Calibration of an anechoic chamber is a complex process designed to ensure that its acoustic performance meets specific standards, such as ISO 3745 or China's JJF 1147-2006 "Specification for the Calibration of Acoustic Characteristics of Anechoic and Semi-anechoic Chambers". The calibration process focuses on several key parameters, including the noise floor level, free-field conditions, acoustic uncertainty, and acoustic uniformity inside the anechoic chamber. Here are some of the main calibration steps and considerations:

1. Noise Floor Measurement

Measure ambient noise: Measure the background noise level inside the anechoic chamber without any external sound source activated. This will provide a baseline value for accurate evaluation of noise sources in subsequent measurements.

Temperature and humidity recording: Record environmental conditions as they may affect the speed of sound and the acoustic properties of air.

2. Free-field verification

Free-field condition check: Use a point sound source to transmit a signal of known sound pressure level at multiple locations, and then measure the received sound pressure level at different distances. According to free-field theory, the sound pressure level should decay according to a specific law with increasing distance.

Sound field uniformity: Ensure that the sound pressure level variation in the measurement area does not exceed a certain allowable range.

3. Uncertainty assessment

System uncertainty: Evaluate the uncertainty that may be introduced by all measurement equipment and calibration processes, including instrument accuracy, operating errors and environmental factors.

Repeatability and reproducibility: Determine the consistency and repeatability of the results through multiple measurements.

4. Other factors to consider

Air absorption: Consider the natural attenuation of sound in the air, especially in the high frequency band.

Ground reflection: In a semi-anechoic chamber, the reflection of the ground must be calculated and corrected.

Sound source directivity: If the sound source is not a point sound source, its directivity will affect the measurement results and appropriate corrections need to be made.

5. Calibration report

Data records: All measured data and conditions should be recorded in detail.

Analysis and conclusions: Analyze the performance of the anechoic chamber based on the collected data and compare it with the standard requirements.

Suggestions and improvement measures: If non-conformities are found, improvement suggestions should be made.

The calibration process is usually performed by a professional acoustic engineer, using precision acoustic measurement equipment such as sound level meters, microphones, and sound sources. The calibration cycle of the anechoic chamber depends on the frequency of use and environmental conditions, but it is generally recommended to be performed regularly to ensure that it continues to meet the standard.

Please note that the specific calibration procedures and details may vary according to different standards and the specific design of the anechoic chamber.