What is the Noise Reduction Index？And How To Test？
What is the Noise Reduction Index？And How To Test？
I. What is the Noise Reduction Index
The noise reduction index (sound absorption coefficient) is a core parameter for evaluating the sound absorption performance of materials or structures. It is defined as the ratio of the sound energy absorbed by a material surface to the total incident sound energy when sound waves strike it. The higher the absorption coefficient, the stronger the material's noise reduction capability. In modern architectural acoustics, industrial noise control, and automotive NVH (Noise, Vibration, and Harshness), the detection of the sound absorption coefficient is crucial for optimizing acoustic environments and enhancing product performance. For example, venues such as theaters and recording studios require high-sound-absorption materials to reduce reverberation, while automotive interior materials must undergo sound absorption testing to improve driving and riding comfort. Detecting the sound absorption coefficient not only guides material research and development but also provides a scientific basis for engineering design and construction. Additionally, the sound absorption performance of materials is influenced by factors such as thickness, porosity, and frequency. Accurate detection helps reveal the patterns of their acoustic properties.
II. What Does the Detection of the Noise Reduction Index Include, and How Is It Detected?
The detection of the noise reduction index typically covers the following key items:
- Normal incidence sound absorption coefficient (0° incidence angle) and random incidence sound absorption coefficient (multi-angle average);
- Sound absorption performance at different frequencies (typically covering the range of 100Hz–6300Hz);
- Noise Reduction Coefficient (NRC, the average of four frequency bands from 250Hz to 2000Hz);The range of values is between 0 and 1. The higher the value, the better the sound absorption effect.
- Correlation analysis between material thickness and sound absorption effect;
- Sound absorption characteristics of composite structures (e.g., perforated panel + porous material combinations).
Mainstream detection methods include:
- Impedance Tube Method:
- Uses the two-microphone transfer function method to measure the sound pressure distribution inside the tube.
- Frequency range: 50Hz–6.3kHz (depending on tube diameter).
- Suitable for small-sized samples (typically diameter <100mm).
- Reverberation Room Method:
- Calculates sound absorption by comparing the reverberation time differences between an empty room and the room with the sample placed.
- Test area ≥10 m², must meet ISO 3741 diffuse sound field requirements.
- Measurable frequency range: 100Hz–5kHz.
- Other Methods: Standing wave tube method (traditional method, lower accuracy), free-field impulse response method, etc.
III. Internationally Common Standards
The detection of the noise reduction index typically covers the following key items:
- Normal incidence sound absorption coefficient (0° incidence angle) and random incidence sound absorption coefficient (multi-angle average);
- Sound absorption performance at different frequencies (typically covering the range of 100Hz–6300Hz);
- Noise Reduction Coefficient (NRC, the average of four frequency bands from 250Hz to 2000Hz);
- Correlation analysis between material thickness and sound absorption effect;
- Sound absorption characteristics of composite structures (e.g., perforated panel + porous material combinations).
Mainstream detection methods include:
- Impedance Tube Method:
- Uses the two-microphone transfer function method to measure the sound pressure distribution inside the tube.
- Frequency range: 50Hz–6.3kHz (depending on tube diameter).
- Suitable for small-sized samples (typically diameter <100mm).
- Reverberation Room Method:
- Calculates sound absorption by comparing the reverberation time differences between an empty room and the room with the sample placed.
- Test area ≥10 m², must meet ISO 3741 diffuse sound field requirements.
- Measurable frequency range: 100Hz–5kHz.
- Other Methods: Standing wave tube method (traditional method, lower accuracy), free-field impulse response method, etc.
By combining standardized detection processes with advanced instruments, the detection of the sound absorption coefficient not only provides reliable data for evaluating material performance but also drives the continuous development of acoustic materials toward broader frequency ranges, higher efficiency, and lighter weight. In the future, with advancements in smart materials and active noise control technologies, detection methods will face more complex testing requirements and technical challenges.
