Open-Air Sound Measurement: Outdoor Acoustics Guide

Outdoor measurement lacks the room reflections and reverberation that dominate indoor measurement. This simplifies some aspects (no room modes, no flutter echo) but complicates others (wind, temperature, ground effects, background noise from traffic and nature). The measurement technique must adapt to these different conditions.

Wind is the primary enemy of outdoor measurement. Turbulent airflow over the microphone diaphragm creates broadband noise concentrated below 500 Hz. A foam windscreen provides 10-15 dB of wind noise reduction. For stronger wind, a dual-layer windscreen (foam inside fur) provides 20-25 dB reduction. Measure wind speed and direction and note them in your measurement log.

Temperature affects sound speed (approximately 0.6 m/s per degree C) and creates refraction. Humidity affects high-frequency air absorption. Rain adds broadband noise. Document all weather conditions with your measurement data.

Sound reflecting off the ground combines with the direct sound to create a comb filter pattern. The first null occurs at a frequency determined by the path length difference between direct and reflected sound. For a source and receiver at 1.5m height separated by 10m over hard ground, the first null is around 1 kHz.

To minimize ground effect, measure at heights well above the ground or use ground plane measurement (microphone on the ground surface). Ground plane measurement eliminates the ground reflection by making the direct and reflected paths equal, producing a cleaner frequency response.

Use a windscreen always, even on calm days. Measure during the calmest period (early morning is usually best). Position the microphone away from large reflective surfaces (buildings, fences). Use A-weighting for SPL measurement to reduce wind noise contribution. Increase averaging time (30-60 seconds minimum) to improve stability. Document weather conditions, time, and location for every measurement.