These mutes are extremely popular - they play very well, they quiet the horn pretty significantly, and they are very compact. This mute is in Aluminum, and also available in plastic as the "Warm-Up Jr."
From the manufacturer:
Practice in a hotel room, and warm up right on the bandstand.
The Warm-up reduces instrument output to a whisper, losing approximately -25dB to -35dB. With this high level of sound reduction, professionals have no hesitation in practicing in a hotel room during their tour, and it becomes possible to do long tones virtually silently backstage, or even gear up for that high section in the pit or on the bandstand.
* Even so, please spare a thought in view of the circumstances.
Balanced intonation and a natural air flow!
Top performance among the warm-up/practice mutes.
With Best Brass’s unique acoustic design, the Sound Transformer “Warm-up” has superior intonation and less back pressure compared to other practice mutes. The fact that many professional players choose the Best Brass Warm-up prove its excellence.
About the mute volume and resistance
Be alright on tour!
Extremely light and compact design.
One of the reasons that the Best Brass Warm-up is endorsed by many professionals is that it's very handy. Warm-up has the most compact size of all practice/silent mutes on the market and can be stored in most bells, so it is convenient to carry around. Now, anybody can Warm-up literally anywhere at any time, a most important thing for top players who everyday travel all across the world.