When they shot a movie scene, why did they snap the two boards
together after they said “Murder scene 2, Take 4” CRACK! Or
“Love scene 3, take 5” CRACK!

What was that device called?
=====================================================
You can catch this in old movies that included the making of
a movie as part of the story!

The device was called clap sticks and it was attached to a slate.
The slate had the scene name and number written on it.

Movies had the early sound track recorded (actually exposed with
light) right on the film along side the visual picture frames.

Most of us know that the sound track and the picture can get out
of synchronization (sync). We have heard this many times on TV
and at the movies (assuming your old enough! – With digital control,
it has become less frequent.)

When they first edit film from different strips, each strip can have
the sound “image” in a slightly different place on the film with
respect to the appropriate picture frame.

The CRACK from the clap sticks is very loud and very sharp. When
the sound is “exposed” on the film audio track it makes a very
distinct and crisp spike image [which you can see].

First, the film editor looks for the particular picture frame where
the closing clap sticks just touch. Then he looks for the sound
“spike” image on the audio portion of the film track. Since he
can literally “see” the exposed spike sound image he can exactly
line it up with the visual clap sticks! Presto! The picture and
sound for that clip will now be in sync and in sync with each
successive splice he/she sets up!

Some studios put a little switch in the clap sticks that set off a
loud buzzing horn to create the visible sound spike.

Quiet on the set! Hooooooooooonk!