Exploring the frequency response of mobile phones applications for sound monitoring in urban green spaces

Citizen science engages non-professional scientists globally in scientific endeavors, fostering commitment to environmental conservation initiatives. This research explores the frequency response of mobile phones for sound monitoring, particularly in the context of urban green spaces. The method involves Laboratory tests at the Brazilian National Metrology Institute, Standardization, and Industrial Quality (Inmetro), evaluating two Android and one iOS devices using the NoiseCapture and Openoise apps. Laboratory tests conform to the frequency response standards outlined in IEC 61672-1, covering the 63Hz to 16kHz range. Following this, the devices captured audio scenes encompassing human, natural, and machine sounds at the University of São Paulo campus. Findings revealed that the frequency response of iPhones functioned as Class 2 sound level meters except for low-frequency bands. Android consistently performed similarly across both applications tested and exhibited acceptable performance in mid-range frequencies. Therefore, when considering citizen science projects, emphasis should be placed on app selection, prioritizing additional features like sound mapping. Field results showed minimal errors from human and transportation sounds but challenges with high-frequency components from birds, insects, and machines. The study concludes the necessity for alternative measurement systems identifying animal sound sources, along with involving non-professional scientists in scientific research.