Anechoic Chambers: Everything You Need to Know about the Quietest Place on Earth!
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A high-performance anechoic chamber with excellent traceability and reproducibility is of utmost importance in EMC (electromagnetic compatibility) testing. The anechoic chamber is no longer a simple evaluation facility but has evolved into a powerful tool for product development.
What Is an Anechoic Chamber?
The word "anechoic" means simply "without echo." The anechoic chamber is a specialized and shielded space designed to resist electromagnetic waves while suppressing the emission of such waves to the outside world. Originating in the early 1950s, anechoic chambers were initially used for experiments and research on radio equipment and antennas. Since the implementation of extensive noise regulations in the 1980s, these chambers have been increasingly used to evaluate noise emissions of electronic equipment.
History of the Anechoic Chamber
Anechoic chambers were first named by Leo Beranek, a notable expert in acoustics. Originally, these chambers were developed as acoustic anechoic chambers. However, as time progressed, the scope of the term was broadened to include RF and sonar anechoic chambers as well. These specialized chambers have the capability to block external noise and suppress reflections caused by electromagnetic waves.
An anechoic chamber is shielded entirely with metal and has radio wave absorbers covering the surrounding walls—five sides (walls plus ceiling) or six sides, including the floor. The size of the anechoic chamber and the type of radio wave absorber to be used are determined by the size and purpose of the electronic equipment being evaluated; 10 m and 3 m anechoic chambers are utilized in EMC testing.
In order to ensure the reproducibility of EMC test results, parameters known as site attenuation are used as a benchmark. This standard indicates whether an anechoic chamber provides an acceptable electromagnetic environment for EMC testing and requires that transmission losses—between the transmitting/receiving antennas and the measuring antennas—fall within specified limits.
Since the EMC Directive went into effect in 1996, noise regulations have been extended to cover large and heavy-duty machinery. With all kinds of household appliances now equipped with wireless technology, products are more complex than ever. As a result, the demand for 10 m anechoic chambers is growing rapidly. Radio wave absorbers for 10 m anechoic chambers are required to have a wide bandwidth and absorption level of at least 20 dB. To achieve these conditions using only a dielectric absorber, the absorber would need to be longer than 5 m, requiring a huge building.
For this reason, professors Kunihiro Suetake, Yoshiyuki Naito, and Yasutaka Shimizu of the Tokyo Institute of Technology developed a composite absorber combining ferrite and dielectric materials in the 1960s. Since then, short-length composite absorbers have become mainstream in 10 m anechoic chambers.
The World's Oldest Wedge-Based Anechoic Chamber
According to Bell Labs, the Murray Hill anechoic chamber, which was constructed in 1947, is recognized as the world's oldest wedge-based anechoic chamber. The internal dimensions of the chamber are approximately 9.1 m in height, 8.5 m in width, and 9.7 m in depth. The exterior walls are made of cement and brick and are about 0.9 m thick to minimize external noise.
The term "anechoic" refers to the absence of echoes. To absorb echoes or reflections, the Murray Hill chamber is equipped with large fiberglass wedges mounted on its interior surfaces. These wedge-shaped absorbers measure 1.37 m long and 0.6 m square at the base. The anechoic chambers of today commonly use the same alternating wedge pattern found in the Murray Hill chamber. The wedge shape was selected to achieve an "impedance match" between the absorber and the surrounding air. It can also be thought of as a waveguide where all incident acoustic energy is internally reflected into the wedge. The alternating pattern was chosen to provide more uniform angular absorption. Above 200 Hz, the chamber absorbs more than 99.995% of the incident acoustic energy. The Murray Hill chamber was once recognized in the Guinness Book of World Records as the world's quietest room.
The Rising Importance of Anechoic Chambers
Electronic equipment, wireless devices, and communication systems generate a variety of electromagnetic waves. The electromagnetic waves generated by one device may negatively affect nearby devices. EMC (electromagnetic compatibility) addresses two issues:
1. Emissions: the need to suppress the generation of EMI (electromagnetic interference).
2. Immunity: the need to resist EMI generated by other devices.
There is an urgent need for countermeasures, starting early in the product design and development stages, that allow a wide variety of electronic, wireless, and communication devices to coexist.
One way to measure the electric field strength of electromagnetic noise generated from electronic devices is at an open-air test site (OATS). However, the disadvantage of outdoor testing is that it is easily affected by adverse weather conditions and is susceptible to electromagnetic waves present in the surrounding environment.
Additionally, because extremely strong electric fields are produced during immunity tests, they cannot be conducted at an OATS due to radio communication regulations. Therefore, the anechoic chamber is becoming increasingly important as a facility that can perform such tests with high reliability and efficiency.
Anechoic chambers have many advantages that make them essential in various fields such as acoustics, radio frequency engineering, and electromagnetic compatibility testing:
- They are unaffected by intrusive electromagnetic waves.
- They provide a stable test environment.
- They contain electromagnetic interference.
- Tests with electric solid fields (immunity tests) can be performed.
- Tests can be kept confidential.
Moreover, anechoic chambers enable tests with strong electric fields, also known as immunity tests, which cannot be performed at outdoor test sites due to radio communication regulations. With anechoic chambers, the electric field strength of electromagnetic noise generated from electronic devices can be accurately measured, allowing for the early detection and suppression of electromagnetic interference.
In summary, anechoic chambers offer multiple benefits, including stable testing environments, immunity testing capabilities, and the ability to conduct tests confidentially, making them essential in various fields such as acoustics, radio frequency engineering, and electromagnetic compatibility testing.
Ferrite: The Predominant Radio Wave Absorber Material
Radio wave absorption materials are classified into three categories: magnetic, dielectric, and resistive. Examples of magnetic absorbing materials include sintered ferrite (hereafter referred to as “ferrite”), soft magnetic alloys, and carbonyl iron compounded with resin. Ferrite, the most common material applied to the walls of anechoic chambers for EMC testing, causes magnetic loss in high-frequency AC magnetic fields. Ferrite-based radio wave absorbers make use of this property. Examples include the absorption of television waves, which are converted to heat to reduce ghosting effects seen on TVs caused by waves reflected off high-rise buildings, and to prevent false images on airborne radar. Thanks to production techniques accumulated over the years, ferrite is being developed further as a major radio wave absorber for EMC testing in anechoic chambers.