: Main Topics vibrations and waves types and effects of waves sound resonance musical sounds

2 Homework: Ch19: RQs: 2, 11, 12, 13, 16, 18. Problem 1. Ch20: RQs: 3, 5, 7, 8, 20, 21, 24. Ch21: Read pp

3 Vibrations back and forth motion regularly repeating cycle time = period of motion, T (seconds) frequency = 1/T (1/s = hertz) amplitude = maximum displacement from non-vibrating position have energy

4 Simple Pendulum mass on light string amplitude of motion is angle of swing “vibrates” when disturbed T, period of motion = cyclic time T is very nearly constant for all oscillations less than ~5 degrees.

5 Waves moving vibrations in fluids and solids wave vibrations have amplitude (A) and frequency (f)

6 Wave Speed speed (v) of waves = length of wave ( ) divided by time (T) for wave to pass by. v = /T but 1/T = f, the wave frequency v = f (a general wave formula)

7 Types of Waves Transverse Longitudinal (e.g. light) (e.g. sound)

8 wave interference waves reaching the same point “add” waves can add constructively to produce higher intensity or, can add destructively to produce lower intensity Examples: noise-canceling headphones, colors on soap bubble.

9 natural frequency lowest frequency an object vibrates with when struck also called “resonant frequency” /

10 Standing Transverse Waves on Strings Nodes (N) are places with zero amplitude Anti-nodes (A) are places with maximum amplitude

11 resonance objects absorb more energy when the frequency of a disturbance hits it at the objects natural frequency example: a guitar string tuned to produce sound of frequency 256 Hz, will resonate when exposed to a sound of 256 Hz.

12 Sound Waves pressure/density waves compressions rarefactions reflects refracts (similar to light) diffracts (bends around corners)

13 sound in air speed increases with temperature at a rate of 0.6m/s per degree C (from 0 to 20C) speed is about 340 m/s (760 mph) at atmospheric pressure and 20 degrees Celsius

14 Sound Speed in gases:~ 300 to 1200 m/s (e.g. air v = T C m/s) in liquids:~ 1100 to 1500 m/s in solids:~ 1800 to 5000 m/s

15 Pitch subjective perception frequency is a measurement Ex. The note A above middle C played on any instrument is perceived to be of the same pitch as a pure tone of 440 Hz, but does not necessarily contain a partial having that frequency. (Wikipedia)pure tonepartial

16 factors affecting frequency length, l string tension, T string density source: Wikipedia

17 Sound Intensity and Loudness intensity = power per unit area “loudness” is the human perception of sound intensity sound level dB (decibels) is a logarithmic scale of intensity

18 hearing “audible range” of humans is defined to be 20 Hz to 20,000 Hz (1 Hz = 1 cycle per second) individual audible ranges vary greatly and depend on age, genetics, and exposure (music, workplace, etc)

19 summary

20 sound quality accuracy with which a device records or emits original sound waves mp3 reduces amount of information in real sounds, can reduce sound quality. quality measured by bit rate in kbits/s “excellent” at kbit/s “very good” at kbit/s, “good” at kbit/s

21 doppler effect the frequency heard by an observer depends on the motion of source and the observer.

22 character of doppler effect let f be the frequency of the source let f o be the observed frequency observer moves toward source: f o increases source moves toward observer: f o increases observer moves away from source: f o decreases source moves away from observer: f o decreases

23 wave speed p.82 7a,b: two crests pass per second  f = 2 cycles/s  T = 1/f = ½ s/cycle 7c: if wavelength = 1.5m and f = 2 Hz, then v = f = (2cycle/s)(1.5m/cycle)= 3m/s 7d: period of wave = 1/f = ½ second