1) You can be right next to the damsel and not hear her whimpers.
2) You can be right outside the door of the room she's in and not hear her mmmmppphh.
3) You can be downstairs (or upstairs) and not hear her cries for help.
4) You won't hear her cries outside the house.
Most people chose 2 or 3, and these are probably the most correct answers. But it is complicated. How thick are the walls? Is the woman tied with her face buried in a thick carpet or blanket or is she tied to a chair where her moans might carry better? Where is the rescuer relative to the damsel? How much ambient noise is there? Etc.
The best a gag can do (without covering her nose) is to reduce the noise the damsel can make below what she can do through her nose. Try it, a lot of noise can be made through one's nose...if the damsel figures it out. Now I heard the chorus of OTN lovers...point taken, but even an OTN gag has its limitations, assuming you want her to continue the beathe. If you want her to breathe, the muting effect of an OTN gag is significantly reduced. Thus, I will only consider non-OTN gags.
So...what is the best gag? FEAR! Nothing need be in her mouth. Consider the real life cases of women/girls kidnapped for long periods and taken into public locations. They are too scared to raise the alarm, as KP-Presents puts it in his stories, because the fear of what might happen overrides the potential reward of rescue. So a rescuer could be feet away and not know the captive they are looking for is there, whether she is bound or gagged or not.
Second on the list is the handgag. I am not a huge fan of handgags (or fear as a gag for that matter). A handgag is a short term solution to what might be a long-term problem. But if the captor is determined, a hand gag can be the most effective physical gag. Pressure on the mouth can be controlled becoming a crushing force, if required. The nose can be pinched at the same time with the same hand to make a point to the damsel. A hand provides a thick OTM gag that can stifle most cries. Combined with the pressure, the damsel's mouth can be well-sealed. Could someone in the same room hear her? Almost certainly, but then there is the fear factor that someone is holding her and what they might do if she did try to make noise. Realistically, if the damsel is well-controlled, I doubt that she would be heard outside the room. Having said that, the handgag is a short-term solution. How long can a captor keep the damsel under control with a rescuer nearby and the desire to escape?
So what is the best solution for a gag that doesn't involve fear or a handgag? A multilayered gag. This would include mouth stuffing, perhaps some sort of cleave gag, and some type (cloth or tape) of OTM gag used together. The cleave gag is optional for me. The stuffing and OTM gag are necessary, IMO. With this type of gag, if done correctly, I think it is possible to keep a damsel quiet enough that they wouldn't be heard outside the bedroom. Certainly someone downstairs would be unlikely to hear anything with this type of gag.
[As a side note, even a small amount of mouth stuffing can significantly reduce the noise made, although a larger stuffing is better. I had better point out the dangers of stuffing the mouth...in real life don't leave the person alone, if you stuff their mouth.]
So what level is next? Stuff gags (without the OTM sealer), ballgags (this includes knotted cloth gags). The problem with these gags is that nothing actually is covering the mouth and a lot of noise can seep out around the edges. Each of these gags, including rubber ballgags, would be greatly improved with either mouth stuffing (yes, even with the ball in the mouth...it is in the front part of the mouth and stuffing is often used with knotted cloth gags) or an OTM gag added. I know, it wouldn't look like a ballgag with an OTM gag over it and ballgag lovers wouldn't like it. Remember, we are talking effectiveness here, not visible appeal of the gags. So, someone downstairs might not hear the woman with a stuff gag or ballgag in her mouth. Certainly, they wouldn't hear her outside.
What about the TV gags...a cloth cleave gag or OTM gag or OTM tape gag. Honestly, if someone is in the house, they will be heard...yes, even the tape gagged damsel...have you ever heard a damsel with her mouth taped cry out on TV or with one of those thin cleave gags? Like almost no gag at all. Yes, I know tape lovers will disagree and with certain types of tape (like microfoam tape, that I have never tried) or different applications (many layers), the gags will be more effective. But the standard duct (duck) tape over the lips isn't that effective. A thick silk scarf OTM (folded with many layers) is a more effective gag...sorry. Of course, the tape is more secure. OTM gags vary so much from bandage wraps to handkerchiefs tied over the mouth that a lot depends on the tightness and thickness of the OTM gag. The effectiveness of a cleave gag obviously depends on the thickness of the gag. (Interestingly, the thicker the cleave, the less secure it is in the mouth and the more difficult it is to knot tightly.)
So is there any point to these TV gags? To a purest, no. But they are useful in reality. A properly applied cleave or OTM probably would prevent the damsel from being heard outside the house. If that is what you need, so what if the gag isn't that effective. It is still doing its job. Also, the damsel might not even realize that she should be able to make noise when she is gagged. Sounds crazy, but she is scared (and has never been gagged before). How would she know that a TV gag isn't that effective. It might take her a while to figure it out in real life.
For fetish bondage, the gag is as much about appeal as it is effectiveness. An ineffective gag that looks good in the damsel's mouth might be better than effective gag that doesn't appeal to the viewer. Some people like that strip of tape over the mouth. Some like the look of a ballgag in a woman's mouth. For me, I like scarf, handkerchief, bandannas and other cloth gags. I like them better with mouth stuffing.
You are welcome to comment. However, please note that I am not claiming that this analysis is absolute. There are many variables that might make one gag better than another in a particular situation. Still, I think that the general conclusions are valid.
As Dennis Miller used to say, "That's just my opinion. I could be wrong"