Anekoik Room

An anechoic chamber is a specialized acoustic test room designed to eliminate echoes or reflections. The term “anechoic,” derived from Greek, means “non-reflective.” These chambers are used primarily to simulate free-field conditions for the propagation of sound and electromagnetic waves. Anechoic chambers serve as essential tools in various industrial tests, research and development processes, and measurements requiring compliance with international standards.

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Anechoic chambers are equipped with high-performance sound and wave absorption materials installed on their interior surfaces. These absorbers are typically wedge-shaped and mounted on all six surfaces (walls, ceiling, and floor), effectively eliminating nearly all reflections from the chamber’s surfaces. The chamber floor is often supported by a transparent platform made of stretched wire mesh; however, semi-anechoic solutions with reflective hard floors are also preferred for practical reasons and to reduce measurement errors.

In the design of sound absorbers, material density and resistance to airflow are critical parameters. To achieve effective absorption at low frequencies, wedge structures may be made multi-layered or incorporate resonator-based designs. For electromagnetic applications, materials such as carbon-loaded foam or ferrite tiles are commonly used.

Anechoic chambers are used for measurements in both acoustic and electromagnetic fields. In acoustic chambers:

• The sound power levels of machinery and equipment can be determined.
• The directional and spectral characteristics of noise sources can be analyzed.
• Sound components emitted by product subsystems can be individually examined.

RF anechoic chambers used for EMC (Electromagnetic Compatibility) testing are designed to:

• Perform emission and immunity tests,
• Provide indoor conditions that can replace open-area test sites (OATS),
• Enable standardized measurements at 3-meter and 10-meter test distances.

Measurements conducted in anechoic chambers are defined by international standards such as ISO 3745 and ANSI S12.35. These standards specify methods for measuring the chamber’s anechoic performance, microphone placement, sound source characteristics, and measurement procedures. The directivity of the sound sources used during chamber validation is critical to the reliability of measurement results.

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Creating fully anechoic environments is not always necessary and can be costly. Therefore, semi-anechoic chambers with reflective floors are widely used. Such chambers offer a more practical and suitable test environment for measuring devices that interact with the ground, such as washing machines, compressors, and electric tools.

The history of anechoic chambers extends back to the mid-20th century, when radio waves and acoustic properties began to be studied scientifically. With the widespread adoption of EMC regulations in the 1980s, major companies shifted toward constructing chambers to replace open-area test sites. One milestone in this field was the first 3-meter EMC anechoic chamber built for IBM in 1982. From the 1990s onward, the use of ferrite tiles and hybrid absorber materials enabled chambers to become smaller while improving performance.

Measurements in anechoic chambers vary depending on the method used and the size of the test object. The two primary measurement approaches defined by international acoustic standards are:

• Hemispherical Method: In this method, the test object is surrounded by microphones positioned on a hemisphere centered on the object. Microphones are placed at specific distances and angles to capture the directional characteristics of the radiated sound. This method is particularly suitable for small and symmetric sources.
• Rectangular Prism Method: Preferred for large-scale machinery. Measurements are taken using microphones arranged around the test object in a rectangular prism configuration. This method is suitable for non-directional or complex source structures.

In both methods, sound power levels are calculated using A-weighted sound pressure levels, octave or 1/3-octave band analyses, and frequency components.

The performance of anechoic chambers depends on the properties of the absorber materials installed on their interior surfaces. The primary purpose of these materials is to absorb sound or electromagnetic waves without reflection. In acoustic anechoic chambers, absorbers typically take the following forms:

• Wedge-Type Absorbers: This traditional design provides high absorption at low and mid frequencies. Longer wedges improve performance at lower frequencies. However, their large volume and space requirements can pose installation challenges.
• Layered and Resonator-Based Absorbers: Resonance structures can be integrated into the base of the material to enhance low-frequency performance, providing additional absorption at lower frequencies.
• Hybrid Absorbers: In electromagnetic chambers, combinations of ferrite tiles and carbon-loaded foam offer more compact solutions. Ferrite tiles are effective in the 30–100 MHz range, while foam-based absorbers dominate at higher frequencies.

