So I have a hard time understanding why the HC-SR04 has a dead-zone of 2 cm.
I understand why, the single-transducer ultrasonic distance sensors have a dead-zone, as they need to switching – But this is not the case with HC-SR04 as it both has a sending and receiving transducer.
Anyway, if this is due to the fact, that the wavelength of 40khz is around 0.8cm and that it needs to produce 8 full impulses (see the datasheet) – Shouldn't it then be 0.8*8 / 2 = 3.4cm?
Best Answer
Probably the physical layout and size of the transducers. Sound gets more directional as frequency rises, the received needs to be able to get the echo. Looks like the transducers are about 3cm apart (judging by the 0.1" connector). The transducers have a polar pattern which is almost certainly the cause of the range restriction.