THE DIFFERENCE BETWEEN FULLY ANECHOIC CHAMBERS AND HEMI ANECHOIC CHAMBERS
The need for noise control and assessment technology has never been greater. Building regulations frequently require accurate assessment of noise at the planning stage and containment to specified levels thereafter.
Development proposals often depend upon accurate calculation of machinery noise impacts, and individual appliances ranging from washers and dishwashers to lawnmowers and farm equipment must be accurately assessed for their operational noise emissions.
Accurately assessing noise characteristics and profiles requires an insulated test environment. The two main types are the fully anechoic chamber and the hemi-anechoic chamber. Chambers of both types take sound exclusion a step beyond ordinary sound-proofed booths or rooms. A fully anechoic chamber is completely insulated in all directions with profiled sound-absorbent materials (anechoic wedges), whilst the hemi-anechoic chamber forgoes this technology in the ground direction in order to provide a hard, albeit floating and sound-insulated floor. The latter is, of course, more convenient for load-bearing and human access and the entry and removal of test subjects and testing equipment. However, fully anechoic chambers can also have cable mesh floors floated above the anechoic wedges to facilitate access and support lighter equipment. Retractable flooring is also an option.
Chambers as small as an oven can be constructed – up to ones large enough to house an aircraft. While the choice of size is determined mostly by the dimensions of the equipment under test, it can also be influenced by the need to maximise the exclusion of specific frequency ranges.
Although the initial design objective is to exclude or minimise acoustically reflective materials of all kinds (for example, mounting equipment on wooden or plastic supports rather than metal or ceramic ones), once this is accomplished it permits the effect of potentially reflective or resonant surfaces to be explored. For example, both types of anechoic chamber are used to explore the noise attenuation characteristics of aircraft fuselage components and car body panels.
The solid flooring of the hemi-anechoic chamber may be chosen specifically to better replicate the intended operating conditions of the machinery under test. For example, some domestic appliances generate unexpected noises because of resonant interactions with the flooring on which they stand. Reflective or absorbent surfaces are easily introduced into the chamber to simulate the equipment’s acoustic interactions with those likely to be encountered in real-world operation.
Deliberate introduction of reflective and absorbent surfaces is also used in the development of sound-generating equipment like speakers, amplifiers and musical instruments to profile how sound frequency ranges will be changed in typical domestic or concert hall scenarios. Professional recording studios are often hemi-anechoic in design.
For purposes that entail the use of sensitive electronic instruments, chambers can also be constructed to provide shielding from RF and other electromagnetic interference.
For most purposes, chambers with 10-20 dBA are adequate, but it is possible to construct them with sound absorbency down to as little as -20 dBA (where zero dBA is calibrated to be the minimum noise level audible to the typical human ear). The IAC Acoustics Microdyne® Schedule 60 is a fully anechoic chamber designed as a room within a room with additional noise-suppression technologies that achieve maximum sound transmission loss. They are ideal for engineers and scientists that need a free-field acoustic environment in which to take the most accurate measurements.