Sound pressure levels in mineral blasting: Measurement errors and microphone frequency response

The method employed for rock removal in mineral exploration is blasting, which involves the use of plastic explosive charges capable of generating extremely high sound pressure levels. These elevated blasting noise have the potential to cause structural damage to homes located near mineral exploration sites. Sound waves with frequencies up to 250 Hz, particularly infrasonic waves, can propagate over long distances while retaining significant energy. Accurate measurement of these high sound pressure levels is crucial for safeguarding inhabited areas adjacent to mineral extraction sites. However, seismograph manufacturers typically use microphones with a cutoff frequency of 20 Hz. The technology utilized in these microphones is outdated, as the pressure equalization vent allows the entry of sound waves with frequencies below 20 Hz, causing the microphone's response to function as a high-pass filter. As a result, measurements taken with these microphones obscure the true sound pressure level (SPL), leading to inaccurate assessments of the safety of residential areas. This article presents and analyzes the frequency responses of seismograph microphones obtained through calibration laboratories during a proficiency testing program conducted in Brazil between 2024 and 2025.