Is to place the target on the ground at the site you are hunting, do a noise cancel and see how far you can lift the coil and still hear the target. this is also a good way both to hear and see how to move the coil to get max depth. you will notice that with a slow sweep back and forth and the target disappears, that a quick wiggle of the coil will get you a few more inches, exactly the way you should try and work up a deep iffy one you hear in the ground.. Settings can affect the air test depth depending on ground conditions a manual lowere sens may get deeper than higher, as well as auto, this especially holds true where there are negative hot rocks, and iron mineralized rock. Not sure where you are in NY but we got quite a variety of ground here, from neutral to very high mineralization, from the sound of your other post it might not even be air interference your getting. you might be in an area with the high iron mineralized rock if your getting sputtering high tones in a null.. if thats the case keep it in manual and ruin around 18-20 and try and ignore the sputtering, it will still lock on to the good stuff....Jim