These are low-cost Chinese ultrasonic (sonar) distance sensor. They work by outputting an ultrasonic pulse and waiting for a return pulse (echo). They don’t have a lot of the features of other ultra-sonic sensors such as variable gain on the receiving amplifier. This is a feature where the sensor turns up the gain to listen for increasingly distance echoes.
The HC-SRO4 does not do well at all with softer clothing, similar in that regard to the Parallax Ping, which we have also found lacking when sensing people. They work fine for sensing walls and even jeans or shoes, but may output more noise than more expensive sensors. You can’t beat the price however.