backyard friendly to me is when the cops don't come.*L*but personally between 70db and under. some townships or areas may have a noise ordinance which could be as low as 50db so it's what they have that counts.
IF I was going to shoot in my back yard and not be heard, I would use either my Daisy 953 or the Daisy 777. These guns are really quite. Especially the 777. I do not shoot my Crosman 2240 because the neighbor 200ft across the street says he can hear it, in his house and I have my garage door down.
Where I live now as long as it is as quiet as a .357 mag I will be fine. I do not care about the neighbors with all the Tejano music.
Yes if your neighbor complains that is not backyard friendly for that person. Mudd what DB would you say those other airguns are?
In my neck of the woods, I'd say anything less than this.