Mutes.

A mute is a device used on string instruments, such as violins, cellos or double basses, to alter or dampen the sound produced by the instrument. The main purpose of a mute is to reduce the volume or change the tonal quality of the instrument. 

Practice Mutes - designed for use during practice, these mutes significantly reduce the volume of the instrument, making it possible to practice without disturbing others. They are typically heavier and more robust than other mutes, often made of rubber, plastic, or a combination of materials. Some are entirely placed over the bridge, while others may have a clamp design that grips the bridge.

Orchestral Mutes - these are lighter and typically used in performances where a more subtle, muted sound is required for expressive or stylistic purposes. They are often made of metal or rubber and are generally smaller than practice mutes.

In Tamsta music store you can find a great choice of mutes for string instruments.

https://tamsta.com/lt/465-surdinos

https://www.petzkolophonium.com/shop/lng/en/accessories/mutes/