Once a chamber is built, you cannot judge it by appearance alone. It has to be accepted through a series of acoustic tests, and the two most important figures are the free field deviation and the noise floor. The free field deviation measures whether the room really comes close to an ideal free field: as the microphone moves along a test path, the sound pressure should fall smoothly with distance by the inverse square law, and the smaller the deviation, the more complete the absorption.
The noise floor reflects how quiet the room is when no source is playing, and it is set together by the isolation, the vibration control and the ventilation system. The lower the noise floor, the fainter the sound the room can measure, and the wider its usable dynamic range becomes.
Acceptance is usually carried out to the relevant standard, such as ISO 3745, across the specified frequency range and several directions, with a report issued by a qualified third party. At handover Anteck works with the customer to complete these tests, so that the real performance of the laboratory matches the figures it was designed to.