Properties of sound

1. Properties of sound • Longitudinal waves travel at different speeds depending on the medium (air @ 25oC 346m/s, water 1490 m/s) • Denser the medium, the faster it travels, energy can transfer faster through molecules closer together. • Loudness of a sound • depends on intensity (the amplitude of the wave. Higher amplitude … louder sound) • Measured in decibels (dB), talking is about 50 dB over 120 dB is painful.

2. Loudness/intensity

3. Properties of sound • Pitch (high or low) is related to frequency. Higher frequency = higher pitch • (Also related to wavelength, indirectly. Bigger wavelength = lower pitch) • Movie: pitch • Humans hear between 20 Hz and 20,000 Hz. Below that range is infrasound, above is ultrasound.

4. Musical Instruments • Rely on standing waves to make sound … only certain wavelengths • Fundamental frequency:the lowest freq. standing wave made by the instrument. It can be determined by the size of the instrument itself. • String instr.: the wavelength of the fundamental frequency is twice the length of string. • Changing the size of the string changes the wavelength and the frequency. • Movie: fundamental freq.

5. Musical Instruments • Wind/brass instr.: change size of wave by opening closing holes, changes size of tube that standing wave is made in. • Wavelength is related to size of the tube. • In an open tube, you get ½ the wavelength, so multiply by 2 to find the fundamental frequency • In a closed end tube, you get ¼ the wavelength, so multiply by 4 to find the fundamental frequency

6. Musical Instruments • Strings make ½ wave, so mult. by 2. • Open tubes make ½ wave so mult. by 2 • Closed tubes make ¼ wave so mult. by 4.

7. Musical Instruments • Most instruments vibrate at the fundamental frequency plus other standing waves…called harmonics. • That’s why everyone’s voice sounds different and why instruments sound different when they play the same note. • Vibrations can cause other things to vibrate at same frequency … called resonance. • Movie: resonance • Waves interfere, called beats

8. How we hear • Outer ear focuses waves, passes them to ear drum (middle ear) which passes them to three bones (hammer, anvil, stirrup) to the cochlea (inner ear) which resonates at certain frequencies. • Hair cells pick that up and transmit to brain.

9. Ultrasound and Sonar • Use speed of sound in medium and time to figure out how far away something is (e.g. bottom of ocean floor) • Movie: sonar • Ultrasound...frequencies beyond our hearing used in medicine (babies).

10. Light: Waves and Particles • Like a wave … light shows interference (reflect, refract, diffract, standing waves) does not require a medium • Like a particle, light can move electrons • So light is described as having a dual nature • Comes in “packets” called photons(more like bundles of energy) • Amount of energy directly proportional to frequency. Higher freq. = more energy. • Indirectly proportional to wavelength. Bigger wave = less energy • Speed of light 3 x 108m/s in vacuum • Speed changes for medium, but not significantly

11. Electromagnetic spectrum • all possible kinds of light, classified according to frequency and wavelength • Radio/TV Waves…lowest energy, used in communication • Microwaves…used in cooking and communication • Infrared Waves… felt as warmth • Visible light…we can see ROYGBIV • Ultraviolet waves…cause sunburn • X Rays… used in medicine • Gamma Rays… used in medicine • Stars, like the sun, give off all of these forms of energy

12. Light Rays • use lines called raysto see where light goes • rough surfaces reflect the rays in all directions • smooth surfaces reflect the light in one direction • the angle light rays come in at equals the angle it’s reflected at. • Angle of incidence equal the angle of reflection (Law of Reflection) • Angles are measured between the ray and an imaginary line perpendicular to the surface called the normal. • movie: Law of Reflection

13. Mirrors • Plane (flat) mirrors form virtual images inside of the mirror. Virtual imagesare from where light appears to come from, not the actual path. • Curved mirrors distort the image, but make realimages. Real imagesform when light rays come together at a certain point • movie: comparing real and virtual images

14. Color • Light hits objects. • Many wavelengths are absorbed by the object and not seen by you. • Wavelengths that are reflected are seen by you as color. • Technically, you don’t see something, you see light bouncing off of something. • Colors can add together to make new colors (e.g. light. White light = all colors of light, black = absence of light) • Colors can subtract to make new colors (e.g. pigments, black paint = all colors of paint, white paint = absence of color) • movie: additive color mixing

15. Refraction • Light bends as it goes from one substance (medium) to another because it changes speed • Compare the bend to the normal, just like reflection • Light bends towards the normal when it enters a denser medium and slows down • Light bends away from the normal when it enters a less dense medium and speeds up

16. Lenses • Mirrors and lenses can be curved out (convex) or can curve in (concave … like a cave). • movie: converging anddiverging lenses

17. Light Rays • There’s an angle called the critical angle, where light will not go back out of something and just keeps bouncing back around inside. • Diamonds are cut like this to keep the light from leaving and fiber optic cables are made so that light does not escape. • movie: Total Internal Reflection