Sensitivity: mV/Pa and dBV/Pa Describe the Same Thing
Sensitivity is the output level a microphone produces for a given sound pressure. ECM capsules are usually quoted in mV/Pa, while datasheets and test reports prefer dBV/Pa: with 1 V/Pa as the 0 dB reference, 1 mV/Pa equals -60 dBV/Pa, so a -38 dBV/Pa part delivers roughly 12.6 mV/Pa. Always state the four test conditions — 1 kHz sine, 94 dB SPL (1 Pa), free field, 0° on-axis incidence — otherwise the numbers are not comparable across vendors or lots. Unit-to-unit spread is normally held within ±3 dB. Push beyond that window and the amplitude weighting in a two-mic beamforming array drifts, which quietly erodes noise-cancellation performance. For line inspection, clamp a 94 dB acoustic calibrator on the capsule and read the output level; there is no need to rebuild the sound field every cycle.
Directivity: Read the Polar Plot as an Attenuation Curve
Directivity describes how sensitivity falls off with the angle of incidence. An omnidirectional capsule should stay flat within ±2 dB from 0° to 180°, while a cardioid shows a pronounced null at 180° with a front-to-back ratio typically specified above 15 dB. Proper measurement needs an anechoic chamber and a turntable stepping in 15° or 30° increments, with every curve tagged by frequency — a clean pattern at 1 kHz says nothing about 4 kHz, where the housing itself starts to shadow and narrow the response. Headsets used for voice calls are especially sensitive to the gap between on-axis sensitivity and side rejection. Once the acoustic port or the wind-noise mesh moves, the polar plot has to be re-measured; carrying over the old curve is how field complaints get missed.
Self-Noise and SNR: Suppress the Room Before You Blame the Part
Self-noise is the capsule's own output with no acoustic input, referred to the input as equivalent input noise and expressed in A-weighted dB SPL. Measuring it requires the ambient floor to sit at least 10 dB below the expected reading, which rules out an ordinary office — use a quiet box or an anechoic chamber. SNR follows directly: 94 dB minus self-noise, so a 30 dB(A) capsule yields 64 dB SNR, a common target for voice products. When self-noise reads high, check supply ripple and ground return first. Noise coupled through the bias resistor into the preamp is frequently misdiagnosed as a defective capsule.
Production Test: Three Items Plus a Weekly Cross-Check
Volume production does not need the full suite on every unit. Keep three checks: single-point sensitivity at 94 dB SPL / 1 kHz, powered self-noise read as RMS output inside a silent chamber, and left-right matching held within 2 dB amplitude and 10° phase. Pull one unit per week for a full frequency sweep and polar re-measurement to catch fixture or housing-tool drift. Store every dataset with its test conditions attached — that baseline is the only thing that makes a customer complaint answerable months later.
FAQ
The datasheet says ±3 dB sensitivity but the customer wants ±2 dB. Is that achievable?
Yes, through binning rather than tighter capsule tolerance. Sort incoming parts by measured sensitivity, then build each unit from a single bin. Matching typically tightens to ±1.5 dB at unit level, and the added cost is sorting labour plus inventory control, not component price.
Why is an anechoic chamber mandatory for directivity testing?
Reflections add to the direct sound and make the 180° null look shallower than it really is, so the front-to-back ratio gets understated. Without a chamber, a short-pulse method with a time gate can suppress reflections, but low-frequency accuracy still suffers and the result is only valid for relative comparison.
Are self-noise and SNR two different measurements?
They are two views of the same data. Self-noise is an absolute figure; SNR is the 94 dB reference minus that figure. SNR is the faster selection metric, while the self-noise spectrum is the better diagnostic — it separates broadband hiss from a discrete tone coming in on a supply rail. ## About Liwei Electronic Shenzhen Liwei Electronic Technology Co., Ltd designs audio and headset electronics with complete chip and PCBA solutions. With over 10 years of experience serving 15+ well-known brands and 300+ customers, we cover voice-call noise cancellation, gaming, live streaming and hearing-protection platforms. Capsule selection, acoustic chamber tuning and test fixture design can all be handled together. Request a quote for a customised audio solution.
