Sound

I. Properties

- Sound Waves - vibrating object moves the air & the moving air vibrates eardrum.

- Sound Waves travel through a medium.

- Air is not the best conductor of sound. Sound can travel farther in water or solids.

- Longitudinal Wave

- Two Parts of the wave, Compression (Molecules close together: Higher Pressure) and Rarefaction (Molecules close together: Lower Pressure)

-Hearing Range: 20 - 20,000 Hertz

- Below 20 Hertz, Infrasonic. Above 20,000 Hertz, Ultrasonic

- Speed of Sound depends on the medium. In air 340 m/s

II. Sound Reflection and Refraction

- Sound bounces off of a surface. If the surface is flat, the reflected angle will equal the incident angle ( This is also true for light). This is the law of reflection.

- The speed of sound depends on temperature. If there is a difference in layers of air or water temperature, the sound wave will change direction.

- changing from one medium to another will also cause the wave to change direction.

- Ultrasound: reflection of waves can locate organs. Moving objects can be seen due to Doppler Effect.

III. Resonance

- Sound Waves, as all waves, transfer energy. The energy is relatively small. ears are sensitive creatures.

- Vibrating object can cause a larger object to vibrate, which will move more air. Sounding board on a guitar or piano.

- Natural Frequency - All objects have a natural frequency of vibration

- Resonance: When an object is forced to vibrate at its natural frequency, the largest amplitude will result and the loudest sound will occur.

- Matched Tuning Forks: Striking one tuning fork can make a second tuning fork vibrate if the natural frequency of the two forks is the same. A property of resonance is that a small amount of energy can cause a large effect.

- Tacoma Narrows Bridge Collapse (movie)

IV. Interference

- When Two waves interact they can constructively add together or destructively cancel each other.

- Demonstration of Interacting Sound Waves (Stereo Speakers in and out of phase)

- Speaker Phasing Cancellation of Bass notes when speakers are out of phase.

- Two sources of single tone sound. Places where things are loud & soft

- Beats: two sound sources produce waves close in frequency. The waves will destructively and constructively add over time causing the sound to become louder and softer. A rhythmic beating sound is produced.

-The Beat frequency is equal to the difference in frequency of the two sounds.

- The Pitch of the sound is the average of the two frequencies.

V. Pitch and Sound Intensity.

- Pitch is related to the frequency of the sound. The higher the frequency, the higher the pitch.

- Intensity is related to the square of the amplitude of the sound wave.

- Hearing Intensity Range: 10^-12W/m² to 1W/m². A very large range.

- Unit of Sound Intensity: decibel. Rated in powers of 10. 0 decibels is the threshold of hearing (10^-12W/m²). 10 decibels, is 10^-11W/m², 20 decibels 10^-10W/m². Increasing by 10 decibels increases the intensity by a power of 10. 60 decibels is a sound intensity that is a million times the threshold of hearing.