Here are a few hints on filing the corks on straight mutes and fixed-cup cup mutes: like the non-adjustable cup mutes of MuteMeister, Humes & Berg, and Ray Robinson, for example. These are as opposed to adjustable-cup cup mutes such as the Shastock Tonalcolor adjustable cup mutes, and the like where the filing of corks is not necessary.
It's crucial that players adjust (file) their own corks for proper slotting of their partials in all registers. As brass instrument bell-chokes differ greatly from model to model, straight mutes and cup mutes ideally need to be adjusted for the specific instrument the player intends to use the mute with.
As many brass instrumentalists already know, once the low F# on a trumpet (or the equivalent note on a trombone, etc) slots well, a straight mute or cup mute will slot correctly in all other registers. Once the low F# slots, it remains there.
With a straight mute, simply file the corks (thus bringing the straight mute further into the bell) just until the low F# locks in nicely. Instructions on exactly how to file these corks is below in the "Filing Details" section.
On cup mutes, this difference in bell-chokes will affect how closely a cup mute rim will come to the edge of any given brass instrument bell. Obviously, what we're trying to avoid here is the cup coming in too close, causing the rim of the cup to rest up against the edge of the bell. What may be a correct cork adjustment on a cup mute for one bell may not be right for another (even with bells on brass instruments of the same make and model).
The new MuteMeister Cup Mute and Ray Robinson Cup mutes, for instance, were designed to be played very close to the bell. This is one of the factors that give them their unique sound. Other fixed-cup cup mutes may be adjusted a little further away from the edge of the bell depending on the quality of sound the player prefers.
On a cup mute, while making your cork adjustment, you can bring the cup rim even closer to the edge of the bell after the low F# locks in, if you'd like. The only thing that will prevent the low F# from slotting on a cup mute is if the corks have not been filed down far enough thus preventing the cup rim from coming close enough to the edge of the bell. In other words, file away until you find your "sweet spot."
Most brass players already know how to file their mute corks, but I'll just go ahead and write this anyway: I suggest using a good 1/2" wide X 6" long flat medium coarse steel file to file your corks with. If you don't already have one, the file can be purchased at any well-stocked hardware store for around $6.
Incorporate diagonal filing strokes to the vertical placement of the cork (at approximately a 45 degree angle) when adjusting your corks. Stop and test your mute intermittently during the filing procedure to listen for that low F#. File your corks just until the low F# (or equivalent) locks in nicely for the minimum degree of correct cork adjustment.
Use an air compressor (or canned air) to blow the cork dust out of your cup when you are finished filing a cup mute, especially if your cup mute is lined with felt or "flocking."