The “acoustics” in this context mean the vibration pattern through the material of the door, which is what’s being detected.
Where the person knocks, how hard they knock, how fast they knock, the humidity and temperature on that day at that moment in time, all affect the vibration pattern. Plus the fact that different doors and door frames are made of different materials.
Remember that the knock is usually on the opposite side of the door from the sensor. That increases the number of variations you can get as well.
It’s pretty easy to set up a sensor that will catch some knocks some times. Really hard to set one up that will catch only the particular pattern you want and not false trigger for anything else.
OTOH, a smart welcome mat with a pressure mat underneath it will catch most visitors 25 pounds and up with very few false triggers. (They may miss visitors in wheelchairs unless you use a very thin mat and move the mat further back from the door.) They’re basically the same physics as a push button door bell.