Mona is waiting for her friend to give her a ride on his new motorcycle. As her friend pulls up the driveway, the motorcycle's brakes squeal loudly. Mona gets on, and then her friend revs the engine. As the motorcycle accelerates, the engine roars loudly. The high-pitched squeal of the brakes was caused by sound waves, and the equally loud but low-pitched roar of the engine was caused by sound waves