Ultrasonic detector - Blind zone

[nForce]

I would like to know why is there a Blind zone in ultrasonic detectors? I know what is Blind zone, but I don't know why is there. (Physics)

Thanks.

[StillTrying]

Minimum sensing distance ?
Here's the audio from a cheap ultrasonic distance measurer, at about 40 cycles of 40kHz the pulse is about 32cm long in the air, so using the same sensor for TX and RX it can't measure less than half the pulse length 32/2 cm, because the echo starts to arrive while it's still transmitting the pulse and not yet listening/receiving.
I've no idea why the 40kHz pulse is that shape.

[nForce]

Ok, but why can't we sense less then some amount of length? Why is there a blind zone?

[ogden]

Ok, but why can't we sense less then some amount of length? Why is there a blind zone?

You measure time delay between chirp and it's echo, by measuring silence period between two. When echo arrives BEFORE end of the chirp, there's no silence between two.

[Berni]

Ultrasonic distance sensors tend to use the same piezoelectric element as both the transmitter and receiver.

To transmit with it a large AC voltage is applied to it making it flex and act as a speaker to create sound, then quickly the AC voltage is removed and a amplifier listens for voltage across the terminals. Some time later the echo sound comes back and the vibration from it flexes the piezo element causing it to act like a microphone and create voltage on the terminals.

So obviously there is a minimum length of transmitting time in order to create a loud enough sound, during that time you can't use the piezo element as a microphone since you are forcing voltage into it. So if a echo comes back before you are done transmitting the sound you will not be able to hear it.

To somewhat get around this a separate transmitting and receiving element has to be used, but even then care must be taken so that the receiver does not directly hear the transmitter next to it and interpret that as an echo. If you need it to operate at short distances then design your system for it, or use a different technology all together.