Brakes most often utilize slots, vents, holes, baffles, and similar devices. Slosh baffles are also mounted on this bulkhead. The metal tube was divided into waterproof compartments with vertical baffles. One popular technique is to have alternating angled surfaces through the stack of baffles. The PBS-1 is a two-chambered silencer using baffles and a rubber wipe. More advanced designs use baffles and expansion chambers to slow escaping gases. Baffles and ends can be riveted into place. Excellent acoustics are achieved by a system of hardwood baffles along the walls and ceilings. These boilers usually incorporated additional feedwater heaters in the updraught above these baffles. Many fuel tanks' baffles (particularly in aircraft and racecars) contain lightening holes. When stripped of baffles and carpets, it serves as an echo chamber during mixdown. The chambers are divided by either "baffles" or "wipes". The armor is typically made from ceramic ballistic plates and aramid baffles. Pliny wrote that a diamond baffles poison, keeps off insanity, and dispels vain fears. It took months to discover that effective baffles must contain a layer of air.