Sound

2. Sound Sound – when vibrations of an object produces longitudinal, mechanical waves that are transmitted via a medium to a detector… our ears Loudness is related to intensity, which is related to the amplitude of the wave and is measured in decibels (dB) Pitch is related to the frequency of sound Determined by how much of a substance is vibrating more matter in vibration  lower frequency less matter in vibration  higher frequency Ex: Both are considered subjective – depends on our personal impressions

3. Human Hearing Audible Range – approximately 20 Hz to 20,000 Hz • Infrasonic: < 20 Hz heard by elephants, other animals? produced by earthquakes, thunder, heavy machinery, auto focusing cameras neat use: a fire extinguisher! • Ultrasonic: > 20,000 Hz heard by dogs, bats produced by whistles, small insects

4. Human Hearing The Human Ear • Outer Ear – channels noise into ear • Middle Ear – eardrum vibrates by the compression & rarefractions, causing bones (hammer, anvil, stirrup) to vibrate • Inner Ear – cochlea contains fluid & nerves-like hairs that are rooted in the auditory nerve – connected to the brain – each different lengths and resiliency (stiffness), so naturally sensitive to a particular frequency – their resonant frequency…

5. Speed of Sound • In air: v = 331 m/s in dry at 0°C +.6 m/s per °C 343 m/s at room temp (20C) since molecules are moving faster at higher temps also it moves faster if more humidity (water) • In liquids & solids, speed not based on density or temp, but elasticity – how quickly structure of substance returns to the original position/shape ex: water: v = 1440 m/s concrete: v ≈ 3000 m/s steel: v ≈ 5000 m/s

6. 1 million times!! • That’s fast, but no where near the speed of light (300,000,000 m/s)! How much faster is light than sound? 3 x 102 vs 3 x 108 = Examples where you can see/hear this: • Fireworks • Thunder/lightning storm (1 mile away / 5 sec) • Airplane • Baseball game • When 3 Rivers Stadium was imploded 10 miles is about 15 km or 15000 m / 330 m/s = 45 sec delay between see it on TV and hear it “rumble past” outside the windows of my South Park home

7. Reflection of Sound: Echoes Created when a sound wave is reflected from a surface • Hard surfaces reflect more clearly than soft • Wall of rock, side of building • Minimum distance helps too, so that reflected sound can be distinguished from the original • Gymnasium vs Concert Hall vs Sound proof room • Sonar is an application of echoes – when an object is capable of emitting a sound, then receiving back the reflected wave of that sound, and therefore able to know about or “see” its surroundings ex: dolphins, bats, submarines, auto focus on camera, EKGs, ultrasounds

8. Doppler Effect The apparent change in frequency of a sound due to the relative motion of the sound source vs the receiver • Higher frequency if approaching each other • Lower frequency if moving apart Examples: sirens, whistles or horns as they move past Applications: police radar gun, astronomy to study movement of galaxies and spinning stars (red/blue shift)

9. Interference of Sound Waves Examples of • Constructive • Shock Waves and Sonic Booms • Resonance boxes • Standing waves in musical instruments • Destructive • Dead spots in auditoriums Can have constructive uses: • Anti-noise technology • Combination of both • Beats

10. Shock Waves and the Sonic Boom When the object creating the sound travels at or above (supersonic) the speed of sound, the sound waves “pile up” causing constructive interference of many waves, resulting in a huge amplitude, and so a huge sound is created – a sonic boom. Mach # is the ratio of vobj / vsnd Doppler effect At rest Mach 1 Supersonic

11. Shock Waves and the Sonic Boom Shock waves are analogous to the bow waves produced by a boat going faster than the wave speed in water. Shock waves form in 3D, up in the air, by an aircraft. You hear the sonic boom, when the shock wave reaches you. A sonic boom is not a single event, but a continuous noise that reaches different places at different times.

12. Pictures of Sonic Booms

15. Shock Waves and the Sonic Boom Aircraft exceeding the speed of sound in air will produce two sonic booms, one from the front and one from the tail. Actually, the object doesn’t even need to produce its own sound to create one – anything traveling faster than sound automatically makes sound and therefore a sonic boom. ex: bullet tip of a whip even the crack of the tip of a twisted, wet towel…

16. Forced Vibrations: Sound Recall, forced vibrations are the vibration of an object caused by another vibrating object that is nearby • Example: a “struck” tuning fork held to a table top • Application: the sounding boards/boxes on musical instruments like piano, guitar, violin, etc • Neat example: resonance boxes

17. Sources of Sound: Musical Instruments They produce sounds in many ways: • vibrating strings - • vibrating membranes - • vibrating metal or wood shapes - • vibrating air columns – The vibration can be started by: plucking, striking, bowing, or blowing… The vibrations are transmitted to the air and then to our ears.

18. Vibrating Strings: A Guitar The strings on a guitar can be effectively shortened by fingering, raising the fundamental pitch. The pitch of a string of a given length can also be altered by using a string of different density. Uses a sounding box to amplify the sound by putting a greater surface area in contact with air.

19. Vibrating Strings: A Piano A piano uses both methods to cover its more than seven-octave range – the lower f strings are both much longer and much thicker than the higher f ones. Uses a sounding board to amplify the sound.

20. Vibrating Air Columns: The Woodwinds A tube open at both ends (most wind instruments) has pressure nodes (b), and therefore displacement antinodes (a), at both ends. They can produce all harmonics

21. Vibrating Air Columns: Organ A tube closed at one end (some organ pipes) has a displacement node (and pressure antinode) at the closed end. Therefore it can only produce odd harmonics.

22. More Interference: Beats Beats are heard when two sounds of slightly different frequencies are made at the same time, near each other. These 2 wavelengths vacillate between being in phase, in constructive interference, creating louder sounds; and out of phase, in destructive interference, creating softer sounds.

23. More Interference: Beats This pattern repeats, creating a single sound that oscillates loud to soft to loud, etc, over time. And this oscillation has its own frequency… the beat frequency, which equals the difference between the 2 frequencies used to produce it.

24. Similar to the idea of beats • The pattern produced by combs with teeth that have slightly different spacing • Holding hands walking side by side but slightly out of step, so at times you’re together, but sometimes you’re not • Practical application: a piano tuner – uses a different fork to test each key – if she hears beats, it’s out of tune and needs adjusted, if no beats, then that key is fine.

25. Natural (Resonant) Frequency Particular frequency at which an object, once energized, will vibrate. • Minimum NRG is required to initiate and sustain this frequency of vibration. • Virtually all objects have a or a few natural frequencies • Depends on elasticity, shape, mass

26. Resonance When the frequency of a forced vibrations (outside source) matches an object’s natural frequency, a dramatic increase in amplitude is caused, called resonance. ex: matched tuning fork boxes Inadvertent applications: well built structures collapsing