Free field in acoustic of anechoic chamber / Hemi-anechoic chamber

Acoustic anechoic chamber dimensions, measurement objects, setting of lower frequency limit to be measured Setting according to the ISO3745: setting from measurable range.

This section explains the measurable range (free field in acoustics) of an anechoic / hemi-anechoic chamber.
The area shaded in green in the above-mentioned plan view indicates the measurable range. 

1. [1] the range λ/4 away from the sound absorbing layer (sound absorbing wedges)
   Wavelength of targeted frequency x 1/4 (approximately 560mm for 150Hz) 
2. [2] the range excluding a radius of 2d0 or 1m from the center point

The calculation method for d0 differs between anechoic chamber and hemi-anechoic chamber. As shown in the diagram above.*
*ISO3745:2012(E) Figure1 – Reference box, centre for the measurement surface (when using the geometric centre of the source) and characteristic source dimensions for application in an anechoic and a hemi-anechoic chamber.
According to the ISO3745:2012, not 2a nor 1 is used as standard, but it has been revised as:
Anechoic chamber -the distance from the center of the object to be measured to the diagonal.
Hemi-anechoic chamber -the distance from the center of the floor surface where the object to be measured is placed to the diagonal.

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