{"id":890,"date":"2025-06-25T16:24:57","date_gmt":"2025-06-25T07:24:57","guid":{"rendered":"https:\/\/acoustic-measurement.com\/?post_type=technology&p=890"},"modified":"2025-06-25T18:59:18","modified_gmt":"2025-06-25T09:59:18","slug":"the-truth-behind-the-0-99-absorption","status":"publish","type":"technology","link":"https:\/\/acoustic-measurement.com\/en\/technology\/the-truth-behind-the-0-99-absorption\/","title":{"rendered":"The Truth Behind the 0.99 Absorption Coefficient : Understanding Cut-off Frequency and the Performance of Acoustic Wedges"},"content":{"rendered":"\n

What Does a Sound Absorption Coefficient of 0.99 Really Mean in an Anechoic Chamber?<\/strong><\/strong><\/h2>\n\n\n\n

Have you ever experienced that sensation of your ears \u201ctightening\u201d the moment you enter an anechoic chamber?
This is a sign that almost all sound energy is being absorbed, with virtually no reflections occurring in the space.
This performance is expressed numerically by the sound absorption coefficient of 0.99<\/strong>, meaning that 99% of the sound reaching the wall is absorbed, and only 1% is reflected<\/strong>.<\/p>\n\n\n\n

The key element that makes this \u201cnear-total absorption\u201d possible is the sound-absorbing wedge<\/strong>, the heart of the anechoic chamber.<\/p>\n\n\n\n

The Relationship Between Absorption and Reflection Coefficients<\/strong><\/strong><\/h2>\n\n\n\n

The sound absorption coefficient (\u03b1) represents how much of the incident sound energy on surfaces like walls or ceilings is absorbed.
Here, r<\/strong> is the sound pressure reflection coefficient.
For example, if r = 0.1<\/strong> (10% reflection), the absorption coefficient is:<\/p>\n\n\n\n

\u03b1=1\u2212r2=0.99\\alpha = 1 – r^2 = 0.99\u03b1=1\u2212r2=0.99<\/p>\n\n\n\n

This means that achieving a coefficient of 0.99 requires suppressing reflected sound to below 10%\u2014a highly demanding condition.<\/p>\n\n\n\n

What Is the Cut-off Frequency?<\/strong><\/strong><\/h2>\n\n\n\n

Does the absorption coefficient of 0.99 apply to all frequencies?
The answer is no<\/strong>.<\/p>\n\n\n\n

Sound-absorbing wedges have a design parameter called the cut-off frequency<\/strong>, which defines:<\/p>\n\n\n\n

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The lowest frequency above which the wedge can maintain an absorption coefficient of 0.99.<\/strong><\/p>\n<\/div>\n<\/div>\n\n\n\n

Sounds below this frequency\u2014such as low-frequency noise under 100 Hz\u2014may penetrate the absorbing material or reflect back into the room.<\/p>\n\n\n\n

Why \u03bb\/4 Design Is Critical<\/strong><\/strong><\/h2>\n\n\n\n

The most critical factor in achieving the desired cut-off frequency is the length of the wedge<\/strong>.
In most anechoic chambers, wedges are designed based on the quarter-wavelength (\u03bb\/4)<\/strong> of the target frequency.<\/p>\n\n\n\n

Example:<\/strong><\/h3>\n\n\n\n
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For 500 Hz:<\/strong><\/p>\n\n\n\n

    \n
  • Sound speed \u2248 340 m\/s<\/li>\n\n\n\n
  • Wavelength \u03bb \u2248 0.68 m<\/li>\n\n\n\n
  • \u03bb\/4 \u2248 0.17 m (17 cm wedge length)<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n\n\n

    So, to lower the cut-off frequency, the wedge must be made longer.
    A 100 Hz cut-off would require wedge lengths of at least 0.85 meters<\/strong>.<\/p>\n\n\n\n

    Performance Requirements from ISO and JIS Standards<\/strong><\/strong><\/h2>\n\n\n\n

    The older standards, such as ISO 3745:2003<\/strong> and JIS Z 8732<\/strong>, required an absorption coefficient of 0.99 or higher<\/strong> for the walls and ceilings of anechoic chambers.<\/p>\n\n\n\n

    However, ISO 3745:2012<\/strong> removed this specific numeric requirement.
    Instead, performance is now judged based on whether the inverse square law<\/strong> is satisfied\u2014i.e., sound pressure decreases in proportion to the square of the distance from the source.<\/p>\n\n\n\n

    Thus, practical acoustic performance<\/strong>, rather than just the numerical value of 0.99, is emphasized today.<\/p>\n\n\n\n

    Can a Room Be \u201cAnechoic\u201d with Less Than 0.99?<\/strong><\/strong><\/h2>\n\n\n\n

    In some cases, the answer is yes<\/strong>.<\/p>\n\n\n\n

    Modern designs include “non-tingling” anechoic chambers that use perforated metal panels over glass wool<\/strong> or melamine-based flat absorbers<\/strong> instead of wedges.
    Even if their high-frequency absorption falls below 0.99, they can still be certified as anechoic rooms\u2014as long as the inverse square law holds true<\/strong>.<\/p>\n\n\n\n

    Sonora’s Acoustic Wedges: Exceeding Cut-off Frequency Limitations<\/strong><\/strong><\/h2>\n\n\n\n

    Sonora\u2019s BF series (BFW, BFB, BFP)<\/strong> is engineered with advanced acoustic design principles and carefully selected materials to exceed traditional limitations regarding cut-off frequency.<\/p>\n\n\n\n

    \u00a0Absorption Based on \u03bb\/4 Principles<\/strong><\/strong><\/h3>\n\n\n\n

    Each wedge is designed to match \u03bb\/4 of the target frequency.
    With multi-layered internal structures and precisely controlled air cavities, high absorption is achieved with compact thickness.<\/p>\n\n\n\n

    Tyvek\u00ae Surface Material<\/strong><\/strong><\/h3>\n\n\n\n

    The surface of the wedge uses Tyvek\u00ae<\/strong>, a nonwoven material known for high water resistance and durability<\/strong>.
    Tyvek allows water vapor to pass but blocks liquid water, making it ideal for humid environments.
    It also minimizes dust generation and enables easier cleaning and long-term stable performance.<\/p>\n\n\n\n

    Reference video (starting at 4:00)\uff1a<\/h4>\n\n\n\n
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