Frustrated with interfering echoes, I design my own PAUT calibration block. The holes are spaced far enough apart and the ends tapered such that reference hole and corner echoes are isolated from each other, allowing for a clean sensitivity calibration up to 6 inches of sound path. Not bad for a 3/4" block.
Proud to announce that same little calibration block is now available from the fine folks at PH Tool. Click here.