what causes that hammer on wood sound when you score a free game in pinball?

[Hoagie_one]

i've always wondered what theyhave inside that gives that loud thunderclap sound when you score a free game?

[HaRuMaN]

Me, too... always makes me jump when I am playing and get the replay.

[Witchboard]

It's called a knocker.  They also used a knocker in Q*Bert when you or the snake would fall off the... whatever it is you were on.

[RayB]

It's a solenoid. Same thing they use to pop the ball out of a pinball kick-hole, or to knock the ball around when it hits a bumper.

I believe it's basically an electro-magnet that when turned on causes an armature to shoot forward. When power is off, the armature moves back.

They use this and make it knock against the wood of the cabinet.

[OSCAR]

Ray is correct, it is a standard pinball knocker assembly.  When the coil is energized, the plunger strikes a plate mounted inside the backbox.  Here is a shot of my Creech, the knocker coil located in the upper left of the backbox and is yellow.

[Hoagie_one]

cool, i learned something

[fredster]

Yeah, it's the same type of coil that runs the pop bumpers in my Genesis.  Mine's mounted under the ball plunger.

[SirPoonga]

Technical term is solenoid.

It's an electromagnet with a pin.  When a current goes through it pushes or pulls the pin.