Hidden sounds, because much acoustic communication in nature occurs in the high ultrasound, above your hearing range, or in the deep infrasound, below your hearing range.

隐秘声音，是因为自然界中很多声音的交流都会在高频超声的频段，超过了你的听觉范围，或是在低频次声的频段，低于你的听觉范围。

It also stated that acoustic communication evolved independently in most major tetrapod groups, with origins in the range of 100 million to 200 million years ago.

研究还指出，在主要的四足动物种群中，声音交流进化彼此独立，起源于一亿至两亿年前。