered":" Introduction When discussing the specifications of an anechoic room, the figure \u201csound absorption coefficient of 0.99\u201d can sometimes take on a life of its own. Seeing this value listed in a catalog for wedge-shaped sound absorbers, some people may assume that the space automatically qualifies as an anechoic room. However, a sound absorption coefficient of […]<\/p>\n","protected":false},"featured_media":0,"parent":0,"template":"","solution_cat":[2],"class_list":["post-1176","technology","type-technology","status-publish","hentry","solution_cat-tax_power","en-US"],"acf":[],"_links":{"self":[{"href":"https:\/\/acoustic-measurement.com\/wp-json\/wp\/v2\/technology\/1176","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/acoustic-measurement.com\/wp-json\/wp\/v2\/technology"}],"about":[{"href":"https:\/\/acoustic-measurement.com\/wp-json\/wp\/v2\/types\/technology"}],"wp:attachment":[{"href":"https:\/\/acoustic-measurement.com\/wp-json\/wp\/v2\/media?parent=1176"}],"wp:term":[{"taxonomy":"solution_cat","embeddable":true,"href":"https:\/\/acoustic-measurement.com\/wp-json\/wp\/v2\/solution_cat?post=1176"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Separation of Test Room Qualification Methods<\/h3>\n\n\n\n
From Absorber Specifications to Measured Sound Field Performance<\/h3>\n\n\n\n
Evaluation Based on Deviation from the Inverse-Square Law<\/h3>\n\n\n\n
The Inverse-Square Law Is the Essential Criterion<\/strong><\/h2>\n\n\n\n
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The \u201cestablishment of the inverse-square law\u201d is the verification of the result.<\/p>\n\n\n\nPractical Points to Consider<\/h2>\n\n\n\n
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Conclusion<\/h2>\n\n\n\n