Data from: Weather conditions determine attenuation and speed of sound: environmental limitations for monitoring and analysing bat echolocation

Echolocating bats are regularly studied to investigate auditory-guided behaviours and as important bioindicators. Bioacoustic monitoring methods based on echolocation calls are increasingly used for risk assessment and to ultimately inform conservation strategies for bats. As echolocation calls transmit through the air at the speed of sound, they undergo changes due to atmospheric and geometric attenuation. Both the speed of sound and atmospheric attenuation, however, are variable and determined by weather conditions, particularly temperature and relative humidity. Changing weather conditions thus cause variation in analysed call parameters, limiting our ability to detect and correctly analyse bat calls. Here, I use real-world weather data to exemplify the effect of varying weather conditions on the acoustic properties of air. I then present atmospheric attenuation and speed of sound for the global range of weather conditions and bat call frequencies to show their relative effects. Atmospheric attenuation is a non-linear function of call frequency, temperature, relative humidity and atmospheric pressure. While atmospheric attenuation is strongly positively correlated with call frequency, it is also significantly influenced by temperature and relative humidity in a complex non-linear fashion. Variable weather conditions thus result in variable and unknown effects on the recorded call, affecting estimates of call frequency and intensity, particularly for high frequencies. Weather-induced variation in speed of sound reaches up to about ±3%, but is generally much smaller and only relevant for acoustic localisation methods of bats. The frequency- and weather-dependent variation in atmospheric attenuation has a three-fold effect on bioacoustic monitoring of bats: it limits our capability (1) to monitor bats equally across time, space, and species, (2) to correctly measure frequency parameters of bat echolocation calls, particularly for high-frequencies, and (3) to correctly identify bat species in species-rich assemblies or for sympatric species with similar call designs.

回声定位蝙蝠（echolocating bats）常被用于研究听觉引导行为，同时也是重要的生物指示物（bioindicators）。基于回声定位叫声的生物声学监测（bioacoustic monitoring）方法正日益广泛地应用于风险评估，并最终为蝙蝠的保护策略提供依据。当回声定位叫声以声速在空气中传播时，会因大气衰减（atmospheric attenuation）与几何衰减发生变化。然而，声速与大气衰减均为可变参数，由气象条件决定，尤以温度与相对湿度（relative humidity）的影响最为显著。因此，变化的气象条件会导致分析的叫声参数出现偏差，限制了我们检测并准确分析蝙蝠叫声的能力。本研究利用真实世界气象数据，阐明变化的气象条件对空气声学特性的影响。随后，本研究针对全球范围内的气象条件与蝙蝠叫声频率范围，给出大气衰减与声速的相关数据，以展示二者的相对影响。大气衰减是叫声频率、温度、相对湿度与大气压力（atmospheric pressure）的非线性函数。尽管大气衰减与叫声频率呈显著正相关，但其同样受温度与相对湿度以复杂非线性方式的显著影响。因此，多变的气象条件会对录制的蝙蝠叫声产生可变且未知的影响，干扰叫声频率与强度的估算，尤其针对高频叫声。气象因素导致的声速偏差最高可达约±3%，但整体影响幅度较小，仅对蝙蝠的声学定位方法有实际意义。受频率与气象条件共同影响的大气衰减变化，会从三方面干扰蝙蝠的生物声学监测：其一，限制我们在不同时间、空间与物种间开展均等的蝙蝠监测能力；其二，无法准确测量蝙蝠回声定位叫声的频率参数，尤其针对高频叫声；其三，难以在物种丰富的群落或叫声特征相似的同域（sympatric）物种中准确识别蝙蝠种类。