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allowable levels are too high to help residents living near truck <br /> routes. <br /> Whatever levels are achieved, maintenance must play a large part <br /> in maintaining lower truck noise levels . The average diesel <br /> truck will probably run over 500,000 miles in its lifetime. Over <br /> this time period, many of the components will be replaced either <br /> due to wear or to modification for individual operator needs. <br /> Consequently, the noise output of many heavy trucks may increase <br /> significantly from their original condition, particularly if <br /> muffler and tire replacements do not provide noise performance <br /> equal to that of the original equipment. For example, the <br /> difference between a faulty or no muffler and a truck muffler in <br /> good condition is approximately 15 dBA-4 <br /> Tire replacement is also important. Tires fall into three clearly <br /> defined categories as noise producers: pocket retread, cross-bar, <br /> and rib. Loudest are the pocket retread. The quietest tires <br /> are rib designs . Loudness produced by Most tires increases when <br /> the tire is half used. The difference between rib and pocket <br /> retreads may be nearly 20 decibels: 4 times as loud.5 <br /> Operation of the truck also makes a difference in noise levels. <br /> On a level roadway, acceleration produces 5 dBA mare than steady <br /> running conditions-6 <br /> Cars <br /> Exterior levels of passenger cars show that the noise of the <br /> 4-6 <br />