The Physics of Sound

1. The Physics of Sound By Ryan Waggoner

2. What is sound? • In the form a “wave” • Must be of a “frequency” that is within the range of hearing • Caused by vibration • Can be defined as an “oscillation of pressure through a solid, liquid, or gas” • Not only physical, but neurological • Can be converted into an electronic wave to be modified by transducers such as microphones and speakers

3. How is sound created? • Vibrations. An object vibrates which makes the air molecules around it vibrate and the process is like a domino effect • Sound does not travel as an object itself. • SOUND IS NOT HEAT • You cannot “see” sound

4. We can imagine sound as a 2-dimensional “wave” • The wave contains three parts that we care about: • Amplitude (soft/loud) • Wavelength • Frequency (pitch)

5. The Voice • Using your voice you can show the following changes of a sound wave: • Frequency (different pitches) • Vocal cords vibrate quicker/slower • Amplitude • Voice gets louder/softer • Caused by the amount of air exerted past the vocal cords • Generally controlled by diaphragm

6. The high and low points on the graph respectively show the areas where molecules of the medium are packed more densely or loosely • Audio engineers exploit this to alter sound waves

7. Direction of Sound • Sound can travel through gases and liquids in longitudinal waves • Moves straight ahead • Sound is transmitted through solids in both longitudinal waves and transverse waves • Transverse waves = alternating shear stress at a right angle from where the sound came from.

8. Timbre – what makes a sound unique? • Imagine a soccer ball. • The vibrations of an object that then result in the vibrating of air molecules have the same effect as a kick, making the air molecules vibrate in different ways • Sound bouncing off of a wall generally doesn’t change the sound that much, although it does effect the reverb on sound and how “full” it can be • Not determined by wavelength, frequency, or amplitude

9. Speed of Sound • There is no set speed of sound like there is a speed of light • Determined by the medium through which sound travels • Temperature and pressure play a role in the speed of sound as well STP in air = 343 m/s Fresh water = 1482 m/s Steel = 5960 m/s

10. Harmonics • A harmonic is a note that has a frequency that is an integer multiple of the “base” frequency. • Given a note of a 25 hz frequency, 50 hz, 75 hz, 100 hz, etc. • Basically synonymous with an “overtone” • Can be heard in choirs or bands when instruments play different but harmonic pitches. A missing note of a chord can be heard, for example.

11. Sound and the Human Body • As sound is all about vibrations, those vibrations are taken by the ear drums and converted to “neural action potentials” • Audio engineers must take into account not only the physical properties of sound waves, but also the fact that sound is a sensory and perceptual event • mp3

12. Auto-tune • Audio processing technique that covers up inaccuracies of pitches • Digitally modifies the sound wave to blend to the nearest true semitone (or half-step) • Has much criticism for not being “real”