Lecture 16 Things that make strange noises October
Lecture 16 Things that make strange noises. October 27, 2004
What’s Wherefore n Exam is distributed. n I normally do NOT curve grades and won’t again, but in this case I think I made some mistakes so I fixed them. n You gan get your “grade” from the syllabus.
Today n Let’s recap some of what we learned. – Sounds – Objects – Tones – Music n In front of us this week – other instruments (very brief) – sounds ……. – then on to § hearing § room acoustics § electricity – microphones – speakers
Let’s recall a few things n. DEMO STUFF
Increasing Frequency Higher Tones Strings L
The String Frequencies
String Frequencies fn = (n+1) f 0
Sound From the Guitar t=0. 005 s t=0. 012 s
OP n calculate f
fundamental frequency
Spectrum Expect: 141, 282, 423, 564
The Answer …. .
Remember the Bar?
Some Facts n All objects resonate at some frequency. n Objects can resonate at different frequencies depending on the structure and the kind of wave we are considering. n A musical sound is something that is “periodic” in time … it repeats itself f times per second with a period of 1/f.
More Bar
This is an example
Another
Fourier’s Theorm Any periodic function can be expressed as a sum of “sine” waves just like the one above.
Example
This can be a sum of sine waves!
Corollary n. Any vibration can be formed by a sum of the appropriate sine waves. n. Strange stuff!
All objects have multiple “modes” of vibration Transverse Longitudinal
When we hit an object n All modes of vibration are usually excited at the same time. n Some die out quickly n Others are sustained for a longer time. n A guitar string has lots of modes all of which are multiples of a fundamental frequency so the tone is harmonic. n R rock through a glass makes a sound which is clearly NON-HARMONIC
What about a drum? ?
First Mode of Drum Bio. Waves, LLC.
Drum
More Thumping
The Drum n Each of these modes are usually excited. n The tension of the drum determines the frequency of each mode. n The modes may NOT be harmonic n Each mode dies out at a different rate. n The player can change the basic “tone” of the drum by changing the tension of the drum head.
Kettle Drum INITIAL Spectrum
The Sonogram
Modes n All modes are excited at first strike. n These vibrations may excite others … resonance. n Each mode decays in a different time. Amplitude time
So … back to the tuning forks
Objects will resonate when n They are in contact with something that vibrates at its resonant frequency. – Buzz in cars is a good example n Sound can cause resonance if it is at a frequency that is the resonant frequency of another object nearby. – It must have enough energy.
- Slides: 34