Cal. Code Regs. tit. 13, § 1025 – Testing Sites.

(a) Laboratory Tests. A laboratory test site shall consist of an anechoic chamber that meets the requirements of ANS S1.13-1971.
(b) Open Field Tests. An open field test site for mechanical siren testing shall consist of a flat paved area at least 15 m (49 ft) in diameter and free of large vertical sound-reflecting surfaces within 15 m (49 ft) of the microphone and siren except for the test vehicle.

Sites for laboratory or field tests of sirens shall comply with the following requirements:

Note: Authority cited: Section 26103, Vehicle Code. Reference: Sections 26103 and 26104, Vehicle Code.

1. Editorial correction of Note (Register 81, No. 44).