Pitch and the Doppler Effect

Last week in class Professor demonstrated the dropping of different rods of metal to show the difference in the pitch made by their clanking with the ground.

Though some people may know that the different sounds were created by different pitches, other may not understand what is causing this pitch to be produced, and what makes one pitch higher than another.

As humans we have the ability to distinguish different pitches, these pitches are associated with the frequency of the sound wave that impinges upon the ear.

As we know from class, frequency is defined as how often the particles of the medium vibrate when a wave passes through the medium. Since waves move longitudinal, the frequency is the up and down motion of the wave. Now that we know what frequency is we can determine which type of frequencies create what type of pitch.

A higher pitch is created by a higher frequency, this means that the there are more up and down motion of the wave per second. Opposingly, a low frequency causes a lower pitch. The reasons why the metal rods depend not only upon the metal of which they are made, but, also their length, diameter, tension, and density. The density of the metals, since all were relatively same length, diameter, and tension is the main causes for the difference in pitch for this scenario. It is important to remember that dense molecules vibrate at slower speeds; the more dense the metal, the heavier its molecules, thus the slower their vibrational speed and lower their frequency, and a vice versa for a less dense metal.

Now, lets think talk a little about the doppler effect, this phenomena explains why Professor had some trouble distinguishing which rods were making which sound when dropped away from him in the hallway. The Doppler Effect explains that if a sound is getting closer to you, either because its source is approaching you or because you are going towards the source, the sound will seem higher than it really is, adversely, if you are heading away from a source or it is going away from you, the sound would seem lower than actual pitch. The Doppler Effect happens because distance directly effects the amount of time that it takes you to hear a sound. A shorter distance will make a pitcher seem higher than it actually is, and a far distance will make the pitch seem lower than it actually is. This is why when Professor Bell dropped the a certain ride in front of lecture the pitch was so high that it hurt our ears, but it was a bearable sound when Brian dropped the same rod farther away from us, in the hallway.

Though there may be some humans who have amazing hearing, they still will not be able to override the effects of the doppler effect. Since frequency/time creates the pitch, the pitch will always change with distance variation, whether it be us getting farther or closer to the source emitting the sound - like us walking towards a stand still tornado siren, or the sound source moving farther or closer to us - like that of a moving train horn, or a moving ambulance with sirens on. Sound, as well as sound waves are a tricky concept, but with the right tools and definitions they can be easily understood. 



http://www.ndt-ed.org/EducationResources/HighSchool/Sound/frequencypitch.htm

http://www.physicsclassroom.com/class/sound/Lesson-2/Pitch-and-Frequency

http://www.ndt-ed.org/EducationResources/HighSchool/Sound/dopplereffect.htm