Material selection is determined by the target frequency range, chamber size, and test requirements.

The design of an anechoic chamber is determined by parameters such as the target frequency range, required measurement accuracy, and the size of the objects to be tested. Key design criteria include:

• Frequency Range: The length of the absorber material and the chamber volume are determined based on the lowest frequency of interest. Lower frequencies require longer absorbers and larger internal volumes.
• Accuracy Class: As defined by ISO standards, measurement classes are “Precision (Grade 1),” “Engineering (Grade 2),” or “Survey (Grade 3).” Precision-class chambers provide more homogeneous free-field conditions and operate with tighter tolerances.
• Structural Isolation: The chamber is constructed using a “room-within-a-room” principle to prevent internal sound from escaping and external noise from interfering with measurements. Sound transmission between inner and outer structures must be blocked, and solid contact should be avoided.
• Vibration and Noise Isolation: The chamber is mounted on floating floor systems (neoprene pads or spring-based systems) to isolate it from structural vibrations. This prevents structural noise sources from affecting measurement results.
• Geometric Shape and Microphone Placement: The chamber’s shape may be designed with non-parallel surfaces to minimize internal reflections. Microphone arrangements follow standardized hemispherical or rectangular prism configurations.

All these parameters ensure high-accuracy measurements in both acoustic and electromagnetic testing.

Anechoic chambers are used across various sectors for product development, quality control, and regulatory compliance testing. Each sector may require different types of anechoic chamber solutions based on specific measurement needs. Major application areas include:

• Automotive Industry: Sound power levels and radiation patterns of subsystems such as engines, fans, and air conditioning units are measured. Electromagnetic immunity tests for vehicle interiors are also conducted in RF anechoic chambers. Such tests are part of type approval processes for products targeting international markets.
• White Goods and Home Electronics: Noise emissions from products such as washing machines, vacuum cleaners, and small household appliances are tested under specific operating modes in anechoic environments. This enables certification of product sound levels in compliance with standards.
• Medical Devices: Electromagnetic immunity tests for sensitive equipment such as magnetic resonance (MR) imaging systems are performed in anechoic chambers. Compliance with safety criteria is evaluated for both electromagnetic emissions and acoustic performance.
• Defense and Aerospace: Directional radiation tests for radar and antenna systems are conducted in anechoic environments. These measurements enhance system accuracy by enabling the study of wave behavior in reflection-free conditions.
• Information Technology and Telecommunications: Electromagnetic emissions from computers and network devices are measured in EMC anechoic chambers to ensure compliance with regulations such as FCC and CE. Antenna characterization is also performed in these environments.

These application examples demonstrate that anechoic chambers are not merely academic tools but integral components of product development and certification processes.

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In Türkiye, anechoic chamber infrastructure has developed to meet growing testing demands in industry and academic research centers. These chambers enable domestic products to meet the EMC and acoustic compliance requirements necessary for international market acceptance. Examples of key institutions include:

• TÜBİTAK UME (National Metrology Institute): Located in Gebze, the center houses both reverberation and anechoic chambers. TÜBİTAK UME is a leading institution in Türkiye for certifying anechoic and reverberation chambers according to ISO standards and providing measurement validation services.
• Istanbul Technical University – OTAM (Automotive Technologies Research Center): OTAM, located at the Maslak campus, has anechoic test chambers for acoustic and vibration testing in the automotive industry. Sound power levels of engines, exhaust systems, and air conditioning units are measured in detail here.
• Hema Endüstri (Çerkezköy): Anechoic chambers used for industrial product development and testing are employed in acoustic performance validation processes tailored to customer requirements.

In addition to these institutions, major domestic manufacturers such as Vestel, BSH, and Arçelik also maintain anechoic and semi-anechoic chambers to control the acoustic characteristics of their products. This infrastructure has become an indispensable part of product development, quality control, and compliance evaluation processes.