I need a circuit to measure the speed of sound in concrete to 1μs or better. This is for a school demonstration where students who study construction will use this circuit to measure the speed of sound in a sample of concrete to determine the quality of the concrete.

I have two 40kHz transducers: one to transmit the pulse and the other to detect the pulse at the other side of the concrete sample that is about 10cm thick.

I have PIC processors to generate the pulse and then detect the pulse.

However when I look at the many commercial Ultrasonic Concrete Testers on

<http://www.alibaba.com/trade/search?sb=y&IndexArea=product_en&CatId=&SearchText=Ultrasonic+Concrete+Tester>

It appears from their specifications that they are operating with khz transducers, not mhz.
They do not mention frequencies higher then 200khz. Perhaps there is a limitation of
good transmission of sound in concrete to khz frequencies because mhz frequencies are attenuated? 

I have a requirement to build a VERY LOW COST student system, and I can only find inexpensive
40khz transducers. The Mhz transducers that I can find are much too expensive for my requirements. 

It appears from the specifications for the commercial devices that they use pulses of 20us to 20ms
and then wait for a receiver detection before sending another pulse. So the shortest pulse would be just
a one complete 40khz sine wave and longer pulses would be several complete 40khz sine waves. Any distortion
may be unimportant because they are not detecting a narrowband frequency, but just the first rise
of the pulse from the receiver?

Does this make sense to anyone? I mean can anybody help me to solve this problem...

Thank you.