Muted Trumpet.

A trumpet with a mute inserted into the bell. This generates a quieter, muffled sound. As the trumpet played without a mute generates a very loud sound, the mute is sometimes  used in the symphony orchestra, in order to better balance the trumpet against the quieter instruments, especially the woodwinds.

In jazz bands, the mute is often pulled in and out rapidly, to produce a 'wow-wow' sound , a sort of wailing, where it starts quieter and gets suddenly louder.