This new mute is designed by Shinji Hamanaga, the inventor of the Yamaha Silent Brass System. It is so small that it fits completely inside the bell, allowing you to store it inside when the horn is in the case. Incredibly lightweight at 60g (75g for bass trombone), you hardly notice it's there. It plays well throughout the entire range, and is very quite. Though pricey, it's much more comfortable that a larger mute to use for long periods of practice.
The first photo shows both the tenor and bass trombone Warm-Up mutes. The second photo shows them in relation to a typical trombone straight mute.
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Photo below shows Warm-Up Mute on left (and warm-up jr. mute on the right).