I wanted to add a video here of the expected results of this before describing it, as I want to demonstrate it first. The first time I witnessed the results of this, I nearly wet myself laughing so hard, and soon after it "went off", those that had "set it off" also were laughing at the novelty of this. Happiness all around :-)
So, with out further ado: Imagine you are coming up onto the porch of front stoop of your friend's house. You reach up to ring the door bell and this is what you hear (play video):
<I am sorry, I can't get my video converter to work, and YouTube is not rendering my video properly at all >
This one is audio only, and poor at that. My mic does not seem to be sounding so good lately....
Step 1: Here Is All You Will Need....
one door bell unit (minus the bell)
one step down transformer (if necessary because of voltage difference of door bell and digital play back module); OR a relay
Depending on the voltage of your doorbell, you may need to regulate it so you don't fry your voice recorder/player.
Some solid and hollow pieces of wood, or door to create your "sound" from. My wife's desk, inside the drawer, was the perfect "hollow" sound chamber for a "knocking" effect (pictured below).
Step 2: Creating the Joke
Set up your recorder/player to record.
Finding the best and loudest sounding "knock" from your wood, or door, record a good solid 4 knocks on the recorder (knock knock, knock knock).
Now, play it back to make sure this is fairly loud. If it is loud enough to hear, while standing outside the front door, while being played just inside the front door, we have success (if it is not loud enough, one may need to add an amp before the speaker, and possibly a speaker also with greater capacity) .
Wire this all up to the existing door bell unit, using either the step down transformer (and any voltage regulation you may need), or the relay (make sure the regular "bell" is not connected).
Now, we are ready for the final step.
Step 3: This Is Best If You Can Observe the Person "ringing" Your New Door "bell"
Now, think of this.
Imagine going up to a person's door, and when you "ring the bell" you get a Knock, Knock, Knock sound ?
Can you imagine how startled people will be at first ? :-)
It is really overly simple in it's application. But the effect is awesome (I think, but then, I have a fairly ODD sense of humor :-).