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

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.

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

Let’s recall a few things DEMO STUFF

Strings L Increasing Frequency Higher Tones

The String Frequencies

String Frequencies fn = (n+1) f0

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

OP calculate f

fundamental frequency

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

The Answer …..

Remember the Bar?

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.

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

All objects have multiple “modes” of vibration Transverse Longitudinal

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

What about a drum??

First Mode of Drum BioWaves, LLC.

Drum

More Thumping

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.

Kettle Drum INITIAL Spectrum

The Sonogram

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

So … back to the tuning forks RESONANCE !

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.