Course:PHYS341/2018/Calendar/Lecture06
Phys341 Lecture 06: Summary and web references
2018.01.15
Textbook: 6.1-6.5
Lecture 6: Resonance and Beats
- Resonance
- In plain language, if you hear a comment that resonates with you, that implies you share some quality with the speaker, and the comment produces a bigger reaction in you than in others who do not share that quality.
- Ditto in vibrating systems
- Possess natural (resonant) vibration frequencies
- Can be excited into motion more easily by a source vibrating at the same frequency
- “Natural” and “resonant” frequency often used interchangeably.
- Sympathetic strings
- Coupled pendula
- Resonance curve
- Graph shows vibration amplitude a of a mass on a spring, driven by a force with varying frequency f.
- Cavity (“Helmholtz”) resonators
- Beer bottles and ocarinas
- Acoustic filters
- Helmholtz resonators have been used to adjust acoustic response and noise output in:
- Concert halls
- Exhaust pipes
- Jet engines
- Beats
- Two tones far apart will either sound as separate tones or merge together into a complex tone
- Two tones too close together to sound different, but not identical will:
- Sound as one for a short while but:
- Periodically the pressure waves will get out of step and cancel each other out
- This beating is occurs regularly with a frequency equal to the difference of the two frequencies contributing
- e.g. tuning a string bowed string instrument (more later), twin-engined aircraft with engines not quite going the same speed (often heard above the skies of Vancouver)
- Beats demonstration
- Two demonstrate on a computer open http://www.szynalski.com/tone-generator/ in two windows and see how the two tones interact with each other.