Impact of Ventilation Airflow on Background Noise Levels in Anechoic Chambers

Introduction

Anechoic chambers are specially designed acoustic spaces that enable free-field sound conditions for precision testing and measurement. However, when ventilation is required, the airflow introduced by the ventilation system can significantly influence the chamber’s acoustic environment, especially background noise levels.

This article explores how ventilation airflow affects background noise in anechoic chambers, referencing ISO standards and real-world measurement data.

What Is Background Noise in Anechoic Chambers?

In this context, background noise refers to ambient noise present in the measurement environment independent of the Device Under Test (DUT). Common sources include HVAC systems, power supplies, and structural vibrations.

Reducing background noise is essential for accurate acoustic measurements and is explicitly defined in international standards such as ISO 3745:2012 and ISO 3744:2010.

Airflow and Its Acoustic Impact

Effects of Ventilation Airflow

Air movement inside the chamber introduces several possible issues:

Turbulent Noise Generation	High-speed airflow causes turbulence in ducts and along chamber walls, creating low-level noise.
Pressure-Induced Vibrations	Airflow fluctuations may apply pressure variations on surfaces, generating low-frequency vibration noise.
Temperature Gradient Effects	Ventilation can create thermal gradients that alter sound propagation characteristics.

Under ISO 3745, the chamber should maintain free-field conditions with a K₂ correction factor ≤ 0.5 dB. However, airflow-induced turbulence and pressure variations can compromise this condition.

Ventilation Systems and Measurement Accuracy

Environmental Correction Value (K₂) in ISO Standards

ISO 3745:2012 (Anechoic Chamber)	Requires K₂ ≤ 0.5 dB
ISO 3744:2010 (Semi-Anechoic Chamber)	Accepts K₂ values in the range of 0–4 dB

Increased airflow velocity may raise K₂, reducing measurement precision.

Ventilation Design Strategies for Anechoic Chambers

1. Low-Velocity Ventilation Systems

• Keep airflow velocity below 1 m/s to minimize turbulence
• Use large-diameter ducts to reduce pressure loss and maintain low velocity

2. Sound-Absorbing Ducts

• Line ducts with sound-absorbing materials to reduce flow-induced noise
• Use porous damping materials in duct bends to suppress turbulence

3. Intermittent Ventilation

• Stop ventilation during acoustic measurements to reduce background noise
• Minimize thermal drift when restarting airflow post-measurement

Measurement Data and Analysis

Methodology

Based on ISO 3745 procedures:

1. Background Noise Level Measurement

• Compare chamber noise levels with ventilation ON vs. OFF
• Calculate K₂ at each microphone position

2. Frequency Band Comparison

• Analyze data across octave bands (63 Hz–8 kHz)
• Note stronger airflow influence below 125 Hz

Results

• Background noise rises significantly when airflow exceeds 0.5 m/s
  Low-frequency noise (≤125 Hz) shows the largest increase
• In some cases, K₂ exceeded 0.5 dB, making the chamber non-compliant with ISO 3745
• Applying duct acoustic treatments reduced low-frequency noise increases by approximately 40%

Conclusion

While ventilation is essential for thermal and comfort control, its airflow velocity directly affects acoustic background noise in anechoic chambers. Low-frequency noise is especially sensitive, and compliance with ISO 3745 requires proactive acoustic management.

Recommended Measures:

• Limit ventilation velocity to ≤1 m/s
• Use sound-absorbing ductwork to suppress turbulence noise
• Suspend ventilation during sensitive acoustic measurements

Proper integration of acoustic and ventilation design is crucial for ensuring accurate sound power measurements in modern anechoic environments.

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