1. Things that make strange noises. October 27, 2004
Lecture 16
Things that make strange noises.
October 27, 2004

2. What’s Wherefore Exam is distributed.
I normally do NOT curve grades and won’t again, but in this case I think I made some mistakes so I fixed them.
You gan get your “grade” from the syllabus.

3. Today Let’s recap some of what we learned. In front of us this week
Sounds
Objects
Tones
Music
In front of us this week
other instruments (very brief)
sounds …….
then on to
hearing
room acoustics
electricity
microphones
speakers

4. Let’s recall a few things
DEMO STUFF

5. Strings L
Increasing Frequency
Higher Tones

6. The String Frequencies

7. String Frequencies
fn = (n+1) f0

8. Sound From the Guitar
t=0.005 s t=0.012 s

9. OP
calculate f

10. fundamental frequency

11. Spectrum
Expect: 141,282,423,564
Huh??

12. The Answer …..

13. Remember the Bar?

14. Some Facts All objects resonate at some frequency.
Objects can resonate at different frequencies depending on the structure and the kind of wave we are considering.
A musical sound is something that is “periodic” in time … it repeats itself f times per second with a period of 1/f.

15. More Bar

16. This is an example

17. Another

18. Fourier’s Theorm Any periodic function can be expressed as a sum
of “sine” waves just like the one above.

19. Example

20. This can be a sum of sine waves!

21. Corollary
Any vibration can be formed by a sum of the appropriate sine waves.
Strange stuff!

22. All objects have multiple “modes” of vibration
Transverse
Longitudinal

23. When we hit an object
All modes of vibration are usually excited at the same time.
Some die out quickly
Others are sustained for a longer time.
A guitar string has lots of modes all of which are multiples of a fundamental frequency so the tone is harmonic.
R rock through a glass makes a sound which is clearly NON-HARMONIC

24. What about a drum??

25. First Mode of Drum
BioWaves, LLC.

26. Drum

27. More Thumping

28. The Drum Each of these modes are usually excited.
The tension of the drum determines the frequency of each mode.
The modes may NOT be harmonic
Each mode dies out at a different rate.
The player can change the basic “tone” of the drum by changing the tension of the drum head.

29. Kettle Drum INITIAL Spectrum

30. The Sonogram

31. Modes All modes are excited at first strike.
These vibrations may excite others … resonance.
Each mode decays in a different time.
Amplitude
time

32. So … back to the tuning forks
RESONANCE !

33. Objects will resonate when
They are in contact with something that vibrates at its resonant frequency.
Buzz in cars is a good example
Sound can cause resonance if it is at a frequency that is the resonant frequency of another object nearby.
It must have enough energy.