这个实验室的墙壁经过特殊处理，形成了一个完全无回声的环境。

The lab walls were specially treated to create an ___anechoic environment.

超声波检查发现了一个无回声的区域，医生怀疑可能是囊肿。

The ultrasound revealed an ___anechoic area, which the doctor suspected might be a cyst.