Originally Posted by GRX (Post 7316864)

All four came with them from the factory. They were to help eliminate vibration which was found to not be a major issue. Reason why newer drums do not have them. If your drums are naked, don't sweat it.

Originally Posted by Keith Seymore

The springs are a "damper" to snuff out any noise response of the drum.

In order to perceive any disturbance you have to have three things: a source, a noise transmission path, and an responder. In the example of the interior boom: the source might be an imbalanced driveshaft, the noise transmission path is the air in side the vehicle, and the responder is the vehicle body. In view of this there are three ways to attack the problem: deal with the source (ie, properly balance the driveline), eliminate the transmission path (take all the air out of the vehicle), or change the vehicle body in some way (ie, change the interior volume, brace the interior sides, or provide additional damping on the body).

Let’s try another: a bell. Input: banging on the bell. Noise transmission path: air. Responder: the structure of the bell. Once again the potential solutions could be: eliminate the input – stop banging on the bell. Eliminate the noise transmission path – get rid of the air. Deal with the responder: change the structure of the bell (dramatically stiffer, or dramatically softer – or add damping).

I think you can see that some solutions are constrained by practicality. We wouldn’t want to elminate the air inside a vehicle (or between us and a bell); similary we might be bounded by performance constraints that limit how much you could change the stiffness of a component.

These examples may look trite, but consider how readily they translate into real life: let’s say you have a pickup truck rear brake moan. Using the same thought process: Input: stick/slip brake shoe behavior against the inside of the drum. Transmission path: air. Responder: brake drum ringing like a bell. Since we already determined we don’t want to eliminate the air around us (!) we could attack either responder or the input. In this case adding damping or changing the structure of the drum would be either increased piece cost or a new design to the drum (redesign of validated and proven brake components is typically frowned upon) we chose to go right to the source and address the input. In this case the stick/slip phenomenon was caused by improperly radiused brake shoes which were engaging the drum on the heel and toe of the shoe. Radiusing the virgin shoes so that the initial contact patch was focused in the center (and migrated outward as burnished in) eliminated the problem and cost no additional money.