Tuning, Centre Slotting - The Harmon Trick

It is very important to always pitch each note in the centre of your instruments slot band.
If you pitch a note too low, or too high there will be a loss of resonance resulting in a significant loss of power and tone. Having the right equipment to help with this centre pitch slotting, i.e. a trumpet that has been designed to have well defined slotting, coupled with a balanced mouthpiece, is obviously beneficial, but not a complete solution in itself.
When you need to be in tune with other players; i.e. when playing in a band or orchestra, it is very important not to get pulled out of the centre slot band for your instrument, hence your instrument's tuning needs to be set correctly to enable you to play in tune and on resonance. Often players hear the tuning note they need to match, and their ear locks on to this enabling them to play ‘in tune’ even though the instrument tuning is not set correctly, hence the note can be forced (pulled) off slot centre. Playing like this will significantly reduce endurance, as the player has to work harder at producing a good sound with sufficient power.
A very neat and simple way to help ensure you are pitching in the centre of each note on the trumpet is to use a good quality Harmon mute with the stem removed. With the stem removed, the Harmon mute will only buzz loudest when you play in the centre of the pitch. If you bend the note up or down from centre, the buzz will get quieter, and if you bend it far enough it will stop completely, making a dull sound.
When tuning up at the start of a rehearsal or concert, I recommend you test your tuning in this way, and re-check occasionally, especially with changes in ambient temperature.