In line with research and measurement demands, the core technical parameters of an anechoic chamber consist of cut-off frequency, free-field range and background noise level. The cut-off frequency refers to the lowest usable frequency that can be measured accurately inside the chamber; the free-field range stands for the maximum acoustic space available for precise measurement; background noise determines the minimum measurable sound level. For acoustic measurements without correction, the background noise of the anechoic chamber shall be at least 10 dB lower than the sound source under test.

To construct a free sound field indoors, sound-absorbing wedges are conventionally adopted as the basic absorptive structure. In accordance with classic acoustic theories, the length of a wedge needs to equal one quarter of the wavelength corresponding to the cut-off frequency. For example, a cut-off frequency of 100 Hz requires 850 mm long wedges, while a cut-off frequency of 50 Hz calls for wedges as long as 1,700 mm. Although certain technical optimizations can moderately shorten wedge length, the scope of reduction is limited. For a 100 Hz cut-off frequency, wedges still need to be approximately 750 mm long even after adopting various optimization techniques. Oversized absorptive structures occupy massive indoor space. Therefore, drastically reducing absorber thickness and developing subwavelength structures to meet sound absorption criteria have long been key targets of technological advancement, and research on acoustic metamaterials and metastructures keeps advancing toward this objective.

For the design and construction of our anechoic chambers, we adopt self-developed, patented fiber-free sound-absorbing structures ASA and BCA. BCA is a planar composite resonant sound absorber with an overall thickness of 350 mm; ASA is an asymmetric sound absorber with a total thickness of 650 mm. Both structures can satisfy the sound absorption requirements of anechoic chambers with a low cut-off frequency down to 50 Hz.

Fundamental principles of room acoustics prove that the geometric dimensions of a room exert a critical influence on its internal sound field. In our anechoic chamber design, the room geometry is optimized based on room acoustic theories, granting the chamber inherent superior acoustic performance.

For every anechoic chamber design, validated simulation software is utilized to carry out sound field simulation and calculation, so as to inspect the deviation between sound field attenuation characteristics inside the chamber and the ideal free field. This practice significantly cuts down engineering risks and ensures the finished anechoic chamber meets the designed acoustic performance indicators.

Semi-Anechoic Chamber of Autoliv Automotive Safety Systems

Semi-Anechoic Chamber of BAIC New Energy

Semi-Anechoic Chamber, Acoustics Center of Zhijiang Laboratory

■ The automotive industry frequently constructs various types of semi-anechoic chambers, including full-vehicle semi-anechoic chambers (with or without dynamometers), pass-by noise test semi-anechoic chambers, powertrain semi-anechoic chambers, engine test semi-anechoic chambers, tire noise test semi-anechoic chambers, and tire pass-by noise test semi-anechoic chambers.

■ The HVAC industry generally builds semi-anechoic chamber groups for noise testing of air conditioning units under operating conditions (indoor side and outdoor side), semi-anechoic chambers for household air conditioners, and semi-anechoic chambers for industrial air conditioner noise testing.

■ The household appliance industry often deploys semi-anechoic chambers for noise measurement of refrigerators, washing machines and other home appliances.