ANSI S1.4: Microphone Requirements and Windscreen Corrections
TL;DR
ANSI S1.4 specifies microphone requirements as part of the complete sound level meter system. The microphone must be omnidirectional within the tolerance limits for the instrument Type. Free-field response microphones are designed for sound arriving from a specific direction (0° incidence); random-incidence microphones are corrected for diffuse-field measurement. Windscreens are required for outdoor measurements to prevent wind-induced noise but introduce frequency-dependent insertion loss (typically <0.5 dB below 4 kHz, rising at higher frequencies). The microphone sensitivity and noise floor determine the lower measurement range.
Directional Response
The microphone and instrument body together must meet the directional response tolerances of ANSI S1.4. At low frequencies (<500 Hz), microphones are inherently omnidirectional. At higher frequencies, the microphone body diffracts sound waves, causing angle-dependent sensitivity variations. Type 1 instruments must maintain ±1.4 dB at 1 kHz for angles up to 90°; Type 2 allows ±2.4 dB.
Free-Field vs Random-Incidence Response
Two microphone response types are defined:
- Free-field (0° incidence): Calibrated for sound arriving perpendicular to the diaphragm. Used when the microphone points directly at the source.
- Random-incidence (diffuse field): Calibrated for sound arriving equally from all directions. Used for reverberant field measurements and environmental noise where sound arrives from multiple directions.
The difference between free-field and random-incidence response is typically <1 dB below 2 kHz and up to 3 dB above 8 kHz. The correct response type must be selected based on the measurement scenario.
Windscreen Requirements
Outdoor measurements require windscreens to suppress turbulence-induced noise. ANSI S1.4 requires that the windscreen insertion loss be documented and within the overall tolerance budget. Typical windscreen effects:
| Frequency | Insertion loss (typical) |
|---|---|
| Below 1 kHz | <0.2 dB |
| 1 – 4 kHz | 0.2 – 0.5 dB |
| Above 4 kHz | 0.5 – 1.5 dB |
Without a windscreen, wind speeds above 3 m/s can add 10-20 dB of low-frequency noise, completely invalidating measurements. Specialized outdoor windscreens ("birds nest" type) handle higher wind speeds.
Sensitivity and Noise Floor
The microphone sensitivity (typically 50 mV/Pa for 1/2" condensers, 4 mV/Pa for 1/4" condensers) and self-noise (typically 15-25 dBA for 1/2", 28-36 dBA for 1/4") determine the lower measurement range. Type 1 instruments typically measure down to 20-25 dBA; Type 2 to 30-35 dBA.
Phone Microphone Limitations
Smartphone MEMS microphones have inherent limitations compared to measurement-grade capsules:
- Self-noise: 30-40 dBA (limits low-level measurement)
- Maximum SPL: 110-120 dB (clips at high levels)
- Frequency response: varies by model, typically rolled off below 100 Hz and above 10 kHz
- Directional response: not characterized to standard specifications
Calibration against a reference source partially compensates for frequency response deviations.
SonaVyx Approach
The SonaVyx SPL meter works with any microphone source — phone mic, USB interface, or professional measurement mic. Calibration corrects for microphone sensitivity. For environmental factors affecting measurements, see the next section.
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Last updated: March 19, 2026