How sound waves detect faults in girders

[sgstudent]

[PLAIN]http://postimage.org/image/4ure9j3sr/[b]1. Homework Statement [/b]

i have a physics questions about sound Sound waves are used to detect faults in girders. a cro is used to detect these faults, a pulse producer and a detector is placed on opposite sides of the detector.

The safe detector has a upwards spike at 0microseconds and at 10 microseconds.

The unsafe detector has a upwards spike at 0 microseconds and 10 mircroseconds and also one additional but smaller upwards spike at 4 microseconds.

why is the the unsafe detector considered unsafe?

Homework Equations

The Attempt at a Solution

my understanding: there is a crack in the girder so when the sound waves is sent in, some of the waves will have to go around it so it causes it to be slower than the sound waves which do pass the crack. because it is slower, so it is shown in the 4 microseconds. meaning that if another pulse is sent out one of the recorded one will be at 10microseconds from the previous one and another which is 4 microseconds ahead of just detected pulse

the image i drew of the CRO is here http://postimage.org/image/4ure9j3sr/

 

[Spinnor]

I think the spike at 0 and 10 are the reflections from the front and back surfaces of the metal. The spike at 4 are reflections at a discontinuity in the metal, the discontinuity being a internal flaw in the metal. See,

https://www.google.com/webhp?hl=en#...w.,cf.osb&fp=a76f04b08dc68692&biw=734&bih=439

Good luck!