The bell covers meet the recommendations of Performing Arts Aerosol Study . | Ideal for the bell size around 8 inch, fits for Standard Trombone, Alto Horn, Baritone Saxophone. | The bell covers are made of elastic material and can be easy on and easy off of instrument bells with no pull tabs. | No measurable effect on the sound of the instruments. The bell covers can easily be washed after each use. | The instrument bell covers are double layered, there is a mesh pocket in the back of the bell cover. Removable filters can be inserted for better protection.