Hi - in my experience it is never going to be silent, but I have not had any problems with neighbours and in a half acre it should be fine.

I think the key point is that subsonics remove the sonic "crack" associated with normal bullet flight. This is the noisiest part from an observer/neighbour standpoint - i.e. not in close proximity to the shooter. From this perspective, you often find that neighbours are not even aware of shots being fired.

However, you still get some of the bang from the powder ignition, as well as muzzle blast, but it is "moderated" and is therefore alot easier on the ears and the sound doesn't seem to travel as far.

Of course you still get the noise of the bullet impact, but this travels the least of the three noises described. To my/the shooter's ears, this can be as loud as the muzzle blast bang, but to someone standing a field away, it is probably much less significant.

I'm sure you would get complaints if you turned your garden into a rifle range, but taking shots at rabbits and squirrels has not been a problem for me. The sound carries much, much less than a supersonic .22LR and is far less than a shotgun blast.

Not very scientific, but I hope you get the picture... Also, I'm referrring to sub-sonics in conjunction with a moderator - but I'm not sure what rules Spain has on moderators. If used without a moderator, the sonic crack would be eliminated, but the muzzle blast bang may be too loud for neighbours in close proximity.