An anechoic chamber is a room specifically designed to prevent reflections or echoes of sound or electromagnetic waves, allowing only direct sound or signals to be perceived. This makes them ideal for situations where precise testing and measurement is essential.

The term "anechoic" was coined by American acoustician Leo Beranek¹, originally referring to acoustic chambers, but is now also applied to chambers used for radio frequency (RF) and sonar testing.

• Acoustic chambers: Designed to minimize sound reflections.
• RF and sonar chambers: Used to eliminate reflections and external noise caused by electromagnetic waves.

The size of an anechoic chamber can vary greatly, from small rooms the size of microwaves to larger rooms the size of airplane hangars², depending on the objects being tested and the frequency ranges involved.

How Acoustic Anechoic Chambers Work

Acoustic anechoic chambers are used to test devices such as loudspeakers that produce intense sound levels. These chambers are critical because it would be impossible to test these sound levels outdoors in populated areas.

Common Testing Applications

• Measuring the transfer function of a loudspeaker.
• Analyzing the directivity of noise emissions from industrial machinery.

Silence in an Anechoic Chamber

Anechoic chambers are known for their extreme quietness. Noise levels inside can be as low as 10-20 dBA. For example, in 2005, the quietest chamber was recorded at -9.4 dBA, and in 2015, a chamber at Microsoft set the world record at -20.6 dBA.

Reflection Reduction Mechanism

Anechoic chambers use wedge-shaped absorbers to effectively reduce the reflection of sound waves. These absorbers are typically made of materials such as foam that are strategically placed to interact with sound waves, dissipating them rather than reflecting them back.

Detailed Process of Reflection Reduction

• Incident Wave (I) The sound wave entering the chamber is called the incident wave. This wave travels through the air toward the chamber's surfaces. I --> Incident Sound Wave
• Impact on the Absorber (W) When the incident wave hits the wedge-shaped absorber (W), it is not merely reflected. The absorber material is designed to dissipate the acoustic energy of the wave by absorbing it, rather than bouncing it directly back. W --> Wedge Absorber
• Partial Reflection (R) Although the incident wave is mostly absorbed, a small portion of it may be reflected due to the structure of the absorber. This creates a reflected wave (R). However, the amount of reflected energy is significantly reduced by the absorber's material. R --> Reflected Sound Wave
• Energy Dissipation in Air (A) The reflected wave (R) propagates through the air gap between the wedge-shaped absorbers (A). The energy of the reflected wave is gradually dissipated due to interactions with the air and the absorbing materials (especially through the viscosity of the air). This process further reduces the amount of reflected wave that could return to the sound source. A --> Air Gap
• Dissipation via the Corners (C) The corners of the absorber are specifically designed to maximize energy dissipation. As the sound wave bounces in the air gap (A), it is quickly converted into heat due to interactions with the absorbing materials. This prevents the reflected wave from being sent back into the chamber, minimizing undesirable reflections. C --> Corner

Thus, the reflection reduction mechanism relies on a series of interactions between the sound waves and the wedge-shaped absorbers. These interactions efficiently dissipate the wave's energy, preventing it from reflecting back to the sound source and allowing for a pure, measurable acoustic environment.

Difference from Full Anechoic Chambers

Unlike full anechoic chambers, semi-anechoic and hemi-anechoic chambers do not absorb energy in all directions. These chambers typically feature a solid floor to support heavier objects such as vehicles or industrial machinery, making them suitable for tests that require a solid surface.

• Semi-anechoic chambers: Solid floor with absorbent walls and ceiling.
• Hemi-anechoic chambers: Typically feature a reflective floor or a flat, untreated floor, which might differ in performance or design.

Purpose of RF Anechoic Chambers

RF anechoic chambers are specifically designed for testing antennas, radars, and electromagnetic interference (EMI). The internal surfaces are lined with radiation absorbent materials (RAM) to absorb RF radiation and minimize reflections.

Radiation Absorbent Material (RAM)

RAM is a specialized material that absorbs electromagnetic radiation (non-ionizing radiation) to reduce the level of reflected RF radiation. The effectiveness of RAM is crucial for accurate measurement in fields like antenna testing.

Performance and Chamber Size

The chamber's ability to absorb radiation depends on its size and the frequency range being tested. Larger chambers are used for lower frequencies (longer wavelengths), while smaller chambers are sufficient for higher frequencies (shorter wavelengths). $\lambda \; =\; v\; /\; f\; (wavelengths\; =\; velocity\; /\; frequency)$

Installation and Security

RF anechoic chambers are often built inside shielded rooms that use the Faraday cage³ principle to prevent external interference from affecting the tests.

Risks Associated with RF Anechoic Chambers

Several risks are associated with RF anechoic chambers, especially during high-frequency tests.

• RF Radiation: Personnel are typically not allowed inside the chamber during testing, as RF radiation (though non-ionizing) can pose health risks, especially at high power levels.
• Fire Hazard: The RAM absorbs RF radiation and may generate heat. Without proper heat dissipation, this could lead to fire risks, especially if high-power antennas are too close to the RAM.
• Trapped Personnel: Safety measures should be in place to prevent personnel from being trapped inside the chamber in the event of an emergency.

Anechoic chambers are not just fascinating from a technical standpoint they also challenge our perceptions of sound and space. From the absolute silence of the world's quietest room to their crucial role in testing cutting edge technology, these chambers provide a unique environment for both scientific exploration and sensory experience.

Whether used for acoustic testing, radio frequency research, or even simulating outer space conditions, anechoic chambers continue to push the boundaries of what's possible, making them an indispensable tool across various industries.

• ^↑ Leo Beranek - Wiki
• ^↑ B-1B in the Benefield Anechoic Facility (BAF), 2016 - Picture
• ^↑ Faraday cage - Wiki

Source: Wikipedia - Anechoic Chamber