A glass pack, sometimes called cherry bomb (a popular brand of glass pack), is a kind of automobile muffler in which the exhaust gas passes straight through the center of the muffler. Packed fiberglass surrounds the exhaust channel and absorbs some of the high-frequency sound. Glass packs are an old, simple, and relatively inexpensive design. They are very effective at reducing back pressure, but not very effective at muffling noise. Thus, they not only make the engine more powerful, they make it sound more powerful.