As a firetruck flies by our classroom, the Doppler Effect is evident in the sound waves that it produces. The Doppler Effect is an increase or decrease in a waves frequency, in this case sound waves, as the source and viewer move closer and further from each other. Us, as students, are the viewers of the sounds that the sirens on the firetrucks who observe this change in frequency. The Doppler Effect explains why the sound that the sirens make gets louder when the sirens are closer and then slowly get quieter and quieter until we can no longer hear them. The highest frequency, and therefore sound, that the observer will hear is when the sound is closest to you. The same effect happens with any kind of car that passes you, but the sirens are most obvious because they are the loudest.