Audio Equalization of Different Soundscape Recorders is Necessary to Perform a Correct Analysis

Passive acoustic monitoring (PAM) is widely used by soundscape and bioacoustics researchers. Nowadays, there is a large number of instruments available to record environmental sound, with different characteristics and costs. Given this availability, using all the devices on hand, even from different manufacturers, can be tempting since it allows to expand the area of study with less costs. Unfortunately, due to their diverse frequency response, this procedure introduces biases in a comparative analysis of the recordings (e.g., the computation of acoustic parameters and indices, and the following elaborations). For these reasons, equalization of audio recording is fundamental to ensure proper analysis. Therefore, in this study, we evaluated the effects of the equalization on the data acquired by two soundscape recorder typologies (Song-Meter-Micro and Soundscape-Explorer-Terrestrial). The evaluation was carried out by computing the eco-acoustic indices. The equalization was performed in a laboratory: the soundscape recorders and a class 1 sound level meter (able to produce a.WAV file) were exposed to a white noise source. The equalization curve was calculated in MATLAB environment with a successful response. As a second step, its effect on the eco-acoustic indices was evaluated by applying it to audio recordings from a park in Milan (Italy).