Transcript Sound
Sound 1 Sound Waves Sound waves travel as compression waves. Another name for compression waves is longitudinal waves 2 Factors affecting speed of sound Temperature – Sound moves through hot air faster than cold air due to the increased speed of the particles. Type of medium – Sound moves well through dense material and material that returns quickly to its original position. 3 Factors affecting speed of sound Thus, sound moves fastest in a solid and slowest in a gas Sound travels more quickly through solids and liquids because the individual molecules are closer together than the molecules in gas Sound waves CANNOT move through a vacuum – they need a medium! 4 Bells in a bell jar… 5 Decibels The decibel (dB) is the unit used to measure sound intensity or loudness. Loudness corresponds to the amplitude of a wave. 6 Decibels Sounds greater than 120 dB can cause pain in human ears. Sounds with an amplitude between 0 dB and 120 dB are called audible. Anything below 0 dB is considered subaudible. 7 Decibel drag races 8 Frequency and Pitch Pitch – how high or low a sound is. High pitch = high frequency Low pitch = low frequency 9 Frequency and Pitch Humans can hear pitches that have a frequency between 20 Hz and 20,000 Hz Pitches above 20,000 Hz are called ultrasonic. Pitches below 20 Hz are called subsonic. 10 Phone Tone Generators 11 Doppler Effect The Doppler effect is a change in the frequency or pitch of a sound that is caused by either the movement of the source or the observer of the wave. Example: The sound from an ambulance siren changes as it approaches the listener 12 Doppler Effect First observed in 1842 by Christian Andreas Doppler The Big Bang Theory - Doppler Costume Video explaining the Physics of the Doppler Effect 13 Resonance Resonance is the vibration of an object at its natural frequency. Example: Windows rattle when the sound from a passing truck matches the window’s natural resonance. 14 Resonance of different things 15 Nodes and Anti-nodes Node – A place where two waves meet and destructively interfere so that the displacement is zero 16 Nodes and Anti-nodes Anti-node – the point of largest amplitude when two waves interfere constructively 17 Music – Natural Frequency A natural frequency exists without any driving source. It is a natural frequency if its waveform has nodes that match up with the ends of the object. The lowest frequency at which this occurs is the fundamental, or the 1st Harmonic. 18 Music - Harmonics Harmonics – a sound wave with a pitch that is a multiple of the natural frequency Overtone – has a higher frequency than the fundamental Octave = ½ or double the frequency of a sound; 8 notes on the musical scale 19 20 Music Consonance – multiple waves combining to form a pleasant sound Dissonance – multiple waves combining to form an unpleasant sound Acoustics – the control of noise and the vibrations that cause noise 21 Toccata Fugue in D Minor 22 Music – open pipe resonators Open pipe resonators – both ends are open Examples: brass instruments, flutes, saxophones It reflects an inverted wave 23 Music - Closed pipe resonators Closed pipe resonators – have one end enclosed Example: pan-flute, blowing across a bottle top, hanging pipes under marimbas, xylophones 24