Yep. Because the Nox runs on multiple frequencies by default, your chance of picking up EMI is greater. But, different EMI is detectable at different frequencies and sometimes you can luck out and find that it's not on one of the frequencies you can select on the Nox.
But, if you're going for deep silver coins, and the EMI is tolerable at a lower frequency with a lower sensitivity, then that's going to be better than 40k. 40k is best for smaller, shallower targets like gold earrings, chains, etc... It's more susceptible to ground mineralization, so you'll want to regularly balance or using auto tracking.
You're just trying to get the EMI down to a tolerable level where you can hear repeatable good signals....you might not be able to eliminate it entirely without reducing sensitivity to ineffective levels. In that case, you're better off going to that 40k frequency and perhaps focusing on a different type of target.
The point I'm trying to make is that sticking with multi-mode, running a lower sensitivity, hearing a little EMI, and going a bit slower could be a better trade off than 40k at a higher sensitivity. As far as finding deeper coins anyway. Really, if you're trying to hit coins at 6 inches or less in a fairly trashy park, then there's no reason to have sensitivity on full blast anyway. It's counter productive.
You can notch out all of the non coin numbers to silence some EMI. You can even quiet down EMI by adjusting recovery speed and iron bias to ignore quick high tone squeaks. The problem with those methods, however, is that the detector is still getting bombarded by EMI and having to process it alongside potentially good signals coming from the ground. You're just not hearing it. The detector does and it's competing with and most likely masking good signals.
Ideally, you want to silence the EMI by avoiding it in the first place. Lowering sensitivity, smaller coil, and/or running a frequency where it's not picking it up, are all solutions where you're not just silencing EMI....your detector doesn't have to process it at all.
