Electret microphone capsules are widely used in various audio applications due to their compact size and reliable performance. However, like all active electronic components, these capsules generate inherent self-noise — a low-level electrical signal present even in complete silence. Understanding and accurately measuring this self-noise is critical for assessing microphone quality, optimizing product design, and ensuring excellent audio clarity in the final application.
Sources of Self-Noise
- JFET Preamplifier Noise:
Electret microphone capsules typically incorporate a JFET (junction field-effect transistor) as a built-in preamplifier. This component generates thermal noise, shot noise, and flicker (1/f) noise, which are primary contributors to the overall self-noise. - Electret Material Noise:
The electret film holds a permanent charge that can fluctuate slightly due to charge leakage or instability, introducing minor noise. However, this is generally less significant compared to the JFET noise. - Power Supply Noise Coupling:
Voltage fluctuations or noise on the power supply line can couple into the capsule output, adding to the self-noise level. Using a clean and stable power source is critical during measurement.
Key Testing Conditions
- Power Supply Quality:
Use low-noise, regulated power supplies or batteries to minimize interference. Batteries often provide quieter power than switching power supplies. - Mechanical Vibration Isolation:
While anechoic chambers block airborne sound, mechanical vibrations can travel through mounting structures and cause noise. Additional damping or vibration isolation is needed to prevent this. - Electromagnetic Shielding:
Shield the capsule and test setup from ambient electromagnetic interference (e.g., WiFi signals, power line noise) to prevent spurious signals. - Measurement Bandwidth:
Self-noise power depends on the bandwidth over which it is measured. Standard audio bandwidth (20 Hz–20 kHz) is commonly used for consistency. - Test Equipment Noise Floor:
Measurement instruments such as spectrum analyzers or oscilloscopes must have noise floors significantly below the capsule self-noise to obtain valid readings. - Weighting Filters:
While A-weighting simulates human ear sensitivity, linear (unweighted) measurements are preferred for true physical noise floor assessment.
Practical Implications
Self-noise sets the lower limit of the microphone’s dynamic range — the smallest sound it can detect over its inherent noise floor. For example, a self-noise level of -60 dBV means the microphone can detect signals 60 dB above its noise floor before distortion or clipping occurs.
Testing self-noise also verifies the effectiveness of design improvements such as low-noise JFETs, electret material quality, and power supply filtering.
Optimization Strategies
- Use JFETs with lower current consumption and noise figures
- Incorporate internal filtering and stabilization components
- Improve capsule structure (e.g., replace copper gate rings with point-contact designs)
- Minimize lead length to reduce antenna-like pickup of interference
Additional Notes on Testing Environment and Procedures
Testing in a fully anechoic chamber is ideal as it completely eliminates reflected sound waves and external noise interference. Mechanical isolation using vibration dampers or suspension mounts is necessary to prevent structure-borne noise.
Power supply noise can significantly influence results; therefore, battery power or ultra-low noise linear power supplies are preferred over switching power supplies. Shielded cables and connectors reduce electromagnetic interference.
The noise power is proportional to the measurement bandwidth. Industry-standard measurements usually cover the audible range of 20 Hz to 20 kHz. While A-weighted filters approximate human hearing sensitivity, raw linear measurements provide a more objective noise floor metric important for engineering and manufacturing.
Why This Matters for Manufacturers and End Users
Low and consistent self-noise is critical for high-fidelity audio capture, especially in professional recording, voice recognition, and communication devices. Variations in noise performance can indicate manufacturing inconsistencies or component degradation.
Our ongoing R&D efforts focus on reducing noise through better JFET selection, refined electret materials, and optimized capsule design — ensuring ECMIC capsules deliver superior sound clarity and reliability.
Accurate testing and understanding of electret microphone capsule self-noise are essential for delivering high-quality audio products. By controlling noise sources, optimizing test conditions, and applying effective design improvements, manufacturers can significantly enhance microphone performance.
At ECMIC, we continuously invest in advanced testing methods and materials to provide low-noise, reliable microphone capsules that meet the demanding needs of global audio applications.
If you want to learn more about our microphone capsules or request technical support, feel free to contact us.
