Simply based on my observations, I believe there are two reasons some feel a detector is "less capable" when nulling takes place. And in my opinion, both have more to do with the user than the machine itself.
First off, I think we all have a tendancy to sweep slower if we are listening to tones opposed to no sound at all. For whatever reason, when we set a group of targets to reject, we are mislead to believe the detector is ignoring those targets. That is not the case. The detector is detecting them. By adding target rejection, we've simply programmed it to not provide an audio response on those targets. This can be proven when you consider the TID still shows ferrous and conductive values on rejected targets. As to why we sweep slower? I suppose human nature tellls us that in order to not "miss anything", we need to register each sound, and decipher them separately. Hearing continued silence does not do that for us.
Secondly, I believe it involoves the user's attention to those changes of sound more than it does the electronic differences of the detector. For example, if you listen very closely with an open screen, you can still hear a very slight lapse in the Threshold sound before the target's audio response is heard. For the sake of discussion, I'll call that a" brief pause of silence". When you pass the coil over a target who's conductive or ferrous properties have been set to reject, it will null. Again, for the sake of this discussion, I'll call that null silence. And if you pass the coil over a target who's ferrous and/or conductive properties have been set to accept and provide a tone, it will provide a tone based on either the conductive or ferrous value of that target. We call that target tone. So let's say you are hunting with some discrimination. You are sweeping along, listening to the sound of the Threshold. When you pass over a target that has been rejected, it will null out. So you have threshold - a brief pause of silence- null silence - another brief pause of silence- back to threhold. When the coil passes over a target who's properties have been set to reject, the lack of any sound (nulling plus entry pause silence and exit pause silence) is longer in duration than if the target you passed over had made an actual tone. Absence of sound is not something that will make you slow down your sweep. On the other hand, if you are hunting with an open screen, every target you pass over will make a sound. Either a tone representing the ferrous properties or a tone representing the conductive properties, depending on which property you have determined you want to hear. If you are running an open screen in TTF, and pass over a target with a ferrous content of 17 or less, that target will make a high tone. If that target has a ferrous value of 18 or higher, it will make a low tone. Regardless of the ferrous content, it makes one tone or the other. Same thing if you were running conductive tones, regardless of the number of tones you chose to listen to. When you pass the coil over a target who's conductive properties have been set to be accepted, you would have threshold - a brief pause of silence - target tone - another brief pause of silence - threshold.
In the TTF setup, I find myself listening to both the high tones and low tones. But having used this setting extensively the past several years, I've trained myself to ignore the smooth sounding low tones. However, if I hear a high tone or even a short bleep of a high tone mixed in with a series of low tones, it causes me to stop and resweep that area from multiple directions. And in areas that I've hunted for years with detectors only capable of detecting a targets conductive properties, I've uncovered many coins that I (and many others) have missed in the past. I believe the vast majority of these targets were missed due to what we commonly call target masking. And these targets were masked because of a sweep speed that was too fast to separate the targets and the nulling effect of running with too much discrimination.
Again, it's just my opinion, based on my habits and observations I've made of others. But I think that listening to a tone, any tone, causes the user to slow down and concentrate more than they would if a good percentage of the targets created a null.
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